Index


Video broadcasting technology page

    Broadcasting

    Nowadays, television broadcasting is an essential access point to information, culture and entertainment. Broadcast stations use a powerful antenna to transmit radio waves to the surrounding area. Viewers can pick up the signal with a much smaller antenna. The main limitation of broadcast television is range.

    The radio signals used to broadcast television shoot out from the broadcast antenna in a straight line. In order to receive these signals, you have to be in the direct "line of sight" of the antenna, or pretty close to it. Small obstacles like trees or small buildings aren't a problem; but a big obstacle, such as the Earth, will reflect these radio waves. To get a perfectly clear signal like you find on cable, you have to be pretty close to the broadcast antenna without too many obstacles in the way.

    Television transmissions normally occupy frequencies from about 50 Mhz to over 800 MHz. Television broadcasts do not use this whole frequency range (there are also FM radio, some radio communication and some cellaular communciation on that frequency range on their own bands).

    Practically all analogue TV broadcasting system use interlacedscanning to save video bandwidth. In broadcasting, the most important one is usually bandwidth, meaning that a change to non-interlaced scanning ("progressive") would mean halving either theframe rate or the number of lines.

    Practically all analogue TV broadcasting systems use a modulation called vestigial sideband (VSB). Vestigial sideband is an AM signal with most of one sideband filteredout to save bandwidth (all you need is the carrier and one sideband to recover the video). This is how broadcasters do this. Simple TV modulation devices usually just use simple AM modulation (it is easier to leave a "Vestige" of the other sideband to prevent extra cost and possible phase shift from needs to be cheap filter affecting the signal quality).

    Here are many different TV transmission systems for terrestrial TVbroadcasts in use. The three main broadcasting standards are:

    • NTSC: This is what is used in USA
    • PAL: This is used in Europe
    • SECAM: This is used in France and some Eastern European countries

    Plase note that those mentioned standarda re video signal standard. They define the video signal itself, but does not define things like broadcasting frequencies and such. There are several variations those general broadcasting standard in use. Many countries used to have years ago have some slight country specific modifications to the systems for various reasons (technical compatibility with some existing system, protectionism, marginal technical improvement etc.).

    For example PAL has no direct connection with broadcastfrequencies (channels) world-wide. Any given hand-held TV receiver you buy locally may or may not receive both picture and sound in the various countriesyou intend to visit because, PAL or not, the local stations may broadcast onfrequencies your receiver cannot receive... and with sound mixed with picture inways your receiver cannot decode. Note that there are about half a dozen variations on "PAL", mostly being differences in things like sound carrier frequency. Do not assume that a PAL tuner/receiver will work correctly everywhere. The receivers designed for different variations have somewhat differently designed electronics in them.

    Although nothing is impossible, modifying a table-top VCR tuner (and the associated demodulating circuitry) to handle different signal standards (PAL,SECAM, NTSC) and different channel assignments (frequencies) and different audio modulation schemes is not going to be easy. There are some multi-standard devices though on market that can be set to work with more than one broadcasting standard. If you want to go from broadcasting standard to another, you generally need to demodulate the video signal, the convert it and modulate it with a new standard modulator. There are standards-converting boxes (PAL-SECAM-NTSC) out there that can do video standard conversion (do your web search and you will find converter products/manufacturers).

    The first color TV broadcast system was implemented in the United States in 1953. This was based on the NTSC (National Television System Committee) standard. NTSC is used by many countries on the American continent as well as many Asian countries including Japan. NTSC runs on 525 lines/frame. For example NTSC TV transmissions in USA use 6 MHz channel bandwidth. The analog RF is 4.2MHz vestigial sideband + 25KHz FM aural carrier for a channel of 6 MHz.

    The PAL (Phase Alternating Line) standard was introduced in the early 1960's and implemented in most European countries except for France. The PAL standard utilises a wider channel bandwidth than NTSC which allows for better picture quality. PAL runs on 625 lines/frame.There are several commonly used PAL versions in use:

    • PAL System B/G: This is used in most Western European countries, 4,43 MHz color subcarrier, 5,5 MHz audio subcarrier, 7 MHz channels spacing in VHF and 8 MHz channel spacing un UHF
    • PAL System I: This is used in Great Britain, 4,43 MHz color subcarrier, 5,5996 MHz audio subcarrier, 8 MHz channel spacing in UHF
    • PAL System N: This is used in Argentina, 3,57 MHz color subcarrier, 4,5 MHz audio subcarrier, 6 MHz channel spacing
    • PAL System M: This is used Brazil, 3,57 MHz color subcarrier, 4,5 MHz audio subcarrier

    The SECAM (Sequential Couleur Avec Memoire or Sequential Colour with Memory) standard was introduced in the early 1960's and implemented in France. SECAM uses the same bandwidth as PAL but transmits the colour information sequentially. SECAM runs on 625 lines/frame.

    All RF-TV channels have a designated 6MHz bandwidth containing the separate video and audio RF carriers. The video carrier is amplitude modulated (AM) by the composite video signal using negative sync modulation. This means the sync tips produce maximum carrier amplitude (0% modulation) and the white peaks produce minimum carrier amplitude (87.5% modulation) (those modulation figures apply to NTSC signals). The video modulation is AM vestigial sideband. Full sideband modulation produces sideband frequencies above and below the RF carrier frequency ranging from 0-4.2MHz (this applies to NTSC, somewhat more for PAL signal). The upper and lower sidebands would equally contain the full 4.2MHz luminance and chroma signal information covering over 8 MHz bandwidth. Vestigial sideband operation limits the lower sideband to approximately .75MHz below the carrier frequency and permits the full 4.2MHz above the carrier frequency.

    The audio carrier is frequency modulated (FM) by the composite audio signal, which may include multi-television sound signals (MTS). The RF carrier is deviated from its resting frequency by +/- 25 kHz producing sidebands ranging from approximately 0-200 kHz above and below the carrier. In NTSC system the RF channel the video carrier is positioned 1.25MHz above the lower edge of the channel. The color subcarrier is positioned 3.58 MHz above the video carrier. The FM modulated audio carrier is positioned 4.5 MHz above the video carrier. PAL broadcasts use the same idea, but somewhat different subcarrier frequencies (4.43 Mhz for color, usually 5.5 MHz for sound etc.).

    Real life example in NTSC world: TV or cable channel 3 has a 6 MHz bandwidth from 60-66 MHz. The video carrier is positioned at 61.25 MHz with lower sidebands limited to ?1.25MHz and upper sidebands extending +4.2 MHz above the carrier (65.45MHz). The color subcarrier is positioned +3.58MHz from the video carrier (64.83MHz). The I color sidebands extend 1.3 MHz below the color subcarrier frequency and the I and Q color sidebands extend +.5 MHz above this frequency. The audio carrier is positioned +4.5MHz above the video carrier or at 65.75MHz.

    TV transmitters needs lots of transmitting power to be able to transmit enough signal to the large coverage area. Analogue TV signal is wide bandwith signal that can easily pick up noise interference, so the actual TV signal that the antenna picks from air needs to be pretty strong to be able to be recevied without too much visible noise on it (when broadcast signal gets weaker as you move more away from transmitterm, you see more noise and less actual picture). For analog signals, the recommended level at the receiver input is more than 1 mV (+60 dBuV, -49 dBm). A typical common antenna systems in Europe aim to provide signal in +60-80dBuV (1-10mV) signal level to the receiver.

    The typical TV transmitter has power of tens of kilowatts or more. The cost of electricity is significant cost in the operation of the transmitter. The published transmitter rating is ERP (effective radiated power), which is the power that the station would have to run to produce the same signal strength using a non-directional transmitting antenna. Because of antenna gain (transmitting a focused beam), the actual transmitter power is much less than the ERP. At UHF, it can be on the order of 1/20 the ERP or even less. This is offset somewhat by the fact that the transmitter is not 100% efficient, and there is some loss in the transmission line. But a 100KW ERP UHF station is typically running around 10KW transmitter power. It's cheaper to buy a big antenna once than pay the power company each month. Many low power and medium power TV transmitters are air-cooled with blowers. VHF TV transmitters are generally air-cooled (even up to 50kw). Very high power transmitters require water cooling and a lot more acreage for heat exchangers, a radiator and huge electrical power supply. Klystrons are special cavity vacuum tube transmitting devices used in high pewer radio and TV transmitters. Klystrons commonly used in Television work today typically have 4 to 5 cavities. Each cavity is individually tuned, and electromagnets are placed between cavities for focusing purposes.

    Measuring the power for complex waveforms such as analog or digital TV is quite complicated. For analog systems with negative video modulation, the peak (rated) power is at the synch pulse, while the active video part is well below this and thus the average power with live program is quite a few dB below the rated power.

    Television history

    At 1928 Baird transmits from London to New York, using his mechanical system.with 30 vertical lines. By 1930 it was clear that mechanical television systems could never produce the picture quality required for commercial success. For this reason mechanical system was rapidly succeeded by the electronic TV systems. The first all-electronic American systems in 1932 used only 120 scanning lines at 24 frames per second Since the mid-1930s picture repetition frequency (field rate or frame rate) has been the same as the mains frequency, either 50 or 60Hz according to the frequency used in each country. This is for two very good reasons. Studio lighting generally uses alternating current lamps and if these were not synchronised with the field frequency, an unwelcome strobe effect could appear on TV pictures. Secondly, in days gone by, the smoothing of power supply circuits in TV receivers was not as good as it is today and ripple superimposed on the DC could cause visual interference. If the picture was locked to the mains frequency, this interference would at least be static on the screen and thus less obtrusive.To determine what electronic system to use, the BBC sponsored trial broadcasts by two systems, one by Baird, with 240 lines, and one by EMI with 405 lines. Scheduled electronic television broadcasting began in England in 1936 using 405-line system (lasted until the 1980s in the UK). Germany made their forst TV broadcasts at 1936 olympics using 180-line TV system. Germany also made their TV broadcasts by the fall of 1937 using a 441-line system. Also fFrance tested TV (455 line system). RCA introduced electronic television to the U. S. at the 1939 World's Fair,and began regularly scheduled broadcasting at the same time (525 line system).In 1940 the USA established its 525-line standard. At year 1941 the 525-line standard, still in use today in USA, was adopted.Russia also produced TV sets before the war (240 and 343 line systems).World War Two interrupted the development of television. Immediately after World War Two production of TV sets started in the U.S-In USA there was TV broadcasts and few throusand receivers at 1945. In the early 1950s, two competing color TV systems emerged: CBS sequential color (used color wheel) and RCA dot sequential system. At 1953 color broadcasting officially arrives in the U.S. on Dec. 17, when FCC approves modified version of an RCA system.It calls this new RCA color system "NTSC" color. The first NTSC color TVs were on the marker at 1954.In Europe the TV broadcasts started to use experiment using 625 line system 1950s. This standard is used nowadays throughout Europe. France also tried 819 line system at the same time (this system was in use to 1980s). The rest of Europe opted for 625 lines, a system devised in 1946 by two German engineers, M??ller and Urtel (it appears that the Russians came up independently with a very similar system). The use of PAL color standard started at around 1967 and is still in use. The SECAM color system (used in France) testing started also at 1967. The TV broadcasting history has not ended. The newst thign is digital television. It is expected that terrestrial television will open up billion-dollar opportunities for those companies and organisations best prepared to embrace this new broadcasting era. At 1996 small digital satellite dishes hit the market. They become the biggest selling electronic item in history next to the VCR.

    Digital TV

    Digital television is a hot topic now.If you have looked at television sets at any of the big electronics retailers lately, you know that Digital TV, or DTV, is a BIG deal right now in the U.S. In Europe Digital TV is also a hot topic, because many countries have started terrestrial digital TV broadcasts and plan to end analogue broadcasts after some years (will take 5-10 years). Satellite TV broadcasts have also shifted very much to digital broadcasts.The main advantage if digital broadcasts are that it does not havethe picture quality problems of analogue TVs (it had it's own videoproblems caused by video compression), it allowes putting more TV channels to same medium (TV channel frequencies and satellites) and it allows new services (like HDTV and interactive multimedia). The digital brodcasts are generally designed to use such modulation that the digital data stream (typically around 20-30 Mbit/s) is modulated to the same bandwidth (around 6 MHz) as the analogue TV broadcasts. The used modulation vary between different media, which means thatdifferent modulation techniques are used in terrestrial transmissions, cable TV and satellite. Different modulations are used because of the different characteristics of those transmission medias. There is not on "digital TV", but several different variations of it in use.The basic technology of digital TV, known as MPEG 2 video compressionand MPEG 2 transmission stream format, is same around the world, butis is used somewhat differently in different standards used in differentcountries.

    USA uses ACTS Digital Televisio Standard, which standardizes NTSC format transmissions, HDTV transmission, sound formats and data signal modulation in use. The ATSC MPEG-2 formats for DTV, including HDTV, uses 4:2:0 samling for video signal. The US system uses a fixed power and a fixed maximum bitrate, at which some bits are always transmitted. That rate is typically 19.3 Mb/sec.

    Europe uses DVB (Digital Video Broadcasting) standard. This standardallows basically normal PAL resolution transmisssion (vasically HDTVcould be added later but is not yet standardized) with several audio formats, digital data rates and digital signal modulation. There are several different variations fo DVB standard for different media:

    • DVB-T for terrestrial broadcasts
    • DVB-S for satellite
    • DVB-C for cable TV
    Those different DVB versions varyon the data signal modulation methods, error correction and frequency bands used. DVB and option for some interactive extra services, but thestandardization of this is not ready here yet(there are fire different incompatible interactive servicessystems in use in different countries and by different broadcasters).

    The process of transmitting digital TV signal is the following: Analog video/audio - digitisation - MPEG compression - Multiplexing ( youcan now call it digital) - Preparation for transmisson - modulation toanalog carrier.Reception process is the following: Demodulation of analogue carrier - Error correction - Demultiplexing - MPEG decompression - DA conversion to get analogue signal (unless you use digital display). The analoguie video signal that gets digitized can be practically from any video source, for example produced with old analogue video production equipment and distributed with a video tape. In high-end system the information is analogue only in the image sensor on the video camera, and from this on the signal gets digitally processed. In many real-life TV production systems the reality is something between those two extremes.

    At least in Europe, the signal level requirements for DVB-T are well below the analog requirements, so the transmitter power is much less than on the analog side. In the NorDig recommendation the minimum received signal level for 64QAM, 7/8 code rate with a Rayleigh fading path and 8 dB receiver noise figure would be -64 dBm. With other code rates, modulations and fading mechanisms, the requirement is lower. Many receivers can perform much better at conditions where there is no fading (a quasi error free less than one uncorrected error/hour signal even at 27 dBuV (-82 dBm) with 64QAM and 8 MHz channel width). For analog signals, the recommended level is more than 1 mV (+60 dBuV, -49 dBm). While the ERP can be at least 10 dB lower than analog, the question of power consumption is more complicated, since COFDM with 64QAM carriers require a quite good linearity, which may affect the efficiency and hence power consumption.

      Digital TV system in use in USA

      The FCC mandate to change our broadcast standards from NTSC analog to ATSC digital broadcasting (DTV) is big bold move, requiring changes in everything from the way the studios shoot video, the format that's transmitted, to the equipment we use to receive and watch broadcastsDTV (digital TV) applies to digital broadcasts in general and to the U.S. ATSC standard in specific. The ATSC standard includes both standard-definition (SD) and high-definition (HD) digital formats. The notation H/DTV is often used to specifically refer to high-definition digital TV. The federal mandate grants the public airwaves to the broadcasters to transmit digital TV in exchange for return of the current analog NTSC spectrum, allowing for a transition period in the interim. At the end of this period scheduled for 2006, broadcasters must be fully converted to the 8VSB broadcast standard. Digital Television ("DTV") is a new broadcast technology that will transform television. The technology of DTV will allows TV broadcasts with movie-quality picture and CD- quality sound and a variety of other enhancements (for example data delivery). With digital television, broadcasters will be able to offer free television of higher resolution and better picture quality than now exists under the current mode of TV transmission. If broadcasters so choose, they can offer what has been called "high definition television" or HDTV, television with theater-quality pictures and CD-quality sound. . Alternatively, a broadcaster can offer several different TV programs at the same time, with pictures and sound quality better than is generally available today. HDTV (high-definition TV) encompasses both analog and digital televisions that have a 16:9 aspect ratio and approximately 5 times the resolution of standard TV (double vertical, double horizontal, wider aspect). High definition is generally defined as any video signal that is at least twice the quality of the current 480i (interlaced) analog broadcast signal. There are 18 approved formats for digital TV broadcasts, but only two (720p/1080i) are proper definition of the term HDTV. The advent of high definition has allowed monitors to read images differently, either in standard interlaced format or progressively. Sets that do not have any decoding capabilities but can display the high-resolution image is often labeled as "HD-Ready" a term that describes 80% or more of the Digital TVs on the market. HDTV displays support digital connections such as HDMI (DVI) and IEEE 1394/FireWire, although standardization is not finished. HDTV in the US is part of the ATSC DTV format. The resolution and frame rates of DTV in the US generally correspond to the ATSC recommendations for SD (640x480 and 704x480 at 24p, 30p, 60p, 60i) and HD (1280x720 at 24p, 20p, and 60p; 1920x1080 at 24p, 30p and 60i). In addition, a broadcaster will be able to simultaneously transmit a variety of other information through a data bitstream to both enhance its TV programs and to provide entirely new services. The technical specifications of USA DTV system is defined in ACTS Digital Television Standards.

      Digital TV in Europe

      Digital TV brodacasting in Europe is done according to DVB standards. DVB technology has become an integral part of global broadcasting, setting the global standard for satellite, cable and terrestrial transmissions and equipment. There are three versions of DVB in use: DVB-S, DVB-C and DVB-T.DVB-T is a flexible system allowing terrestrial broadcastersto choose from a variety of options to suit their various service environments. This allows the choice between fixed roof-top antenna, portableand even mobile reception of DVB-T services. Broadly speaking the trade-off in one of service bit-rate versus signal robustness.

      DVB-T network is very flexible. Having many transmitters all on the same frequency is not a problem for the used COFDM based system. COFDM has been chosen and designed to minimise the effects of multipath in obstructed reception areas. In fact multipath signals can significantly improve the overall received signal with no adverse effects. These properties are particularly valuable for radio cameras and mobile links. DVB-T because of its unique design which allows single frequency networks (SFN). This means that many transmitters along the planned routes can transmit on the same frequency. It is also possible to use simple gap fillers that amplify and retransmit the signal. In-air digital TV broadcasts in Europe use DVB-T. 8 MHz of bandwidth may be used to provide a 24 Mbps digital transmission path using Coded Orthogonal Frequency Division Multiplexing (COFDM) modulation (theoretical maximum 31.67 Mbits for 8 MHz bandwidth). In cases where less bandwidth is available (6 or 7 MHz), the data rate is somewhat lower (around 20 Mbit/s).

      DVB-C does the same function as DVB-T, but the modulation used in this system is optimized to operate well in cable TV networks. The modulation used in DVB-C is QAM. Systems from 16-QAM up to 256-QAM can be used, but the system centres on 64-QAM, in which an 8MHz channel can accommodate a physical payload of about 38 Mbit/s. Digital cable TV in Europe uses DVB-C. The DVB standard for the cable return path has been developed jointly with DAVIC, the Digital Audio Visual Council. The specification uses Quadrature Phase Shift Keying (QPSK) modulation in a 200kHz, 1MHz or 2MHz channel to provide a return path for interactive services (from the user to the service provider) of up to about 3Mbit/s. The path to the user may be either in-band (embedded in the MPEG-2 Transport Stream in the DVB-C channel) or out-of-band (on a separate 1 or 2MHz frequency band).

      DVB-S is the satellite version of DVB. Satellite transmission has lead the way in delivering digital TV to viewers. Established in 1995, the satellite standard DVB-S is the oldest DVB standard, used on all six major continents. QPSK modulation system is used, with channel coding optimised to the error characteristics of the channel. A typical satellite channel has 36 MHz bandwidth, which may support transmission at up to 38 Mbps (assuming delivery to a 0.5m receiving antenna) using Quadrature Phase Shift Keying (QPSK) modulation. 16 bytes of Reed Solomon (RS) coding are added to each 188 byte transport packet to provide Forward Error Correction (FEC) using a RS(204,188,8) code. For the satellite transmission, the resultant bit stream is then interleaved and convolutional coding is applied.

      The core of the DVB digital data stream isthe standard MPEG-2 "data container",which holds the broadcast and service information.This flexible "carry-all" can containanything that can be digitised, includingmultimedia data. The MPEG-2 standards define how to format the various component parts of a multimedia programme (which may consist of: MPEG-2 compressed video, compressed audio, control data and/or user data). It also defines how these components are combined into a single synchronous transmission bit stream. The process of combining the steams is known as multiplexing. The multiplexed stream may be transmitted over a variety of links, standards / products.Each MPEG-2 MPTS multiplex carries a number of streams which in combination deliver the required services. A typical data rate of such multiplex is around 24 Mbps for terrestrial brodcasts.

      European DVB systems currently transmit only standard definition TV signals and set top boxes also handle only normal TV resolution. It would be possible to transmit HDTV signals on DVB data stream, but those broadcasts have not yet started in any wide scale. There is one satellite broadcater that broadcasts HDTV DVB signals in Europe (some cable TV operators carry that signal on their cable).

      Many DVB-T integrated TV sets, and some set top boxes, in the Europe come with a Common Interface slot - which is pretty much the same form-factor as a PC Card (aka PCMCIA) used in PC laptops. This CI slot accepts a Conditional Access Module, in the same way that DVB-S receivers do, which implements at least one (some can do more than one) decryption algorithm. This CAM may also, itself, have a smart card slot to accept a consumer subscription card to authorise decryption - you plug your smartcard into your CAM and your CAM into the CI slot in your receiver/IDTV. Some DVB receivers have an integrated CAM (in the case of some receivers this is implemented purely in software, with no extra hardware required) rather than a CI slot to plug in a 3rd party device. With these type of receivers you just plug in the smart card and don't have to worry about CI slots and buying CAMs. So there is an interface standard for DVB - but different broadcasters can chose different encryption schemes, requiring different CAMs for decryption.

      Here is a list of several DVB standards and related specifications:

      • EN 300 744: Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television.
      • TS 101 191: Digital Video Broadcasting (DVB); Mega-frame for Single Frequency Network (SFN) synchronization.
      • N 50083-9: Cable distribution systems for television sound and interactive services; Part 9: Interfaces for CATV/SMATV headends and similar professional equipment for DVB/MPEG-2 transport streams.
      • ETR 290: Digital Video Broadcasting (DVB); Measurement guidelines for DVB systems.
      • TR 101 190: Digital Video Broadcasting (DVB); Implementation guidelines for DVB terrestrial services; Transmission aspects.
      • ISO/IEC 13818-1: Information technology ? Generic coding of moving pictures and associated audio information: Systems.
      DVB Standards and related documents are published by the European Telecommunications Standards Institute (ETSI). These include a large number of standards and technical notes to complement the MPEG-2 standards defined by the ISO.

      There are few different standard how interactive TV functionaly is implemented in DVB-systems in use in differenct countries. DVB-MHP is one gaining some acceptance. Multimedia Home Platform (MHP) is the open middleware system designed by the DVB Project (www.dvb.org).

    Cable TV

    Cable TV network is a system designed to deliver broadcast television signals efficiently to subscribers' homes. To ensure that consumers could obtain cable service with the same TV sets they use to receive over-the-air broadcast TV signals, cable operators recreate a portion of the over-the-air radio frequency (RF) spectrum within a sealed coaxial cable line. CATV is comprised of multiple TV channels (and usually radio channels also) transmitted over a single cable, with each channel occupying a different frequency range. Several vide channels (thens of them) may be carried over a single cable. Cable TV is a transmission system can be viewed as abroadband cabling system that supports transmission of multiple services over a single cable by dividing the bandwidth into separate frequencies, with each frequency assigned to a different service. Each TV channel (or other service) uses a different frequency range. Cable TV signals occupy the freqs that are used for public service(police and fire, etc.) and for this reason the cable TV companies arerequired by law to maintain their cables to prevent leakage, so they doregular checks. If they find that the cable TV signal is gettingoutside the cable, they will take necessary action to stop it. If they find that your equipment/wiring is causing it, they will really take action, which means disconnecting you and possibly subjecting you to other action (possibly legal consequences that can get expensive).

      Traditional cable TV networks

      Traditional coaxial cable systems typically operate with 330 MHz or 450 MHz of capacity, whereas modern hybrid fiber/coax (HFC) systems are expanded to 750 MHz or more. Logically, downstream video programming signals begin around 50 MHz and uses the frequencies up from that. Each standard television channel occupies around 6 MHz of RF spectrum, just like in normal TV broadcasts. Thus a traditional cable system with 400 MHz of downstream bandwidth can carry theoretically the equivalent of 60 analog TV channels and a modern HFC system with 700 MHz of downstream bandwidth has the capacity for some 110 channels. In practical applications the number of usable channels is somewhat less than that. The cable television (CATV) industry has come a long way since it began using community antennas to receive broadcast signals and distribute them to homes on twin-lead wire (later coaxial cable). The earliest cable systems were, in effect, strategically placed antennas with very long cables connecting them to subscribers' television sets. Because the signal from the antenna became weaker as it traveled through the length of cable and it was split to many receiver, cable providers have to insert amplifiers at regular intervals to boost the strength of the signal and make it acceptable for viewing.For the first 40 years, the vast majority of technological advancements in the CATV industry were driven by engineering requirements to improve signal quality, expand systems to cover larger geographical areas and deliver more channels. Today CATV industry competition focuses on making CATV the primary carrier of digital sound and video plus voice and data communications to homes and offices.

      Cable TV technology

      CATV systems typically utilise the 54 - some upper frequency region(330, 550, 750, 850 MHz), without breaks, to distribute TV channels. The frequency range used in modern bidirectional cable TV networks can cover frequencies from 5 MHz to 1000 MHz.

      In a perfect world, the video quality you perceve on your TV is the same quality as the video signal source. In the real world, this is not usually the case. The video signal is usually degraded by a factor of dB from the source to the TV. The type and length of cable used, the number of splitters, the type of video amplifiers, the quality of the incomming video signal, all influence picture quality and dB differently. The key to good picture quality is to evenly amplify the video signal to all video devices without exeeding the 15.5dBmV maximum allowed by the FCC and not going below 1 mV signal level. The typical receivers are designed to work well in this signal level range. Lower signal levels cause easily noisy picture and higher signal levels can cause signal distortion problems. In Europe the signal levels are usually described in different units dBuV. For example regulations in Finland ask for 60-80 dBuV (=0-20 dBmV) to be available on the subscriber antenna outlet. When designing a video distribution system or adding a new component to an existing system, one must try to take into consideration all factors that influence dB.

      There are many different components used in cable TV system:

      • The video amplifier is one of the most important components for producing a good quality picture. This device amplifies the video signal on the cable. Amplification is needed because cable and signal splitters attenuate the video signal on the cable. The amplifier is used to compensate those losses. Amplification needs to be done where signal is strong enough, because no amplifier can help the signal when it is buried below the cable noise. Amount of noise in signal plays an important part in the quality of cable TV a video signal. Cable amplifies their hardline coax trunk and feeder lines at intervals varying from every 600 feet out to every 1,200 feet - amplifiers are placed as a function of cumulative cable losses, signal splits along the trunk or feeder after an amplifier, and "tap" or "directional coupler loading". Cable TV amplifiers require power and every couple of miles of plant there is usually a pole mounted / ground mounted humoungous looking power supply. It takes 117/240VAC and turns it into 30, 60, or 90 VAC (DC to coax installed outside is no-no) which is then diplexed onto the coaxial cable to run throughout the plant to be a power source for the solid state amplifiers. The amplifiers generally operate internally at 30V DC or less. The cable TV amplifiers at the end user premises typically use the household power (230V AC or 120V AC). Many cable TV equipment components are powered through a mains power supply that supplies 24V DC power to the amplifiers. Some equipment have built-in mains power supply.
      • Coaxial cable is the medium which carries the video RF signal. Cable TV systems are built using 75 ohm coaxial cable. A typical RG6 Coax Cable attenuates the cable TV signal 4-6 decibels per too feet (30 meters) of cable. In some applications also RG-59 cable is used (it is more lossy than RG-6).
      • A splitter is a small device that has one input (the 75 ohm load) and 2 or more outputs, each driving a separate 75 ohm load. Essentially they are transformers that split the power in the input signal to multiple outputs, while maintaining the 75 ohm impedance. Every time you split an RF signal with a splitter, you drastically decrease the signal's strength. Splitters are used when cable TV video signal needs to be split to multiple viewers. A typical 1x2 Way Splitter splits the input signal to two output, maintains 75 ohm impedance on all connections and attenuates the video signal typically 4-6 decibels. 1x4 Way Spitter works in the same way but has four outputs and attenuates the signal 7-9 decibels.
      • A combiner is simply a splitter hooked up backwards. It combines the channels on two or more separate cables or signal source onto one cable. The only drawback to this piece of magic, is that the signal on the cables being combined cannot have any channels in common with each other (if they contain signal at same channels/frequencies the resulting signal on that channel would be trashed).
      • Taps are similar to splitters, but are "wound crooked" so that the outputs are not equal in signal strength. The "through" output of a tap may only reduce the signal level by a very small amount, while the "tap" output is a small fraction of the signal level. Taps are primarily used in complex commercial distribution installations. A typical applicaton for taps are to take out individual outputs from a strong signal main distribution line that goes through many taps. Other uses for taps are measurement applications where you can have a suitable high attenuation (high attenuation to tap out, very low on the signal going through) tap where you connect our measuting instrument to that tap output. Connecting and disconnecting he measuring equipment to that output does not affect considerably the main line operation, and the tap can be kept on the line all the time (the pass-through attenuation is neglegtable). When looking at measurement reasults you just multiply the reading you get with the tap attenuation and you know the signal strength on main line.
      • A Terminator is a small cap that screws on the end of an open coax connector. It contains a small 75 ohm resistor. A Terminator is used to prevent video signal bounceback and ghosting. All connections in your system should be terminated either by a video device or a terminator. An
      • Attenuators are simple "one in, one out" devices that reduce the signal strength. Attenuator pad reduces signal level (usually expressed in decibels). This component is usually necessary to equalize video signals with different signal levels before they are combined to the same cable. Attenuation pads are sometimes needed also when some signals are too strong to some sensitive devices. Attenuators come in various sizes and are useful when tuning up the video distribution system. Typical attenuator pad values are -3dB, -6dB, -10dB, and -20dB. There are also adjustable attenuation pads with typical attenuation range of 1-20 dB.
      • Antenna outlet is the connector on your wall. In the simplest case it can be just a coaxial connector terminating the cable coming from the main antenna line tap. The antenna outlets in USA have typically F connectors, while the coaxial IEC/DIN 45325 connectors (9.5 mm diameter) is the most commonly used antenna outlet connector in Europe. The antenna outlets in most European countries have two output connectors, one male IEC/DIN 45325 connector for TV signals and one female IEC/DIN 45325 connector for radio signals. A modern antenna outlet has typically attenuating directional coupler (1-12 dB attenuation) and filters to filter different frequencies to different outputs.
      • A Tilt Compensator attenuates lower frequency video signals, to compensate the fact that typical cables attenuate higher frequencies more than lower frequencies. Also video signal amplifiers sometimes amplify lower frequency signals much more than higher frequency signals. A Tilt Compensator is usually needed for every 250ft run of RG6 coax cable. This kind of compensator typically attenuated the low frequencies around 12 decibels compared to the highest frequencies. Nowadays tilt compensators when used are usully built into amplifiers designed for cable TV signals.
      • Modulators are devices that convert composite video signal and audio signals to to RF signal similar to TV broadcasts. The operation of RF modulator is similar to TV broadcast transmitter, the only main difference is that the output power of the typical RF modulator is very low (typically in milliwatts level). The signal from a RF modulator is typically suitable to be fed to a TV receiver or cable TV amplifier input. The modulators could be built to work on specified channel frequency or their channel can be altered with modulator controls. A tpyical use in cable TV head-end is that a set of modulator takes in the audio and video signals from different program sources (signal from local studios, signal from playbac VCRs, signals from satellite receivers etc.). A typical cable TV broadcasting head-end has many RF modulators transmitting ant different frequencies, and signals from them are combined together to form the cable TV channels that you see. Sometimes modulators are used also used at residential premises. The concept of "in-house" channel generation, together with the new cheaper and more reliable digital modulators, is opening up many new possibilities in residential video distribution. Also digital TV broadcasts used modulators, in this case those modulators take in digital video data stream and convert it to RF signal.
      • Channel converters are devices that take in the signal from TV receving antenna, receive one channel and give out RF signal at other frequency out. Channel converters are used to receiver on-the-air broadcasts and convert them to different channel frequency used for them in the cable TV (there are good reasons to use different freuqencies on cable and on-air broadcasts). A typical channel converter takes in one TV channel information, mixes it with a local oscillator to form intermediate frequency (IF) signal, filters that IF signal and then modulates that signal with another local oscillator to form the RF signal at wanted frequency. Typically the input and output frequencies, as well as the signal amplification, are are adjustable on modern equipment. A typical cable TV head-end has one channel converter per on-air channel that is converted to cable. The outputs from the channel converters are combined toghether with the signal from the RF modulators to form the channels on the cable TV system.
      • Fiber converters are devices that are used to convert the cable TV broadcast signal to light signal that can be transported through a fiber optic link. Many modern cable TV systems use fiber optic links to carry the cable TV signals long distances (for example from head-end on down-town to cable TV distribution amplifier rack somewhere on the suburb may kilometers away). The fiber optic transmitter consists of a powerful infrared laser transmitter and a high speed analogue modulator that can modulate the whole cable TV RF signal to the light signal on the fiber (the signal is amplitude-modulated to light signal). The receiver on the other end of the fiber consists basically just from a very high speed photodiode followed by cable TV amplifier. The cable TV fiber transmitter part is very expensive special device that must be carefully tuned to work well (the modulator must operate at "linear operation area" to avoid signal distortion). The fiber optic receiver can be a quite simple device and is much less expensive than the transmitter part. Fiber optic transmission systems can carry analog and digital signals in the form of light waves.
      The whole cable TV system must be well impedance matched system. It must also be designed in such way that different devices connected to different cable TV outlets do not interfere with each other. The terminal isolation provided to each subscriber terminal shall not be less than 18 decibels (the exact decibers vary somewhat from country to country, usually in 18-22 dB range) and shall be sufficient to prevent reflections caused by open-circuited or shortcircuited subscriber terminals from producing visible picture impairments at any other subscriber terminal. The isolation is provided by correct design of the cable TV system. The antenna outlet usually has a considerable part in to guarantee the isolation between different outputs. Modern antenna outlets include typically directional coupler, some attenuation and filters. For example typical European antenna outlets have output connectors are according to IEC standard. One outlet supplies all TV frequencies and othet outlet has the radio frequencies (the outlet has needed filters built into it). Some outlets have nowadays a separate data outlet (passes wideband signals out, passes return channel data signals from cable modem also well).

      The cable TV system needs to be continuoisly maintained to guarantee that it does not have excess signal leakage out of the system. FCC requirements say radiation must not exceed a few microvolts per metre at a distance of ten metres from the lines, equipment. Leaks are concern being potential interference to air and safety communications.This maintaining process can contain continumous measurement drive-outs, measurement flyoves, monitoring of reverse band monitoring of cable data services signal quality. One system to test this is is to have test equipment modulate a carrier (such as at 108 or 133 MHz) with an (FM) warble. The carrier is carried throughout the system and the service trucks have a mobile receiver tuned to that frequency. If they hear their inside-of-cable plant "warble" they know they have a leak. Radiation is very easily traced - a handheld or smaller battery operated TV set, an FM radio with "TV audio" frequency coverage and a whip antenna (or rubber duckie) and look for the TV-FM signals sent to cable. Radiation occurs because one (or more) of three conditions exist: there is a break/crack in the line, a connector is loose or there is a resonant line section someplace. The crack acts like a "slot antenna" and RF energy, resonant with the crack, literally leaks or radiates away from the line (most cracked line radiation occurs above 100 MHz). A loose connector typically radiates low band signals, seldom at frequencies above 200 MHz. A resonant line section can be very difficult to pinpoint. In practical world it is very hard to keep all the leakage out, because is practically not possible to install coax cabling system in the house with shielding better than 65 dB (the individual components are typically designed for 80-90 dB shielding).

      The inner conductor of the coaxial cable consists generally of a solid, drawn copper wire. A speciality is the so-called copperwelded conductor, a copperclad steel wire, which is applied in small drop cable types for its high tensile strength. Due to the skin effect the thin copper cladding is decisive for attenuation with high frequencies.High qualtity polyethylene (PE) is is used as the insulating material in CATV coax cables. This material guarantees low dielectric loss with high frequencies. Maintaining the samelevel of transmission properties the outer diameter of the cable can be reduced considerably with the use of a compound dielectric of PE and air (foam PE). There are two versions: physical foam material, i.e. the socalledcellular PE insulation, and the chamber construction with PE discs and a PEtube, the so called Bamboo insulation. The outer conductor consists of pure copper in best cables. In outdoor installations exclusively outer conductors of copper are applied, i.e. as welded tube. However, for certain applications, it is recommendable as outer conductor for its low cost: i.e. for drop cables installed in buildings aluminium laminated plastic foils combined with a tinned copper braid are used. The quality of the electromagnetic screening is determined by the outer conductor of the cable. Real conductors with a finite conductivity radiate up to some 100 kHz of electromagnetic energy in the lower frequency range, with higher frequencies there is, however, a sufficient screening for all practical applications.

      Generally the screening efficiency values used in CATV cables achieve approx. 80 dB for cables with low optical coverage (aluminium laminated plastic foils combined with a 35% optical coverage tinned copper braid) and 90 dB or more for cables with high optical coverage (aluminium laminated foils and copper braid with optical coverage of 80 %). There are some cabling shielding proposals: the screening classes A with 85 dB and B with 75 dB of the european andinternational standard proposals. The outdoor cables in line and distribution networks are generally provided with an UV-stabilized polyethylene sheath allowing direct burial or use as aerial cable. Indoor cables are generally provided with a PVC or FRNC sheath. because the needed flame resistance. Cables with FRNC sheath are preferably used for indoor installations, since their flameresistance is considerably higher than that of PVC and they are non-corrosive. The cables are usually identified by sheath materials in different colours and a sheath marking. This marking can be applied on the outer sheath by means of embossing, inkjet or sintering.

      Essential properties of CATV coaxial cables are their characteristic impedance and its regularity, their attenuation as well as their behaviour concerning the electrical separation of cable and environment, i.e. their screening efficiency. Another important factor is the D.C. resistance, because the supply voltage for repeaters and other active components in a distribution network is partially transmitted via the coaxial cables.

      Cable television companies provide broadcast and video programming to subscribers. In recent years, many companies have upgraded their systems to provide new cable services such as digital television, Internet access through a cable modem, and telephony. Upgraded systems typically use fiber optic cable based long distance signal transmission because optic fibers have large capacity, are reliable and and transmissions over them are not susceptible to interference by outside signals. Fiber optic transmission systems nowadays carry both analog and digital signals in the form of light waves. One fiber can carry on best cases many different signals on different wavelengths. The cable TV networks that use both fiber optics and coaxial wiring are called Hybrid Fiber Coaxial systems. The cable TV distribution network is constructed following a tree and branch structure. The cable TV signal from a main hub is first brought to an optical node through a glass optical fiber where the cable TV signal is Amplitude Modulated (AM) to an optical carrier frequency. After demodulation using Optical-Electronic (O/E) devices in the optical node, the cable TV signal is carried over a few branches of coaxial cable network to every subscriber. The root of each tree and branch distribution network is at the optical node. The main branch of the distribution network consists of distribution coaxial cables (usually cable types 500-F and 625-F). The distribution cable is connected to subscribers through a device called a Tap and drop coaxial cables. The common drop cable types are RG-6 and RG-59. Drop cables are also used for in-house TV signal wiring.

      Proper in-house cabling is the last part of cable TV network. In a new construction, coaxial cables are connected from a central location near the TV signal source, be it a cable TV or a satellite dish, to every room where an in-house TV wiring connection should be available. This configuration forms a star topology. A multiport splitter is located at the center of the star. Depending upon the number of rooms to be served, some times an amplifier is inserted between the video source and the multiport splitter to raise the signal level, compensating signal losses caused by branch splitting. Assuming that signal separation loss is 15 dB between two output ports of every splitter stage, the minimal signal separation loss is 15 dB between coaxial cable outlets. Enough signal separation is needed to avoid the possibility of a problem in one outlet (broken cable, noise generating recevier etc.) would cause the signal on other outlets to become unuseable.

      For in-house wiring installed by cable TV companies on existing homes, splitters are usually randomly installed at the cable TV entrance point and some other convenient splitting points. This configuration forms a star daisy-chain topology where splitter is used at every cable branch point.

      Cable TV standards and regulations

      There are some standards on European cable TV systems.

      • EN 50083-1:1993 Cabled distribution systems for television, sound and interactive multimedia signals -- Part 1: Safety requirements
      • EN 50083-3:1998 Cable networks for television signals, sound signals and interactive services -- Part 3: Active wideband equipment for coaxial cable networks
      • EN 50083-4:1998 Cable networks for television signals, sound signals and interactive services -- Part 4: Passive wideband equipment for coaxial cable networks
      • EN 50083-5:1994 Cable networks for television signals, sound signals and interactive services -- Part 5: Headend equipment
      • EN 50083-6:1997 Cable networks for television signals, sound signals and interactive services -- Part 6: Optical equipment
      In USA FCC standardizes the operation of cable TV networks.

      Bidirectional cable TV

      To fullfill the data communication needs the cable TV network needs to be bidirections. While CATV is moving toward adequate downstream bandwidth to get signals to the customer, one of the remaining problems is the upstream bandwidth to carry signals from the customer to the cable system headend or interconnect with other networks for telephone, data, and interactive services. The available bandwidth for these upstream signals is small, ranging from 5 MHz to 42 MHz, and it is sensitive to the ingress of other frequencies. To deliver data services, like cable modem service, over a cable network, typically one television channel (in the 50 - 750 MHz range) is typically allocated for downstream traffic to homes and another channel (in the 5 - 42 MHz band) is used to carry upstream signals. A single downstream 6 MHz television channel may support up to 27 Mbps of downstream and upstream channels may deliver 500 Kbps to 10 Mbps. Some European cable TV systems use a larger bandwidth for upstream signals. For example using frequencies from 5 MHz to 65 MHz for upstream traffic gives much more bandwidth to this direction. The downside of this approach is that some of the lowest frequency TV channels cannot be used anymore in the cable TV network.

      Digital cable TV

      The TV brodcasting in cable TV networks is becoming nowdays also digital. This is usually called "digital cable". To understand this, let's comparethe analogue and digital cable first.In an analog cable you have the picture data encoded as analog voltagedifferences (relative) to a reference and a reference level. It is onseparate frequency carriers to separate signals. In a digital cable youhave the separate frequency carriers, but the data in encoded as pulsecodes which must be interpreted into values.Usually one carrier carries lots of data, enough to carry more than oneTV channel and accessory services. Generally few TV channels(typically 3-5) and some extra data are multipexed to this one carrier. In this way one carrier (which takes about same bandwidth as oneanalogue channel) can transfer more than one TV channel. Basically the digital cable system will work with the same cable TV insfrastructure (same cable, amplifiers, signal splitters etc.) as the analogue cable TV. Generally when cable TVs start offering digital cable service, they just send those signals to their existing cable TV network just to some unused channel freuquencies in the cable TV. So the cable TV network will carry at the same time both analogueand digital cable TV signals at the same time. The process of transmitting digital cable signal is the following:Analog video/audio - digitisation - MPEG compression - Multiplexing ( youcan now call it digital) - Preparation for transmisson - modulation toanalog carrier.Reception process is the following:Demodulation of analogue carrier - Error correction - Demultiplexing - MPEG decompression - DA conversion to get analogue signal (unless you use digital display).

      Wireless cable

      Wireless Cable is a broadband service that delivers addressable multichannel television programming, Internet access, data transfer services, and other interactive services over a terrestrial microwave platform. Multipoint Multichannel Distribution Service (MMDS) is often used as a synonym for "Wireless Cable." Multichannel Multipoint Distribution System, or MMDS, spectrum, has been in use for analog TV since the 1960s. The original idea was that educational institutions would use these frequencies for long-distance learning. But this part of the spectrum, with the capacity for roughly 30 analog TV stations, was later deployed by private companies planning to compete with cable franchises.

      • How Does Wireless Cable Work? - Wireless Cable is a broadband service that delivers addressable multichannel television programming, Internet access, data transfer services, and other interactive services over a terrestrial microwave platform. Multipoint Multichannel Distribution Service (MMDS) is often used as a synonym for "Wireless Cable."    Rate this link
      • Wireless cable makes a surprise comeback - Wireless cable is an odd business that's been suffering a slow death for the better part of the 1990s. The wireless-cable industry, based on the Multichannel Multipoint Distribution System, or MMDS, spectrum, has been in use for analog TV since the 1960s. The industry looked promising in the early '90s and launched some of the hottest public offerings in pre-Internet times. Nowadays the wireless-cable spectrum is useful because it is wide enough to carry high-bandwidth data applications, including video-over-data and voice-over-data.    Rate this link
      • Wireless Cable TV FAQ - Wireless cable is a name given to a service that is called Multichannel Multipoint Distribution Service (or MMDS). Wireless cable uses Super High Frequency ("SHF") channels to transmit satellite cable programming over-the-air instead of through overhead or underground wires.    Rate this link

    Satellite TV

    Conceptually, satellite television is a lot like broadcast television. It's a wireless system for delivering television programming directly to a viewer's house. Both broadcast television and satellite stations transmit programming via a radio signal. Satellite television broadcast signals from satellites orbiting the Earth. Satellite television systems transmit and receive radio signals using specialized antennas called satellite dishes. The television satellites are all in geosynchronous orbit, meaning that they stay in one place in the sky relative to the Earth. Each satellite is launched into space at about 7,000 mph (11,000 kph), reaching approximately 22,200 miles (35,700 km) above the Earth. Those satellites are put to such orbit that that they stay always in the same direction when you look at them from the ground, so the people who want to receive signals from the just aim their satellite dish to the satellite they want and they can keep the aiming same all the time.

    Today most satellite TV customers get their programming through a direct broadcast satellite (DBS) provider or other digital satallite broadcasting provider. . The provider selects programs and broadcasts them to subscribers as a set package. Basically, the provider's goal is to bring dozens or even hundreds of channels to your television in a form that approximates the competition, cable TV. Modern satellite provider's broadcast is completely digital, which means it has a goof picture and sound quality. The two major providers in the United States use the MPEG-2 compressed video format. Also European digital satellite broadcasters use MPEG-2 technology. TV signals are transmitted at many different frequency bands. Early satellite television was broadcast in C-band radio -- radio in the 3.4-gigahertz (GHz) to 7-GHz frequency range. Digital broadcast satellite transmits programming in the Ku frequency range (12 GHz to 14 GHz ).

    The basic idea on the receiving side is simple: The viewer's dish picks up the signal from the satellite (or multiple satellites in the same part of the sky) and passes it on to the receiver in the viewer's house. The receiver processes the signal and passes it on to a standard television.

    A satellite dish is just a special kind of antenna designed to focus on a specific broadcast source. The standard dish consists of a parabolic (bowl-shaped) surface and a central feed horn. The satellite dish is a very direction antenna that needs to be carefully adjusted to point exactly to the satellite. To be able to make the right adjustment, you need to first know to what satellite you want to point your dish to (know the direction and height). Then you need certain tools to make the adjustment. Usually a compass is a good tool to know the right direction to point to. For height there is often some kind of scale in the satellite dish mounting hardware. First you point your satellite dish to approximately to right direction, and then use a satellite signal strength meter to find the exact direction that gives the strongest signal. This signal strength meter is a simple meter (can have analogue or digital display or just audio signal) that is used to maximize a satellite signal between the LNB amplifier and the receiver. It just measures the signal strenght at 950MHz - 2,050MHz frequency range, -25 to -75 dBm signal amplitude and is typically powered by satellite receiver (same 18-18V power as used by LNB).

    The central element in the feed horn is the low noise blockdown converter, or LNB. The LNB amplifies the radio signal bouncing off the dish, filters out the noise (radio signals not carrying programming) and converts the signal to frequency range that can be tranported practically through normal coaxial cable (converts many GHz frequency to below 2 GHz frequencies). The LNB passes the amplified, converted and filtered signal to the satellite receiver inside the viewer's house. The reception heads, the LNBs, have made enormous progress, with average noise factors in the range of 1.5 to 1.8 dB in 1990 and 0.8 to 1.1 dB in 1998, compared to 3 to 5 dB in 1977. This ensures the same service area with much smaller transmission power (50 to 100W instead of 200W at satellite) at identical or smaller antenna dimensions. Different satellite systems can use different signal polarisations. Circular polarisation is nowadays preferred over the linear polarisation in digital direct broadcasting satellite system in USA. This is because, for circular polarisation, the orientation of the reception head (LNB) around the propogation axis is unimportant, and therefore does not require any precise adjustment. This point is especially important for motorised antennae which, with linear polarisation, require a polarisation adjustment for each different satellite. Linear polarisation is traditionally used by telecommunication satellites and also used by some satellite TV systems (allows more channels for smae frequency band because there are choises for vertical and horizontal polarisation). There are different LNBs for different satellite bands. In many satellite TV applcations nowadays so called "Universal LNB" devices are used in Europe. Here are general specifications of such devices (from SHP catalogue 2004 page 472):

    • Freuqency in: 10.7-11.70 and 11.7-12.75 GHz
    • Frequency out: 950-1950 and 1100-2150 MHz
    • Local oscillator: 9.75 and 10.60 GHz
    • Amplification: 58 dB
    • Phase noise: less than -75 dBc (@10kHz)
    • X-polarization: greater than 23 dB
    • Switching voltage for vertical polarisation: 11.5-14.0 V
    • Switching voltage for horizontal polarisation: 16.0-19.0 V
    • Tone switching: Lo=0kHz, Hi=20kHz
    • DC current: typically 140 mA, max. 200 mA
    • Mounting: 40 mm diameter
    Prices of this kind of devices typically vary from 10 Euros to over 100 Euros depending on the features (for example LNBs with outputs for multiple satellite receivers are more expensive than single output models).

    The signal traveling through the cable from the satellite dish to the receiver is very deferent then a standard TV signal used by the cable companies or what you would receive with a roof top antenna. The signal from the dish to the receiver is typically in 1-2 GHz frequency range (where normal TV broadcasts and cable TV are all blow 1 GHz frequency). It takes a good quality cable to transfer this kind of hifh frequency signals well. Practically every manufacture calls for RG-6 cable to be used during the installation of satellite system. This is a very good cable to use, although not the only option. Not all RG-59 can be used for a satellite dish installation, but there is good RG-59 that can be used just fine and really low quality RG-59 cable that should not be used. Besides the cable signals from the dish, this coaxial cable generally also carries the power (typically 14-18V DC) to the LNB.

    The end component in the entire satellite TV system is the receiver. The receiver has four essential jobs: extracts the data stream from incoming satellite signal, extracts the individual channels from the larger satellite data stream, de-scrambles the encrypted signal (you need to have deryption keys generally supplied in smartcard by the satellite provider), takes the digital MPEG-2 signal and converts it into an analog format that a standard television can recognize. Some receivers also keep track of pay-per-view programs and periodically phones a computer at the provider's headquarters to communicate billing information. In addition to the receiving functions the satellite receiver unit also sends the power to LNB (typically 14-18V DC) and possibly some control signals (DISEQ standard to control LNB switchers etc.).

      Satellite receving equipment technology

      The conventional LNB known as the "Marconi (polarisation) Switching LNB" responds to the supply voltage to change the polarisation. If the supply voltage going up the dish cable is less than 15 volts, the LNB will "see" only vertically polarised transmissions. If the voltage is more than 15 volts, it will "see" only horizontally polarised transmissions. The "Universal" LNB switches polarisation with voltage but it also switches its internal oscillator for "High Band" when it "hears" a 22kHz tone. Specificallly, the oscillator changes from 9.75 GHz to 10.6 GHz. An alternative use for the 22kHz tone is to control an external switching box whichfeeds signals from one of a pair of LNBs into the receiver. When the box "hears" a 22kHz tone it swaps to the other LNB. ToneBurst is a method for controlling the simplest DiSEqC switches. The satellite tuner sends special 22 kHz signal to the antenna cable. When the switch destects this signal, it changes the switch position (no signal means one position, 22 kHz bursts present mean another position). Now this swithing is becomign digital. DiSEqC is an open standard that stands for Digital Satellite Equipment Control. DiSEqC messages are sent as sequences of short bursts of 22KHz tone modulated on the LNB power supply carried by the coax cable from the LNB input on the receiver (the master). Messages comprise a number of digital bytes of eight bits each. DiSEqC system has been designed primarily to meet the problem of two-satellite, two-band systems with ease. It has dedicated outputs to select polarity, satellite position and frequency band. In DiSEqC 1.0 system the satellite tuner sends digitally modulated 22 kHz audio tone to the antenna cable. This modulated signal then controls DiSEqC 1.0 compatible devices like antenn signal switchers. The newer DiSEqC specification (1.1, 1.2, 2x) have extra functionality to the system.

      • DiSEqC Specifications - Full specifications and associated documentation for the DiSEqC (Digital Satellite Equipment Control) system, which is a communication bus between satellite receivers and peripheral equipment using only the existing coaxial cable. DiSEqC can be integrated into consumer satellite installations to replace all conventional analogue switching, providing a standardised digital system with non-proprietary commands and enabling switching in multi-satellite installations.    Rate this link
      • What is DiSEqC? - DiSEqC is an open standard that stands for Digital Satellite Equipment Control.    Rate this link
      • Diseqc 1.2 to standard 36 V DC Motor interface - A Diseqc 1.2 compatible positioner for old 24/36V DC Motor positioner.    Rate this link

    Video protection systems

    Cable and satellite TV protection is designed to protect the broadcaster's program material so that only the people who pay for viewing those protected channels can view them. Owning and/or using your own video protection decoding circuit is illegal in many countries. Just building your own cable decoder box is also not a generally good idea, beachus there are tens of different cable decoding box systems and many different protections schemes in use, so every cable decoder plan works only on a very limited number of cable TV networks and it is hard to get to know which cable TV decoder works in which network unless you build and test. And there is no guarantee that any of the circuits below has worked well in any cable TV network. Basically those following circuit have only informational value to electronics hackers who are interrested in cable TV decoding electronics.

      Cable TV

      Many cable TV operators use their cable to send both normal unencrypted TV channels (basic cable TV service) and encrypted channels (extra pay channels) to the customer usin same cable. Those pay channels are encrypted so that they can be viewed with only a proper decrypting device (you usually get it from the cable operato when you subscribe to the pay channel service). Those pay channels are usually encrypted using quite simple analogue encryption techniques. Those encryptions prevent normal viewers to view those channels. There are many ways and means of scrambling a video signal. Unfortunately, very few methods survive the combined requirements of being able to pass through non-linear transmission media, prevent TV's from holding lock, and deceive the human eye into not recognising scenes even though they are grossly distorted and corrupted. It is also desirable to remain compatible with all video equipment and be tolerant of other devices that can (and do) distort the signal. You might have heard of decrypting boxes which allow which allow you to view pay channels for free (without paying to cable TV operator). Such devices really exist, but is another thing to make such box to work. First using such box is a fraud, steal of a service you are not paying for (more or less illegal in most countries). If you think that you want to be a criminal and steal the service, the thigns do not get easy here. The problem of getting or building a right type of cable decrypting box that works for you is problematic. There are mor than half dozen different major encryption systems widely used. And there are many variations of the different encryption systems. When the cable TV operator does not tell you what encryption you use and most cable TV decoder plans are poorly documented what system they work with, so it is usually not a good idea to try to build such thign unless you know very well what you are doing and you are prepared to take the risks (like that the circuit might not work and you could get gaught of stealign cable service which can get you to problems). Due to different scrambling systems, you might find it hard to determine what kind of cable decoder you can use with your cable TV connection (if you plan to do this).

      VCR copy protection

      VCR copy protection systems are designed to prevent consumers to copy video material to their own VCR. The most commonly used and best known video protection system for this is called Macrovision. Macrovision is an analogue copy protection of Videotapes, DVD movies and pay-per-view television (Analogue signal). Its purpose is to make it harder to record such signals with a VCR. Generally the protection must be disabled or removed, before recording by a VCR is possible. Macrovision is the most commonly used antitaping process for VHS video tapes and digital video systems.Around 95% if VCRs on the market do not record Macrovision protected signal with any usable quality. If you try to copy a Macrovision-protected tape, the copy becomes crappy and not worth seeing, the picture changes brightness with time and the color is distorted. Not all TV's work very well with this form of protection e.g. the top of picture is distorted and it might look like the tape is damaged. Macrovision is based on using a hyper-brightsync pulse (to confuse the AGC of a VCR) and also does things to the chromato further confuse such. Macrovision works due to the differences in theway VCRs and TVs operate. The automatic gain control circuits within a TV are designed to respond slowly to change. Those for a VCR are designed to respond quickly to change. The Macrovision technique attempts to take advantage of this by modifying the video signal so that a TV will still display it but the VCR will not record a viewable picture. CGMS is Copy Guard Management system for NTSC systems. A method of preventing copies or controlling the number of sequential copies allowed. CGMS is transmitted on line 20 for odd fields and line 283 for even fields for NTSC. For digital formats it is added to the digital signal conforming to IEEE 1394.

      Circuits


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