Index


Video circuits

    Video distribution

      Basic video amplifier circuits

      Video amplifiers are most often used when there is need to amplify an attenuated video signal or there is need to split a video signal from one video source to many video devices.

      • Driving Video Lines - When does a trace or a wire become a transmission line? Bandwidth, characteristic impedance, ESD, and shoot-through considerations for selecting the proper video driver, receiver, mux-amp, or buffer. This application note discusses on those topics and gives recommendations for suitable video line driving components.    Rate this link
      • Fast, Stable Wideband FET Amplifier - a gain-trimmable wideband FET amplifier with very good speed, high input impedance, and excellent dc stability under all conditions, more than adequate performance for driving video cable, high-speed data converters, etc.    Rate this link
      • Transconductance Amplifiers Simplify Wideband Techniques - This article describes the unique architecture used in the MAX435/MAX436 transconductance amplifier and how this applies to traditional applications as well as new ones. Sample circuits are shown using the MAX435/MAX436 as a phase splitter, an impedance transformer, a coaxial cable driver, and as a twisted pair cable driver for distances over 5000 feet.    Rate this link
      • Positive Feedback Terminates Cables - positive feedback along with a series output resistor can provide a controlled output impedance from an op-amp circuit, with lower losses than would result from using an actual resistor    Rate this link
      • Positive Feedback Terminates Cables - Positive feedback along with a series output resistor can provide a controlled output impedance from an op-amp circuit, with lower losses than would result from using an actual resistor. The circuit is useful when driving coaxial cables that must be terminated at each end in their characteristic impedance, which is often 50 ohms.    Rate this link
      • Positive Feedback Terminates Cables - Positive feedback along with a series output resistor can provide a controlled output impedance from an op-amp circuit, with lower losses than would result from using an actual resistor. The circuit is useful when driving coaxial cables that must be terminated at each end in their characteristic impedance, which is often 50 ohms.    Rate this link
      • Video amplifier circuit from discrete transistors    Rate this link
      • Video Circuits Collection - many video amplifier circuits in pdf format, includes video transmission over UTP (AN57)    Rate this link
      • Video signal emphasis - With this circuit we can amplification selectively the high signal of picture frequencies [Video] with result bigger clarity than this. This circuit has adjustable gain and frequnecy response equalization.    Rate this link

      Coaxial cable video transmission

      engineers are aware they must match impedances to avoid reflections when driving transmission lines. this is especially true in video, with its wide range of component frequencies. while most applications span a few octaves, video covers six or more. transmission lines have a characteristic impedance (zo) with which they should be driven and terminated; and, in video, the most popular is the 75 ohm coaxial cable. this give the 150 "back terminated" unit load the driver sees due to the series 75 ohm source resistor, (ro in figure 2) and the 75 ohm line. only dissipative elements (resistors) can be relied on for matching1 over such wide bandwidths. the use of resistors creates a loss. the driver must compensate with added gain. that's why most video drivers have a fixed gain of two, though some are settable. when video signals are transported through long distances, those long coaxial lines needs to be equalized, restoring their frequency response to the required bandwidth for the application.

      Differential video signals on UTP cable

      Normally video transmission is done using 75 ohm coaxial cables, but unshielded twisted pair or UTP cable is a very inexpensive interconnection when compared to a coaxial cable connection. This section gives you circuits for transmitting video signals over unshielded twisted pair wiring. Modern unshielded twisted pair wiring (CAT5 or better) can be used to transport hogh quality video signals when this is done in the right way. Also lower quality cables will do in the applications where some signal quality degration is allowed (for example telephone wire pairs in some CCTV applications). The secret to sending signals over UTP is to balance them well in order to limit both radiation and noise pick up. This kind of unshielded twisted pair wiring method is used in many CCTV applications nowadays to use existing in-house twisted pair wiring instead of installing new coaxial cable for the CCTV camera. Video signal can be adapted to UTP wiring using a special balun transformer between BNC video connector and the wisted pair wiring. This converter converts the unbalanced audio signal to balanced signa which nicely travels through the cable. A similar transformer can be used on the other end of the cable to convert the video signal back to unbalanced format which fits to BNC connector. There are both passive solutions (balun transformers) and active converters available on the market for this application. Generally this kind of adapters can be used for both NTSC and PAL video signals. Simple UTP baluns just convert an unbalanced coax signal to a balanced signal for use on the twisted pair. Some companies add gain with active converters to extend the distance. There are difference how well different products perfom. Some products provide inadequate noise immunity, some perform very well. Some products provide also ground loop isolation and surge protection.

    Video switching

    Most commonly seen video switch type is video multiplexer.Video multiplexers route video from several sources to a single channel. Low-end consumer products use CMOS analog switches and multiplexers, such as the 4066 and 4051. Unfortunately, these devices have a series on-resistance that ranges from approximately 100. to 1 k., a resistance that is not constant with video level and that appears in series with the signal. The traditional way of solving this problem is by buffering the analog-switch outputs with transistor stages. With this approach, the characteristics of the CMOS switch and the buffer stage degrade video performance. Better approach exists. There are special video multiplexing ICs and video amplifiers with controllable outputs that can be used to implement very high quality video multiplexers.

    • MOSFETs reduce crosstalk effects on analog switches - Some cost-effective analog multiplexer/demultiplexer ICs, such as the CD4053 and CD4066, find frequent use as signal distributors. These digitally controlled analog switches have low on-resistance. However, with all channels in the same package, crosstalk can be annoying and unavoidable. The circuit presented in this article provides a cost-effective and viable method of solving this problem.    Rate this link
    • 48-input?16-output crosspoint IC eliminates the need for multiplexer amps - multiple camera inputs while providing playback and multiple loop-through to multiple monitors. To provide video loop-through or monitor outputs, these systems often require additional multiplexer amplifiers that can drive standard video loads. Thus, one or more external multiplexer amps often follow a crosspoint-matrix switch. As an alternative, you can employ a 32?16 nonblocking crosspoint-matrix switch, whose 16 2-to-1 multiplexers eliminate the need for extra multiplexer amps. The MAX4358 IC is a fully buffered, 32-input?16-output nonblocking crosspoint switch that includes 16 additional buffered analog-video inputs (OSDFILL) intended for the insertion of OSD (on-screen-display) information.    Rate this link
    • Build A Video Switcher - Automatically or manually switch the outputs from up to four cameras into one video monitor, pdf file    Rate this link
    • Printer port controls video multiplexer - four-channel computer controlled video-signal multiplexer for video frequencies as high as 300 MHz    Rate this link
    • Remote Controlled AV Switch with S-Video, Composite Video, and Audio - The three main devices which make up this design are the Panasonic PNA4613MOOYB IR sensor (available from DigiKey), the PIC16F84 microprocessor, and the Linear Technology LT1204 4-input Video Mux.    Rate this link
    • S-Video Source Selector - Connecting a DVD player to the S-video input on a television produces an amazingly sharp picture. However, what if you have more than one device that generates S-video but only one input on your TV? Then you'll need this simple switch to select between the two sources.    Rate this link
    • Low-Cost, Two-Chip Voltage-Controlled Amplifier and Video Switch    Rate this link
    • S-Video Source Selector - Connecting a DVD player to the S-video input on a television produces an amazingly sharp picture. However, what if you have more than one device that generates S-video but only one input on your TV? Then you'll need this simple switch to select between the two sources. This is a simple mechnical switch design for S-video signals.    Rate this link
    • Video multiplexer uses high-speed op amps - This circuit shows a video multiplexer configuration using high-speed op amps in a video-multiplexing application.    Rate this link
    • VISW 8 x 4 Stereo audio/video router - VISW module (mono) was designed years ago, primarly as main AV crossing switch for ATV repeaters. Years after, the Win95/98 VIPS package was written to enable simple use at home with help of PC as main controller.    Rate this link
    • Zoomkat's "El Cheapo" Parallel Port Video/Audio Switcher - Homemade web controlled parallel port video switcher made from Radio shack parts. ). This one is set up for 3 cam video/audios in and one out to capture card and audio card. The schematic shows for one parallel pin to switch a cam video/audio on/off using 2 resistors and 2 transistors. The combined output of 3 of these are connected at the video out and audio out RCA connectors for single seperate outputs to the video and audio cards.    Rate this link
    • Zoomkat's "El Cheapo #2" Parallel Port Video/Audio Switcher - This is version #2 of a homemade web controlled parallel port video switcher made from Radio shack parts. It can be used with other webcam programs, but you will need to make accomidations with a webserver CGI setup for web control. This switcher is set up for 4 cam video/audios in, and one video/audio out to capture card and audio card.    Rate this link
    • Zoomkat's "El Cheapo #3" Parallel Port Video/Audio Switcher - This is version #3 of a homemade web controlled parallel port video switcher made from Radio shack parts, using the 74HCT259 8-Bit Addressable latch switch. . It can be used with other webcam programs, but you will need to make accomidations with a webserver CGI setup for web control. This type switcher should work for up to 40 cams (using 5 chips) video/audios in, and one video/audio out to capture card and audio card. The schematic shows the wiring for four parallel port pins to switch 8 cams video/audio on/off using 2 resistors and 2 transistors per cam.    Rate this link

    Video mixing circuits

    A video mixer is a video production component that allows video signals from several different sources to be selected and combined. Basically mixing video sources is just summing the video materials from two video signals together. When mixing video signals from different sources, you need to ensure thatthose two video sources are in sync with each other.A good video mixer includes a frame synchronizer. This device delays one video source so that it starts at the same moment as a second source.Even more sophisticated is a frame synchronizer with time base corrector, or TBC, which not only synchronizes two pictures, but also cleans up their timing. This is important since VCRs, especially consumer models, are subject to some time base error, which causes wobbly lines and other picture flaws.A clear, steady picture requires an accurate, unchanging time base.A good commercial video mixer usually include a frame synchronizer / time base corrector, to insure that both sources are in perfect harmony and provide smooth, clean cuts between different sources.Some basic transitions can be performed without a video mixer by electronically-generating a source to match the timing of the moving video source. Such a signal is called genlocked because it is generated in a way that locks its timing to the video source. Genlocked images are usually solid color backgrounds, titles, or computer-generated graphics. Since the genlocked image is created in step with the moving video source, it is possible to perform fades, wipes, and other transitions between them. A genlocked system can fade from one scene to a solid color then fade from the color to the next scene. The term "genlock" is also used to refer to synchronizing a video signal to a house reference.When multiple pieces of video equipment are used together, there is a great advantage to having them all locked together so that every video frame from every source starts at the same instant. This is done using TBCs equipped with a reference, or sync, input. A standard reference video signal, often called house black or black burst, is routed to each reference input. This is how professional video studios are normally built.

    • Using the GT4122 & GT4124 Video Mixer ICs - The GT4122 and GT4124 are broadcast quality monolithic integrated circuits specifically designed to linearly mix two video signals under the control of a third channel.    Rate this link
    • Video Effects Unit - Digital AV mixers are available commercially, however these normally cost as much as a good video recorder, which is rather expensive for home use. The Video Effects Unit presented here has a more modest specification, with a more modest price tag! The unit enables the picture to be wiped to black at the end of a track, and then restored at the beginning of the next track. The recording video recorder would be paused once the picture is black, and the pause released just before fading in the next section. This circuit was originally published in Electronics in Action, December 1993    Rate this link

    Video modulators

    Video modulator is a circuit which converts composite video signaland audio to an RF signal which can be connected to TV antenna input. In video modulators audio+video is modulated onto a radio frequency (VHF or UHF) carrier in the range of 30 MHz to 900 MHz. This means that using a video modulator you can fed the normal videosignal you get from video out connector (RCA or BNC) to a signalwhich can be connected to antenna input connector (F-connector orIEC antenna connector) of another video device.Basically a video modulator is a very low power TV transmitter.Most analogue TV systems use a modulation called vestigial sideband.Vestigial sideband is an AM signal with most of one sideband filteredout to save bandwidth (all you need is the carrier and one sideband torecover the video). This is how broadcasters do this. In simple devicesit is usually easier to leave a "Vestige" of the othersideband to prevent phase shift from the filter affecting the signalquality.

    Video signal processing

      Video faders

      • Circuit facilitates video fading - Turning potentiometer P1 adjusts the image brightness from normal video to a black image. With the P2 potentiometer ganged to P1 , the sound also varies accordingly. The objectives in building this circuit are to use inexpensive, readily available components and to obtain batteryless operation.    Rate this link
      • Simple Video fader - This is a very basic circuit for fading video signal. You can use this between two video equipments. The circuit is so simple that the impedance matching and some other things are not exactly correct, so might not always work reliably with all equipment. Anyway it is so simple and easy to build that it does not hurt much to try.    Rate this link
      • Video amplifiers make low-cost fader - bandwidths handle SVGA or Super Mac (up to 70-MHz) video signals without degradation, fading is voltage controlled    Rate this link
      • Video fader preserves synchronization - The common video effect "fade to black" is usually accomplished by increasing video signal attenuation to the point where the picture disappears, leaving a black screen. As the composite signal is attenuated, the signal's sync amplitude becomes too small to synchronize the picture properly, and the picture rolls and tears. This circuit shows a simple video "volume control" that operates on the picture but leaves the sync unchanged, allowing a smooth fade to black while maintaining video fidelity.    Rate this link

      Sync related video signal processing

      • Digital sync-tip clamping: a new approach to video-signal conditioning - digital sync-tip clamping allows an ac-coupled ADC to digitize analog video without restoring the horizontal timing    Rate this link
      • Video Amplifier with Sync Stripper and PC Restore - This document is an application note of 200 MHz video amplifier, sync stripper and DC restorer in pdf format.    Rate this link
      • Video circuit clamps under all conditions - Many video-circuit clamps operate well in the presence of a composite-video signal but cannot achieve a clamp level with signals other than composite video or in the absence of an input signal. This circuit, developed for the ADC1175 (a popular and inexpensive, high-performance, 8-bit, 20M-sample/sec ADC), provides the normal back-porch clamp function to the input of the ADC in the presence of a composite-video signal. The circuit further ensures that the voltage presented to the ADC is within its correct operating range in the absence of an input signal and forces any signal other than composite video to be within the ADC's input common-mode range.    Rate this link
      • Video Circuit Clamps Under All Conditions - Many video-circuit clamps operate well in the presence of a composite-video signal but cannot achieve a clamp level with signals other than composite video or in the absence of an input signal. This circuit, developed for the ADC1175 (a popular and inexpensive, high-performance, 8-bit, 20M-sample/sec ADC), provides the normal back-porch clamp function to the input of the ADC in the presence of a composite-video signal. The circuit further ensures that the voltage presented to the ADC is within its correct operating range in the absence of an input signal and forces any signal other than composite video to be within the ADC's input common-mode range.    Rate this link

      Video parameter adjustment

      • The KD2BD ATV AM Video Modulator - This video signal processing circuit that allows adjustment of video gain, video bias, sync and sync level. The first section of the video modulator performs a video level clamping function. The clamped video is then level shifted. Video is amplified. FM subcarrier audio at 4.5 MHz is also injected. Clamped video is also fed into an LM311 voltage comparator that serves as a video sync detector.    Rate this link

      Reconstruction filters

      Originally, video filters were passive L-C circuits surrounded by amplifiers, but the increased gain-bandwidth product (GBW) of modern op-amps makes it possible to combine them with R-C circuits to achieve smaller, more accurate designs.

      • Active Filters for Video - Originally, video filters were passive L-C circuits surrounded by amplifiers, but the increased gain-bandwidth product (GBW) of modern op-amps makes it possible to combine them with R-C circuits to achieve smaller, more accurate designs. Active filters developed a bad reputation because of problems getting repeatable results until sensitivity analysis methods provided solutions for these problems in the 1960s.    Rate this link
      • SDTV Reconstruction Filter with Adjustable Group Delay - This application note describes a 5.25MHz, 3-Pole, Butterworth filter using a Sallen-Key realization. It has gain of 2V/V, for driving a 75 back-terminated coax to an overall gain of 1. Such filters are used for video reconstruction of RGB and component video (Y, Pb, Pr) signals following a DAC to remove the higher frequency replicas of the signal, and as an anti-aliasing filter before an ADC.    Rate this link

      Image sharpness processing and enhancement

      • Video equalizer sharpens VCR images - sharpens picture images without introducing the shadows, ringing, and noise often observed with commercial video equalizers    Rate this link
      • Video Signal Edge Enhancements - This circuit is based on the claims that the edge enhancement technique would improve the quality of standard TV images. This circuit adds information to the edges of the objects and was reported to bring out more detail. Accordign the designer this circuit gives only a marginal improvement. Still, it is an interesting circuit with which someone might experiment. This circuit is designed for NTSC video in mind.    Rate this link

      Video signal stabilizers

      Time base correction

    • Homebuilt digital video TBC/standards converter - This project is a timebase corrector/standards converter with the following features: Two CVBS inputs and one Y/C (SVHS) input; CVBS, Y/C and RGB outputs available simultaneously. This device supports PAL BGHDIN, PAL M, NTSC M, NTSC-Japan, NTSC 4.43 and SECAM input standards. Supports PAL, NTSC and SECAM output standards. Conversion of 60Hz NTSC to 50Hz PAL is possible. The 525->625 line conversion is done using line interpolation. Device supports also freeze, inverted video and mirro image effects. Device has built-in black burst and colorbar features.

    Digital video technology

    • Amiga Frame Grabber Documentation - This document describes the hardware and software for an Amiga based video digitizing system.    Rate this link
    • Closed-Caption Decoder with serial output - based on a PIC16C71, an ?lantec EL4581C sync separator, and an LM393 dual comparator    Rate this link
    • Dirt Cheap Frame Grabber V2.02 - This is a very cheap and simple but bad quality video digitizer for PC. The purpose of the DCFG is to simply provide a very simple method to grab video pictures and display them on a computer monitor. The DCFG requires at least a 80286 or better and a VGA display to run.    Rate this link
    • Homebuilt Video digitiser Mk1 - A "video-digitiser" captures television pictures from a TV set, camera, or video recorder, etc., and forwards them to a computer for display, storage, or general manipulation. This document describes a home-built digitiser which interfaces to an EPP (or bi-directional) parallel port on IBM PCs.    Rate this link
    • Homebuilt Video digitiser Mk2 - A "video-digitiser" captures still frames from a TV set, video camera, or video recorder, etc., and forwards them to a computer for display, storage, or general manipulation. This document describes the MarkII version of a home-built digitiser which interfaces to an EPP parallel port on IBM PCs. MkII supports colour captures (PAL/NTSC decoding in software).    Rate this link
    • Software-based PAL colour decoding - This page explains the issues concerning the decoding of colour from broadcast-standard television pictures, and presents software algorithms and a Windows-based application capable of colourising digitized PAL-encoded still-frame images with very high quality. This is an interesting project in practical image-processing!    Rate this link

    Combining many signals to one cable

    In some video applications there is need to combine more than one signal to one cable. ?ne common application are surveillance cameras. Because remotely located video-surveillance cameras do not always have a ready source of power, it is convenient to run both the power and the video signal through one coax cable. One way to accomplish this task is to use an inductor to present a high impedance to the video and a low impedance to dc. The problem with this method is that the frequency spectrum of a monochrome video signal extends down to at least 30 Hz (and composite video even lower, down to 15 Hz). For example, a 0.4H inductor has an impedance of only 75 ohms at 30 Hz, which is approximately the minimum necessary impedance. Large inductors have their downsides (large series resitance, self-resonance usually below the 4-MHz), thus usually this kind of functionality is usually accomplished using active electronics (a gyrator, or synthetic inductor). Simple inductors pass the power and capacitors pass the signal only approach works well on higher frequency applications, for example antenna ampliifers powered through cable or in the satellite dish systems.

    TV antenna circuits

    Most commonly asked antenna circuit might be TV antenna amplifier.Antenna amplifies can work in helping weak TV signal reception - within their limits. Most TV/FM boosters are simple, broadband VHF amplifiers. They provide an extra amplification stage for the tuner. This kind of amplifiers amplify anything entering to them that is within their operation frequency range. This means that they amplify the signal, but they will also amplify the noise. Most designs have pretty good noise characteristics, but they may belacking in other areas. In particular, some are easily overloaded by stronglocal signals (e.g. TV stations and public service band stations caninterfere). When this happens, the FM signals can become badly distorted.If you are subject to multipath problems, the booster make make them worse.

    • Building your own antenna isolators - A ground loop in your AV system caused by antenna connection or TV cable is very common if you have your computer connected to the same system. This type of ground loop problem can be solved by using suitable isolator between your AV system and the antenna cable. Antenna isolator is very convient way to solve antenna connection related ground loop problems.    Rate this link


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