PC monitor technology and interfaces

Monitors general information

PC monitor system is complicated system, but luckily for us it's one that's easy to understand. The following description is centered on traditional analogue CRT PC monitors.

The video adapter in PC sends the signals from it's image memory at fixed rate (usually configurable) through the DAC (digital to analog converter) circuit to the monitor connector on the graphics card. The DAC converts numeric pixel color values to voltage levels for red, green, and blue which are sent to monitor through the monitor cable. Most monitors today use the traditional CRT, which works on the same scientific principle as a television set. This vacuum tube produces an image when an electron beam strikes the phosphorescent surface inside the monitor. Normal PC VGA monitors nowadays are so called "non-interlaced" monitors. The computer requires a "video Interface" sometimes referred to as a video card to communicate with your monitor. Your monitor is the single most important component of your computer system if you want to get good picture quality (also the graphics card can contribute to this).

The visual quality, depends on the quality of your monitor. Consumers have now become more concerned about the visual quality. The flat screens, high resolution, high refresh rates, and recently the USB and solid state screens top the list of desirable features. The multimedia monitor includes oudspeakers of some sort, maybe a microphone and in some cases a camera for video conferencing all in the same box as the monitor.All analog monitors can produce thousands of colors, it is inherent in the design. The limitation on color registration is directly related to what is available in the interface card and the mode selected. There are practically infinite number of colors possible with the analog monitors (although they can not properly display all natural colors correctly).

Resolution is the number of pixels the graphics card is describing the desktop with, expressed as a horizontal by vertical figure. Standard VGA resolution is 640 x 480 pixels. The commonest SVGA resolutions are 800 x 600 and 1024 x 768 pixels. A typical PC monitor is designed to accept signals at wide resolution and frequency range. When you change the seolution on refresh rate on monitos, just the scan frequencies that are changing to accomodate the new timing/pixel format. The focus (which is sort of the electron beam width, at least as it is seen at the screen) MAY be altered slightly as well, if the monitor has the capability of storing adjustments for that and other parameters (geometry, convergence, etc.) for specific timings, although it is VERY unusual for focus to be included in this. Please not that the monitor physical dot pitch can't change - that's a fixed physical parameter of the CRT itself - but the physical dots on the screen (or the holes in the shadow mask) really have nothing at all to do with the logical pixels of the image, other than being one of the things which ultimately limits the resolution. The scan frequencies do not necessarily change at all when you change the resolution. What happens is that the signal as seen on the VGA plug has (for example) 1024 discrete values between 2 consecutive line syncs as opposed to 800 discrete values and 768 line syncs between frame syncs as opposed to 600 (assuming non interlaced). Typical PC CRT monitor an display all resolutions from the lowest up to the highest supported resolution well. If you have a modern flat panel display, things can be different. On TFT monitors they specify a "recommended" resolution that the TFT works best at and when not run at this resolution they get seriously blocky and in some cases unreadable text.

Refresh rate, or vertical frequency, is measured in Hertz (Hz) and represents the number of frames displayed on the screen per second. Too few, and the eye will notice the intervals in between and perceive a flickering display. The world-wide accepted refresh rate for a flicker-free display is 70Hz and above (preferably 75 Hz or more). The flicker is strongly dependent upon visual angle, because eye peripheral vision response is faster than the higher resolution centerof field vision. The bigger the monitor, or the closer you are to it, the worse the flicker will be, so you will need higher refresh rate to get "flicker free" picture. CFF (Critical Flicker Fusion) also depends on illumination levels. The CFF frequency is lower at lower illumination levels. As the height of the picture increases, it is necessary to increase the number of horizontal lines to create a smooth line-free display image. To do this, the monitor and the interface card increase the frequency of the repetitive horizontal scan rate.

In order to consistently reproduce the video information at a high resolution, the monitor must have a wide video bandwidth. In order for the term to be meaningful for comparison purposes, the bandwidth expressed in mhz. must be within +- 3dB You might see a term "sync signal" sometimes.All computer monitors require a "sync" signal which determines the resolution of the display. Some monitors require the sync signal to be a separate electrical connection, some monitors require the sync signal to be mixed in with the green video signal (sync on green). Some monitors support both separate sync and sync on green. PC VGA card uses separate sync signals and PC monitors are designed to accept at least this sync format.

The term "dot pitch" is the measurement in milimeters of the distance between two adjacent phosphor color elements. There are two color phosphor systems in use today in CRT monitors: triad dot shadow mask (most monitors) and aprature grille (used in the trinitron tube from SONY). NEC has developed a hybrid mask type, called slotted mask, which uses elliptically-shaped phosphors grouped vertically and separated by a slotted mask.

Here are some guidelines for suitable resolutions for different monitors:

If you use a higher resolution exceeding these guidelines, a very good monitor may deliver adequate pictures but you can also run into a poor quality picture. Keep in mint aldo what frequencies and resolution your monitor can handle. Trying to use frequencies and resolutions that the monitor was NOT designed to support can severly damage your monitor. To make the monitor installation easy, VESA has produced several standards for plug-and-play monitors. Those standard features (like DDC) should in theory allow your system to figure out and select the ideal settings, but in practice this very much depends on the combination of hardware.

Here is an overview of different video display resolution standards and de-facto standards in use (not all lf them used in PCs):

Computer Standard Resolution
VGA 640 x 480 (4:3)
SVGA 800 x 600 (4:3)
XGA 1024 x 768 (4:3)
WXGA 1280 x 768 (15:9)
SXGA 1280 x 1024 (5:4)
SXGA+ 1400 x 1050 (4:3)
WSXGA 1680 x 1050 (16:10)
UXGA 1600 x 1200 (4:3)
UXGAW 1900 x 1200 (1.58:1)
QXGA 2048 x 1536 (4:3)
QVGA (quarter VGA) 320 x 240 (4:3)
Analogue TV Standard Resolution
PAL 720 x 576
PAL VHS 320 x 576 (approx.)
NTSC 640 x 482
NTSC VHS 320 x 482 (approx.)
Digital TV Standard Resolution
NTSC (preferred format) 648 x 486
D-1 NTSC 720 x 486
D-1 NTSC (square pixels) 720 x 540
PAL 720 x 486
D-1 PAL 720 x 576
D-1 PAL (square pixels) 768 x 576
HDTV 1920 x 1080
Digital Film Standard Resolution
Academy standard 2048 x 1536

In the late 1980s concern over possible health issues related to monitor use were raised. In Sweden this resulted a standard MPR1 to be developed. This was amended in 1990 to the internationally adopted MPR2 standard, which called for the reduction of electrostatic emissions with a conductive coating on the monitor screen. In 1992 a further stricter standard, entitled TCO (TCO92), was introduced by the Swedish Confederation of Professional Employees. Other relevant monitor safety standards include: ISO 9241 part 3 (the international standard for monitor ergonomics), EN60950 (the European standard for the electrical safety of IT equipment) and the German TUV/EG mark (monitor has been tested to ISO 9241 part 3, EN60950, MPR2 and German standard for basic ergonomics ZH/618). TCP99 is the latest iteration of the standard TCO99 give rgulations on screen refresh rates. To reduce eye fatigue caused by image flicker, the minimum required refresh rate is increased to 85Hz for displays of less than 20in, with 100MHz recommended, and to a minimum of 75Hz for 20in or greater.

Power consumed by the monitor can also be a significant figure. In 1993, VESA initiated its DPMS standard, or Display Power Management Signalling, which allowed a DPMS compliant graphics card to turn the montor to standby more or suspend modes which consume considerably less power than normal operation. EPA Energy Star is a power saving standard, mandatory in the US and widely adopted in Europe, requiring a mains power saving mode drawing less than 30W. In 1995, TCO was expanded with a range of conditions to cover environmental issues. TCO95 became the first global environmental labelling scheme. Over and above TCO92, the product may not contain cadmium or lead, the plastic housing must be of biodegradable material and free of brominated flame retardants and the production process must avoid use of CFCs (freons) and chlorinated solvents.

Monitor interfaces

Analogue VGA interface

Modern PC graphics cards even nowadays still use the old 15 pin VGA connector (known ad HD15 connector). The connector carries the video signal in RGB format. The sync information is carried through separate horizonal and verticna sync signal wires. This whole video signal format (video and sync) is generally referred as RGBHV signal format.The signal levels in RED, GREEN and BLUE signal are 0.7V peak to peak video signals terminated to 75 ohm load in video card and monitor ends. All other signals are TTL level signal (around 0..0.7V means logic 0 and 3..5V means logic 1). Analogue video signals are carried through 75 ohm coaxial conductors. The video signal carried vy VGA connector can also be carried with separate 5 coaxial cables with BNC connectors (some monitors and video projectors use this kind of interface and adapter cables for this are readily available). The pinout of video signals on VGA connector:

In addition to video signal, the VGA connector has some monitor identification pins (pins 11, 12 and 15) that allow PC video cards to determine what type of monitor is connected to the graphics card. The originaal plan used to such that the monitors grounded some of those pins to tell that the monitor is there and what type of monitor is there. Modern plug&play monitor systems have changed their use in such way that pins 11 and 15 are used for DCC data communications between computer and monitor (pin 12 = DDC DATA, pin 15 = DCC Clock). The extra control signals are generally carried through separate wires (all inside one cable main shield). Pin 5 is sometimes referred as GND TEST and sometimes just ground.Pins 4 and 9 are not generally used. Not all connector pins are used in VGA cables. Generally pins 9 has been removed because is is used in many devices as key to stop plugging in full 15 pin connectors. In some cales also pin 15 have been removed for compatibility with all VGA computers (also older ones, because pins 9 and 12 pins were removed in early VGA cables and blocked in old VGA cards).Here is one common wiring used:
Pin New VESA DDC     Old VGA
1 Red Red
2 Green Green
3 Blue Blue
4 No Connect Reserved
5 Ground Ground
6 Ground Red Ground
7 Ground Green Ground
8 Ground Blue Ground
9 No Connect No connect
10 Ground Ground for syncs
11 No Connect Monitor ID 0 (ground)
12 DDC DAT Monitor ID 1 (no connect)
13 Horizonal Sync Horizonal Sync
14 Vertical Sync Vertical Sync
15 DDC Clock No Connet
The video signals carried in VGA connector are designed to be matched to 75 ohm load and use coaxial cable. At least the RGB signals on the cable used to connect VGA signals must have 75 ohm coaxial construction to guarantee good quality high resolution image. A typical high quality VGA monitor cable or VGA extension cable has three three 75 ohm mini coax cables to carry RGB signals and 9 other wires (typically 24 AWG) to carry other signals like syncs and monitor identification. The whole cable has a good metallic shield around all of those wires. Some very high quality cables use five 75 ohm coaxial cables and for RGBHV signals and just few extra wires. In applications where monitor ID signals are not needed, just five 75 ohm coaxes are used to transfer VGA signals. Typical features of flexible mini coax cables (from http://www.drakausa.com/pdfsHHT/AVprcise.pdf for extra flexible miniature coax):
  1 MHz:   0.6 dB/100ft
5 MHz: 1.3 dB/100ft
10 MHz: 1.8 dB/100ft
30 MHz: 10.2 dB/100ft
100 MHz: 17.1 dB/100ft
Some attenuation figures of high quality VGA extension cable (for reference of typical features):
 10 MHz:  1.6 dB/100ft
50 MHz: 4.0 dB/100ft
100 MHz: 6.1 dB/100ft
200 MHz: 9.8 dB/100ft
300 MHz: 13.0 dB/100ft
400 MHz: 15.9 dB/100ft
The connector uses in VGA connections is HD 15 connector. This ubiquitous connector is convenient, low cost, and most importantly, adopted by IBM, but technically not the best possible connector. The connector was originally selected to be good enough for the signals existing in the early days of VGA interface and was more than good enough for this use, but VGA connector has it's limitations at high resolutions. Does anyone know the impedance of a 15-pin VGA connector? Unfortuantely the HD15 connector used does not match to 75 ohm impedance (in reality the impedance of a typical VGA connection is about 100 ohms). Even though the connector impedance is not exactly right, the primary issue centers on the limited length of the connector interface, so it does not significantly hamper performance in systems we most often deal with.This HD15 connector is still used, because this ubiquitous connector is convenient, low cost, and most importantly, adopted by IBM. It is still with the limitations considered "good enough". And in practice one VGA connector on the route from the graphics card to the monitor does not cause too much problems for picture quality. The primary issue for this centers on the limited length of the connector interface. Because of the limited length, it does not significantly hamper performance in systems we most often deal with. Because there is no no significant effects, hence the popularity of the VGA connector as a low-cost, general interface for the PC even nowadays. The problems of connector impedance mismatch becomes visible if you happen to have more than one VGA connector on the route to monitor and you run high frequency video signal (high resolution at high refresh rate). Impedance mismatch degrades the picture quality. You can see the impedance matching problems usually when you use devices like VGA monitor switch boxes, VGA extension cables etc.

Digital Visual Interface (DVI)

Digital Visual Interface (DVI) is the standard interface for high-performance connection between PCs and Flat Panel Displays, Digital CRT Displays, Projectors, and HDTV. DVI Cables deliver the high-performance, high-bandwidth interface needed for video displays of today, and leaving headroom for the products of tomorrow.

DVI standard is defined by DDWG (Digital Display Working Group). DVI most commonly used digital video interface with PCs in DVI. It comes in theree version: DVI-A, DVI-D and DVI-I. The difference on those is that DVI-D support only digital signals, where DVI-I includes both digital and analogue video signals (analogue signals are same as used by VGA interface). DVI supports hot plugging of DVI display devices.

DVI-A format is used to carry a DVI signal to an analogue display, such as a CRT monitor or an HDTV. Basically this interface has same signal as VGA connector has, but uses different shape connector. DVI-A can transmit a higher quality picture than standard VGA, because the connector user matches better to the needs of transported high frequency video signal than the old 15-pin VGA connector.

DVI-D is a digital only connector version of DVI interface. DVI-D is the leading connector standard for digital only connection. It comes in two flavors: Single Link and Dual Link. The primary difference between Single Link and Dual link is that each supports varying resolution levels. DVI-D uses LVDS signaling for digital signal and supports cable length up to 5 meters (longer distances are possible with repeaters every 5 meters). In case of longer transmission distances are needed, you need to either have DVI repeater every 5 meters or use a special converter that converts DVI signals to fiber optics and back. Some manufacturers seem to make also 10 meters long DVI-D cables, but because those are longer than standard permits their operation is no guaranteed (causes unreliable operation and signal transmission errors on many equipment, but can work on some equipment). The DVI-D Single Link supports resolutions up to 1920x1080. For gher resolution there is a dual link version also available.

Within the DVI system, parallel data from the computer graphic memory is serialized (similar to digital television) and transmitted differentially over a minimum of four twisted pair wires:a red channel, green channel, blue channel, and clock channel at about 165 mega-pixels/second per channel (1.65 Gbps on the basic system). The RGB data are not simply serialized and dumped onto the cables. Encoded sync information is carried along and the data is scrambled using a specific routine that minimizes errors during transmission from source to destination. The system operates on 3.3 volts and can operate at lower voltages. The twisted pair differential swing is about 1.0 volt peak-to-peak.

The DVD-D dual link configuration provides enough bandwidth for resolutions up to 2048 x 1536, and is designed for digital use only. In dual link system the number of wires used to transport red, green and blue component data is doubled (giving total 7 pairs of wire used to transport data). The DVD-D dual link uses DVI-D 24-pin connectors and supports digital signal only. To support those high resolutions, very high data rates are needed in the cable. DVI achieves up to 9.9-Gbps dual-link or 4.95-Gbps single-link data speeds.

DVI-I format is an integrated cable which is capable of transmitting either a digital-to-digital signal or an analog-to-analog signal.DVI-I can supports both digital DVI-D signals AND analog (RGB). The connector has a few more pins than digital only DVI-D. Many graphics cards manufacturers are offering this connector type on their products, so this can be connected to either digital or analogue display device. The signals from DVI-I connector can be adaped to analogue VGA signal by using a simple connector adapter (usually comes with graphics card, can be bought separately). DVI-I format is an integrated cable which is capable of transmitting either a digital-to-digital signal or an analog-to-analog signal. Make sure that you know what format each part of your equipment is before you purchase any DVI cables. Only equipment with a DVI port labeled 'DVI-I' will accept both a DVI-D and DVI-A source signal.

The physical cable used to do DVI connection has different conductor types depending on the signal they carry. The digital signals are carried through twisted pairs that have 100 ohm +/- 15% impedance (usually separately shielded twisted pairs). Analogue video signals are carried through 75 ohm coaxial conductors.

Transmission of the TMDS (transition minimized differential signaling) format combines four differential, high-speed serial connections (in its base configuration) transmitted in a parallel bundle. When the DVI specification is extended to the dual mode operation, greater data rates for higher display resolutions are possible, but now there are seven parallel differential, high-speed pairs. Cabling and connection become extremely important. The DVI cable and its termination is very important. The physical parameters of the twisted pairs must be highly controlled. Specifications for the cable and the receiver are given in fractions of bit transmission time, so the requirements depend on the clock rate or signal resolution being used. Transferring the maximum rate (1600 x 1200 at 60 Hz) for a single link system means that one bit time (10 bits per pixel) is 0.1 (1/165 MHz), which is only 0.606 nanoseconds. Ten bit times describe one pixel in this system. The DVI receiver specification allows only 0.40 x bit time, or about 0.242 nanoseconds intra-pair skew (within the twisted pair). A cable for DVI-D should be evaluated on its insertion loss for a given length. The DVI transmitter output eye pattern is specified into a nominal cable impedance of 100 ohms. A normal signal swings +780 mV to -780 mV. The minimum positive signal swing is +200 mV and the minimum negative swing is -200 mV (total swing of 400 mV). When the signals are combined in the differential receiver, the resulting signal level is two times the swing value. But, for the cable situation, we must assume minimum performance on the transmitter side and best sensitivity on the receiver end. The receiver must operate on signals as low as +75 mV to -75 mV, or a total swing of 150 mV. This means that under worst-case conditions, the cable attenuation can be no more than 8.5dB at 1.65 GHz (10 bits/pixel times 165 MHz clock). As you can imagine, maintaining this type of performance on twisted pair wires is relatively difficult. The nominal DVI cable length limit is 4.6 meters (about 15 feet). Electrical performance requirements are similar to serial digital. Signal rise time (0.330 nanoseconds), cable impedance (100 ohms), far end crosstalk (FEXT) of no more than 5%, and signal rise time degradation (160 picoseconds maximum) are the key parameters highlighted in the DVI specification regarding the physical connection.

Cable for DVI is application specific because maintaining these specifications is no easy feat since the actual bit rate per channel is 1.65 Gbps. And, we're talking twisted pair cable here. Upgrade your system's video performance by connecting VGA- or DFP-configured monitors to fast DVI cables.Use DVI to eliminate resolution or color changes and pixel-lock adjustments in laptop-to-projector connections, too.

29 pin DVI Connector PinOut and Signal Names
Pin # Signal Name Pin # Signal Name Pin # Signal Name
1 TMDS Data2- 9 TMDS Data1- 17 TMDS Data0-
2 TMDS Data2+ 10 TMDS Data1+ 18 TMDSData0+
3 TMDS Data2/4 Shield 11 TMDS Data1/3 Shield 19 TMDS Data0 Shield
4 TMDS Data4- 12 TMDS Data3- 20 TMDS Data5-
5 TMDS Data4+ 13 TMDS Data3+ 21 TMDS Data5+
6 DDC Clock [SCL] 14 +5 V Power 22 TMDS Clock Shield
7 DDC Data [SDA] 15 Ground (for +5 V) 23 TMDS Clock +
8 Analog vertical sync 16 Hot Plug Detect 24 TMDS Clock -
C1 Analog Red -- -- -- --
C2 Analog Green -- -- -- --
C3 Analog Blue -- -- -- --
C4 Analog Horizontal Sync -- -- -- --
C5 Analog GND Return: (analog R, G, B) -- -- -- --