|Video Signal Formats
The purpose of this article is to explain the main differences between the various different Video Signal Formats. RGB, Component, S-Video and Composite are terms that are commonly heard, but what do they mean and which one should you use.
First the basics A video signal originates in one of two ways.
At its final destination it must recreate the original image by emitting Red, Green and Blue light. A TV displays this on a Cathode Ray Tube (CRT) which emits light when a beam of electrons hits a phosphor coating on the face of the tube. If you look closely at your TV with a magnifying glass you will see one of the two images below.
|It is how these signals are processed, stored and transmitted to the display that ultimately decides how good or bad the picture will be.
Starting with the best and working down the list, here are the definitions of the different formats and how many cables are required to convey the signal to the distant end. Typically used connectors are also shown although other connector types may also be found on equipment.
|RGBHV||5 cables (5 x BNC connectors or 15 pin High Density D-Type connector)
A PC outputs RGBHV from its VGA connector (I wont mention any digital formats as that is not relevant to this topic). That is the PUREST form of ANALOGUE video you will find as each of the 5 signals is transfered discretely.
|4 cables (4 x BNC connectors or SCART)
Discrete colour signals but a COMPOSITE SYNC (S) signal containing H & V pulses. Many items of domestic video equipment that claim to output RGB actually output RGB+CompositeVideo rather than RGB+CompositeSync. For a TV this is no problem but some monitors can get upset if they expect true composite sync pulses.
Sync On Green
|3 cables (3 x BNC connectors)
As RGBS except that instead of a separate Sync, the Sync signal is sent on the GREEN colour signal just like Composite Video. This format is used by some Graphics Workstations.
|3 cables (3 x BNC connectors or 3 x Phono connectors aka RCA Jacks)
A black and white composite video signal containing Luminance LUMA (Y) Brightness information and composite sync. Cr and Cb are two signals containing matrixed colour information to extract the Red/Blue from the picture information in the Y signal. Once the red and blue is removed the only information left is green. This format is used by many DVD players although UK display equipment rarely has inputs for Component Video
|2 cables (4 pin MiniDin connector, SCART or 2 x BNC Connectors)
A black and white composite video signal containing Luminance LUMA (Y) Brightness information and composite sync. CHROMA (C) contains ALL the colour information. S-Video outputs are becoming commonplace on domestic AV equipment and almost all AV amplifiers support the switching of S-Video in addition to Composite Video
|1 cable (SCART, Phono [usually yellow] or BNC connector)
Almost the lowest of the low. A Composite Video signal contains all of the brightness, colour and timing information for the picture. Because of this there can be noticable artefacts introduced into the picture.
In order for the colour information to be combined with the brightness and timing information it must be encoded. There are three main colour encoding systems in use throughout the world with some of them also having variants.
All three colour standards are incompatable although many modern TV sets are multi-standard and can display almost any signal.
|RF||Can have many signals on 1 cable. (Coaxial Plug or F Connector)
The lowest of the low. Composite Video and usually Audio as well, modulated to a much higher frequency but can enable multiple signals to be distributed over the same cable by choosing different carrier frequencies. This is the method used for mass distribution of TV signals either via Terrestrial Aerial, Cable TV feed or Satellite distribution. If the carrier frequency is not carefully chosen different signals on the same cable can cause harmonic interference to each other causing strange patterning on the screen.
|Which signal type is best ?
Without doubt the answer is RGBHV but that doesnt neccesarily mean that is the best one for you to use. For starters... it may not be an option available to you.
|Which signal should I use ?
This depends on a number of factors
For domestic use, S-Video would tend be the most commonly supported quality format. Almost all AV amplifiers support S-Video switching whereas very few support any higher formats such as Component or RGB.
TV sets normally only have one input that will support RGB but often have several that support S-Video. DVD Players generally have the widest range of output formats.
|Can I change the signal format ?
YES Converters are readily available to convert an RGB signal into S-Video. A well regarded converter is the RGB 2 S-Video from JS Technology. Several KAT5 customers are using these units to convert RGB signals so that they can distribute them around the home as S-Video sent over Low Cost CAT5 cabling.
|Can I change the signal to a HIGHER format ?
You CAN... but there is nothing to be gained
ANY form of signal conversion will cause some degradation of the signal. If you attempt to Upconvert a signal it may well look worse after you have finished. Some of the detail has already been lost in the down conversion and the best you can hope for is a signal that looks the same.
As stated at the begining of this article, the display device ultimately has to display an RGBHV signal so the TV or Projector already has circuitry that will do that conversion for you at no cost. It is in a TV manufacturers interest to ensure that this conversion is a flawless as possible and an awful lot of effort goes into the circuit design. Due to the high production volumes for TV's, top quality components become more affordable to the manufacturer. By comparison, an external converter will have much lower sales and will almost certainly be very expensive or of inferior quality.
|KAT5 AV Distribution
S-Video is the ideal format to be distributed around the home over CAT5 cable using KAT5 AVS modules. The four pairs of the CAT5 cable carry the Luminance (Y) and Chrominance (C) signals and the Left and Right Audio channels.
This gives a vastly superior picture than that obtained from RF distribution with the added attraction of Stereo Audio. If the source is Dolby Pro-Logic encoded material then surround sound will be available to any TV sets capable of reproducing it.
|The Final Decision
The final decision is down to the user. Wherever possible you should use the highest possible standard but take into consideration the source material as well. There is no point using your highest quality input for a Digital TV receiver if the channels you watch have such low bitrates that the picture suffers badly from pixellation. It will just make it more obvious. Save that input for your best source such as DVD.