The full range of choices in such connections are RF, composite, S-Video, Component Video and RGB. These are described, briefly, in a moment. But first we must understand the underlying video signal.

A standard CRT colour TV set needs three colours to display its picture: Red, Green and Blue. That is also how TV cameras work, using RGB receptors. Had TV been colour right from the start I have few doubts that all TV signals would use the RGB signals as their basis. But it wasn't. TV started out as black and white. So TV transmissions then used just one signal: luminence. The carrier was simply modulated with the brightness of the scan line at a particular point. Synchronisation signals were imbedded in this signal to make the scan lines line up correctly, and to mark the start of each new frame.

When colour was introduced, in order to avoid making those millions of B&W TVs obsolete, it was decided to keep the luminence signal as it was and encode the colour information in a different way so that it would simply be ignored by B&W TVs. Thus the luminence (or brightness) signal was derived from all three colours. This luminence signal was then electronically subtracted from the red and the blue signals to create two colour difference signals. All these can be electronically remixed to re-create the original RGB signal with virtually no loss of quality.

So how does this relate to DVDs and cables? These descriptions should help you understand the cabling systems available.

RF	Some people with older TVs that do not have A/V inputs, or those who require signal switching convenience, connect their DVD player to the TV via a VCR. While this won't work with any degree of quality for many VCRs due to interference from Macrovision copy protection of the DVD, for those few models which work okay the VCR radio frequency modulates the composite video signal and feeds it to the TV's antenna input. This is in effect emulating a TV broadcast, minus possible reception problems. Read about "composite" to find what this means.
Composite	How do you get the three signals (luminence, red colour difference and blue colour difference) into one signal suitable for broadcasting or sending down a single strand of wire? By matrixing them together. This is what happens in composite video connections. All three signals become one and it's up to the TV to do the best job it can separating them out again. In fact, the two colour difference signals are first matrixed together and then these combined are matrixed into the luminence signal. In practice this means that the two colour difference signals interfere with each other and with the luminence signal.
S-Video	This system keeps the luminence signal separate from the two colour difference signals by providing it with its own bit of wire. The two colour difference signals are still matrixed together, but at least they can't interfere with the luminence signal.
Component	This is the stage at which the RGB signal has been processed to yield luminence and the two colour difference signals. Three separate conductors are used in component video to avoid all matrixing. Component video is the native format for DVD: that is how the pictures are stored on the discs. Thus the DVD signal is actually matrixed down for the S-Video output and further matrixed down for the composite video output, both of which steps have to be reversed by your TV.
RGB	This, of course, is the original recording format used by studio video cameras and the telecines used to digitise film for ultimate DVD rendering. And it is the format used by CRT display devices to actually produce a picture in your home. European TVs and DVD players tend to offer this signal standard with connection via SCART cables. Remember, though, for DVD use it is not an inherently higher standard because the picture actually spends its time on your DVD in component format.

Consider, for a moment, what happens to a video signal from the time it enters the video camera during a live broadcast of, say, the cricket until it appears on your TV screen. The camera captures an RGB picture. This is converted to component, then down-converted to S-Video, then further down-converted to composite, then radio frequency modulated for transmission, then, when it gets to your home, RF demodulated, upconverted to S-Video, then further up-converted to component video, then converted to RGB ready to fire the guns at the back of your TV.

Now consider DVD (leaving out MPEG compression). The RGB picture is converted to component, delivered to your home on a shiny disc, then converted back to RGB.

Now does all this processing make a difference? Definitely, and quite clearly with certain pictures. How much difference depends in large part on the quality of your TV's input and signal processing circuitry.

I present here a series of twelve pictures, shot with a digital camera at very close range to a Philips 68cm 29PT9418 Flat Screen TV. The picture is the bouncing concentric circles test pattern in Title 15 of the Video Essentials test DVD. I used a Sony DVP-S725D DVD player to supply the signals in all cases. In each of three cases, without moving the camera, I shot the picture four times. The first shot is an RF feed to the TV via an Akai VCR. The next is via composite video. The third is via S-Video while the last is via component video. The Australian version of the Philips TV is not equipped with an RGB input so I was unable to capture any differences due to this. Note that this DVD is NTSC rather than our own, potentially better, PAL. Still, it illustrates the point. Note also that the shots for component video are shifted slightly to the left in comparison to the others. This appears to be due to a different sync alignment on either the TV or DVD player. Certainly the camera was not moved. The DVD was in pause for all shots.

Concentric Circules This is where the interference between the colour difference and the luminence signal becomes obvious. Note, there is little difference between the RF and composite shots, nor between the S-Video and component shots, but a gulf in the form of rainbowing between the two categories.
RF
Composite
S-Video
Component
Diagonal Lines Again those nasty rainbows appear for RF and composite video feeds. Note that this picture is supposed to show fine lines rising diagonally from left to right. This indeed is what it looks like sitting at a normal distance from the TV ... at least with S-Video and component. The differences between the rainbow patterns of the RF and composite shots are because the rainbow was swirling around, even though the DVD picture was paused.
RF
Composite
S-Video
Component
Colour Bars But what difference is there between S-Video and component? Colour clarity in finely spaced situations. Notice the clear rendition of the colours in the vertical bars on the component shot and the relative muddiness on the S-Video shot. Note also the jagged nature of the darker lines on the RF and composite shots.
RF
Composite
S-Video
Component

The moral of this story? When you buy a new TV, insist on S-Video at the very least. And preferably strive for component video.

© 2000 by Stephen Dawson