Which Connector?

Taming the Composite, S-Video, Component and RGB Jungle

In the days before DVD, this topic wasn't relevant to the vast majority of home theatre enthusiasts. Most of us would have used the RF (antenna) output of our VCR to connect up to our TV. Some would have gone a little further and connected up using the composite output because it looked better. A small, very fortunate minority of us, had to grapple with the intricacies of hooking projectors up to video sources.

Things have changed. DVD has brought a new level of readily-accessible video quality into the home environment, and along with it, several unfamiliar ways of connecting up to your video display device to access this additional quality.

Before we can discuss the nitty-gritty of the various video connectors that you will find on the back of your DVD player, we need to briefly discuss just what is recorded on a DVD and a little about how it got there in the first place.

What comes out of a TV camera?

A TV camera outputs a video signal that is split into the three primary colours; red, green and blue (RGB). The entire colour spectrum can be represented by varying intensities of these three colours. This signal needs to be modified before it can be further processed or broadcast. Why?

Problems with the RGB signal

The RGB signal has two specific problems associated with it in the professional video world. Firstly, it has a very high bandwidth. Secondly, the colour and the black and white picture information are combined within the RGB signal. This is dealt with in the professional video world by converting the RGB signal into a component signal, also referred to as a YPbPr or YCbCr signal. The Y component of this signal is the black and white information contained within the original RGB signal. The Pb and Pr signals are colour difference signals, which are mathematically derived from the original RGB signal. For our purposes, it is sufficient to understand that the Y signal contains full bandwidth black and white picture information, and the colour difference signals contain bandwidth reduced colour information.

It is important to realize that component video output and RGB video output are not the same and are not directly compatible with each other, however, they are easily converted either way using a transcoder. Note that transcoders are generally professional-level video equipment and priced accordingly. You should expect to pay around $1000 US for an external transcoder, so this is an impractical solution for all but those of you with high end projection equipment and/or a high end DVD player. Here is an example of the sort of equipment that you are looking at.

What's recorded on a DVD?

DVD stores a component video signal in digital format. Since this is the native video format that is stored on DVD, this is also the best format to use to display the picture, if your equipment is capable of dealing with this type of signal. In Australia, virtually no equipment exists that is compatible with a component signal, though there is some that is compatible with an RGB signal. Many DVD players are capable of converting their native component signal to an RGB signal, but this varies on a player-by-player basis.

Problems with the Component signal

As discussed above, DVD stores its video information in the component form, but unfortunately the great majority of us cannot take advantage of this format. The designers of the DVD format anticipated this, and made allowances for it in the specification. All DVD players are capable of downconverting a component video signal into a more suitable format for display on the current generation of consumer display devices. The first such downconversion step is to S-Video, which is a connector that will always be found on any DVD player.

S-Video

Remember that the component video signal is split into three parts; black and white information (Y), and two colour difference signals (Pb and Pr). The S-Video connection keeps the all-important black and white (Y) information separate, and combines the colour difference signals into a single colour signal (C). Instead of three separate signals going to the display device, there are now two separate signals.

As you would expect, combining the two colour signals results in a degradation of the colour information. In the grand scheme of things, this is a fairly minor degradation, and you still get an exceptionally good picture from this signal.

Composite

What about if your display device doesn't have an RGB, component or S-Video input? Well, then we downconvert another notch to composite video. As its name suggests, composite video is a single video signal that is a composite of the black-and-white information (Y) and the colour information (C). This is the same type of signal that at least some of us will have been using prior to the advent of DVD to connect up our laserdisc players or VCRs.

Composite video signals have a number of unavoidable image problems because of inherent limitations of the PAL and NTSC systems. The problem is, once the colour (C) and the black and white (Y) information have been put together, they can no longer be perfectly separated due to fundamental design limitations of the two systems. Whilst a detailed description of these image problems is beyond the scope of this article, there are two specific artefacts which I will mention which are readily demonstrable.

Dot crawl. Colour Bars Test Pattern This occurs on the boundaries between two colours where you can see moving blocks of incorrect colour information. The simplest way of demonstrating this artefact is to look at a test pattern on your TV. If you have access to either Video Essentials or A Video Standard on DVD or Laserdisc, take a look at a colour bar pattern. On the Video Essentials DVD, this is located at Title 10, Chapter 10, amongst other places. In particular, look at the vertical edges between the colour bars. You will notice that the edges are smeared, with little blocks of moving colours throughout in a regular pattern. If you then compare this same test pattern via an S-Video input, you will see that these blocks have disappeared, and the edges of the colours are sharp and clear. What applies to a test pattern also applies to normal images that are displayed with composite video. They, too, will exhibit dot crawl at the boundaries of different colours on the screen.

Cross-colouration. Cross Colour Test Pattern You may be familiar with this artefact when watching a black-and-white image on your TV, particularly if it is an older model. Fine lines in the image result in a purple colour being displayed by the TV. If you look at the test pattern shown on the right (Title 17, Chapter 23, Video Essentials) via a composite video output, you will readily see if your TV is prone to this artefact. If you look at the same test pattern via an S-Video input, you will not see this artefact.

Conclusion

In a nutshell, if your display device can support it, a component or RGB video connection will give you the best possible image from DVD. An S-Video connection is not far behind in quality. A composite connection, however, will result in a perfectly acceptable, but less than optimal result from your DVD player.