There are different formats to transmit a video signal. The most common are composite and s-video. A closer description including the advantages and disadvantages of these formats can be found in the cable-FAQ. However, a TV set doesn´t work internally wether with composite nor s-video, but with RGB. In the tube of the TV the color picture is composed by three single pictures in the colors red, green and blue. So the supposition seems very likely that one gets the best picture quality if a RGB signal is supplied to the TV without any roundabout way. But is that possible at all with a graphics card and does one really get the best picture quality? This article tries to clarify these questions.

RGB Mode

Since version 6.5 TVTool offers the possibility to choose the output format RGB. If you have a look on the connectors of your graphics card you will find a s-video output and, dependent on how generous the manufacturer was, a composite output. But there is no RGB output. That means you have to pick up your soldering iron and make it on your own! Principially three lines would be enough for the RGB transmission, one for red, green and blue. In this case the needed synchronisation signal was packed into the green signal. But most TVs on the market are not able to filter the sync signal out of the green signal. So a fourth line is necessary to transmit the synchronisation signal.

On the picture above there is a GF2-MX card where the RGB output is already put up. As you can see just a another cinch connector was added (black connector on the left). If TVTool is set to RGB mode the sync signal will be output on this connector. The color signals RGB are found on the composite connector and on the s-video connector. So if you got a card with s-video and composite out you are lucky as there is not much to do to get an RGB output.

Graphics cards with a s-video output only require two new connectors, as there are 4 signals required at all. But what makes the construction of an RGB output really unpleasent are not the additional connectors, but the fact that you have to solder the missing lines directly to the TV-chip! I guess this is the 'out' for many interested users. Before we come to the schematics let´s find out if the amount of work is worth it.

Is RGB really an improvment?

As I already mentioned TVs do work with the RGB technic. Actually the standards composite and s-video are compressed RGB signals. Composite compresses the signals R,G and B into one signal, s-video compresses it into two signals, the luminance and the chrominance signal. The compression is lossy of course, so the picture quality with composite is worse than s-video. How does it look with RGB?

To find that out the graphics card shown above is connected to a 100 Hz Sony Triniton TV with 29 inch picture size. Now the RGB input is enabled. The picture which is presented to the viewer is really better than s-video. It is the best picture what I have seen with Chrontel chips so far. But, it is not dimensions better than s-video. You can see that the picture is more clear and the sharpness is a bit better than s-video. Especially texts give this impression. Then a DivX movie is started. Also here you can see that there is an improvment compared to s-video. On the other hand, this is not an advantage only. The artifacts of the DivX compression are also visible more clearly. Unfortunately it is difficult to show the results here on the homepage with frame grabbs, because no semi-professionel video card has a RGB input. Below you find three photos (!) of the TV screen instead where you can see the differences more or less.

screen shots -> click to enlarge

Results: RGB is a big improvment compared to composite, which was to be expected. Also compared to s-video there is a slight improvment noticeable. But as you have to solder wires directly to the TV chip the conversion to RGB is a bit risky without enough experience. And the cabling is more expensive and more complex as you need four instead of two lines. So IMHO the RGB mode is useful for these groups of users:

The requirement is a TV which can handle RGB mode at all of course. Usually you can find that out via the on screen menu. Sony TVs for example signals the RGB mode with three dots above the source icon.

Go on with the directions for the construction of the RGB output.