A short
history of S-video: In the beginning TV's were black and white. They
just received a luminance signal (luma: how bright something is) Then
people wanted color. But how to add color and still have all the old
black and white TV's still work? The answer was to introduce a
chrominance (chroma) signal along side the luma signal. The black and
white TV's still get the luma signal and the color TV's will receive
both. Hence we get what we now call composite video. If you look the
word composite up in the dictionary you would see that it means you are
blending several things together. Unfortunatly, the composite signal
tends to be a bit blurry. So Commodore decided on the C64 to seperate
the luma and chroma signals. They called it LCA back then and it worked
by having two seperate RCA jacks. Later in the 90's the S-Video (stands
for seperated-video) connectors started to become popular. The picture
is clearer because the signals are being carried on seperate wires.
It is possible to
merge the luma and chroma signal into a single composite signal with a
very easy design, as you can see on the left. Basically both grounds go
straight through. The luma signal goes straight to the tip of the RCA
jack. The Chroma signal also goes to the tip, but passes through a
capacitor first. I didn't have a 470pf available, but I used a 580pf
and it worked fine. Guess what? You already have this circuit built
into your DTV unit. So the easiest way to get an S-video signal out of
it is to remove the capacitor which links the two signals together.
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This is the video circuit in the DTV (specifically the Hummer) and I have anotated the picture with instructions. |
Now there are two ways of adding this to your system. One way is by creating an S-video connector on your DTV. But if you do this, you will sacrifice having a composite signal. You can create a cable (see above) to convert S-video into composite so you will still have that option. (This is the approach I took) Or you could create some kind of toggle switch that you could take the capacitor in and out of the loop which would allow you to have both composite and svideo on the same unit.
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Here is another example for you. This one from the game Atomino. |
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A Side-by-side comparison using a blown up image from Thrust Concert. |
To Sum it all up: This hack isn't for everyone. First of all, it won't work on my 1084 monitor (which doesn't matter, see next paragraph) but works on every TV and video capture card I've tried it on. The results are a mixed bag. On my video capture card the improvement is small, maybe 20% better. On my bedroom's 20-inch television the improvement is dramatic. If I swtich between a composite cable and S-video there is a very noticable difference. However, on a few other televisions I've tried the advantage seems less. I suppose a lot of it has to do with how good the comb-filter (if it even has one) is on the TV's composite input. So your mileage will vary. I would say if you just play the occasional game on your DTV, I wouldn't worry about it. But if you are going to be sitting in front of it writing programs for hours, I'd highly recommend the hack.
The main reason I decided to do the hack is that I'm going to have to retire my Commodore 1084 monitor. It makes a high-pitched whining noise that bothers my wife and she says it has to go. So I'm going to be buying a 13 or 14 inch color TV with S-video input to replace it. I figure a TV isn't going to be as sharp as my 1084 in composite mode so the S-Video will probably make a more significant difference there.