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Quorten Blog 1

First blog for all Quorten's blog-like writings

S-Video explained

2019-02-12

Categories: media-prog   apple-ii  
Tags: media-prog   apple-ii  

So, for quite some time, I’ve been doing a nifty trick with S-Video and composite video. I have this adapter plug that has a S-Video plug on one end and a composite video plug on the other end. I have no idea how it works, or whether it was intended to be bidirectional or not, but experimentally, I found that it is bidirectional. So, any time I wanted to connect an S-Video output to a composite video input or vice versa, I would use that cable, without thinking exactly how it works. However, especially when working with Apple II video output, I did prefer to convert composite video to S-Video because I noticed that television equipment displays S-Video with a higher resolution than composite video, even with the same video input signal. Now, this makes me wonder. How exactly does S-Video work? I consult Wikipedia, and lo and behold, I find the schematic diagram for my adapter cable right in there.

20190211/https://en.wikipedia.org/wiki/S-Video
20190211/https://en.wikipedia.org/wiki/File:S-video-composite-adapter.svg

S-Video and composite video adapter schematic

S-Video is really simple in concept, and when coupled with the converter cable, really powerful in practice. The primary goal of S-Video is to improve video quality by carrying the luma and chroma signals separately. This allows each signal to be conveyed with higher fidelity. By contrast, when television equipment wishes to display composite video, it must filter out the chroma signal from the luma signal, effectively reducing the resolution of the chroma signal.

So, how does the composite video and S-Video conversion cable work? Simple. You pass through the luma signal directly to the composite video signal pin, but the chroma signal must go through an approximately 470 pF capacitor. The grounds of each signal are wired to the common ground of the composite video. Wow, so that’s how they made that conversion cable so small and bidirectional. To convert from S-Video to composite video, you simply mix the signals together, with a capacitor to process the chroma signal. To convert from composite video to S-Video, you simply separate out the chroma signal using a capacitor and pass through the combined chroma-luma signal to the luma.

Now, here’s the catch. With this video signal converter cable, you realize something that unfortunately, many video equipment designers have not. You don’t need to filter the chroma signal out of the luma signal. In fact, black and white televisions work exactly by displaying the combined chroma and luma signal. The chroma signal shows up a a random dot pattern that is otherwise negligible. So, if you wire in your composite video output into our television’s S-Video input, you get to bypass the low-pass filtering under all circumstances, yet pass through the same video signal. This gives you a perceptively sharper image. Especially in the case of Apple II 80-column monochrome text, the image is literally sharper, and the difference makes or breakes whether you can reliably read the text in 80-column mode. So, now you know the story of the mysterious S-Video and composite video conversion cable.