15

It's difficult to find a modern LCD monitor that can display a 15 kHz signal through VGA, so I'm wondering if S-Video is just as good when displaying 240p and 480i? I've looked at screenshot and video comparisons online, but I can't see a difference.

  • As a side-note, I recently built an RGB to S-Video converter using an AD724. It was for my Amiga 500. I also used the popular RGB -> VGA upscaler that you see on the YouTube videos for my Amiga. I was deeply disappointed with the upscaler. It never looked good and had artifacts all the time. My converter that I built (using S-Video) looks amazing. It also helps to have a good display. I used a 42" Plasma TV and a Commodore 1084S. The 1084S looks better despite the 14" screen. I've also tried NTSC<->PAL converters. They're crap too. At least in my experiences. – cbmeeks Nov 23 '16 at 18:12
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S-Video relies on colour transformation from RGB to YUV, and then takes the U and V and modulates them using a colour subcarrier. The TV has to undo all these steps in order to get the original RGB signal. If the subcarrier frequency is not in phase with the pixel clock (as will be the case if using different crystals), then moving artifacts will show up in the final image. Those moving artifacts are of course not visible in a screenshot.

There are upscalers that can take a 15kHz RGB signal (as generated by the Commodore Amiga, for example) and convert it into HDMI along with the audio. There are similar converters that do the same from RGB to VGA. Look for "arcade rgb to vga"

  • If U and V are low-pass filtered to (say) 3 MHz before QAM, will the artifacts still move, or even be that noticeable? – Damian Yerrick Mar 15 '18 at 16:26
3

The answer will depend a lot on what application you have in mind. There is a clarity difference, but how much it will matter will depend on how large of a display the output is going to as to how much you'll notice. On a 20" desktop monitor, you probably won't notice a huge difference, on a 50" TV the difference will be more pronounced.

The other factor that may matter depending on what you are using it for, is displaying 80 column text. You stated 240p or 480i, but you didn't specify horizontal resolution. If you're using, for example, a 640 x 240p resolution and dealing with a lot of text display (say application use over gaming), the difference will matter more. 80 column text is certainly readable on S-Video but the clarity will be better on a 15khz VGA signal if that's going to be a primary use for it.

  • 1
    A monochrome signal has exactly the same bandwidth and quality for S-Video and VGA. – pipe May 24 '16 at 7:26
  • True, but S-Video isn't just mono. You generally don't really want to just throw away the colour signal for the sake of clarity (Ok, occasionally you might, but a standard S-Video cable isn't wired that way). Separating the luma and chroma helps a lot, but its still not the same bandwidth as having three separate R, G, and B signals. – mnem May 24 '16 at 8:05
  • About bandwidth: If two signals (U and V) are modulated into a QAM signal, and no other signals are on the same wire pair, the theoretical bandwidth of the two signals equals the QAM carrier frequency. So the 3.38 MHz of Rec. 601 UV would appear to fit in the 3.58 MHz of NTSC S-Video. – Damian Yerrick Mar 15 '18 at 16:32
1

Component video would seem a better choice than SVGA. I don't have any circuit designs handy, but using four video op amps and some resistors it should be possible to convert most forms of "VGA" video into a component video signal that should be usable on many kinds of LCD screen.

  • A lot of component inputs on modern TVs won't accept a 240p input on the component video input though, they expect 480i or better. Its a good idea to test the TV you plan to use first by hooking up a known 240p composite video output to the Y (Green) jack and see if you get a (mono) picture to confirm that it supports it. – mnem Nov 22 '16 at 6:10
  • @mnem: Some sets do a much better job than others at handling the various not-quite-right formats collectively called "240P" from composite or S-video connectors [e.g. 228 chroma by 262 lines; 227.5 by 262, 227.5 by 263, serrations present or absent, etc.] Are component video inputs worse than S-video in any of those regards? – supercat Nov 22 '16 at 15:26
  • For the TVs that handle 240p signals at all, some handle the oddball signals better than others. Assuming your TV accepts a 240p input over component, I'd say its about the same as S-video in that regard ... variable. – mnem Nov 22 '16 at 16:50
  • 2
    My Vizio VX32L TV, for instance, displays 240p S-Video reasonably but displays corrupt 240p component. It has this issue with both my PlayStation 2 and my Wii. – Damian Yerrick Mar 15 '18 at 16:34

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