Posted By
MMS
on 2020-06-28
03:56:56
 |  Re: Quality of S-Video image from a Plus/4 - mine looks bad
I just checked the C64 Wiki, thanks the info.
Actually the signal levels also not explained there, "just do it like this", but they do not explain, why.
The Composite could look better, as the standard (and the TV) also expect 1V peak signal, and this is what the Plus/4 provides too. On the other hand, the mixing of the synch, color and luminance could be problematic as the equipment and it' component ages (it is valid for the Commodore and the TV set too)
https://en.wikipedia.org/wiki/Composite_video#/media/File:Composite_Video.svg
I think this is the value what the project team (or Commodore) copied, due to some resons the SVHS ( S-Video) standard makers decided to use lower voltages for the signals.
Actually if you check the standard, the Luma line also needs the a resistor, even a higher valued one.
The Luminance (Y) signal peak is fixed 700mV at 100% white, while the chroma could increase to 850mV. (Well, actualy I am not 100% sure that 300/330 Ohm are the correct values, but some other ppl seems tried it out, so I believe they are correct, but the C64 Wiki also mentioned 2000 Ohm.
As I see it is more about the TV's circuit's tolerance (what does it do with above standard signals) than about the difference between the standards.
Maybe two independent fine potmeters with 300-3000 Ohm value could be the best solution on both Luminance and Chroma signals.
The cheapo solution is a normal liner potmeter, up to 1K or 2K or 2.5K
https://www.hestore.hu/prod_10028894.html
As I see, there are 3000 Ohm helipots, allow very fine adjustment from zero up to 3000 Ohm.
https://www.hestore.hu/prod_10036580.html
@siz may able to provide a proper value to close the gap between the 1V to 0.7V on the luminance, but it is beyond my humble skills, I did not count anything similar in the last 25 years. (75 Ohm should be the cable, but on DC it is zero, just on AC it is measurable (capacitance and impadance of cable together)
The quality of the cable (shielding) also plays a major part here. When I used cheap single, rigid AV cable juts for the video, I had much worse picture than with a properly shielded cable. But the picture with SVideo was always worse than the composite, but I've never applied any resistance to the Y/C signal (yet)
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