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Off topic - S-video to composite conversion

I've seen a number of ways of doing this.

  • Small capacitor from C to Y. Around 470 pF seems to be popular and works relatively well. But there seems to be excessive color noise compared to using the composite output of the same source (which presumably had to do the same conversion). Increasing the cap value decreases color noise but smears the picture.

  • Direct connection from C to Y. Apparently, some cheap cables do this (based on an ohmmeter, unless there is something else in there that doesn't show up as a cap or resistor!). Works well on some sources with decent resolution and minimal color noise, won't even sync on others.

  • Rs in addition to the cap, but reduces video level.

  • Active approaches?

So, is there an optimal conversion scheme???

Thanks.

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Reply to
Sam Goldwasser
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Truly no need for any active circuitry. The Bosch encoder from the mid '80s used passive mixing and was a broadcast product. Capacitor technique preferred.

GG

Reply to
stratus46

I did some more experiments using the composite and S-video outputs of a Cyberhome CH-DVD 300S DVD player feeding an RCA ColorTrak 2000 (circa

1984) TV as well as a small LCD monitor.

Best (lowest color noise): Composite output.

Next best: Direct Y to C connection using $3 eBay S-video adapter. (Virtually the same with home-built adapter that wasn't well shielded.)

Worst (highest color noise): 470 pF between Y and C.

The B/W resolution seemed similar for all three. There was almost no color noise with the composite output. The color noise shows up at edges and especially with lettering like titles and credits, or where there is fine detail.

Comments?

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Reply to
Sam Goldwasser

I thought about that more after I said capacitors. The ideal situation would be to use a summing amplifier. Nearly as good would be converting the chroma to current output rather than voltage using a transconductance amplifier and then resistively sum the chroma and luma. What exactly was in the home built combiner? Just a 'Y' cord?

None of that touches digital. HD across the LAN cables works great with the 'TV' computer connected DVI to a 50" DLP.

GG

Reply to
stratus46

(Virtually

the

I've done the home-built with the capacitor, as well as with Y and C shorted together, or with a small resistor between them.

The IC solution like Intersel ISL59114 has the summing amp approach along with a low pass filter on both Y and C. I wonder if the LPF is part of the missing link here. Except that the commercial S-video adapter has nothing inside as far as I can tell.

Let met get into S-video before jumping into digital. :)

Thanks!

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Reply to
Sam Goldwasser

The filtering is done in the Y/R-Y/B-Y section before modulating the subcarrier so it's just fine to sum it together.

GG

Reply to
stratus46

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