[Coco] S-Video

[John Kowalski]

Hi Chris,

>and if I [disconnect?] the Chroma connection on the S-Video, blue disappears

On mine, blue alone still generates a Luma signal although it is darker than
red or green alone (green being brightest).  Technically, this is normal
because the NTSC standard gives different weights to red, green and blue -
blue getting the short end of the stick because it's perceptually the
darkest color.

That aside, I *do* think the AD72x maybe goes a little too far with the
blue.. to the point where it appears a bit fuzzy and undefined compared to
red and green.  Because the chroma and luma are generated inside the chip
itself, there isn't much you can do to tune the blue.

Two things you can try:
-Change the resistors feeding the blue input so that the incoming blue has a
slightly higher drive than red and green.
-Add a seperate Luma generator and use that instead of the AD72x Luma output.

I've tried both.  The first option improves the blue definition a bit, but
at the cost of giving white and/or grey colors a slight blue tint.
The second option would allow you to adjust/mix the levels however you want
without chroma shift, but would require more parts on the board - 5 or 6
resistors, 1 transistor and 2 XOR gates.

Honestly, the improvement wasn't significant enough for me and I eventually
removed them to just use the AD724's Chroma and Luma as-is.


At 08:38 PM 12/04/2006 -0500, CoCoList for Color Computer Enthusiasts wrote:
>John:
>
>    The display from my RGB to S-Video adapter looks just like your
>picture. Blue is a little faint and a little blurry. I did some checking
>and if I the Chroma connection on the S-Video, blue disappears
>completely!! Looks like there is _no_ luminance for the blue.
>
>    Can you check on your adapter?? (ISTR, it's a separate jack.) Let me
>know.
>
>    Maybe someone here knows about s-video signals.
>
>Christopher R. Hawks
>HAWKSoft

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