[Coco] S-video Prospects...

[John Kowalski]

Hi Chris,

The reason I changed the capacitor on the clock crystal was because I just
got a black & white display when I used the recommended capacitor value.  I
suspect my crystal may have been a bit off and my monitor simply did not
lock onto the NTSC colorburst frequency until I tweaked it a bit.

None of the changes to capacitors/resistors will have an effect on the
horizontal centering of the CoCo's display.   Did you try hooking up the
regular video out (from the back of the CoCo3) to your monitor?  Is the
first 1/2 character in the 80 column display still cut off?   I'd be willing
to bet that yes, it still is.

The problem isn't the circuit, but rather the CoCo's video generator.  They
messed up the video's horizontal timing when they designed the GIME.  CoCo3
users have had to suffer the first 1 or 2 characters in the 80 column screen
getting cut off on their TV for a long, long time.

Anyway, it depends on your monitor.  My CoCo's monitor has a horizontal
position knob and I can adjust the centering.  My TV does not, but it
displays okay anyway.  But, my PC monitor (which also has an S-video input)
does not let me adjust it and cuts off the first character in the 80 column
screen.

In a case like that, there is only one thing you can do - and that's add
some extra circuitry to the S-video board to delay the HSYNC line.  (We
actually want to make HSYNC 2% earlier, but I think that's very complicated
to do.  Making it 98% later will effectively be the same thing.)

Maybe someone on the list has a good simple idea how to go about doing that.
I'm thinking it will require a timer or two generate a new HSYNC pulse,
using the original HSYNC pulse as reference/start timing.



At 10:14 PM 25/01/2006 -0600, CoCoList for Color Computer Enthusiasts wrote:
>John & Stephen:
>
>    I built the RGB to S-Video adapter and it works great, EXCEPT...
>On the 80 column screen (and maybe the 40 column, it's har to tell), I'm
>missing a few dots at the beginnig of the lines. About 1/2 a character on
>the 80 column screen. Either of you see this??
>
>    My circuit is right off the data sheet, except for the 75 ohm
>resistors on the R, G, abd B lines. I used 33 and 47 ohm resistors:
>
>     33
>---/\/\/--------||-----
>Coco       |           ad724 
>           /
>           \ 47
>           /
>           \
>           |
>          GND
>(1.2v * 47 / 75 = 7.5v)
>
>    Does the same with 2  75 ohm resistors.
>
>John:
>    What did the display look like before you changed the crystal's
>capacitor???
>
>Christopher R. Hawks
>HAWKSoft


                                         John Kowalski (Sock Master)
                                         http://www.axess.com/twilight/sock/