[Coco] CoCo2 black and gray video only -- need help!

[Dave Philipsen]

The colorburst frequency of 3.579545 is derived from the 14.31818 crystal (14.31818 / 4 = 3.579545). I’m pretty sure any frequencies generated in that RF modulator are solely for generating the VHF carrier and associated signals for the TV. I assume you’re getting a clean picture on channel 3 or 4. The problem of it being in black & white is related to the colorburst which is on each video line. All of that timing is taking place in the SAM and VDG. And I don’t believe the VDG itself is very picky about the exact frequency it runs at. I have seen spec sheets speaking of using a 16 MHz crystal so it can create video at that frequency albeit not according to the NTSC color standard.

Dave


> On Oct 22, 2021, at 10:58 PM, Keith Monahan via Coco <coco at maltedmedia.com> wrote:
> 
> Thanks Dave.
> 
> I just checked the 6847T1 spec sheet...... Yeah it's saying 3.579535 to 3.597555. And I'm 3.577817. So I'm out. I was searching for some NTSC spec value, but couldn't find it. I guess more important is THIS CHIP that's actually consuming the clock is saying it's too low.
> 
> https://www.techtravels.org/wp-content/uploads/2021/10/colorburst_annotated.jpg
> 
> Is that the crystal there?
> 
> The problem isn't so much the cost of the crystal. It's that I have to unsolder the whole can to get to it!
> 
> Thanks
> Keith
> 
> 
> 
>> On 10/22/2021 11:40 PM, Dave Philipsen wrote:
>> The NTSC spec says  +3.579545 MHz ± 0.0003 per cent which is about +/- 10 Hz so you’re quite away off if you’re 1728 Hz slow. From Digi-Key the price of a new crystal is less than a dollar. I bet that it’s just old and has drifted.
>> -Dave Philipsen
>>>> On Oct 22, 2021, at 10:03 PM, Keith Monahan via Coco <coco at maltedmedia.com> wrote:
>>> 
>>> Dave,
>>> 
>>> Thanks much for the reply!
>>> 
>>> I used my scope to sample pin 1, clock output, out of the MC1372P at 2GS/s, in averaging mode, with 64k averages.
>>> 
>>> I'm getting a period of 279.5ns, or 3577817.53 hz. So 1728hz slow. I don't know this stuff that well. Is 1.7khz on the color burst a big deal?
> 
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