>> Hi, All!
>>
>> Before Christmas I asked you all for ideas and suggestions
>> on a video capture device I was planning to design.
>>
>> Within the holiday I had a working design. At least almost working.
>>
>> In short this is how the design works:

>... interesting details on a video sampler omitted ...

Yes, but I said "in short"..   :-)

>one thing which comes to mind is why you don't just get the whole field
>between two VSYNC, leaving HSYNC processing to the PC (you just have to
>make the HSYNC/VSYNC pulses store an "invalid" value into the RAM, e.g.
>0 or 255 (or, if you also have a parity bit...). That might simplify
>the hardware, and make the design less critical (you would then
>just use the PIC to drive the transfer to the PC I guess...) so that
>you could use a slower PIC.

I thought of this, but I have a possible application for this device which
needs all the speed I can get, and therefore to save space (in RAM) and
time (on the PC and the datatransfer) I went with this solution.
Of course I can make some SW on the PC to correct for the drift but I'd prefer
to get it right from the start.

>As for your drift problem: isn't it just an effect of the tolerance of
>the transmitter and your xtal ? you drift 400ns every 134 lines or
>8576us, which is approx 45 ppm -- maybe you can put a variable
>capacitor in the oscillator and tune it so as to reduce the drift.

That's an interesting thought... I didn't think of that.
However, after some thinking I don't think this is the problem either.
The 10 MHz xtal oscillator is probably (worst case) of the 100 ppm type and
when divided down to 5 MHz I suppose it will be approx 50 ppm.

If I'm not totally mistaken the 100 ppm figure of the oscillator
is the maximum deviation from its frequency. Over time (and temperature)
it might vary up to 100 ppm. In my application it would have to drift
approx 90 ppm in 20 mS, and that's not very likely, I think.

Please correct me if I'm wrong here. I'm not a crystal osc expert....

>In any event, would you like to make schematics & sources available ?
>sounds like an interesting project.

I might do this when the project is finished. I have some SW to make
first, and I don't have much time....

But remember, it does not sample colors (my CCD camera is B/W)
and the resolution is not very high. (approx 270x300)

In my application it's sufficient.


-Oyvind         oyvind.kaurstad@nofac.abb.no