Re: Tweak PC clocks

Jan Panteltje wrote:
On a sunny day (Thu, 15 Oct 2009 11:19:01 +1100) it happened Sylvia Else
<sylvia@xxxxxxxxxxxxxxxxxxx> wrote in
<0034a649$0$24451$c3e8da3@xxxxxxxxxxxxxxxxx>:

It doesn't really need to be phase locked. Merely adjusting the frequency up and down at intervals would be sufficient, provided the tweaks are sufficiently small (and it should be possible to make them small) that the image size isn't affected in a noticeable way.

You need gen lock:-)


BTW, it appears from further more precise measurements that I was wrong about the video card being synched to the system clock. Indeed, the frame rate drifts a bit.

Which begs the question of how it's supposed to issue sync interrupts properly. Then again, my card (a Matrox G450) seems unable to do that - interrupts get lost frequently.

Sylvia.

It is a rather complicated subject.
In case of TV, and assuming you are in the US and I think the
NTSC frame rate was 29.9700 Hz, and your monitor sync 60 Hz...

No, I'm in Australia, with 25Hz frames and 50Hz refresh - nominally.

The video is stored in a buffer in the PC, usually 2 levels deep (one is written
to while the other one is read out), and the graphics card reads from one
of those buffers at the time.. at it's own rate (60 x per second).
Here is a link for Nvidia cards, that explains some of those issues:
http://http.download.nvidia.com/solaris/1.0-8762/README/appendix-v.html

I'm working lower down - writing directly to the frame buffer in synch with the display. It works fine except for the slight frame rate mismatch which forces me to drop frames or duplicate them. I'd like to avoid doing either.


I dunno about Matrox, never used those, I use Linux an Nvidia.

It has a particularly good composite video out. With the Nvidia card composite video out, I could never get rid of extraneous noise. It appears that Nvidia didn't really care about it - just included it as a selling point.

Sylvia.
.

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