On Fri, 2009-09-11 at 17:04 -0400, Devin Heitmueller wrote:
> On Fri, Sep 11, 2009 at 4:56 PM, Andy Walls <awa...@radix.net> wrote:
> > + /* FIXME: hard-coded for NTSC support */
> > + format->fmt.vbi.sampling_rate = 6750000 * 4 / 2; /* FIXME: ??? */
> >
> > The BT.656 Pixel clock for both 60 Hz and 50 Hz systems is 13.5
> > Mpixels/sec. Unless you switch to PAL or NTSC square pixel timing for
> > some reason, I would expect this particular number to stay the same for
> > both PAL and NTSC.
> >
> > A pixel in BT.656 consists of 1 Y sample and 1 U or V sample. So, in
> > the VBI active line regions, I would have expected 2 * 13.5 Msamples/sec
> > for VBI data. I don't know if any app in userland actually pays
> > attention to this number though....
> >
> > Regards,
> > Andy
>
> Hey Andy,
>
> Well, when I said "hard-coded for NTSC support", I was referring to
> the entire block of code that followed, not just the one on the
> sampling rate. I expect the line count and start offsets to be
> different for PAL as well.
Ah, OK. Yes they are.
>
> In reality, the "???" was because I actually didn't know what the
> proper value needs to be. I copied it from another driver without
> fully understanding its role, and changing it caused userland to break
> (zvbi-ntsc-cc in this case).
> Worth noting that we are actually delivering 8-bit VBI info, as
> opposed to 16-bit yuyv (there are some USB bandwidth issues I'm
> actively working and doing 16-bit put me over the limit), so I'm not
> confident that the generalization for ITU656 applies.
OK. You can always work backwards to get the number:
For example, if your hardware uses the BT.656 pixel clock of 13.5
Mpixels/sec and BT.656 pixel count of 720 pixels per active region of a
horizontal line, then:
720 pixels/line active region / 13.5 Mpixels/sec =
53.3333 usecs / line active region
And so your VBI sample rate is:
n VBI samples/line active region / 53.3333 usecs/line active region
with n being whatever the driver provides per VBI line.
Ah and now I see you're only providing 720 samples per line. So yes,
13.5 Msamples/sec is the right number.
Mea culpa.
Regards,
Andy
> As I get the PAL support working, that area will certainly get some cleanup...
> Cheers,
>
> Devin
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