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bttv 2.6.16: wrong VBI_OFFSET?

On Thu, 4 May 2006, Michael Schimek wrote:

Strange, my message doesn't appear in the list archive, but it looks like it
made it through.

> On Thu, 2006-05-04 at 01:47 -0700, Trent Piepho wrote:
> > The first bit after the run-in should start at 27.452 us, or 768 pixels at
> > Clk*2 from 0H.  It appears to start about 566 pixels in.
>
> Typo? 27.452 µs * 28636363 Hz = 786.

Yes, typo.  I should have said the offset appears to be 220 pixels.  The mid
value of the rising edge of the first bit is at pixel 566 for almost all
stations.  I found a couple that start a little earlier, about 555.  In order
for the offset to be 256, the pulse would have to start by 530 and nothing is
anywhere near that.

> Btw there seems to be a bug in the cx88 driver capturing only 288 DWORDs
> per line instead of 400, and another one returning wrong line numbers.
> I'm still waiting for confirmation because tvtime seems to have another
> problem with this driver.

Yes, I found those bugs as well when I wrote a simple program to make those
pictures of line 21 data.  For some reason, someone added a " -1" to all the
start lines that shouldn't be there (at least for NTSC).  The driver is also
set to capture just 288*4 = 1152 pixels, which isn't enough.  63.556 us *
28.636363 MHz = 1820 pixels - 220 pixel offset = 1600 pixels.

tvtime still doesn't work when these driver bugs are fixed.  The VBI code in
tvtime is stupid, and doesn't check the VBI format _at all_.  It just assumes
it will get what the bttv driver has historically provided.  After fixing
tvtime, I was able to get it to decode captions somewhat.  I think the decoder
tvtime (and everything else, it's all the same code cut and pasted) uses
doesn't work very well.  I will have to port the decoder I wrote for the
pre-V4L1 bttv driver and see if it works better.  I know it worked much better
back then, as I could get nearly flawless captions from VHS tapes.

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