bttv 2.6.16: wrong VBI_OFFSET?

[Trent Piepho <xyzzy (at) speakeasy.org>]

On Wed, 3 May 2006, Edgar Toernig wrote:
> | Hmm... played around with this.  It seems, the real offset is dependent
> | on hscale.  Samples start earlier/later when capture window is resized.
> | Strange...  Good luck you driver writers ;)
> |...
> | Btw, (186-64)*2 = 244  (186 = recommended hdelay for PAL).

I did some more tests with different parameters:
http://www.speakeasy.org/~xyzzy/vbi

According to my tests, changing HSCALE, HDELAY, and BGDELAY have no effect on
vbi start offset.  It always appears to start about 220 pixels at Clk*2 from
0H.  Changing HSFMT has the expected effect for the bt878, but doesn't appear
to have any effect on the cx23880, unless something extra must be done for a
change in HSFMT to take effect.  Changing VBI_HDELAY has the expected effect.

For NTSC, the burst gate delay is set to 93 Clk*2 pixels.  /HRESET width is
128 Clk*2 pixels.  93+128 = 221, observed value of vbi delay.  But changing
bgdel has no effect.. oh well.

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