Re: bttv driver bug: VIDIOC_ENUMSTD returns wrong NTSC value

[Trent Piepho <xyzzy@xxxxxxxxxxxxx>]

On Sun, 27 Jan 2008, Robert William Fuller wrote:
> Trent Piepho wrote:
> > On Sun, 27 Jan 2008, Mauro Carvalho Chehab wrote:
> >> On Sat, 26 Jan 2008 22:30:07 -0300
> >> Cláudio da Silveira Pinheiro <taupter@xxxxxxxxx> wrote:
> >>> I'd like to know if this is an invalid bug or if it will be fixed upstream, or
> >>> if I should send a patch.
> >>> bttv reports NTSC as 36864  0x9000  1001000000000000
> >>> ( V4L2_STD_NTSC_M | V4L2_STD_NTSC_M_KR )
> >>>
> >>> videodev2.h says NTSC is 45056  0xB000  1011000000000000
> >>> ( V4L2_STD_NTSC_M |  V4L2_STD_NTSC_M_JP | V4L2_STD_NTSC_M_KR )
> >> ENUMSTD returns a bit mask of the supported video standards. In this case, bttv is known to
> >> work with both American (V4L2_STD_NTSC_M) and Korean NTSC versions (V4L2_STD_NTSC_M_KR).
> >
> > AFAIK, the only difference in Korean NTSC is the encoding of stereo audio.
> > I don't think adding a norm for it was the correct thing to do.
> >
> >> Probably, it would also work with NTSC/M format in Japan (V4L2_STD_NTSC_M_JP),
> >> but maybe some special configuration would be needed for the audio decoder
> >> chips. For us to add also V4L2_STD_NTSC_M_JP, we would need a positive feedback
> >
> > This isn't the case.  I know I've said this before, but NTSC-JP has a
> > different black level.  American (and I think everyone else's) NTSC has a
> > 7.5 IRE pedestal, but not in Japan.
> >
> > The bt848 chip does support NTSC-JP, I guess no one has bothered to add it.
> > It could even be usefull outside of Japan.  I've read that most DV cameras'
> > analog outputs don't have the proper black level and use 0 instead of 7.5.
>
> I have a lot of Japanese laserdiscs, not DVDs, but laserdiscs.  You
> know, the big LP sized things....  Would NTSC-JP support give me better
> video for capturing these?

Good question.  I don't know.  It depends on where the pedestal is
generated, on the disc or by the player.  For DVDs, the pedestal isn't
encoded on the disk, it's generated by the analog output section of the
player.  So you would use NTSC-JP with a Japanese player, regerdless of
what region the DVD is for.

But laserdiscs are basically an analog recording of a composite video
signal, so it's quite possible the pedestal is part of the disc.  That
would be my guess.  In which case you should use NTSC-JP for a Japanese
disc.

Besides just being too dark, what should happen if you capture set to
NTSC-M with an NTSC-M-JP signal, is that blacks from 0 IRE to 7.5 IRE will
all be clipped to a Y value of 16.  This will mean details in dark areas
will be lost (not just too dark).  If you take a frame and pump up the
brightness in the Gimp or set your monitor to max brightness, so the black
areas are now gray, they'll just be a uniform gray, the detail simply
wasn't captured.

You can "fix" the image being too dark by just increasing the brightness,
but there is no way to restore the lost detail.  (other than to adjust the
analog gain manually, which I don't think the bt8x8 can do, or maybe if
using the full range luma capture mode makes it possible to capture luma
values below 16, one could convert digitally post-ADC)

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