[Ffmpeg-devel] closed captioning bug?
Mon Apr 24 05:50:17 CEST 2006
Cyrus A wrote:
> Luca Abeni wrote:
>> On Sat, 22 Apr 2006 02:51:19 -0400
>> Cyrus A <lists at rooseveltmedia.com> wrote:
>>>> ok, ive not the faintest idea whats wrong but does chaning the
>>>> resolution / encoding to /dev/null using -vcodec copy help?
>>>> btw, are there any distortions in the video vissible?
>>>> maybe the diskaccess for encoding is causing some buffers of the
>>>> capture hw to overrun in that case hdparm / pci settings
>>>> might have some effect
>>> I think it's ALSA. I just tried ffmpeg with "-an" and captions work
>> I had similar problems in the past, and I do not think the cause is
>> alsa. In my opinion, it is the A/V synch code in ffmpeg.c that does
>> not work well with video4linux + oss input.
>> I think you have 2 possibilities:
>> 1) go in ffmpeg.c:do_video_out(); about 10 or 20 from the beginning
>> of the function you see something like if (vdelta < 1.1) ...
>> else if (vdelta > 1.1)
>> Change 1.1 with something bigger (around 5 should be enough, if I
>> remember well), and it should fix your problem.
>> This is clearly a hack :)
>> 2) the correct solution: use v4l2. Which brings the question: why is
>> it not used? I suspect there is a problem in the check in configure
>> (which might be too conservative, to avoid breaking the build).
>> You are using fedora, right? I might have a patch somewhere that
>> could help... I'll search for it, and I'll send it later.
> The vdelta hack didn't work. I tried -5, 5 and -15, 15. The first line
> read "if (vdelta < -1.1)" which is what I'm assuming you meant in your
> hack (you had it without the negative sign). If not, please let me know.
> I'm not sure how to force ffmpeg to use v4l2. The configure output
> says nothing about it. Any help on how to force that change would be
> greatly appreciated.
> In the meantime, I'll try the final alsa 1.0.11 version from sources.
> It might be a long shot, but at least it's something I can do while I
More info here: I downgraded (wiped and reinstalled) to FC3. An old
version of ffmpeg (probably early 2005) seems to work (at least for a
while) when trying to capture closed captions.* I also tried the CVS
version of ffmpeg and the caps were just as screwed up as before.
* The caps will work fine for up to 5 minutes (maybe more) with ffmpeg
driving /dev/audio and /dev/video0. However, they eventually get garbled
again for some reason. Maybe the caps go bad when I use my USB KVM
switch? I have no idea. I'm going to try doing them without KDE or GNOME
running and no USB switching over the next 24 hours. We'll see how they
BUG SUMMARY: Basically, there is a bug in the latest ffmpeg for
capturing video and audio. I've tried different kernels, different
distros, various codecs, etc and as long as /dev/audio and /dev/video0
are both driven at the same time by ffmpeg, the closed caps are messed
up. Otherwise, they're fine. zvbi-ntsc-cc does them fine, tvtime does
them fine, even ffmpeg with "-an" does them fine. ffmpeg with "-ad" +
"-vd" = BROKEN.
I hope someone (with more time and expertise than myself) is able to fix
this bug soon. I would be extremely grateful.
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