Hi all,
I've searched the list archives and googled a bit for my problem, but
didn't find anything that makes sense to me.
I am trying to encode one video and one audio stream into an AVI. The video
is H264 at 25 fps and the audio is raw 16bit PCM at 44100 Hz. The actual
encoding goes fine and the file plays back, but there is a severe confusion
about the time_base of the audio stream which messes up the final duration
of the file (actually only the audio stream).
I am filling out all the time_base fields. My first question would be why
there are two time_base fields, one in the AVStream structure and then
another one in the AVCodecContext structure (AVStream->codec). I've looked
a bit at the av_write_header() function, and I don't get it at all. For
example, there is code like this:
/* init PTS generation */
for(i=0;i<s->nb_streams;i++) {
int64_t den = AV_NOPTS_VALUE;
st = s->streams[i];
switch (st->codec->codec_type) {
case CODEC_TYPE_AUDIO:
den = (int64_t)st->time_base.num * st->codec->sample_rate;
break;
case CODEC_TYPE_VIDEO:
den = (int64_t)st->time_base.num * st->codec->time_base.den;
break;
default:
break;
}
if (den != AV_NOPTS_VALUE) {
if (den <= 0)
return AVERROR_INVALIDDATA;
av_frac_init(&st->pts, 0, 0, den);
}
}
So it uses time_base.num from AVStream, and time_base.den from
AVStream->codec for vieo streams?
In any case, what happens is that I set the audio stream timebase to
"1/44100", both in AVStream and AVStream->codec. Additionally, I set
sample_rate to 44100.
After calling av_write_header(), the AVStream time_base has magically
changed to 1/2000. I have no idea where that comes from. It seems to be the
AVI muxer doing it, since when I comment out the call to write_header() of
the muxer in av_write_header(), it doesn't change. From looking at the AVI
code, I don't see where it happens.
Anyways, when I prepare the audio packets, I calculate the PTS like this:
fPacket.pts = (encodeInfo->start_time
* fStream->time_base.den / fStream->time_base.num) / 1000000;
encodeInfo->start_time is the chunk start time relative to the stream start
(0) in 1/1000000 seconds. fStream is the AVStream structure.
So what happens is that even though the time_base changed, I should still
be calculating the PTS correctly. The problem is that
AVStream->codec->time_base was not adjusted. It's still 1/44100. The
resulting file reports a much too long duration as a result. I've
calculated that the reported duration pts is correct for a 1/44100
timebase, but of course the reported AVStream time_base when I open the
file is 1/2000.
The interesting thing is that when I do not set any of the time_base values
for audio streams, but only the sample rate, the same thing happens.
Initially, the stream has 1/90000, the codec has 0/1 and after calling
av_write_header, the stream is 1/2000, codec still 0/1, but the resulting
file reports a wrong duration for the audio stream (would fit for 1/44100).
What am I doing wrong? :-) Am I even supposed to calculate the packet PTS
which I pass to av_write_frame()? Thanks for any and all help!
Best regards,
-Stephan
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