[Ffmpeg-devel] Audio bitrate

[Michael Niedermayer]

Hi

On Mon, Nov 06, 2006 at 05:28:24PM +0100, Michel Bardiaux wrote:
> Michael Niedermayer wrote:
> >Hi
> >
> >On Mon, Nov 06, 2006 at 04:49:46PM +0100, Michel Bardiaux wrote:
> >>Seems wrong in some cases:
> >>
> >>ffmpeg -i utc_1434.mpg -vn -acodec adpcm_ms -y -ar 8000 msadpcm.wav
> >>FFmpeg version SVN-r6917, Copyright (c) 2000-2006 Fabrice Bellard, et al.
> >>  configuration:
> >>  libavutil version: 49.0.2
> >>  libavcodec version: 51.24.0
> >>  libavformat version: 51.6.0
> >>  built on Nov  6 2006 16:13:12, gcc: 3.3.5 (Debian 1:3.3.5-13)
> >>Input #0, mpeg, from 'utc_1434.mpg':
> >>  Duration: 00:00:57.0, start: 92564.370489, bitrate: 668 kb/s
> >>  Stream #0.0[0x1e0]: Video: mpeg1video, yuv420p, 352x288, 104857 kb/s, 
> >>25.00 fps(r)
> >>  Stream #0.1[0x1c0]: Audio: mp2, 48000 Hz, mono, 32 kb/s
> >>Output #0, wav, to 'msadpcm.wav':
> >>  Stream #0.0: Audio: adpcm_ms, 8000 Hz, mono, 64 kb/s
> >>Stream mapping:
> >>  Stream #0.1 -> #0.0
> >>Press [q] to stop encoding
> >>size=     235kB time=59.8 bitrate=  32.2kbits/s
> >>video:0kB audio:235kB global headers:0kB muxing overhead 0.018285%
> >>
> >>Should be 32kbits in the 0.0 line.
> >>
> >>With pcm_s16le, the bitrate is correct. But I dont see at once where it 
> >>is set. "Use the source, Luke", all right, but if somebody just *knows*...
> >
> >ffmpeg.c audio_bit_rate default i guess
> >changing that of course wont help ...
> 
> No. And after a bit of grepping, it appears that the only reason the 
> message is right when pcm is used, is because avcodec_string 
> deliberately ignores bitrate in the codec and recomputes. So, even 
> though the output is right, the internal state is *not*. Which is rotten 
> for those using the libs instead of the command line.
> 
> >
> >you could try to set AVCodecContext.bitrate in adpcm.c to the correct value
> >but iam not sure if this is free of sideeffects
> 
> IMHO the init method of a codec with a fixed bitrate, should force the 
> correct value. Unfortunately, the init method is called LONG after 
> avcodec_string is used. Maybe we need to add a method for codecs; one 
> that should be called to change desired bitrate, framerate, number of 
> channels, or to retrieve bitrate.

add a list to AVCodec similar to supported_framerates

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

In the past you could go to a library and read, borrow or copy any book
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