[Ffmpeg-devel] Audio bitrate

[Michel Bardiaux]

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.

> 
> [...]
> 


-- 
Michel Bardiaux
R&D Director
T +32 [0] 2 790 29 41
F +32 [0] 2 790 29 02
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