Properties of ALAC in mp4

Frank Lai frank.franklai at
Wed Jun 8 19:40:35 CEST 2011

The alac format info is from

Test an alac file encoded by iTunes. The bitrate is correct. But test
another alac file encoded by FFmpeg, the bitrate is 1411 kbps.

Found a link discussing this issue
Looks like FFmpeg did not fill the bitrate value correctly. But other
applications (except iTunes) can report the correct bitrate, so there should
be another way to calculate the bitrate.

Best Regards,
Frank Lai

2011/6/9 Lukáš Lalinský <lalinsky at>

> On Wed, Jun 8, 2011 at 6:45 PM, Frank Lai <frank.franklai at>
> wrote:
> > Hi all,
> > The attached patch is for read properties from "alac" atom when the audio
> > codec in mp4 is ALAC.
> Hi Frank,
> Thanks for the patch. It seems to work fine, except for bitrates. What
> is the source that you used for the format info? I've encoded a small
> file consisting only of silence and all applications report it to have
> 3 kbps. Your code reports 1411 kpbs, which is what I get when I
> convert it to a wav file. Is it possible that it's the bitrate of the
> uncompressed audio data?
> Lukas
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