ALAC codec I am not sure it's really lossless

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ALAC codec I am not sure it's really lossless

kurdy
Hi,

I did some conversions from flac to m4a using alac codec the output file was so small I did following test:

From a flac file I did avconv -i f.flac -acodec alac f.m4a
then back to flac  avconv -i f.m4a -acodec flac f.flac

after using metaflac tools I checked parameters like md5 and some other values and  there are different.

I did the same using aiff as output format and everything is OK.

Is alac codec really lossless ?
or
Is my tests which are not significant ?


Thanks for advice

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Re: ALAC codec I am not sure it's really lossless

Claudio Freire
On Fri, Jun 1, 2012 at 3:58 PM, kurdy <[hidden email]> wrote:
> From a flac file I did avconv -i f.flac -acodec alac f.m4a
> then back to flac  avconv -i f.m4a -acodec flac f.flac

Better try some other reference format (pcm ideally), as flac may have
multiple representations of the same waveforms.
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[Libav-user] Re : ALAC codec I am not sure it's really lossless

kurdy
This post was updated on .
Hi, 

As flac is lossless the number of frame and md5 code of the track
should be the same.


But I did the test you propose, I extract from the
flac the wav and I launch:

avconv -i file.wav -acodec alac file.m4a

and surprise ! I got an Apple lossless audio



What I was suspecting is true  the conversion from flac to m4a using alac codec create an acc 128 !!!


Conclusion the command below create an acc even using codec alac I think this a bug.   

avconv -i file.flac -acodec alac file.m4a 

Many people using avconv or ffmpeg to create Apple lossless from flac got acc 128

My version is avconv


version 0.8.1-4:0.8.1-0ubuntu1, Copyright (c) 2000-2011 the Libav developers

 Regards
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Re: Re : ALAC codec I am not sure it's really lossless

Claudio Freire
Glad you managed to solve it, but:

On Sat, Jun 2, 2012 at 5:34 AM, [hidden email] <[hidden email]> wrote:
As flac is lossless the number of frame and md5 code of the track should be the same.

That's false. MD5 processes bytes, not samples, and as gzip can represent the same text in multiple way, with different MD5 each time, the same happens with other lossless codec.

Lossless != stable. PCM is both lossless *and* stable, so that's why you ought to compare PCM only.


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Re: Re : ALAC codec I am not sure it's really lossless

Carl Eugen Hoyos
In reply to this post by kurdy
kurdy_ch-forum@... <kurdy_ch-forum@...> writes:

> version 0.8.1-4:0.8.1-0ubuntu1

This is an intentionally broken version of FFmpeg with several
hundred regressions, some of them security relevant, we
therefore cannot support it.
Please see http://ffmpeg.org/download.html for supported versions.

Carl Eugen

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[Libav-user] Re : Re : ALAC codec I am not sure it's really lossless SOLVED

Reinhardt Willy
Hello,

Using the last version from http://ffmpeg.org/download.html  it wotks fine.

The version is:
> ffmpeg version 0.10.2-4:0.10.2-0ubuntu0jon1

Thanks

@Claudio I was talking about stream MD5 have a look to flac documentation.


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[Libav-user] Re : Re : ALAC codec I am not sure it's really lossless

kurdy
In reply to this post by Carl Eugen Hoyos
Hello,

Using the last version from http://ffmpeg.org/download.html  it works fine.

The version now is:
> ffmpeg version 0.10.2-4:0.10.2-0ubuntu0jon1

Thanks

@Claudio I was talking about stream MD5 not a file MD5 calculated afterward. Have a look to flac documentation.


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Re: Re : Re : ALAC codec I am not sure it's really lossless

Claudio Freire
On Mon, Jun 4, 2012 at 12:38 PM, [hidden email]
<[hidden email]> wrote:
> @Claudio I was talking about stream MD5 not a file MD5 calculated afterward.
> Have a look to flac documentation.

Sweet, I didn't know flac had such a header. Nice.
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