AAC encoder produces low-quality output

[Adam Klobukowski]

ffmpeg mangles sound when compressing to aac.

command:
ffmpeg -i file.mkv -vcodec copy -acodec aac -ab 384000 -strict experimental -f mp4 file.mp4

GIT ffmpeg (installed using this guide: https://ffmpeg.org/trac/ffmpeg/wiki/UbuntuCompilationGuide) does not have this problem with the same command used.

[Daniel Letzeisen (dtl131)]

Do you know what audio format the .mkv file uses? If you use a different .mkv file or even try converting from a plain audio file, does the issue still occur?

[Adam Klobukowski (adamklobukowski)]

Original stream was:
Stream #0:1: Audio: ac3, 48000 Hz, 5.1(side), s16, 384 kb/s (default)
And I was converting it to:
Stream #0:1: Audio: aac (@[0][0][0] / 0x0040), 48000 Hz, 5.1(side), flt, 384 kb/s (default)

I do not have any plain audio for testing :/

[Reinhard Tartler (siretart)]

On So, Okt 21, 2012 at 17:54:59 (CEST), Adam Klobukowski wrote:

> Original stream was:
> Stream #0:1: Audio: ac3, 48000 Hz, 5.1(side), s16, 384 kb/s (default)
> And I was converting it to:
> Stream #0:1: Audio: aac (@[0][0][0] / 0x0040), 48000 Hz, 5.1(side), flt, 384 kb/s (default)
>
> I do not have any plain audio for testing :/

The ac3 encoder included in libavcodec is known to be of poor quality.
Unfortunately, Ubuntu does not enable the wrappers for external ac3
encoding libraries that would be available. The reason is that the
license of resulting binary package would not allow redistribution.

I know the situation sucks, and hope that we soon find someone who would
be willing and capable to work on the libavcodec ac3 encoder.

--
Gruesse/greetings,
Reinhard Tartler, KeyID 945348A4

[Daniel Letzeisen (dtl131)]

@reinhard, I do not see where the user is encoding ac3. It looks like s/he is decoding ac3 and enconding aac.

[Reinhard Tartler (siretart)]

On So, Okt 21, 2012 at 20:14:48 (CEST), Dave Lentz wrote:

> @reinhard, I do not see where the user is encoding ac3. It looks like
> s/he is decoding ac3 and enconding aac.

damn, I meant to write aac instead of ac3.

I'm sorry for the confusion

--
Gruesse/greetings,
Reinhard Tartler, KeyID 945348A4

[Adam Klobukowski (adamklobukowski)]

If Ubuntu's ffmpeg can't produce correct aac files, this encoder should better be disabled.

[Reinhard Tartler (siretart)]

Adam, the produced AAC file are correct in the sense that a compliant decoder is able to decode them. It's rather that the encoder's quality is poor.

I don't see how a low quality encoder is better than none at all. We could add a warning message, though.