[FFmpeg-devel] AAC ALS (lossless) and SLS (scalable lossless)

Alex Converse alex.converse
Thu Mar 19 20:02:11 CET 2009

On Wed, Mar 18, 2009 at 9:49 AM, Alex Converse <alex.converse at gmail.com> wrote:
> On Tue, Mar 17, 2009 at 10:09 PM, Jethro Walters
> <turkeyman121 at yahoo.co.uk> wrote:
>> Hey guys.
>> Was doing a little research on both AAC ALS and SLS. ALS is the official lossless format for AAC, and a completely separate codec to AAC. SLS is the extension to AAC that adds a lossless reproduction layer on top of the lossy AAC file, sort-of like WavPack's Hybrid lossy/lossless mode.
>> Is there any plan to implement these formats in ffmpeg / libavcodec? I feel that having one or both in ffmpeg would give them more chance of moving more into the mainstream, but there seems to be little information around about both formats. Anybody know anything further? Before you ask, no I'm not volunteering to code these, as my coding skills are in dire need of improvement.
>> Regards,
>> Jethro Walters
> At this point ALS has picked up very little steam in the wild and
> probably won't be seen in FFmpeg anytime soon. A subset of SLS is
> currently getting a push under the name HD-AAC. I imagine if it takes
> off FFmpeg will get support for it. Right now we can play the LC core
> of SLS files.

To expand on my previous answer a little bit since you were talking
about helping to popularize the formats...

ALS doesn't do anything particularly interesting in my opinion. It's
seems to be a result of NIH syndrome. There is nothing it does that
couldn't be solved by a FLAC mapping for MP4 (well except for the fact
that FLAC probably doesn't use nearly as many patents owned by the
interests that run MPEG). There seems to be no good reason to push to
popularize ALS. Though if someone really takes interest in it, there
is no format too obscure for FFmpeg.

SLS is a much more interesting format because of it's hybrid WavPack
like nature. Now I'm sure you are thinking ALS is like FLAC, SLS is
like WavPack what's makes SLS interesting? SLS is interesting because
it's lossy layer is already widely proliferated in the wild and there
are high quality tools for it. A file that can be quickly stripped to
AAC is far more useful on a number of players and devices than a .WV
WavPack lossy file.

That is why recently I've started work on an HD-AAC (subset of SLS) decoder.

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