[Flac-dev] A-law and mu-law

[Brian Willoughby]

On Nov 19, 2011, at 16:42, Giulio Paci wrote:
> So the problem would be suboptimal compression due to suboptimal  
> assumption about the input signal, right?

The problem is more that FLAC should not be a collection of code to  
read every possible file format in existence.  That would be a  
duplication of at least two other audio file format conversion  
utilities, and quite bug ridden, at best.  Instead, FLAC supports the  
two major lossless file formats plus totally raw data (no container  
or formatting besides the raw samples).  Between those three options  
you have everything you really need for lossless audio compression.

The only totally new file format that would make sense as an addition  
is CAF, and only because it allows larger files and is also lossless.

> What I do not understand is how the format of a FLAC format would be
> affected by supporting A-law and mu-law files as input (and thus
> output). Despite of suboptimal performance, is it possible to treat  
> 8bit
> *-law samples as 8bit linear PCM files and write down the original
> format information in the output while decoding?

If you were to treat *-law data as 8-bit, then you would have a  
couple of problems.  First of all, I don't think that FLAC supports 8- 
bit.  It's really rather pointless to develop lossless compression  
algorithms for 8-bit audio.  More importantly, reinterpreting *-law  
as 8-bit linear PCM would introduce nonlinear distortion that would  
affect the compression algorithms.  FLAC would still be lossless, but  
it probably wouldn't be able to compress distorted audio as small as  
clean audio.

> For my needs converting them to 16bit linear PCM is an option, but I
> will need to keep track of the original format outside the file, which
> is something that I would prefer to avoid.

I think the biggest problem here is that you have two types of sound  
files or audio data in your overall collection.  All of the A-law and  
mu-law files are not up to the same lossless recording quality as an  
8-bit AIFF, for example.  Trying to shoehorn two different quality  
levels of recordings into the same file format doesn't really make  
much sense.  You're probably better off by archiving the *-law files  
with a slightly modified organization to reflect the different  
quality level.

Unless conversion to 16-bit lossless audio followed by FLAC  
compression shows substantial savings in space, then you're probably  
better off just leaving the *-law files as they are.

Brian