[Flac] audio data encoding in FLAC: how complex ?

[Brian Willoughby]

Hi Phil,

There is one sample per code.

I'm not sure what you mean when you use the term 'interpolated'  
because there is no interpolating done in FLAC or Rice Coding.

Rice Coding is simply used to change from a fixed number of bits per  
sample, such as 16 or 24, to a variable number of bits per sample.   
Hopefully, the most common 16-bit or 24-bit samples will have the  
shortest codes, while some will necessarily have more than 24 bits  
but will be very rare.  Overall, fewer bits will be used, even though  
there is a one-to-one translation from each sample to its Rice Code.   
Because of the oscillating nature of audio, samples near zero are  
more common.  Also, FLAC uses differential coding rather than  
absolute values, and since each subsequent sample is usually very  
close to the previous sample, the Rice Coding uses short bits  
sequences for small differences and larger bits sequences for larger  
differences.  But this is all my interpretation over the years, so  
please don't quote me.

Perhaps you should write some programs of your own to explore the  
topics, including things outside FLAC, and then you might be better  
prepared to describe and document the specifics.  I would welcome a  
chance to read whatever you write, but I think you've got a bit of  
work ahead of you.

Brian Willoughby
Sound Consulting


On Mar 9, 2009, at 07:53, porte64 at free.fr wrote:
What is unclear to me is how many audio samples encoded with a fixed  
parameter (denoted 'M' in the wiki, which i presume is equivalent to  
the 'Rice parameter' invohed here):
http://flac.sourceforge.net/documentation_format_overview.html

But things are a bit more complicated because the wiki says: "audio  
codecs [...] use a Rice code after the linear prediction step". But  
it does not tell us about how many points/samples get interpolated,  
and what criteria are used.

So i guess that in the end, for every frame, 2 points of the  
interpolating line and a Rice parameter are stored, and, at each  
point/sample, the distance to this line is encoded as a series of  
residues ?!

On the whole, it *seems* to me that it boils down to simple  
operations (after all, it's just basic school arithmetic)
but it's really hard to put all the pieces together and it's a pity  
(and may be dangerous for maintenance over years -- where project  
poeple may change) that it's not documented besides the source code.

It would enjoy writing a note on the whole process but there are too  
many unknowns.
Sorry, i am feeling a bit desperate, having spent several nights in a  
row trying to understand the encoding process globally from the  
source code.

Phil