[Flac-dev] Re: Lossless AMI ADPCM

Juhana Sadeharju kouhia at nic.funet.fi
Fri Jun 29 12:01:17 PDT 2001

>From:	Josh Coalson <j_coalson at yahoo.com>
>I'm copying the flac-dev list to see if anyone has any
>feedback also...

I'm supposed to be there myself since yesterday but have not got
the first digest yet.

>First, the results they show are for compression of data
>that has already been lossily quantized to fewer bits per
>sample, e.g. u-Law and A-Law are logarithmic quantizations
>of 16-bit data to 8-bit.

I thought the author has two models, A-law and AMI ADPCM, which
both he extends. AMI ADPCM starts with 16-bit samples, but I'm not
sure if A-law is involved in that process.

>Second, the average ratio (assuming the table describes
>ratios, since the omitted the units) for 44.1kHz audio
>is 3:1.

I though they are bits/sample. The text says "from the table one
can see that the software works better for audio": average for audio
is 2.75, and average for audio/speech is 3.17. So, 2.75 is better
only if it is the number of bits (or such).

>They only vaguely mention the sources for the
>material.  I can choose material that gives those ratios
>even for linear PCM.

Yeah, and it is supposed to be a scientific paper. Considering
that the author has e-mail address, I would have expected him
to check against Shorten which he even references.

>expensive.  Standard autocorrelation->Levinson-Durbin
>will be too slow.  So they use RLS, which has stability

His RLS seems to be stable enough, up to 5000 samples, and fast.
Would the RLS make FLAC run faster?

>I have done some tests with long kernels and it does
>not buy very much extra compression.  Most of the slack
>can be taken up with better entropy coding.

But if anyone understands the further details of the algorithm,
I would like to code and test it out. I have written down as much
details as I could --- they are available via e-mail for those who
didn't got the entire first mail on this topic. Perhaps it would be
a waste of time, but the author of that paper should have mentioned
the audio sources in the first place. Bad science.



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