[asterisk-bugs] [Asterisk 0008126]: [patch] G.711 codec woes

[noreply at bugs.digium.com]

A NOTE has been added to this issue. 
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http://bugs.digium.com/view.php?id=8126 
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Reported By:                fossil
Assigned To:                murf
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Project:                    Asterisk
Issue ID:                   8126
Category:                   Core/CodecInterface
Reproducibility:            always
Severity:                   minor
Priority:                   normal
Status:                     ready for testing
Asterisk Version:            SVN 
SVN Branch (only for SVN checkouts, not tarball releases):  1.2 
SVN Revision (number only!): 44743 
Disclaimer on File?:        Yes 
Request Review:              
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Date Submitted:             10-09-2006 20:21 CDT
Last Modified:              08-06-2007 11:39 CDT
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Summary:                    [patch] G.711 codec woes
Description: 
There is a *number* of problems in the a-law and u-law core transcoders
(most severe first):

1. a-Law decoder does not add the rounding error to the linear samples
output;
This results in a stable amplitude drop in the decoded signal overall, but
the negative phase portion of the signal is even more adversely affected:
the amplitude drop actually accumulates with consequtive transcodings (see
attached test patch). If the call encounters 127 tandem a-law transcodings
(a-alaw -> slin -> a-law -> slin -> ...), the entire negative portion will
be reduced to http://bugs.digium.com/view.php?id=#0.

2. Lookup table-driven slin->law coding rounds the negative values the
wrong way;
The breaks in linear value sequences do not happen where the table-driven
slin->law system expect them to. This results in certain negative linear
values to be encoded incorrectly (see attached test patch), which isn't
such a *big* problem, but a problem nonetheless.
There is no one-liner fix for this issue. To fix this, for example, we
could generate only half the slin->law table, for positive values only.
This table would contain half-cooked law bytes, so that the sign could be
added later to the values, along with the post-coding transform (NOT for
u-law and XOR 0x55 for a-law). In this case, AST_LIN2MU() would look
something like this:

inline unsigned char AST_LIN2MU(short sample)
{
  unsigned sign = ((unsigned)sample & 0x8000) >> 8;
  unsigned char law = __ast_lin2mu[(sample & 0x7fff) >> 2];
  return ~(law | sign);
}

3. slin->a-law and slin->u-law functions handle value -32768 incorrectly;
This is not really a problem when using a lookup table system because the
slot of -32768 is overwritten later, but for the sake of correctness...

4. alaw.c:linear2alaw() is less than optimal;

5. slin->law lookup table generation code is less than optimal;
There is no reason to enumerate all the possible values between -32768 and
32767 when most of the results are overwritten later.

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---------------------------------------------------------------------- 
 murf - 08-06-07 11:39  
---------------------------------------------------------------------- 
OK, I've uploaded 8126.finalpatch-1.4 and 8126.finalpatch-trunk for 1.4 and
trunk, for those who would like to test on these versions.

They aren't really "final", in that the Makefile will NOT be set up to
compile these in test mode by default. This is just for the sake of
testing.

Looks good to me, and takes care of all the points in your list of
problems!

(Although, the probability of having a situation that would require 256
transcodings (or more) that would demonstrate the transcoding deficiencies
are pretty low, methinks, but still... having the codecs be lossless will
make them of higher quality.) 

Issue History 
Date Modified   Username       Field                    Change               
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08-06-07 11:39  murf           Note Added: 0068498                          
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