[Ffmpeg-devel] [PATCH] fix 2 ac3 bit allocation bugs

[Justin Ruggles]

Hello,

In my attempts to adjust AC3 bit allocation parameters, I have come
across 2 bugs.  They do not currently affect FFmpeg's output since they
only involves table values which are not used right now, but I think
they should be fixed anyway...especially since the tables could be
reused in an FFmpeg AC-3 decoder.

bug #1:
I ran into problems when increasing the bandwidth setting.  I found the
issue to be the last table entry in bndtab.  It should be set to the
next starting bin number, not zero.

bug #2:
The floorcod table entries are copied directly from the spec, but when
using floorcod of 7 it gives buggy output.  Also, the value of 63488
doesn't make sense (it makes the masking curve flat...i think).  Anyway,
I found the answer in the spec.

A/52A page 60, last paragraph:
"...each step is definied exactly in terms of fixed-point integer
operations and table lookups.  Throughout the discussion below, signed
two's complement arithmetic is employed..."

So the table needs to be changed to signed 16-bit.  This makes
floorcod[7] = -2048, which encodes and decodes properly.

Thanks,
Justin
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