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

Justin Ruggles jruggle
Sun Sep 3 18:00:28 CEST 2006

Michael Niedermayer wrote:
> Hi
> On Sat, Sep 02, 2006 at 06:28:51PM -0400, Justin Ruggles wrote:
>>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.
> patch looks ok (assuming regression tests pass)
> [...]

applied. and yes, regression tests pass since neither table value is
currently used.

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