[FFmpeg-devel] [PATCH] G.729A (floating-point) decoder and ACT demuxer

Michael Niedermayer michaelni
Sun Feb 24 18:18:57 CET 2008

On Sun, Feb 24, 2008 at 09:39:21PM +0600, Vladimir Voroshilov wrote:
> Hi, Reimar
> this is reply on g729a decoder related issues.
> On Sun, Feb 24, 2008 at 12:39 AM, Reimar D?ffinger
> <Reimar.Doeffinger at stud.uni-karlsruhe.de> wrote:
> > [...]
> >  > +/**
> >  > + * L1 codebook (10-dimensional, with 128 entries (3.2.4)
> >  > + */
> >  > +static const float cb_L1[128][10] = {
> >
> >  These constants btw. look very much like they were designed with a
> >  16-bit fixed-point implementation in mind...
> L1 L2 L3 - are three different codebooks mentioned in specification.
> Exact values are not described anywhere (or i was not searching carefully),

2.4      Speech coder description
The description of the speech coding algorithm of this Recommendation is made in terms of
bit-exact fixed-point mathematical operations. The ANSI C code indicated in clause 5, which
constitutes an integral part of this Recommendation, reflects this bit-exact fixed-point descriptive
approach. The mathematical descriptions of the encoder (clause 3), and decoder (clause 4), can be
implemented in several other fashions, possibly leading to a codec implementation not complying
with this Recommendation. Therefore, the algorithm description of the ANSI C code of clause 5
shall take precedence over the mathematical descriptions of clauses 3 and 4 whenever discrepancies
are found. A non-exhaustive set of test signals, which can be used with the ANSI C code, are
available from the ITU.

> current values are got from floating-point reference code.

As written in the spec, the fixed point implementation conains the bit exact


Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

If a bugfix only changes things apparently unrelated to the bug with no
further explanation, that is a good sign that the bugfix is wrong.
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