[FFmpeg-devel] [PATCH] G.729A (floating-point) decoder and ACT demuxer
Michael Niedermayer
michaelni
Sun Feb 24 18:48:59 CET 2008
On Sun, Feb 24, 2008 at 11:36:05PM +0600, Vladimir Voroshilov wrote:
> On Sun, Feb 24, 2008 at 11:18 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> > 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
> > values.
> >
>
> C.5 ANSI C code
> [...] The algorithmic description given by the C code shall take
> precedence over the texts contained in the main body of the full
> version of G.729, Annex A or this annex. As of the approval of this
> text, the current version of this ANSI C code is Version 1.01 of 15
> September 1998. More recent versions may become available through
> corrigenda or amendments to G.729. Please ensure to use the latest
> available version from the ITU-T website. [...]
>
>
> So constants can be got from floating-point reference code too, right?
5 Bit-exact description of the CS-ACELP coder
ANSI C code simulating the CS-ACELP coder in 16 bit fixed-point is available from ITU-T. As of
^^^^^^^^^^^^^^^^^^
the approval of this version of this Recommendation, the current version of this ANSI C code is
Version 3.3 of December 1995. More recent versions may become available through corrigenda or
amendments to this Recommendation. Please ensure to use the latest available version from the
ITU-T website. The following clauses summarize the use of this simulation code, and how the
software is organized.
--------------
C.3 Overview
The full version of G.729 provides bit-exact fixed-point specification of an algorithm for the coding
^^^^^^^^^^^^^^^^^^^^^
of speech signals at 8 kbit/s. Annex A is a reduced complexity version interoperable with the full
version of G.729. Exact details of these specifications are given in bit-exact fixed-point C code
available from ITU-T. This annex describes and defines an alternative implementation of the full
version of G.729 and Annex A based on floating-point arithmetic.
--------------
Why would we choose the floating point approximation if we can have an
exact integer implementation ?
[...]
--
Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
Dictatorship naturally arises out of democracy, and the most aggravated
form of tyranny and slavery out of the most extreme liberty. -- Plato
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