[Ffmpeg-devel] Fixed point arithmetic RealAudio G2 (cook) decoder

Michael Niedermayer michaelni
Wed Mar 7 17:49:16 CET 2007


On Wed, Mar 07, 2007 at 05:06:17PM +0100, Guillaume Poirier wrote:
> Hi,
> Ian Braithwaite wrote:
> > I've modified ffmpeg's cook decoder to operate with fixed point.
> > I have an ARM based box I'm using as an audio media player, so now
> > it can stream radio from the BBC, too.
> > 
> > 
> > Regarding speed, the fixed point version runs about half the speed
> > of floating point on my Athlon. On the ARM, it runs about 25 times
> > faster than (soft) floating point, good enough for real time (25% load).
> > (I've used C's integer multiplication, I've not looked into assembler
> >  optimisations for specific CPU's.)
> That's always wiser to first work at source-level before writing any
> line of ASM... ;)
> > As regards quality, I've compared the fixed and floating point outputs
> > on a 46s sample. 95% of the output values are identical, the rest
> > differ by +-1.
> Is the difference hear-able?

+-1 at 16bit, no unless the whole signal is very silent then with sufficient
volume you can hear anything, or short awnser no you cant hear it under
realistic circumstances

> > Is there any interest in taking this code back into ffmpeg? I can
> > see in the archives that fixed point comes up occasionally, but it
> > doesn't seem to attract much attention.
> I know that Rich is always interested in integer implementation for
> his slow K6-III+. You're gonna make at least one user happy ;-)

not only rich, 25x speedup for ARM is interresting, though we of course
cannot unconditionally replace the float decoder so the patch must be
clean to be accepted in svn, clean hear means that common code
is not duplicated and that both float and integer can be selected by the
users (at compile time is ok)

Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

There will always be a question for which you do not know the correct awnser.
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