[vorbis] Low bitrate encoding

[jaromil]

On Tue, Jan 09, 2001 at 11:49:09AM +0100, Olaf van der Spek wrote:

> > i am currently working on down/upsampling routines for the next major
> > release of muse (2:1/1:2, is that meant here?), as far as i get
something
> > useful i'll notice - next 2 weeks. found really useful to peek into
the
> > sourcecode of SDL (www.libsdl.org) src/audio/SDL_audiocvt.
>
> Is downsampling (from 44100 hz to 22500 hz) something else then just
taking
> the average of two samples from the read buffer and putting that in the
> write buffer?
>

that's not exact... more than averaging it's a stretching operation.
assuming 16bit samples i would do it like that:

/* downsampling >>1 */

for(i=src_length>>2;i;--i) {
        dst[0] = src[0];
        dst[1] = src[1];
        src += 4;
        dst += 2;
}

/* upsampling <<1 */

for(i=src_lenght>>1;i;--i) {
        src -= 1;
        dst -= 2;
        dst[0] = src[0];
        dst[1] = src[0];
}

o at the end is really simple
it's not the first time i see this algorithm, 
i trust it more since i saw sam lantinga using it.

i guess it can be done in asm with mmx lubrification...

jrml

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