[vorbis] Low bitrate encoding

[Marshall Eubanks]

Olaf van der Spek wrote:

> ----- Original Message -----
> From: "jaromil" <jaromil at dyne.org>
> To: <vorbis at xiph.org>
> Sent: Tuesday, January 09, 2001 1:14 PM
> Subject: Re: [vorbis] Low bitrate encoding
>
> > 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:
>
> This means you just discard half of the samples.
>

Averaging is much better than decimating as it reduces aliasing.

>
> >
> > /* downsampling >>1 */
> >
> > for(i=src_length>>2;i;--i) {
> > dst[0] = src[0];
> > dst[1] = src[1];
> > src += 4;
> > dst += 2;
> > }
>
> What about this?
> for(i=src_length>>1;i;--i) {
> dst[0] = src[0] + src[1];
> src += 2;
> dst++;
> }
>
> >
> > /* upsampling <<1 */
> >
> > for(i=src_lenght>>1;i;--i) {
> > src -= 1;
> > dst -= 2;
> > dst[0] = src[0];
> > dst[1] = src[0];
> > }
>
> Would it be better to interpolate the second value?
>
> >
> > so 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
>


--
                                 Regards
                                 Marshall Eubanks

T.M. Eubanks
Multicast Technologies, Inc
10301 Democracy Lane, Suite 410
Fairfax, Virginia 22030
Phone : 703-293-9624
Fax     : 703-293-9609
e-mail : tme at on-the-i.com     tme at multicasttech.com

http://www.on-the-i.com http://www.buzzwaves.com

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