[vorbis] Downsampling

[Wayne Nyhlen]

> It is commonly said here that if I want to make AM
radio-quality
> stuff at very low bitrates, a good way is to
downsample.
> I downsampled a song to 11025Hz mono and encoded
with -q 0,
> the result is about 18kbps and is at least radio
quality.
> The downsampler I used is from Edinburgh speech
tools, named
> ch_wave. `sox' performs terribly, so I didn't use
it.

> However, I heard some unpleasant sound at relatively
high
> frequencies in the *downsampled WAV file*, but not
in the
> encoded OGG file. So I assume I need some even
better
> downsamplers. Is there any on Linux?

As I understand it, when you downsample or upsample
with sox, you still need to apply a low pass filter
to the output to get rid of the artifacts.  I think
the oggenc does this by default which is why you
don't hear it in your ogg file.

I have never seen a good example of what values to
use with a low pass filter in sox.  Does anyone have
any idea of common values to use for down/up sampling?

Peter

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