[Vorbis] very low bandwidth encoding

[Andy Baxter]

I'm trying to encode audio tracks as ogg vorbis at very low bandwidths for
peer to peer broadcasting. I.e. need to have two streams uploading from
modem users, which means the bitrate needs to be around 18-22 kbps.

It's not easy to get good quality at this bitrate. The recipe I'm using at
the moment is:

 oggenc Mon-Sep-20-04-0\:3\:27.wav -b 20 -M 22 --resample 17000 --downmix
 -o test.ogg

which is OK, but still has a tinny sound with some tracks.

If anyone knows how to get good sound at this bitrate, I would appreciate
some advice. e.g. any preprocessing that's worth doing.