What is the best way to get good quality 5.1 audio at low bitrates (as in under 256 if possible) using a codec that can be muxed into some kind of video container and played using a directshow filter (so things like mpc would not be an option for example (in any case it doesn't do 5.1)) and which has a downmix option so that the 5.1 can be played in a 2 speaker system without losing any of the channels?

Being a ISO compliant stream is not really an issue (though ideally it would be nice).

The source file is a 5.1 AC3 at 384 or 448 kbps.

I've tried re-encoding the AC3 down to 256 kbps but the quality is really not good enough (it has an upper frequency cutoff of 12 kHz for one thing); AC3 is really aimed at higher bitrates I guess.

Vorbis doesn't have a downmixer at the moment so that's not an option (I hear it should have one in the near future though).

Does AAC have a directshow filter? Can it be muxed into avi or can it only muxed into mp4? What kind of bitrates should I be looking at to get a 5.1 track with quality roughly equivalent to -q 2 vobis say?

Sorry if this thread is a little off the audio-only topic, but in video forums people seem to have really bad ears because they've all recommended that I use AC3 at 256...

AAC would be your best bet. The main issue is that you can't play AAC streams embedded in AVI files. AVI doesn't like VBR.

Nic already released a =crappy= AAC DS filter. It usually doesn't work with AVI files.

He said he would release a working one as soon as better container formats (OGM, MCF) are released.

Until then, AC3 seems to be the only solution.

Regards;

Roberto.

Here's the AC3 filters for whoever needs them after reading this post, by the way