I'm just wondering what people recommend as a minimum bitrate for encoding AC3 5:1 audio. I've been doing 128kbit stereo encodes for a while now and have been generally happy with them, and would like to keep the resulting output roughly equivalent, or slightly better. I am, however, very space constrained so don't want to waste it. I also can't easily perform my own listening tests at the moment as all the equipment I have isn't surround capable, so I can only compare the stereo downmixed results, which I fear might not be adequate to know exactly what I'm doing.

My working theory is that (as AC3 does global bit allocation and shares high frequency data between channels) it shouldn't need quite as much as 3 times the bitrate required for stereo, so 320kbits or maybe even 256kbits should be adequate. Any comments?

At 128kbps for a stereo signal, you will obtain way better results with mp3 than with ac3.

448 bps is the most used bitrate for 5.1. Now if you are space constrained, 384 bps is doable. Under that limit, you might have some really audible problems. But if you are happy with ac3 128 for stereo, you are not really sensible to those. Be careful with your choice, you might regret one day…