I sometimes notice "crap" coming out of my rear speakers, but at other times the sound seems fine ... and I was wonderin' ...

I know the quality with which you encode your OGGs (and MP3s) has much to do with what we "hear" ... but does the quality setting affect the ability to decode/extract music information for the rear speakers ???

The "crap" I hear doesn't seem to have much to do with whether I have chosen some "ambient" effect, or whether I have chosen Dolby Prologic. Has someone tested or otherwise posted some info on this?

Can you be more specific. For instance is it a crackling sound that you hear?

what exactly is the sound coming from rear speakers when playing a normal cd in pro logic? i have noticed that about the same sound comes from the rear speakers as when you use Vocal Cut (remove center material) and listen to the result in stereo.

To be a bit more specific ... it sounds like a "garbled crackle", somewhat like you'd expect of distorted clipping. I don't usually employ Prologic of any form ('cept for DVDs), and instead use my amp's alternatives for ambient effects ... e.g., arena, studio.

I seem to notice this with respect to MP3s I have downloaded ... with the worst examples being older encodings. I cannot be more specific, but it did raise the question in my mind about "encoding quality" ... ears versus enough information for decoding for multiple speakers.

... just an additional curious query:

I thought there may be problems relative to the subject to this thread with regard to OGG q settings and stereo coupling. I read in one place the that stereo is de-coupled for a setting of '5', but in another place it is setting '6'. Which is it?

The big problem here is that inaccuracies in encoding the stereo image can result in the Dolby Surround processor extracting strange-sounding artefacts from the stream which shouldn't really be there.

I doubt you'll see problems with Ogg at either Q5 or Q6. Officially, the Ogg help states that Q6 performs lossless stereo coupling. Unofficially, I've found Q5 to be fantastic at encoding surround recordings (or recordings with unintended surround info.)

MP3 in general is terrible at retaining such information. @128kb/s it's horrendous.

AAC is fantastic at the job, even better than Ogg.

MPC is pretty good, but only at higher quality levels.

If you'd like to see some tests and a discussion about the matter, have a look at this discussion.