MP3 "joint stereo" issue

[Hans]

Hi Paul,
I'm recording to wav 96 kHz, 16 bit. I have noticed, that when I save the tracks (after splitting tracks etc) as mp3's with the option sample rate "as recorded" and highest mp3 quality ("0 VBR"), and no max/min bitrate, the generated mp3 file is 48 kHz, and - according to iTunes - "joint stereo". I'm not concerned with the 96 to 48 kHz downsampling as this should be well beyond my ear. The "joint stereo" probably is as well (?), but I'm curious why it's not "stereo"? As far as I understand, the "joint stereo" is a way to diminish filesize and for low bitrate files by simplifying the stereo image (some of the sound actually being reproduced in mono?). I can't see a way to force to save the mp3's as true stereo. BTW: When ripping a cd using another audio software like windows media player, it also results in "joint stereo" according to iTunes, whereas using iTunes AAC format results in "stereo". Is this something to be concerned with in relation to whether the stereo image is really compromised or not?
Thanks,
Hans

[Paul Sanders (AlpineSoft)]

Hi Hans,

I'm no expert on this - we use LAME to encode our MP3 files and that does all the heavy lifting - but I believe that joint stereo is the de-facto standard for MP3 files at the sort of bitrates used for music.  I don't think there's any loss of quality in doing this and my own collection, which was encoded at VBR 0 at around 200 kbps, sounds 'perceptually transparent' to me.

If you want to follow this up. a good place to ask is Hydrogen Audio.  The LAME developers hang out there and they can certainly answer this question much more competently than I ever could.  As for AAC, I'm not sufficiently familiar with it currently to say whether the  stereo encoding scheme it uses is superior to that used by MP3, but it is a more modern compression format so it is certainly possible.  We intend to support AAC in VinylStudio soon, but because of licensing issues we might only offer this on the Mac.

48kHz is the bandwidth limit for MP3 files.  Are you recording from a NAD PP-3?  There's no point going above 48kHz if you are - that's the limit of the DAC in that particular unit (and most other USB devices).