You mean compressed onto the CD?

You'll get as many answers as there are people who mix CDs.

Personally I use 256kbps CBR MP3s (at 256 I use straight stereo, less than that - joint stereo) - gets me 6-10 albums onto one... I use them for the car... less disc changing and I'm not worried about my music collection getting stolen or melting.

Bren R.