In list.cypherpunks you write:

Tough to do. A CD samples at 44,100 Hz. Nyquist says you can only resolve the original frequencies up to 1/2 the sampling rate. So a CD cuts off, of necessity, at 22,050 Hz. Many people can hear beyond 22 KHz, and can notice the CD cutoff effect. (analog recordings taper off as the analog response diminishes) Not a lot of room inband.

Actually (speaking from personal experience) many many recordings are now done at a sampling rate of 48 instead of 44.1 (actually it may be around 50/50 or so, from my experience). Many CD replicators these days are thrilled to get recordings done at 48 instead of 44.1, which gives even more room. Another possibility, if you want to get even more into detail is to encode the ID in digital format on the CD. This will give an audible sound (ever put a CD-Rom in a regular CD player by mistake?), but a mirror of that sound played at the same time will effectively set that sound to nothing (cancelling it out). You can't hear it, but a machine can still decode the digital info. This way you can set it at a higher frequency where if it happens to cancell out a brief second of music, the listener won't notice (unless you are "Jamie Summers" :) ). Alex F =-=-=-=-=-=-=-=-=-=-=-=-=- Alex F alexf@iss.net Marketing Specialist Internet Security Systems =-=-=-=-=-=-=-=-=-=-=-=-=-