ATIP Absolute Time In Pregroove.

All CD-R and CD-RW discs have a pre-cut spiral groove that wobbles slightly. The groove keeps the write head tracking properly, and the wobble (sinusoidal with a frequency of 22.05KHz) provides timing information to the recorder. The wobble is frequency-modulated with a +/-1KHz signal, which creates an absolute time clocking signal, known as the Absolute Time In Pregroove (ATIP).

It can be read to get some information about the disc.

The ATIP signal also holds some information about the disc's construction and manufacturer.