This article is from the CD-Recordable FAQ, by Andy McFadden (email@example.com) with numerous contributions by others.
ATIP is an acronym for 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).
In the lead-in area, which is at the start of the disc, the ATIP signal
can be read to get some information about the disc. The only really useful
bit of information is the number of blocks on the disc, which is determined
by the length of the pre-formed groove.
The ATIP signal also holds some information about the disc's construction
and manufacturer, but see section (2-33) for some comments about their
usefulness. http://www.orangeforum.or.jp/e/reference/index.htm used to
have ATIP information, but the "Disc Identification Method" link is now