With CD-RW recordable media, the grooves are prepared with a pregroove
. This pregroove already has the exact timing information baked into it as Absolute Time in Pregroove (ATIP) for the various areas. This includes the length of the spiral track, the start of the lead-in and lead-out, the lowest and highest recording speed, the power ratio between erase and write and some other values. Via the ATIP code, the CD drive can determine the position of the read head and thus the rotation speed.