The plastic substrate of a CD-R
contains a faint, pre-pressed section called the wobble track. The wobble track is located in the pregroove and contains the Disc Application Code
and other information about the type and size of the CD-R.
The wobble track uses frequency modulation encoding rather than standard CD encoding, as this can be easily added as part of the CD manufacturing process without a seperate burn step.
Some CD readers, audio and otherwise, are sensitive to whether they are reading a CD-R or not. They recognize the difference by attempting to read a wobble track.