What about cd r?

A blank CD-R is not "empty", it has a pregroove with a wobble (the ATIP), which helps the writing laser stay on track and is used to ensure the data is written to the disc at a constant rate. As well as providing timing information, the ATIP (absolute time in pregroove) is also a data track containing information about the CD-R manufacturer, the dye used and media information (disc length etc). The pregroove is not destroyed when the data is written to the CD-R, a point which some copy protection schemes use to distinguish copies from an original CD.