Optical discs with the exception of the DVD-RAM have their data encoded on a single spiral, or a groove, which covers the surface of the disc. In the case of recordable media, this spiral contains a slight sinusoidal deviation from a perfect spiral. The period of this sine curve corresponds to the wobble frequency. First of all, it is used as a synchronization source to achieve constant linear velocity while writing a disc. The frequencies quoted all assume that the disc is being written at the 'x1' speed. The frequencies are appropriately higher for faster writing speeds.
DVD+R and DVD+RW have a constant wobble frequency of 817.4 kHz, but encodes its addressing information by periodically inverting the phase of the wobble signal (bi-phase modulation) to encode an exact address of the location on the spiral track (Address in Pregroove). The practical upshot of this arrangement is that the recording drive can navigate to an exact location on the DVD+R(W) disc whereas it cannot do so with the DVD-R(W).
BD-R and BD-RE discs utilise Address in Pregrove.
HD DVD-R and HD DVD-RW uses the land pre-pit system of the common or garden DVD-R(W).
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