CD-RW - Speed Specifications

Speed Specifications

Like CD-R, CD-RW have hardcoded speed specifications which limit the allowable recording speeds to certain fairly restrictive ranges, but unlike the former they also have a minimum writing speed under which the disks cannot be reliably recorded, something dictated by the phase change material's heating and cooling time constants, and the required laser energy levels.

Since the CD-RW discs need to be blanked either entirely or "on the fly" before recording actual data, writing too slowly or with too low energy on a high speed unblanked disc will cause the phase change layer to cool off before blanking has been achieved, preventing the actual data from being reliably written.

Similarly, using inappropriately high amounts of laser energy will cause the material to get overheated and become "insensitive" to the actual data, a situation which is typical of slower discs used in a higher powered faster spec drive.

For these reasons, in general older CD-RW drives lacking appropriate firmware and hardware cannot handle newer, high speed CD-RW discs (poor forward compatibility), while newer drives can generally record to older CD-RW discs, provided their firmware can set the correct speed, delay and power settings for the task.

The actual reading speed of CD-RW disks, however, is not directly correlated or bound to its speed spec, but depends first and foremost on the reading drive's capabilities, as with CD-R discs.