Although none of the drivers has implemented this function so far, some drives are capable of head-speed selection, and hence this is a capability that should be standardized through a function in the device-operations structure. This function should select the speed at which data is read or audio is played back. The special value `0' means `auto-selection', i.e., maximum data-rate or real-time audio rate. If the drive doesn't have this `auto-selection' capability, the decision should be made on the current disc loaded and the return value should be positive. A negative return value indicates an error. (Although the audio-low-pass filters probably aren't designed for it, more than real-time playback of audio might be used for high-speed copying of audio tracks). Badly pressed CDROMs may benefit from less-than-maximum head rate.