TABLE OF CONTENTS The data path of an optical recording system is effectively a communications channel with certain bandwidth and noise characteristics. Data to be recorded must pass through this channel and, on replay, must emerge unaltered. The vehicle to navigate the data through the channel is a recording code (also known as a modulation code or channel code). The recording code converts data to an appropriate format to pass through the channel and be recovered as the original data on replay.
The frequency response of the channel falls off sharply at high frequencies. Therefore, if the signal representing raw data contains high-frequency components then these will not be recorded. Because the binary data to be recorded cannot be predicted it is likely that the signal representing the data contains a broad spectrum of frequencies ranging from very low to very high. Consequently, the recording code converts the frequency spectrum of the signal to be recorded to a form that matches the response of the recording channel.
An important function of a recording code is to enable the regeneration of a timebase for effective re-timing of the signal during playback. There will be differences between the speed of the disk during record and playback. It is essential, therefore, to have a timebase signal that is accurately synchronized to the rate at which data are read from the disk. In effect, the recording code combines the data to be recorded with a clock signal on recording.
