EFMThe Modulation Method for the Compact Disc Digital Audio System

The modulation method employed in the Compact Disc (CD) digital audio system codeveloped by Philips N.V. (Eindhoven, The Netherlands) and Sony Corporation (Tokyo, Japan) is described. This method, called eight-to-fourteen (EFM), is an 8 (data bit) ---> 14 (channel bit) conversion block code with a space of 3 channel bits for every converted 14 channel bits which is used to connect the blocks. These 3 channel bits, called merging bits, are selectable, enabling the suppression of the low-frequency contents of the frequency spectrum. First some of the major conditions are listed which are required of the modulation method used for recording/reproducing digital audio signals on an optical disk. The various parameters of EFM as a modulation method are explained in the second part, proving the suitability of EFM for optical disks. An actual example explains the method in detail; a frequency spectrum is also given to enhance understanding. EFM is well matched with the error-correction method CIRC employed in the CD. The combination of these two methods plays an important role in stably reproducing a 2-channel 16-bit audio signal on a 12-cm-diameter optical disk for more than playing time, single sided.