As We See It

Sidebar: Pulse Code Modulation

Digital PCM (Pulse-Code-Modulation) encodina converts the continuous energy fluctuations of the conventional (analog) audio signal into a series of pulses which are either completely On or completely Off. All of the analog information is retained as variations in either the duration or the repetition rate of the pulses. Thus, all subsequent amplification can be done by devices acting as switches, which are either On or Off. Signal degradation can occur only as a result of imperfections in the circuits which do the PCM encoding and subsequently, the decoding back to analog form.

Recording, too, is done as a series of pulses—dots on a laser-scanned disc, or magnetized/non-magnetized signals on tape. Perfect speed regulation of the recorder/player is obtained by using a high™ speed frequency-stabilized electronic "clock" that counts the pulses coming from the playback and keeps them on a constant timebase.

PCM recordings can be copied with absolutely no degradation of signal quality, because instead of trying to re-record the original pulses as they come off the disc or tape, those pulses are used to produce another set of absolutely identical pulses, which then go onto the copy. In other words, a commercial pre-recorded PCM tape would not be a compromised copy like a conventional cassette, or even a 1-to-l copy of a master tape, but a virtual replica of the original master PCM tape.

There is one easy way in which the original sound can be compromised by PCM encoding/decoding, and that is if the "sampling rate"—the repetition rate of the pulses—is not rapid enough to resolve out every detail of the original signal. Both of Mitsubishi's PCM machines, their professional unit and the audiophile cassette, have a 47.5kHz sampling rate. Only the ears of Stereophile-type listeners will be able to determine whether or not that is high enough.—J. Gordon Holt