PCM Adaptor - How A PCM Adaptor Works

How A PCM Adaptor Works

High-quality PCM audio requires a significantly larger bandwidth than a regular FM-modulated analog audio signal. For example, a 16-bit PCM signal requires an analog bandwidth of about 1-1.5 MHz (compared to about 15-20 kHz of analog bandwidth required for an analog audio signal), and, clearly, a standard analog audio recorder could not meet that requirement. One solution arrived at in the early 1980s, was to use a video tape recorder, which is capable of recording signals with this high bandwidth, to store the audio information, but a means of converting the digital audio into pseudo-video was necessary. Such an audio recording system therefore includes two devices, namely the PCM adaptor, which converts audio into pseudo-video, and the video tape recorder itself. A PCM adaptor has the analogue audio (stereo) signal as its input, and translates it into a series of binary digits, which, in turn, is coded and modulated into a monochrome (black and white) video signal, appearing as a vibrating checkerboard pattern, modulated with the audio, which can then be recorded as a video signal.

This video signal can be stored on any ordinary analog video tape recorder, since these were the only widely available devices with sufficient bandwidth. This helps to explain the choice of sampling frequency for the CD, because the number of video lines, frame rate and bits per line end up dictating the sampling frequency one can achieve. The sampling frequency of 44.1 kHz was also adopted in the Compact Disc, as at that time, there was no other practical way of storing digital sound than by a PCM Converter & video recorder combination. The sampling frequencies of 44.1 and 44.056 kHz were thus the result of a need for compatibility with the 25-frame (PAL countries) and 30-frame black and white (NTSC countries) video formats used for audio storage at the time. (Note that neither PAL nor NTSC was itself used, the black and white version of video storage was used with no color subcarrier.)

Most video-based PCM adaptors record audio at 16 bits quantization, and a sampling frequency of 44.1 for NTSC countries (or 44.056 for PAL countries) kHz. However, some of the earlier models, such as the Sony PCM-100, recorded 16-bits quantization as well, but used only 14 of the bits for the audio, with the remaining 2 bits used for error correction, in case of dropouts or other anomalies being present on the videotape.

A PCM adaptor can only store a single stereo signal, and is not capable of studio multi-track recording.