A PCM adaptor
is a device used for recording digital audio
in the PCM
format, which in turn connects to a video cassette recorder
(acting as a transport
) for storage and playback of the digital audio information.
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. The obvious answer, ca. 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. Such an audio recording system therefore includes two machines, namely the PCM adaptor and the video tape recorder. 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 NTSC
)-standard monochrome 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......