PCM adaptor
High-quality PCM audio requires a significantly larger bandwidth than a regular FM audio signal.
For example, a 16-bit PCM signal requires an analogue bandwidth of about 1-1.5 MHz, and, clearly,
a standard analogue audio recorder could not meet that requirement. The obvious answer, at that time,
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 modulated into a pseudo-video signal. The pseudo-video signal can be stored on any ordinary analogue 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. As we all know CD's sampling frequency, 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 NTSC and PAL color video formats used for audio storage at the time.
The Sony 1600 was the first commercial video-based 16-bit recorder, and continues in its 1610 and 1630 incarnations.
Key parameters: sixteen bits quantisation and the sampling frequency is 44.1 (or 44.056 for NTSC) kHz. The PCM adaptors
could only store a single stereo signal, and could not be used for studio multi-track recording. Much later we have
witnessed the advent of dedicated professional digital multi-track recorders such as Mitsubishi’s ProDigi
format and Sony’s DASH format. These recording machines accommodated the obligatory 44.1 kHz, but also 48
and 32 kHz as sampling rate.
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