PCM adaptor
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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.
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[edit] 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 analogue 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 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 coded and modulated into a NTSC (or PAL)-standard monochrome video signal.
This 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. 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 NTSC and PAL color video formats used for audio storage at the time.
Most video-based PCM adaptors record audio at 16 bits quantisation, and a sampling frequency of 44.1 (or 44.056 for NTSC) 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.
[edit] Models of PCM adaptors
The Sony PCM-1600 was the first commercial video-based 16-bit recorder (using a special U-matic VCR for a transport), and continues in its 1610 and 1630 incarnations. The 1600 was one of the first systems used for mastering audio compact discs in the early 1980s by many major record labels.
Several semi-professional/consumer models of PCM adaptor were also released by Sony, including:
- SONY Digital Audio Processor PCM-F10 (the first consumer-marketed model),
- SONY Digital Audio Processor PCM-F1 (which was sold with a companion Betamax-format VCR, the Sony SL-2000 or SL-F1, for recording & playback),
- SONY Digital Audio Processor PCM-100,
- SONY Digital Audio Processor PCM-501ES,
- SONY Digital Audio Processor PCM-601,
- SONY Digital Audio Processor PCM-701.
Technics also made a portable PCM adaptor as well, the SV-100, and a version with a built-in VHS videocassette transport, the SV-P100. Nakamichi as well manufactured a PCM adaptor, the DMP-100.
dbx, Inc. also manufactured a PCM adaptor, the Model 700. It differed from the above listed models in the fact that it did not use PCM, but rather delta-sigma modulation. This resulted in a higher quality digital recording with more dynamic range than what standard PCM modulation could offer. Like a standard PCM adaptor, the Model 700 also utilized a VCR for a transport.
[edit] The obsolescence of the PCM adaptor
A few years after the PCM adaptor's introduction, Sony introduced in 1987 a new cassette-based format for digital audio recording called DAT (Digital Audio Tape). DAT was a much more portable and less-cumbersome format to use than a PCM adaptor-based system, since DAT no longer relied on a separate video cassette recorder. Instead, DAT recorders had their own built-in transport using a small cassette unique to the format. DAT used tape 4 millimeters ( .157 inches) in width loaded into a cassette 73 mm × 54 mm × 10.5 mm (2.87 in. x 2.12 in. x 0.41 in.) in size. The audio data was recorded to the tape in the same fashion that a VCR connected to a PCM adaptor would record to a videotape, by using helical scan recording. In essence, DAT was a modernized and miniaturized version of a PCM adaptor-based system.
DAT could only record 2 tracks of audio for stereo at a time, much like a PCM adaptor, but the smaller size of the equipment and media, as well as being able to accept multiple sampling rates (the standard 44.1 kHz, as well as 48 kHz, and 32 kHz, all at 16 bits per sample, and a special "LP" recording mode using 12 bits per sample at 32 kHz for extended recording time) gave DAT many advantages over PCM adaptor-based systems.
About the same time of the introduction of DAT, digital recorders capable of multi-track recording (as opposed to only two tracks for stereo that a PCM adaptor or DAT could record) such as Mitsubishi’s ProDigi format and Sony’s DASH format also became available on the professional audio market. Machines for these formats had their own transports built-in as well, using reel-to-reel tape in either 1/4", 1/2", or 1" widths, with the audio data being recorded to the tape using a multi-track stationary tape head.
Formats like ProDigi and DASH were referred to as SDAT (Stationary-head Digital Audio Tape) formats, as opposed to formats like the PCM adaptor-based systems and DAT, which were referred to as RDAT (Rotating-head Digital Audio Tape) formats, due to their helical-scan process of recording.
Like the DAT cassette, ProDigi and DASH machines also accommodated the obligatory 44.1 kHz sampling rate, but also 48 kHz on all machines, and a 96 kHz sampling rate on the last-generation units.
