PCM adaptor

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 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 (CCIR 625/50 countries) and 30-frame black and white (EIAN 525/60 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 14 bits quantization, and a sampling frequency of 44.056 kHz for EIAN countries (or 44.1 kHz for CCIR countries.) 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.

Models of PCM adaptors

The Sony PCM-1600 was the first commercial video-based 16-bit recorder. The 1600 (and its later versions, the 1610 and 1630) used special U-matic-format (aka "3/4"" or "three-quarter") VCRs also furnished by Sony for transports, such as the BVU-800DA, VO-5630DA, and the later DMR-2000 and DMR-4000, which were based from the industrial VO-5850 and the broadcast BVU-800 video machines respectively. These were all in essence modified versions of existing Sony U-Matic video recorders adapted for use with the 1600-series adaptors by way of disabling the chroma and dropout compensation (DOC) circuits of the VCRs, which would hinder the proper recording of the monochrome-video-based digital audio data from the 1600-series adaptors if enabled. Editing was accomplished by using a 1600-series adaptor and 2 or more of these VCRs with a DAE-3000 editing controller. The 1600-series were the first systems used for mastering audio compact discs in the early 1980s by many major record labels, with the final U-matic 1600-format digital audio tapes being sent to CD pressing plants to be recorded to a glass master disc used for making the replicated CDs.

Several semi-professional/consumer models of PCM adaptor were also released by Sony:

Technics also made a battery-powered portable PCM adaptor as well, the SV-100, a hi-fi component adapter, the SV-110, and a version with a built-in VHS videocassette transport, the SV-P100. All the Technics (Panasonic) PCM adapters are limited to 14-bit resolution. Other makes and models of PCM adaptors offered on the market were the Nakamichi DMP-100, the Sharp RX-3, the Sansui PC-X1 and the Hitachi PCM-V300.[1]

dbx, Inc. also manufactured a pseudo-video 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.

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, integrated, 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.

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 about the same time as the introduction of PCM encoder/decoders made for use with video tape recorders. 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. ADAT also became available in the early 1990s, which allowed eight-track 44.1 or 48 kHz recording on S-VHS cassettes.

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. They overcame the problems that made typical analog recorders unable to meet the bandwidth (frequency range) demands of digital recording by a combination of higher tape speeds, narrower head gaps used in combination with metal-formulation tapes, and the spreading of data across multiple parallel tracks.

Despite obsolescence, hobbyists are still capable of using modern day DVDs or Blu-ray discs as a transport medium for video-based encoding of digital audio streams, though DVD audio has superior digital audio on its own, as well as the DVD's ability to store audio in "data mode", especially compressed formats such as MP3.

References

  1. ↑ Heitarō Nakajima (1983). Digital Audio Technology. Tab Books. p. 268.
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