PCM adaptor


A PCM adaptor is a device that encodes digital audio as video for recording on a videocassette recorder. The adapter also has the ability to decode a video signal back to digital audio for playback. This digital audio system was used for mastering early compact discs.

Operation

High-quality Pulse Code Modulation audio requires a significantly larger bandwidth than a regular 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. A standard analog audio recorder cannot meet this requirement. One solution arrived at in the early 1980s, was to use a videotape recorder, which is capable of recording signals with higher bandwidth.
A means of converting the digital audio into video format was necessary. Such an audio recording system includes two devices: the PCM adaptor, which converts audio into pseudo-video, and the video tape recorder. A PCM adaptor performs an analog-to-digital conversion producing series of binary digits, which, in turn, is coded and modulated into a black and white video signal, appearing as a vibrating checkerboard pattern, 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, that sampling frequency of 44.1 kHz was thus 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 and 30-frame black and white video formats used for audio storage at the time.
It is simplest if the same number of lines are used in each field, and, crucially, it was decided to adopt a sample rate which could be used on both NTSC and PAL equipment. Since NTSC has a field rate of 60 Hz, and PAL has a field rate of 50 Hz, their least common multiple is 300 Hz, and with 3 samples per line, this yields a sample rate that is a multiple of 900 Hz. For NTSC the sample rate is 5m × 60 × 3, where 5m is the number of active lines per field, which must be a multiple of 5, and for PAL the sample rate is 6n × 50 × 3, where 6n is the number of active lines per field, which must be a multiple of 6. The sampling rates that satisfy these requirements – at least 40 kHz, no more than 46.875 kHz, and a multiple of 900 Hz are thus 40.5, 41.4, 42.3, 43.2, 44.1, 45, 45.9, and 46.8 kHz. The lower ones are eliminated due to low-pass filters requiring a transition band, while the higher ones are eliminated due to some lines being required for vertical blanking interval; 44.1 kHz was the higher usable rate, and was eventually chosen.
Audio samples were recorded as if they were on the lines of a raster scan of video, as follows: analog video standards represent video at a field rate of 60 Hz or 50 Hz, which corresponds to a frame rate of 30 frames per second or 25 frame/s – each field is half the lines of an interlaced image. Each of these fields is in turn composed of lines – a frame of 625 lines for PAL and 525 lines for NTSC, though some of the "lines" are actually for synchronizing the signal, and a field comprises half the visible lines in one vertical scan. Digital audio samples were then encoded along each line, thus allowing reuse of the existing synchronization circuitry – as video, the resulting images look like lines of binary black and white dots along each scan line. The line frequency was 15,625 Hz for PAL, 15,750 Hz for 60 Hz NTSC, and 15,750/1.001 Hz for 59.94 NTSC, and thus to record audio at the required over 40 kHz required encoding multiple samples per line, with 3 samples per line being sufficient, yielding up to 15,625 × 3 = 46,875 for PAL and 15,750 × 3 = 47,250 for NTSC. One wished to minimize the number of samples per line, so that each sample could have more space devoted to it, thus making it easier to have a higher bit depth and better error tolerance, and in practice, the signal was stereo, requiring 3 × 2 = 6 samples per line. However, some of these lines were devoted to synchronization: specifically, the lines during the vertical blanking interval could not be used, so a maximum of 490 lines per frame could be used in NTSC, and about 588 lines per frame on PAL
Most video-based PCM adaptors record audio at 14 bits quantization, and a sampling frequency of 44.056 kHz for EIAN 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

The Sony PCM-1600 was the first commercial video-based 16-bit recorder. The 1600 used special U-matic-format VCRs also furnished by Sony for transports, such as the BVU-200B, 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 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. The BVU-200B packaged with the PCM-1600 also was modified to have its video head switching point moved to the vertical blanking interval of the digital audio-bearing video signal being recorded to prevent errors or interference with the digital audio data. 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 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.
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.

Obsolescence

A few years after the PCM adaptor's introduction, Sony introduced in 1987 a new cassette-based format for digital audio recording called DAT. 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 in width loaded into a cassette 73 mm × 54 mm × 10.5 mm 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 gave DAT many advantages over PCM adaptor-based systems.
Digital recorders capable of multi-track recording 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, and DTRS performed a similar function with Hi8 tapes.
Formats like ProDigi and DASH were referred to as SDAT formats, as opposed to formats like the PCM adaptor-based systems and DAT, which were referred to as RDAT 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 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.