The first PCM digital audio processor intended for home use that I ever tested weighed about 50 pounds and cost more than $5,000. That was back in 1978, when Sony introduced their PCM-1--a massive-looking unit that could be hooked up to a VCR for PCM (digital) audio recording and playback. Back then, a multi-national committee was still at work at setting standards for this kind of product.
About a year later the PCM-10 was introduced. It was still just about as heavy and just about as bulky as its predecessor, but it conformed to the newly agreed-to standards for PCM processors.
Fig. 1-Frequency response. left (top) and right channels.
Fig. 2-Unweighted S/N analysis in 14-bit mode (A) and 16-bit mode (B).
More recently, other PCM processors have appeared-all at considerably lower cost-including the renowned PCM-F1 from Sony, a portable, battery-operated unit that became popular with audio professionals and with audio hobbyists.
Used with any portable VCR, the PCM-F1 made it possible to record musical events in the field with hitherto unheard-of fidelity and accuracy. Remote recording events, which formerly required hundreds of pounds of reel-to-reel recorders, mixers, amplifiers and mixing consoles, could now be handled with a small VCR plus the tiny PCM-F1 processor and a couple of well-placed microphones. No wonder the PCM-F1 found favor more with the pro audio folks than with the home users for whom it had originally been intended.
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Manufacturer's Specifications
Frequency Response: 10 Hz to 20 kHz, ± 0.5 dB.
Harmonic Distortion: 14-bit format, less than 0.007%; 16-bit format, less than 0.005%.
Dynamic Range: 14-bit format, greater than 86 dB; 16-bit format, greater than 90 dB.
Channel Separation: Greater than 80 dB.
Input Levels: Line, 0.24 V for-10 dB reference level; video, 1 V peak to-peak.
Output Levels: Line, 0.24 V for-10 dB reference level; monitor, video, and copy outputs, 1 V peak-to-peak; headphones, 0.003 to 0.9 mW into 32 ohms, continuously adjustable.
Power Requirements: 120 V a.c., 60 Hz, 33 watts.
Dimensions: 17 in. W x 3 1/4 in. H x 14 7/8 in. D (43 cm x 8 cm x 35 cm).
Weight: 13 lbs., 3 oz. (6 kg).
Price: $750.
Company Address: Sony Drive, Park Ridge, N.J. 07656.
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Not content with that accomplishment, Sony went on to introduce, a couple of years ago, the PCM-701, a processor which sold for only $1,200 and which occupied no more space than a typical amplifier. Now, further integration of circuitry plus the experience gained in other digital-audio areas (such as CD and professional digital recording equipment) has enabled Sony to produce the lowest-priced PCM processor ever, the PCM-501 ES. Besides being considerably less expensive than any previous PCM processor, the new PCM-501 ES incorporates a host of features that serious recording enthusiasts will find useful. For one thing, the unit offers both 14-bit and 16-bit recording and playback configurations. Both formats conform to the technical specifications of the EIAJ; thus, any tape recorded using another digital processor that conforms to either EIAJ format can be played back using this unit. Furthermore, the PCM-501 ES allows you to copy tapes digitally without having to decode them first from digital to analog. If a tape recorded in the 16-bit format is duplicated using this digital tape-copy function, the copy can automatically be converted to the more common 14-bit format.
Sony incorporated both formats because the 16-bit mode offers somewhat wider dynamic range and lower distortion, while the 14-bit mode has somewhat better error-correction capability. Specifically, burst errors of up to 32 horizontal video-format lines can be corrected in the 14-bit mode, while the 16-bit mode only allows correction of burst errors up to 16 lines long. As is true of other digital systems, the PCM-501 ES can compensate for burst errors too large for it to correct so that the errors are not perceptible. Interleaving disperses dropout errors by changing the sequence of information blocks or "words" in the recording. When restored to the original order during playback, the words containing errors are therefore placed between error-free words, so linear interpolation can mask the errors.
Another new feature found on the PCM-501-ES is called Optimum Video Condition (OVC) control. This is used in conjunction with the associated VCR's tracking control and a series of indicator lights on the PCM-501 ES, to obtain the optimum relationship between the VCR and the processor regardless of tape speed set on the VCR. Furthermore, when adjusting the VCR's tracking control, one of the processor's level meters can serve the added function of showing when optimum VCR tracking has been attained. Finally, an Auto Playback Mute circuit, when activated, will cause the system to mute in the presence of frequent dropouts or dusty tape.
Control Layout
A "Power" switch and headphone jack are found at the left end of the all-black front panel of the PCM-501 ES. A rotary headphone-output level control is next to the phone jack; to its right is the "OVC" control described earlier. Two pushbuttons are used to select either 14-bit or 16-bit digital resolution; to their right are three pushbuttons arranged in a vertical row and labeled "Copy," "Auto PB Mute" and "Tracking." The "Copy" button is pushed when you want to make a digital copy of a digital tape using two VCRs. The "Tracking" button converts the peak program LED meter into a tracking meter, as described above; the function of the "Auto PB Mute" button has also already been described.
The large, easy-to-read display to the right of these three buttons serves primarily as a dual-channel record-level metering system (or as a tracking-control indicator). To the left and right of the LED banks are indicator lights which denote 14-bit or 16-bit resolution; activation of the built-in emphasis/ de-emphasis circuits, playback muting, and the "Copy" feature, as well as activation of a "Rec Mute" button located just to the right of the display area. This button allows you to interject a signal recording level of zero to create blank spaces or pauses between musical selections. Dual-concentric, calibrated control knobs at the right end of the panel serve as master recording level controls.
The rear panel of the PCM processor has stereo pairs of line-in and line-out jacks, video in and out jacks, the "Copy Out" jack used for copying a tape from one VCR to another, and a monitor-out jack. The latter allows you to monitor an ordinary video signal connected to a TV monitor without having to change connections when you want to use your VCR for its primary purpose, watching recorded television programs.
Measurements
In order to determine the difference, if any, between 14-bit and 16-bit operation of this PCM processor I decided to measure all characteristics of the machine in both operating modes. The first thing I discovered is that there was no measurable difference in frequency response when switching from 14-bit to 16-bit operation. Overall record/playback frequency response for both channels, shown in Fig. 1, was well within the ±0.5 dB tolerance specified by Sony.
Figures 2A and 2B show unweighted signal-to-noise ratios obtained for each digital resolution format. For the 14-bit mode I measured 84.0 dB of S/N, while in the 16-bit mode S/N increased to 88.1 dB. Adding an A-weighting network, the S/N figures improved still further, to 86.9 dB for the 14-bit mode and 90.7 dB for 16-bit operation (see Figs. 3A and 3B). A very slight improvement in harmonic-distortion levels was measured, too, when I switched digital resolution from 14 to 16 bits. For the former mode, I measured a THD of only 0.006%, which decreased to an even lower 0.004% when I switched to 16-bit.
One of the things you have to get used to when using a digital processor for audio recording is the fact that "0 dB" is truly the maximum recording level reasonable. Go above it and you will be in trouble immediately. To illustrate the point, I used the distortion plotting feature of my Sound Technology 1500A tester to see what would happen at higher recording levels. The results for both 14-bit and 16 bit operation are shown in Figs. 4A and 4B: At +2 dB recording level, THD had already jumped up to about 2.7%. Yet, at 0 dB recording level, distortion had been so low that this instrument was incapable of registering a reading. I had to use my dedicated distortion analyzer to read the 0.004% and 0.006% numbers mentioned above.
Channel separation is plotted in Fig. 5. Even at 16 kHz, separation still ranged between 69.4 and 73.7 dB (depending upon which channel was being measured). At mid frequencies, separation was between 90.6 dB and 92.0 dB, again depending upon whether I measured left-to-right or right-to-left separation.
Fig. 3-A-weighted S/N analysis in 14-bit mode (A) and 16-bit mode (B).
Fig. 4-Third-order distortion vs. level in 14-bit mode (A) and 16-bit mode
(B). As is typical of digital systems, distortion is negligible below 0 dB,
but begins rising swiftly as soon as that limit is passed.
Fig. 5-Separation vs. frequency.
Use and Listening Tests
One of the problems I have run into with earlier PCM processors is their inability to record signals faithfully when the associated VCR was operated at its slowest tape speed (either Beta III or VHS EP). The first unit to overcome this problem was Sansui's TriCode PCM processor (see Audio, January 1984.) The Sony PCM-501 ES is the second unit to successfully use the slower VCR tape speeds for digital audio recording. This means that with a Beta VCR you can get up to 5 hours of superb digital audio on a single cassette tape and 8 hours' worth with a VHS recorder.
Furthermore, the OVC control really lets you optimize performance in these slow-speed modes so that as little interpolation as possible is necessary to fill in for missing data. The tracking indicator is also a great help in optimizing the performance of the VCR in relation to the processor.
The "Copy Out" feature will appeal to audio enthusiasts who want to make copies of their homemade, digitally recorded musical performances. Since there doesn't seem to be a great deal of prerecorded software available for use with PCM processor NCR combinations, I don't yet see the present need for this feature.
In my own experiments, I transcribed sample selections from some of my favorite CDs, both in the 14-bit and 16-bit formats. During certain soft passages, I was able to detect a slight difference in residual noise levels, but only when I turned up listening levels to unrealistically loud settings. As for tonal quality, in A-B tests between the original CD and its digitally recorded equivalent on VCR tape, I confess that I could not tell the difference. The only time I thought I heard a difference was when I deliberately experimented with some off-brand, "bargain" videotape I had lying around the lab from an earlier experiment. Some of this tape was so bad that, with the Playback Mute Control activated, I did hear moments of muting when error rates exceeded the error-correction capabilities of the system. With playback muting off, under the same conditions, there was no audible muting, but I did detect occasional changes in timbre when making direct comparisons with the CD source material. I can only attribute these changes to a relatively large number of interpolations which the system was being called upon to perform.
After becoming accustomed to using high-quality, three head reel-to-reel and even cassette tape decks, it's a bit frustrating to have to wait for playback to find out how well a recording came out, but that's the way all PCM processor/ VCR combinations work. Just as you can't monitor a video recording using any home VCR, so too is it impossible to monitor a PCM audio recording made on VCR tape. Under these conditions, it's especially important to stay below that 0-dB indication on the level meters of the PCM-501 ES. Exceed that point and you are going to hear horrendous levels of distortion during playback, since you will have come up against the insurmountable level limit imposed by any digital audio recording system-the point where you run out of "numbers" to describe the loudest sample of the music you are recording. That's the point when the sample you record consists entirely of ones: 14 of them in the 14-bit mode or 16 of them if you use the higher resolution format.
Perhaps some day, when standards are agreed upon for a dedicated Digital Audio Tape (DAT) recording system, provision may be made for the kind of as-you-record monitoring system that many of us enjoy with our analog recorders. Until that day comes, if you have a need or desire to record music and speech digitally, the least expensive way is with a VCR, and a PCM processor such as Sony's beautifully engineered and produced PCM-501 ES.
(Adapted from: Audio magazine, Sept. 1985, Leonard Feldman)
Also see:
Sony PCM-701 Digital Audio Processor (Apr. 1983)
Sony PCM-1 Audio Unit (Mar. 1980)
Sony PCM-2500 Digital Audio Tape Recorder (Feb. 1989)
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