Digital Doctor: A look at DAT and MiniDisc

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[article orig. from Audio magazine 100 anniv. issue, May 1997]
With DAT mostly relegated to professional use, DCC dead, and Mini-Disc just hanging on (at least on this side of the Pacific), recording enthusiasts may have to be content with analog media for the time being. But that needn’t mean much sonic sacrifice, thanks to advances in tape and hardware technology. Under the proper circumstances, you can make an analog cassette that is virtually indistinguishable from its CD equivalent.

Yet this isn’t so easily accomplished. You must use the best possible tape (preferably metal), set the recorder to exactly match that tape each time you make a recording, and use the most advanced noise-reduction system as well as Dolby HX Pro. All this and you get a recording al most equal to what you’d get by just popping a tape into a digital deck and pushing the record button. And even these finicky procedures don’t apply to the problems of old tapes that were recorded under less than ideal conditions.

Now Pioneer has applied digital technology to analog tape reproduction.

This technology ,which Pioneer calls Digital Processing System (DPS), seemed impressive in the early public demonstrations of the company’s CT-W606DR ($285) and CT-W616DR ($300) dual-well dubbing decks.

Recently I’ve had a chance to use one of the first production samples of Pioneer’s CT-W616DR. Naturally, this model does the things that older models did, such as noise reduction and tape matching, but it does them in the digital domain. Matched 20-bit A/D and D/A converters take the deck’s analog input, convert it for processing, reconvert it, and then feed it to the recording head. (Pioneer’s literature suggests that it would be possible to feed a digital signal directly from a CD player to the processing circuits, but there’s no provision for doing that on the CT-W616DR.)


--- 32 This deck digitally improves the sound of old analog cassettes.

= = = =

PCM --- Some history…

When did you first read in Audio about pulse-code modulation (PCM)? Circa 1982, when the CD introduced it to consumer electronics? Or when we first covered it, in 1947? You probably wouldn't have heard about it anywhere before that. The story in our October 1947 issue covered PCM's first public demonstration, earlier that month. For that demo, music and speech were transmitted from Bell Telephone Laboratories in Mur ray Hill, N.J., to a meeting of the Institute of Radio Engineers in New York City. The system used seven-bit coding and a sampling rate of about 8 kHz-hardly hi-fi but fine for its intended use, long distance telephony.

As the demo was in 1947, tube A/D and D/A converters were used. Our story described the special tube used for A/D conversion: "All the 128 codes available in the seven-pulse code group are perforated in appropriate order in a special plate inside the tube. This plate is so placed that a beam of electronics can sweep across the seven elements of any one code group. Which one it actually sweeps across is determined by the position of the beam, and this in turn is determined by the amplitude of the signal at the time of sampling. If the beam goes through a hole in the perforated plate, the pulse is an on-pulse; if the beam is blocked because there is no perforation at that point, it is an off-pulse."

= = = = MiniDisc (MD) drives on . . .[from Audio magazine, May 1997!!!] = = = =

When the MiniDisc (MD) format was first announced, the obvious com parison was with DCC. They came out at virtually the same time, and both offered digital alternatives to the Compact Cassette. But DCC failed miserably, whereas MiniDisc is still with us.

Now the obvious comparison is with laserdisc. Laserdisc has survived for nearly 20 years, largely sustained by the faith and efforts of a single company (not Philips, which developed it, but Pioneer). MiniDisc’s survival, too, is largely based on the efforts of a single company, Sony, and Sony has more muscle to put behind a format than Pioneer. Mini- Disc is also sustained, as laserdisc has been, by popularity in Japan.

But Sony, though MiniDisc’s originator, is not the format’s only friend. Our most recent Annual Equipment Directory (October 1996) also listed MD recorders from JVC and Sharp. At the 1997 Con sumer Electronics Show, Denon introduced its first home MD recorder, and Sharp announced 11 MD products.

MiniDisc is hardly new to Denon, however; the company makes several MD machines for professional use. That may sound odd if you think of pro recorders only in terms of super-fidelity studio mastering or if you’ve failed to keep track of how far MiniDisc’s sound has progressed. Its sound quality is now plenty good enough for radio, and radio studios love its ruggedness, easy editing, and instant cueing. Reporters with MD portables can edit their discs with the aid of laptop computers, then send them via telephone or as E-mail attachments.

And MiniDisc is good for more than audio. Sony has long since formalized a 140-megabyte MD-ROM standard for compact computer storage (though I don’t think anyone’s espoused it). And one of Sharp’s 11 new MD products this year is a digital camera that can store 2,000 VGA-quality still images, or 365 images accompanied by digital sound.

In theory, MiniDisc is as universal a system as cassette is. MiniDisc portable players and recorders, boomboxes, car stereo units, and home decks are avail able; when the format first appeared, you could even buy commercially recorded discs. MiniDisc’ sound is rather good, its convenience is unparalleled, and prices are now affordable. What stands in the MiniDisc’s way is that everyone already owns portables, boomboxes, car stereos, and home decks that take cassettes, plus libraries of cassette soft ware—and cassette recordings are still not difficult to find.

I don’t think Mini-Disc will ever make it big, at least outside of Japan. But despite initial skepticism, I’m beginning to think that it will make it.

= = = =

In recording, DPS handles the sophisticated tape-matching functions Pioneer calls Super Auto BLE XD (not available in the less expensive model), plus Dolby B and C noise reduction. (Curiously, Pioneer has omitted Dolby 5, which was included in earlier models.) In my listening tests, these worked well and produced fine recordings. But Pioneer’s analog predecessors worked excellently, too. Truthfully, I didn’t hear much difference.

It’s when playing tapes that the digital additions come into their own. In playback, DPS equalizes the spectral balance of old tapes (another feature missing from the CT-W606DR). When tapes are copied from one well to the other, DPS also reduces hiss in the quiet passages between songs. But the spectral balancing and hiss reduction are both digital versions of existing functions. What’s really new is that DPS removes noise from existing recordings.

Like most dual-well decks, the two new Pioneers can copy at regular or double speed, but they also include a Tape Duplication Noise Suppressor (TDNS). This circuit senses the end of a track and fades the signal until the next track begins, reducing noise between selections, where it’s most noticeable because there’s no signal to mask it. The TDNS system seemed very effective in killing the noise, but I was sometimes aware that a tiny bit of the following selection’s first note had been clipped. Most of the time, I switched TDNS off.

The 616 also includes Pioneer’s Frequency Level Expander (FLEX). Despite its name, this is actually an automatic equalization circuit designed to detect when the overall balance of a musical signal varies from preprogrammed norms; it then makes whatever corrections are necessary. I found FLEX effective with very old tapes, which tended to be boomy and whose high frequencies may have been partially erased by magnetized transports over the years. While the system didn’t totally flatten response, it did make a number of cassettes sound considerably better.

But the big news with DPS—and the feature that made me want to get my hands on the CT-W616DR and try it out—is its after- the-fact digital noise-reduction capability, which Pioneer claims can yield an SIN ratio of 90 dB. Details on this feature are still fairly sketchy, probably because Pioneer’s patent is still pending. Essentially, the DPS circuitry divides the frequency spectrum into several narrow bands, analyzes ‘whether there is signal or just noise in each, and attenuates the bands it decides contain only noise. The system seemed to consider anything below a certain level as noise; when I recorded music that peaked at about —20 dB on the level meters, the circuits eliminated everything but the peaks. Similarly, if the noise level was too high, the circuits read the noise as signal and left it in.

Except for these extreme situations, this digital noise reduction worked quite well with a broad range of recordings. I put it through its paces with a variety of tapes; most of these were prerecorded, but I included some old dubs as well. A couple of the tapes were nearly 30 years old, others only a year.

The newer tapes didn’t benefit greatly from the process. The noise might have been lowered a bit, but these tapes were al ready so quiet that I didn’t hear much improvement. On the other hand, the digital noise-reduction circuit didn’t degrade the sound—which was fortunate, because unless you go through an unintuitive procedure to cancel it each time you power up the 616, the digital circuits are always on. With older tapes, however, the effect was startling. The digital noise reduction seemed to eliminate the noise on tapes recorded 10 to 15 years ago, which contained a notice able amount of hiss even though they had been Dolby-encoded. And eliminating the noise did not affect the music. Occasionally, as I switched the circuit in and out, I noticed a slight sort of “phasiness” on the processed signal, although it was never obtrusive in normal, extended listening.

Removing the noise sometimes revealed other flaws, hitherto masked. Dolby mis- tracking, which causes high-frequency anomalies, was fairly common. However, tapes that exhibited this problem often sounded better with the CT-W616DR’s Dolby noise reduction switched off; the FLEX circuits were then quite effective in restoring spectral balance.

A couple of old tapes demonstrated the system’s effectiveness most dramatically. First, my wife’s old copy of Neil Young’s Harvest, from 1968 or so, was made without noise reduction and had been tweaked in the bass, presumably to get some low-end out put from the car systems of the day. The re suit was basically unlistenable until the DPS noise reduction virtually eliminated the hiss and FLEX toned down the bass (or maybe goosed the treble); only then was it possible to sit back and appreciate the music.

The second dramatic example was an atrocious copy of The Moody Blues’ Days of Future Passed, duplicated in 1972. Without DPS, the start of the long cymbal crescendo that opens the album was lost in the murk, emerging only as it rose to full level. When the noise was eliminated, I heard what sounded like an intermittent connection in an audio cable and possibly the shuffling of feet in the studio. Not an improvement, perhaps, but interesting nonetheless.

I was also keen to find out whether DPS could eliminate vinyl surface noise from records dubbed to cassette. Here, DPS was less successful, but in fairness, it wasn’t de signed to do this. Tape noise included in a vinyl record—not such an uncommon phenomenon with non-audiophile recordings of the ‘70s—was handled pretty much like any other hiss, but most of the surface noise remained. My greatest success was with jazz pianist Ahmad Jamal’s classic album But Not for Me. My copy of this LP had always contained large dollops of tape noise, but the years have not been kind to it and it has become crackly as well. The DPS circuits couldn’t begin to handle the record’s surface noise, and it was difficult even to hear the hiss under it. But when I tamed the physical noises by playing the record with its grooves wetted by distilled water, the hiss emerged and was easily dealt with by the CT-W616DR.

Pioneer’s new technology is not for all occasions, so it’s fortunate that it’s been incorporated into an otherwise fine cassette deck. But when you encounter the kind of noise it was meant to combat, DPS works like a charm.

Adapted from 1997 Audio magazine article. Classic Audio and Audio Engineering magazine issues are available for free download at the Internet Archive (archive.org, aka The Wayback Machine)





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