RUBBING IT IN (AND OUT)
Okay. Better sit yourself down. Take a few deep breaths and try to relax. This won't be easy for either of us. You know that expensive CD player you just bought, the player you spent three months shop ping for, the player that cost you $500? I'll try to break this to you gently: It's a piece of obsolete junk.
Well, not exactly-but they've come up with something better, a new and improved CD. Specifically, it is a recordable/erasable disc which uses a combination of vertical magnetic re cording and laser optics. Its name:
Magneto-Optical Disc (MOD). The boys in the prototyping laboratories have these MOD things up and running, and their market introduction is only a matter of time . . . .
Magnetic storage has been around for well over a half-century; it's a great way to record and erase data but it suffers from some basic problems such as medium and head wear. In addition, magnetic storage has hither to used longitudinal recording, in which the magnetic particles are laid flat on the medium; this limits the density of particles and hence the amount of information stored in an area.
Optical storage is a newcomer whose longevity (of medium and pick up) and data density are powerful as sets. Thus far, only read-only optical media such as CD have been widely commercialized. That's because the optical properties of materials are not as easily changed as their magnetic properties.
Magneto-optical storage promises to merge the record/erase properties of magnetic materials with the high density and contactless pickup of optical materials, using a number of very clever technological tricks.
Fundamentally, MOD recording is the same as any magnetic recording, except that vertical (sometimes called perpendicular or VR) recording is used. In a vertical medium, magnetic particles are placed perpendicularly to the tape surface; this allows much greater particle density and shorter re corded wavelengths, and hence greater recording density. Vertical recording actually becomes more robust as recording density increases; as the cylindrically shaped particles are packed more tightly, they must be made thinner, which increases their magnetic strength.
However, this great recording density is underutilized by conventional magnetic heads. Their recording flux fields cannot be focused sufficiently; in other words, the recorded area is far larger than necessary. This is where optics, plus a nifty trick, are introduced. In a MOD system a magnetic field is used to record data, but it is about a tenth the strength of conventional recording fields. By itself it is too weak to affect the orientation of the magnetic particles. However, the pro cess utilizes a unique property of magnetic materials: As they are heated, their coercivity suddenly drops close to zero at their Curie point. In other words, at that temperature (about 200° C) the magnetic particles are easily oriented by a weak field.
A laser beam, precisely focused through an objective lens, is used to heat a minute spot of magnetic material to its Curie point. At that temperature only those few particles on that minute spot are affected by the magnetic field from the recording coil, and a very high-density recording results. As in any digital magnetic storage, saturation recording is used. In the case of MOD recording, the aligned particles are reverse-oriented perpendicularly, as shown in Fig. 1.
The funny part is this: We don't really use the magnetic information itself.
You see, reading the data from the MOD uses another trick: The Kerr (or Faraday) effect, which characterizes take another deep breath-the rotation of the plane of polarized light as it passes through a magnetized material.
What this boils down to is that the reverse-oriented regions will reflect laser light differently from the way the un-reversed regions do. To read the disc, we shine a focused laser on the disc and monitor the angle of rotation of its reflection (about 0.3°), as shown in Fig. 2. An optical analyzer (such as a polarizing beam splitter) distinguishes between the rotated and un-rotated light, and converts that information into a beam of varying light intensity. Data is then recovered from that modulated signal.
To erase data, a reversed magnetic shield is applied to the MOD along with the laser heating spot, as shown in Fig. 3, and new data is written. Both erasure and recording can be accomplished in one pass, as with conventional magnetic media.
Fig. 1--Recording a Magneto-Optical Disc (MOD).
Fig. 2--Reading a MOD.
Fig. 3--Erasing a MOD.
A MOD disc would retain the protective properties of other optical media, with the recording layer sandwiched between a transparent substrate and a protective layer. The laser light would shine through the substrate, using re fraction to put surface dust and scratches out of focus with respect to the interior data. Although several magnetic materials could be used, the ultimate selection will be based on orientation properties and long-term stability. System designers are currently researching amorphous thin-film magnetic materials with coercivity of about 2,500 oersteds.
An important aspect of any recording medium is its compatibility with media from other recorders. To achieve this within the high tolerances of a MOD, blank discs will be manufactured with prerecorded non-erasable addressing. The method, called hard ware address sectoring, uses a grooved disc in which address information is physically formed in the groove and detected by light-beam reflection. Using this system, any MOD player will automatically track both ad dress and data information contained on any MOD disc. By superimposing the hardware-addressing information on the recorded data signal, playing time (75 minutes) is not sacrificed.
The optical head and magnetic coils of a MOD system will require sophisticated engineering both in terms of their own design and in terms of the hardware and software to control them.
In addition, a complete signal-encoding chain will have to be contained in every player. However, much of the hardware can be borrowed from CD technology; also, MOD emulates the CD format's sampling rate and word length. Which brings us back to that playback-only device which you recently purchased . . . .
Okay. I know you're feeling pretty despondent. Your new CD player will soon be outclassed, and--oh no! what about all those discs you bought? Well, here's some good news. The MOD system would be upward-compatible with the CD. Because much of the electronics are identical, all MOD players could also play CDs, in the same way that a cassette machine can record tapes or play prerecorded ones. They could share a common optical head, and even the difference between CD and MOD would be automatically detected, owing to the differences in reflectivity.
Go ahead and buy as many CDs as you want; they will remain just as they are-the medium for prerecorded optical reproduction. Of course, if you want to record MODs or play already recorded ones, you'll need a MOD recorder. While many people will certainly get one, I suspect that CD players will keep going strong, just as another great prerecorded software transport, the turntable, has persisted.
Naturally, you still might be harboring some anger toward the manufacturing companies and their habit of constantly improving on their self-pro claimed perfection. In that case, you'll enjoy the MOD/CD-versus-DAT battle they'll create for themselves. These two recordable/erasable digital audio formats should make good competitors. MOD, combining the best of magnetic and optical storage, would appear to have the advantage. For example, a DAT cassette will undergo deterioration after 200 erasures, whereas a MOD disc probably could be erased/ recorded more than 108 times without difficulty. Of course, if (unlike DAT) MOD is given the ability to record--not just play-at a sampling rate of 44.1 kHz, it would probably destroy DAT with a single laser zap.
(adapted from Audio magazine, Nov. 1986)
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