The Mechanics of Sony's MiniDisc--Beyond the Caddy (Dec. 1992)

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Much has been written about digital data compression, or (if that term offends the audio purist) bit-rate reduction. It has been said that digital data compression is the wave of the future, as far as audio and video are concerned indeed, the almost simultaneous introduction of DCC (Philips' Digital Compact Cassette) and the MiniDisc (by Sony) confirm that premise, since both use proprietary forms of data compression known, respectively, as PASC and ATRAC. There is, however, a great deal more innovative technology involved in both of these formats beyond data compression.

Having recently gone on a press trip to Sony's facility in Japan, I now know much about the mechanical and physical aspects of the MiniDisc format and about its specific method of data compression. It is these mechanical and physical characteristics of the MiniDisc that I will address in this discussion.

The MiniDisc has been positioned as a record/playback format especially suited fo- portable applications, Walkman-type miniature players and mobile or car audio systems. The biggest problem in using optical discs, such as CDs, in portable applications has been skipping or mistracking caused by shock or vibration. This difficulty has been virtually eliminated in the Mini-Disc system thanks to the use of a memory chip. The memory acts as a buffer, holding digital data equivalent to approximately 10 seconds of playing time before passing the data on for conversion into analog signals for play back. If the players pickup s jarred from its position on the disc, the semi conductor memory will continue to supply data to maintain uninterrupted playback. Because the position of the laser pickup is constantly monitored, using address locations that are pre sent in both recordable and prerecorded MiniDiscs, the laser is able to quickly resume its correct position.

To fill the buffer memory, the Mini-Disc player must read data from the disc at a faster rate than the decoder uses that data. In practice, this is a reading rate of 1.4 megabits per second, nearly five times the rate of playback. Since data read into the buffer when it's full would be lost, the system only reads data at intervals during normal play. Though the data reaches the buffer in bursts, it's clocked out of the buffer in a steady stream, so playback is not affected. If a shock interrupts the position of the opt cal pickup reading the MiniDisc, however, the amount of data read in subsequent intervals is increased to make up for the data lost while the pickup was incorrectly positioned. Once the buffer memory's con tents are replenished, the pickup resumes rearing the signals at regular intervals.

Types of MiniDisc

Two types of discs have been developer for the MiniDisc system. Designed specifically for prerecorded music from record companies, playback-only MiniDiscs are similar to CDs in many ways. The lead-in area is on the inner circumference of the disc.

This is followed by the program area, while the lead-out area is on the outer circumference of the disc. Information is recorded as pits in the disc's substrate in much the same way as with full-size CDs. The discs can be stamped out and mass-produced with an injection molding machine. The en tire front of the caddy that protects and holds the prerecorded disc can be used for graphics, since the protective shutter on this caddy opens only on the back side.

One of the most important features of the MiniDisc is complete random access capability. That's not surprising on playback-only MiniDiscs; they are recorded like CDs--complete with digital addresses for each selection or track--for quick and easy access to any point on the disc. On the other hand, random access is harder to achieve on discs whose final recorded sequence is unknown at the time of manufacture and, indeed, can be changed many times. Thus, the recordable discs must be pre-grooved, for tracking and spindle-servo control in recording and playback. Sony's technique was to give these pre grooves a wobble, a slight shift in groove position that creates subtle ad dress marks at 13.3-millisecond intervals (Fig. 1).

Sony has devoted part of the record able disc's inner circumference to a user table of contents (UTOC) area whose data structure is more like that of a computer floppy disk than a nor mal CD. This allows the track numbers and addresses to be edited in seconds (far more quickly than in systems like DAT, where the track numbers are written at the points where each selection actually begins). It also speeds and simplifies automatic renumbering. For example, if you edit out track 3, tracks 4 and 5 will be renumbered 3 and 4; if you combine tracks 4 and 5, track 6 will then be designated track 5.

Recordable MiniDiscs, unlike the prerecorded type, employ magneto-optical (MO) technology. As shown in Fig. 2, the lead-in area is on the inner circumference of the disc, followed in this case by the user table of contents area, the program area, and the disc's lead-out area on the outer circumference. Since a magnetic recording head and a laser are used on opposite sides of the disc (more on this shortly) the shutter must open on both sides of the caddy (Fig. 3). The recordable MiniDisc's unique layer structure, along with the pre-groove configuration, is represented conceptually it Fig. 4. This MO layer construction has been engineered to enable Magnetic Field Modulation overwriting. According to Sony, it has already been proven both to handle more than one million over-recordings without degradation and to provide excellent long-term storage characteristics. It seems unlikely that anyone would erase and re cord a MiniDisc so many times!

Fig. 1--The pre-grooves molded into recordable MiniDiscs for tracking control have a wobble; see text.

Fig. 2---Cross section of a recordable MiniDisc.

Fig. 3-Exploded view of a recordable MiniDisc and caddy; note that the sliding shutter uncovers windows on both sides of the disc.

Fig. 4-Layered construction of the recordable MiniDisc.

Principles of MO Recording

Magneto-optical discs can be re-recorded countless times. As the name implies, the recording system uses both optical and magnetic technology.

The recordable layer in an MO disc is made of a magnetic material. A laser beam heats a tiny area of the record able layer to its Curie temperature (typically, about 400° F), which demagnetizes it. If a magnetic field is applied to that spot as it begins to cool, the spot will acquire a north or a south magnetic polarity, corresponding to a digital zero or one.

On conventional MO discs Used for computer data storage, all previously recorded signals must be erased before new data can be recorded. Previous systems have used one of two methods to accomplish this: Either two lasers are used, one for erasing and one for recording (much like a tape recorder with separate erase and re cording heads), or a single laser is used, with old data erased in the first rotation and new data recorded during a second rotation.

Unlike these conventional approaches, the MiniDisc Magnetic Field Modulation overwrite system writes new signals over old ones. This system uses a magnetic head on one side of the disc and a laser beam on the other side in the same corresponding position (Fig. 5). With the disc situated between the magnetic head and the laser, the laser brings the spot beneath it up to the Curie point, which dissipates its previous magnetic orientation. As this spot on the disc moves away from the laser, it cools to a temperature below the Curie point and a new magnetic orientation corresponding to the input signal is created by the magnetic head.

The Dual-Function Optical Pickup

The playback-only MiniDisc is re corded and read in much the same way as a CD. Its data stream is represented by pits molded into its base layer and coated with a reflective layer.

A 0.5-mW laser beam directed at the pits is reflected back when it strikes the smooth surface between pits. But where it strikes a pit, the light is diffracted, reducing the amount of light reflected back. A pair of photodetectors reads these light fluctuations, which are then decoded into the binary "0" and "1" signals, as shown in Fig. 6.

Recordable discs are different. The data is represented by differences in the polarization of the reflected light, not by differences in its intensity. A phenomenon known as the Kerr effect rotates the plane of polarization slightly forward or back, depending on the polarity of the magneto-optical recorded signal (Fig. 7). A polarizing beam splitter varies the distribution ratio of the reflected light according to the direction of polarization: A photodetector on one side of the beam splitter will receive a larger share of light reflected in the forward direction, while a detector on another side receives a larger share of light polarized the opposite way. The differences between the electrical outputs of the two photodiodes are used to re-create the binary zeros and ones. The fact that light reflected from the disc changes its polarity according to magnetic orientation is fundamental to the MiniDisc's recording system.

Sony recognizes that success it marketing the new format will require the simultaneous release of MiniDisc software by record companies along with the introduction of MiniDisc hardware. Like CDs, playback-only Mini Discs carry subcode data as well as audio data, which simplifies production from master tapes made for CD. (The MiniDisc does, however, have additional subcode sectors that can be used for such information as lyrics or explanatory text.) MiniDisc also uses the same basic pressing system as CD. Therefore, a significant advantage of the MiniDisc is that CD mastering systems now in use, including the 3/4-inch U-Matic cassette media used for mastering, can also be used in Mini-Disc production.

From what I was able to observe during my short visit, Sony has explored all aspects of this new technology (work began on the MiniDisc concept back in 1986). At Sendai, about 180 miles north of Tokyo, there was even a production line turning out blank recordable MiniDiscs, with much of that production line already fully automated.

How successful the MiniDisc format will be remains to be determined. It is to Sony's credit that they are not claiming "CD-quality sound" for this new product. Rather, they are saying that under the conditions in which MiniDisc is likely to be used, listeners will be hard-pressed to tell the difference be tween CD sound and MiniDisc sound.

Many of us attending the technical sessions at Sony headquarters brought along some of our favorite CDs for auditioning. The digital output from a CD player was alternately heard after normal D/A conversion and after being passed through a MiniDisc-for mat A/D converter (which compressed the digital data as it would be com pressed in a MiniDisc) and D/A converter. For some of the music, most of us could not detect significant differences in sound quality. With other recordings, most of us could detect a difference, but not a difference that could be said to have seriously de graded the musical quality.

Fig. 5--Magnetic Field Modulation overwrite system.

Fig. 6--In prerecorded MiniDiscs, as in CDs, digital signals are represented by pits and lands that cause variations in reflected light intensity.

Fig. 7--In recordable MiniDiscs, digital signals are represented by magnetic poles that rotate the polarization of the reflected laser beam.

All of us attending the sessions were, I believe, impressed with the sophisticated technology involved in the creation of this new optical disc format.

I'll have to reserve judgment about MiniDisc's ultimate sound quality for a later date, when I get my hands on a MiniDisc recorder/player for more definitive listening tests.

[adapted from: Audio magazine Dec. 1992]

Also see:

Digital Doctor: A look at DAT and MiniDisc (May 1997)

Telefunken Digital Mini Disc (June 1981)

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Updated: Thursday, 2022-05-05 21:39 PST