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MINI DISC IS HERE! At long last, Mini Disc has been released onto the Australian market. Developed by Sony, Mini Disc comes in two forms –playback only & recordable. Both types have high quality sound, random access & up to 74 minutes playing time, all from a disc only 64mm in diameter. By LEO SIMPSON Sanyo’s MDG-P1 player is particularly compact & weighs a mere 250 grams. With shockproof memory & rapid track access, it will be the ideal player for people on the move. The Mini Disc has been a long time coming but will prove well worth waiting for. When the compact disc was released, a little over 10 years ago, it caused a sensation and now it has completely displaced the vinyl record from the marketplace. Mini Disc is claimed to have the same quality sound as the compact disc and is likely to eventually displace the analog compact cassette. Mini Disc is small, absurdly so. It looks like a tiny version of a 3.5-inch floppy disc except that the disc itself is a mere 64mm in diameter. The disc housing is 72 x 68mm and just 5mm thick. Like a 3.5-inch floppy, it has a metal shutter and this slides across to expose the disc to the laser pick-up when it is being played but covers it at other times. The play-only version of the Mini Disc uses the same opti­cal technology for data storage as the compact disc but the recordable version is a combination of optical and magnetic storage technology. Advantages of the new format Sony has really worked hard to address all the disadvantag­es of current tape recording formats while keeping the advantages of compact disc: long playing time, quick random access and excellent sound quality. The Mini 16  Silicon Chip Disc is aimed squarely at the analog compact cassette, a 30-year old medium which has been developed and refined so that it is now far removed from the original dictation machine cassette. The main market for the compact cassette is as a music medium for cars and Walkman-style players and its disadvantages are well known and are responsible for its steady decline. The sound quality of cassettes is perceived by most people to be poor (and it is poor when used in most run of the mill players), it has no random access facility and it cannot cope with shock or vibration. Nor can it accommodate the contents of the longest CDs which exceed 70 minutes in playing time. Mini Disc, the new medium, has been developed to hit the compact cassette where it hurts. It has excellent sound and can accommodate up to 74 minutes playing time, enough to record the contents of any CD. It has the same quick random access to any selection as a CD player and it goes one better – it has excel­lent resistance to shock and vibration. It is possible that the Walkman style Mini Disc player could be dropped and still the listener would hear no disruption to the music! And since the Mini Disc is just as convenient to handle as a floppy disc, it is less susceptible to damage than a CD. Anoth­er advantage compared to analog tapes is that it is not possible to record over a playback-only (pre-recorded) Mini Disc (because of the different formats). Finally, since the Mini Disc uses a non-contact method for recording and playback, Sony claim that, in principle, it can be played and re-recorded at least a million times. Since that could take more than eleven years, it is a claim that is not likely to be put to the test. New technology So how does this radically smaller recording medium achieve all these advantages? Sony has borrowed freely from current computer technology to produce the Mini Disc but as we shall see, Mini Disc will have important ramifications for personal comput­ers in the near future, since it stores far more than current floppy discs – up to 100 times more, in a much smaller format. Let’s talk about the playback-only Sony’s MZ-1 Walkman portable Mini Disc machine is tiny but is a high performance stereo recorder which also provides all the playback functions you would expect to find on a CD player. Mini Disc first, since it is the most closely related to CDs. Like the CD, the recording information (digital data) is stored in the form of pits which are read off the rotating disc by the laser pick-up. The data is read into a one megabit dynamic RAM chip at 1.4 megabits/second but since the following decoder circuitry only requires the information at a rate of 0.3 megabits/ second, the RAM chip acts like a large data buffer. This means that even if the Mini Disc player is jarred sufficiently hard for the laser pick-up to lose its place on the disc, it has plenty of time in which to find its correct position and resume playback. In the meantime, there is no disruption to the music. In effect, the laser pick-up could take up to three seconds of being disrupted and then resuming operation and still there would be no interruption to the music. Selective compression While the method of data storage on the Mini Disc is essen­ tially the same as the compact disc (ie, 44.1kHz sampling fre­quency, 16-bit A-D and D-A conversion), its smaller diameter means that it could only store about 10 minutes of stereo music if it used the same linear recording technique. Rather than increasing the pit density, which could lead to problems of reliability, Sony has adopted a system of data compression. Called ATRAC (Adaptive TRansform Acoustic Coding), it is similar in some aspects to the PASC data compression method used in the digital compact cassette (DCC). While CD uses 16 bits of data for every 0.02ms sample, re­ gardless of the signal amplitude (or even if there is no signal), ATRAC analyses the digital data for waveform content and encodes only those frequency components which are audible. Two psycho­a­coustic principles, “threshold of audibility” and “masking”, are taken into account in identifying those signals which are audi­ble. As most readers are aware, the sensitivity of the ear varies widely October 1993  17 120 SOUND PRESSURE LEVEL (dB) AUDIBLE 60 NOT AUDIBLE 0 200 20 1k FREQUENCY (HERTZ) 5k 20k Fig.1: this diagram illustrates two psychoacoustic principles on which the ATRAC data compression system relies – masking & the threshold of hearing. Sounds below the threshold of hearing are not recorded & frequencies which are close together can mask each other. 120 SOUND PRESSURE LEVEL (dB) AUDIBLE 60 NOT AUDIBLE 0 200 20 1k FREQUENCY (HERTZ) 5k 20k Fig.2: this diagram shows how the sound spectrum in Fig.1 would be recorded by ATRAC & thus a lot of data storage is avoided. with frequency, being most sensitive to frequencies around 3kHz to 4kHz, as shown by Fletcher-Munson curves. At higher and lower frequencies, the sensitivity of the ear is progressively reduced. Therefore, sounds below the threshold of audibility can be removed without affecting the reproduction at all. “Masking” is a less well-known principle whereby a loud sound can mask a soft sound at an adjoining frequency, provided it is within a range called the “critical bandwidth”. The closer the two frequencies, the greater the masking effect. Thus, only those signal components which are deemed to be audible are ENCODER MUSIC SIGNAL INPUT NON-UNIFORM FREQUENCY-TIME SPLITTING encoded. As a result, the signal can be represented with adequate resolution with only 20% of the data which would be required under the 16-bit linear recording method used by compact discs. This data economy allows 74 minutes of music to be stored on a 64mm disc and as we have seen, because it is read off the disc much faster than is needed by the 16-bit D-A converter, it confers a high degree of resistance to shock. Figs.1 & 2 help illustrate the ATRAC encoder principle, while Fig.3 illustrates the recording and playback process of Mini Disc. Another important facet of the ATRAC data compression system is non-uniform frequency and time MiniDisc BIT ALLOCATION DECODER NON-UNIFORM FREQUENCY-TIME ALLOCATION MUSIC SIGNAL OUTPUT Fig.3 shows the recording & playback chain of a Mini Disc. The music signal is encoded & compressed & the data must be reconsti­tuted after being read off the disc by the laser. 18  Silicon Chip splitting. In some ways, this is similar to the frequency band splitting used by the PASC compression system for digital cassettes (DCC). Both are an attempt to overcome the limited data storage of the media without unduly compromising sound quality. For most of the time in such a band splitting system, the signals will be recorded and subse­quently reconstituted with virtually no degradation. However, there will be times when the signal is especially com­plex and this will lead to some form of bandwidth reduction or perhaps a reduction in signal-tonoise ratio or perhaps both. Is it as good as CD? This will be a key question among hifi enthusiasts and the answer seems to be, at this stage, that in the environment it is intended to be used, in cars and Walkmans, Mini Disc will be virtually indistinguishable from CDs. Sony also state that CD will continue to be preferred as the quality sound source in homes. We take that to mean that CD still has the edge but we have had no chance to listen for ourselves at this stage. Nor have we had any chance to make measurements to test the efficacy of the ATRAC data compression system. As far as frequency response and dynamic range specifica­tions are concerned, CD and Mini Disc appear to be identical, depending on the player; ie, frequency response from 20Hz to 20kHz ±0.5dB and a dynamic range of 96dB. At this stage though, we have seen no figures for harmonic distortion and linearity. So far then, we have only discussed the playback version of Mini Disc, which is the format used for pre-recorded discs. Com­pared with CD, Mini Disc represents yet another big step in miniaturisation, a process which has continued unabated ever since integrated circuits were introduced. Even if Mini Disc was available only in play­back form, it would still be a big enough step forward in technology but when we look at the recordable version, we are looking at a whole new ball-game. It will have far-reaching implications for sound technology and computer data storage. Recordable Mini Discs Recordable Mini Discs use a combination of magnetic and optical storage technology. Whereas the playback Playback of recordable discs While the laser and magnetic head RECORDING HEAD WRITING SIGNAL 0 1 0 1 0 OLD NEW DISC ROTATION RECORDABLE MiniDisc CROSS SECTIONAL VIEW OBJECTIVE LENS LASER Fig.4: recordable Mini Discs use a combination of optical & mag­netic technology & hence need a laser & magnetic recording head used together during the recording process. act in concert to record the disc magnetically, the magnetic head is not used in playback; the laser is. How’s that again? Magnetic fields are not read; light polarisation is. It works like this: Upon striking the magnetic layer of the disc, the light from the laser pick-up will be reflected back in one of two direc­tions, depending on the plane polarisation, and this varies in accordance with the magnetic orientation. The fact that light is reflected not from pits but according to magnetic orientation is central to the record/ playback capability of a Mini Disc. This is demonstrated in Fig.6. Notice how the polarisation axis changes according to the magnetic orientation (north or south). Just how this is achieved is a mys- tery at this stage as Sony in Australia were not able to furnish any additional information. Dual function laser pick-up Since playback-only and recordable Mini Discs are read in different ways, they cannot be played back with the same laser pick-up. For this reason, Mini Disc players make use of a dual function pick-up. It is based on the conventional CD player pick-up but incorporating a polarised beam splitter to detect magneto-optical playback signals as well. It has two photo­detec­ tors, one for each type of disc. Fig.7 shows the set up for playback-only discs while Fig.8 shows the arrangement for playback of recordable discs. Natural­ly, the user is unaware of all this electronic jiggery-pokery and PR E-G RO OV E Mini Disc has a shutter on only one side of the disc, allowing access for the laser pick-up, the recordable Mini Disc shutter exposes both sides of the disc. One side of the disc is read by a laser pick-up in the conventional way but in the record mode a laser head and magnetic head are used on both sides of the disc. This is Sony’s Magnetic Field Modulation Overwrite System - see Fig.4. The MO system employs the magnetic head and the laser head together to erase and record the digital information and this is where it gets very clever indeed. On a normal floppy disc or cassette, the orientation of magnetic fields on the recording medium is simply changed by the recording head. But even if the Mini Disc is exposed to quite intense magnetic fields alone, its data will not be affected. It must be heated beyond its Curie point of 180°C and then the orientation of the stored magnetic field can be changed from north to south or vice versa. A couple of diagrams will help to explain the principles of the recordable disc. Fig.5 shows the various layers of the disc. As with CDs and play-only Mini Discs, the recordable version is based on transparent poly­ carbonate but whereas CD has three layers – polycarbonate with pressed in pits, a metallisation layer and a protective layer plus the label – the recordable version has six layers. Above the polycarbonate substrate, there is a magnetic layer sandwiched between two dielectric layers. For recording, a magnetic head works in conjunction with a laser, with the magnetic head above the disc and the laser below. As the disc rotates, the laser heats up a particular spot. At the same time, the magnetic head creates a magnetic field corres­ pond­ing to the data signal, at the spot at which the laser is fo­ cused. The laser heats the spot to the Curie point (180°C) which dissipates its existing magnetic orientation and allows it to take the orientation being applied by the magnetic head. As this spot on the disc moves away from the laser and cools below the Curie point, it retains its new magnetic polarity and the next spot is processed. PROTECTIVE LAYER REFLECTIVE LAYER DIELECTRIC LAYER MAGNETIC LAYER DIELECTRIC LAYER POLYCARBONATE SUBSTRATE 1.1  0. 5m Fig.5: whereas a playback Mini Disc has only three layers, the recordable version has six with the transparent polarising mag­netic layer being the key to the whole process. October 1993  19 Fig.6: this diagram shows the function of the magnetic layer. Its magnetic orientation affects the way in which it polarises laser light at 780nm. POLARISATION AXIS MAGNETIC DIRECTION S MAGNETIC DIRECTION N the two types of discs appear to behave identically during play­back. At this early stage, the technical information is pretty sketchy and the details of the recording and playback of Mini Discs were not available at the time of writing. We hope to publish more on this subject as the information comes to hand. However, we can briefly allude to some intriguing aspects which will be fully explained in the future. The pre-groove signal LASER 1 0 0 0 1 0 1 0 1 PREMASTERED MiniDisc CROSS SECTIONAL VIEW DISC ROTATION OBJECTIVE LENS ANALYSER PD1 OUTPUT PD2 LASER Fig.7: playback of a pre-recorded Mini Disc is essentially the same as with a CD, with a laser reading the pits. 1 0 0 0 0 1 1 What’s available 1 0 RECORDABLE MiniDisc CROSS SECTIONAL VIEW DISC ROTATION OBJECTIVE LENS ANALYSER PD1 OUTPUT PD2 LASER Fig.8 this diagram demonstrates the dual function laser pickup which reads differ­ences in light polarisation rather than differences in light intensity. 20  Silicon Chip One aspect of the recordable disc which we found particu­larly intriguing is the way in which the laser pick-up is continu­ally informed of its position on the disc, so that even if it is jolted away from its correct position, it can quickly find its way back. This is achieved by what Sony refer to as the “pre-groove” signal. This is depicted in Fig.9. It apparently updates the position information for the laser pick-up every 13.3 milli­seconds. There is another interesting difference between the play­ back and recordable versions of Mini Disc. The diagram of Fig.10 shows that the surface of the Mini Disc is devoted to data. There is a lead-in area followed by the program area and then the lead out area. As with CD, the Mini Disc is played from the inside out and it spins at anywhere between 400 and 900 RPM (faster than CD) to give a relatively constant linear velocity of 1.2 to 1.4 metres/second. However, with the recordable Mini Disc, an area called the UTOC – User Table of Contents – is interposed between the lead-in area and the program area. This UTOC appears to function in a similar way to the File Allocation Table (FAT) of a computer hard disc. Sony has announced the release of three products for Mini Disc. The first of these is the MZ-1 Walkman portable Mini Disc recorder. As its name suggests, it can play and record Mini Discs. Measuring 114 x 139 x 43mm and weighing approximately 690 grams with its rechargeable battery fitted, it offers the same playback and random access facilities as a CD player. As a recorder, the MZ-1 offers automatic or manual gain control and has facilities for disc and track titles which are shown on the liquid The MDS-101 incorporates all the features found in the MZ-1 Walkman recorder & features a comprehensive infrared remote control & styling to match Sony’s FH mini hifi range. crystal display. Each recording can be time and date stamped, which could be handy for those using the MZ-1 in professional or semi-professional applications. Recording on a Mini Disc is no more complicated than stor­ing a file on a floppy disc. You just press the record button and the MZ-1 automatically records on a blank portion of the disc. Alternatively, you can erase a selected track and all tracks will re-number. And there are other interesting possibilities such as dividing, swapping and combining tracks, all of which were un­heard of with analog cassette recorders. The MZ-1 incorporates SCMS (Serial Copy Management System) which allows single generation copies from digital systems incor­porating a digital output, which means there is minimal signal degradation. The frequency response is quoted as being from 20Hz to 20kHz. The recommended retail price of the Sony MZ-1 is $1499. Also released by Sony is the MDS101, another Mini Disc recorder intended for use in the home and matching the styling of Sony’s FH range of mini hifi equipment. The MDS-101 has all the facilities of the 22  Silicon Chip Mini Disc is built like a 3.5-inch floppy disc. It has a rigid outer case & a shutter which slides back to expose the disc for playback or recording. Playback-only discs have a shutter opening on one side only; recordable discs have a doublesided shutter. MZ-1 Walkman plus a comprehensive infrared remote control and a bigger liquid crystal display. It measures 225 x 75 x 285mm (W x H x D) and has a recommended retail price of $1799. Finally, there is the MDXU1 Mini Disc car player which incorporates an AM/FM tuner. To provide even more shock resist­ ance, this player has a 4-megabit DRAM which stores about 10 seconds of music. Some pundits have joked that the only way you will ever hear this player mistrack is if you have an impact great enough to trigger the car’s airbag in your face! The recommended retail price of the MDXU1 is $1999. PRE-GROOVE LASER SPOT Sanyo’s Mini Disc player While many electronics companies have indicated that they ultimately will have Mini Disc products, Sanyo is one of the very first to have a player available. It is the model MDG-P1 which weighs just 250 grams. It has high speed track access, the shock­ proof memory feature and a liquid crystal display to show track and time information. The Sanyo MDG-P1 will be on sale during October at a recom­ mended retail price of $1399.00. Sanyo will also release Mini Disc recorders and players for cars. Mini Disc players will also be available from Sharp and Aiwa, while TDK has already announced the availability of recordable Mini Discs in 60-minute and 74-minute ver­sions for $19.95 and $23.95. Sony’s recordable discs will have the same prices. DISC SUBSTRATE Fig.9: Mini Discs have a “pre-groove” layer underneath the entire program area of the disc & this informs the player of the laser pick-up’s position every 13.3ms. LEAD-IN AREA PROGRAM AREA LEAD-OUT AREA Music titles available According to Sony, some 200 music titles are already avail­able and this should increase to around 500 by Christmas. The titles are mostly popular but some classics are included. They will retail at $29.95, the same as premium priced CDs. Initially, all Mini Discs sold in Australia will be pro­duced by Sony MUSIC DATA Fig.10: music data on the Mini Disc is spread over a tiny area. The diameter of the lead-in track is only 29mm & the track pitch is 1.6 microns. plants in Austria, Japan and the USA. However, plans are under way to add Mini Disc production to the new Sony CD plant at the Huntingwood Estate, west of Sydney, to provide for the SC Australian and export markets. Mini Disc For Computers Not only has Sony borrowed from computer technology in developing the Mini Disc but the resultant product is likely to be very important for computers in the future. Sony has announced the development of standards for Mini Disc (MD) DATA which will be available in three formats, all of which will be useable in a single drive mechanism. The first of these will be pre-mastered (MD-ROM), intended for electronic publishing and software distri­ bution. Second, there will be a recordable MD intended for data storage appli- cations and thirdly, there will be the Hybrid MD which will be a disc which is partially pre-recorded, while the remainder will be recordable by the user. This is seen as being suitable for interactive applications. The overwhelming advantage of the MD DATA format, as it is presently known, is that it offers a capacity of 140 megabytes and data transfer rate of 150KB/sec. This rivals hard disc stan­dards. The disc could store up to 2000 still images and the transfer rate is sufficient to allow full motion video to CD standard. On a more mundane level, it will probably find wide application in personal and laptop computers. A new file system which determines how data is encoded has been developed for the MD as part of the overall standard. It is claimed that this will facilitate compatibility between computers with different operating systems. The manufacturing technology for MD DATA is identical to that for audio Mini Discs, which will keep costs low. But to avoid confusion, the MD DATA discs will be encoded in such a way as to make them unplayable or recordable on audio players. October 1993  23