Sony MiniDisc Technology - how it worked

Sony MiniDisc Includes:
Sony MiniDisc - the basics     MiniDisc technology - how it worked    

Many new techniques and technologies were developed by Sony to enable the MiniDisc to provide the level of performance required from a portable music player.

A new optically based disc system was developed. Although this many basic ideas an concepts that had been used in CD technology, the MiniDisc was quite revolutionary in very many respects.

Not only was the disc storage system advanced, but the Sony engineers also developed a new technology for the encoding and compression scheme.

Magneto-Optical Storage

Unlike CDs, which rely solely on optical data reading, MiniDiscs used a magneto-optical recording process.

As the name MiniDisc implies the actual disc is quite small. The actual rotating disc measures only 64 mm diameter and this is housed in a plastic cartridge measuring 72 x 68 x 5 mm. This forms part of the overall disc and gives it an appearance very similar to that of a 3¼ inch computer disc.

There are two types of disc: playback only and recordable:

• Playback only:   This type can only be used for playback. Once manufactured there is no way of recording onto them. These discs would normally be used for prerecorded albums available from the shops.

  The front of the disc is clear and can be used for artwork. On the reverse side there is a shutter which protects the disc when it is not in a player. When the disc is inserted into a player the shutter opens to reveal the playing surface of the disc. This enables a laser to read the data stored on the disc in the form of pits and bumps virtually identical to those on a normal CD.

  When reading a disc the laser tracks from the centre where there is a lead in area, to the outside. This is another similarity to the CD, but of course it is the exact reverse to the old vinyl discs.

• Recordable discs:   It is in developing the recording technology that Sony has devoted a large amount of research effort. The fact CDs could not be used for recording until recently was one of their major drawbacks. As a result it was recognised that it was very important that the Mini Disc should have this facility. Without this it was felt the system would have little chance of success.

  The recordable discs are somewhat different, allowing the discs to be overwritten an unlimited number of times. In terms of visible differences they have a shutter which reveals a section of both sides of the disc. In fact this is an indication of the fact that a different process is used to store data on these discs.

Recording process

To achieve the recording capability it was necessary to employ a system which, although different, would be compatible and use many of the same components. The solution which Sony devised is called Magnetic field Modulation Overwrite (MMO).

One of the keys to the operation of the system is the disc itself. This consists of four main films on the top of a plastic base. The films are very thin and have to be very uniform if they are to perform correctly. The key layer consists of a material called Terbium Ferrite Cobalt. This is located between two layers of silicon nitride with aluminised layer on top. It is within the terbium ferrite cobalt where the actual data is stored.

The manufacture of these discs presented a number of problems which were successfully overcome. The thin layers in the disc are deposited onto the plastic substrate using a process called sputtering. In its basic form this technique involves evaporating atoms in a vacuum and then depositing them onto another surface under the action of an electric potential.

In an improved system developed jointly by Sony and Materials Research Corporation in the USA, enables very uniform layers to be deposited. The system gives the lowest particle contamination level of any sputtering system to date. This leads to much higher production yields and reduced production costs. As a further benefit the system gives a ten fold increase in speed over other systems which are available.

When recording the laser scans the underside of the disc as it rotates and causes the magnetic material in the disc to be heated to 180°C which is above its Curie temperature. At this point the material looses any magnetism it previously possessed. As the disc rotates the laser spot moves away from this point on the disc and it begins to cool down. As this occurs the magnetic material takes on the magnetic orientation of its surroundings. As a magnetic head is placed above the disc the flux which it sets up is the flux which will be taken up by the sections of the disc as they cool down. In other words the laser acts as the key to enable the recording to take place and the magnetic head writes the data onto the disc. In this way digital data can be transferred onto the disc, simulating the pits and bumps of the prerecorded material.

This writing process proved to be very reliable. It had a very large power margin and this minimised the distortion of the data on the disc. This means that there is much less jitter on the data than on other systems. In addition to this the basic MMO system is very resilient to any disc tilt which may occur. Again this can cause data jitter. However as the laser is only used to raise the temperature of the magnetic layer it has less effect on the magnetic spot shape. The result of this is that there is much greater reliability when the disc is played back. It also means that the same recording density as a CD can be achieved.

MiniDisc playback

It is obviously necessary to have a reliable, robust playback scheme that can be used for both re-writeable and read-only discs without any major changes.

The playback system use the same read operation even though they operate in slightly different ways.

In fact the technique used operates as a dual function scheme and it is based around the one that was developed for CDs and has operated reliably for many years.

For pre-recorded material a 0.5 mW laser is focused onto the surface of the disc. The light is reflected back off the surface and detected by two photodiodes.

The level of light reflected then indicates the presence or absence of a pit. If the light is reflected directly back this indicates the absence of a pit.

If there is a pit then the light will diffracted and a much lower level will be detected by the diodes.

These light level variations then correspond to the digital data which can then be processed by the electronics.

To read a recordable disc, the process is somewhat different, although exactly the same pick-up is used. The light from the laser which is polarised strikes the disc and it is reflected back. However the polarisation of the light is rotated slightly in a forward or reverse direction depending upon the magnetisation of the disc at that point.

The reflected light passes through what is called a polarisation beam splitter. This separates the light of different polarisation and distributes it to the two photodiodes. In this way the light reaching the photodiodes is dependent upon the amount of polarisation shift when the light is reflected and hence the magnetisation on the disc. Each photodiode converts its light energy into electrical signals and hence the digital data recorded on the disc is read.

Random access playing

One feature of the CD which has been widely used is the ability to select tracks quickly and easily. Accordingly this facility has been built into the Minidisc, but including this was not as easy to do as might be expected.

There is little problem when prerecorded discs are used because the same system used on CDs has been incorporated. The start and finish points of the tracks are known and these are stored in a directory on the disc. This directory, called the table of contents (TOC) stores all the relevant information. When a particular track is required, the player refers to the TOC and moves to the correct position.

The system for achieving random access on recordable discs is rather more complicated. The system uses a "pre-groove" to give the location information. These pre-grooves are microscopic grooves which are pressed into the surface of the disc at manufacture and they give location information at intervals of 13.3 milliseconds.

To enable the player to find the correct start and finish points the locations are stored on the disc in a reserved area called the User Table of Contents (UTOC). This is very similar to the directory on a computer disc where all the locations of the programmes are stored.

By using the system, random access of tracks can be performed virtually as fast as on recordable ones as on the premastered ones. This can be achieved to an accuracy of 13.3 milliseconds which is more than adequate for most applications.

Buffer memory

One of the major problems of the CD is lack of portability. Although portable CD players can be moved whilst they are playing, any shock can cause them to skip or jump. As a high degree of portability is necessary for the Minidisc it was necessary to incorporate a method of overcoming this problem.

As the laser has to move to track as the disc is played or recorded, it could clearly not be fixed to overcome the effects of shock and another more ingenious solution had to be sought. The actual method used was to incorporate a memory into the system. The data can be read from the disc faster than it is required by the decoding system. In this way it is possible to read data ahead of when it is required and store it in the memory. If the laser is displaced by a jolt the memory will start to empty as it continues to give out data to the decoder, until the laser finds its position again. When this happens the memory will start to fill up again ready for another interruption in the data from the disc. In this way the music is not interrupted, despite any jolts displacing the tracking of the laser.

Some figures are useful to illustrate the system. In current Minidisc systems one Megabit memories are used. the data can be read in at a rate of 1.4 Megabits per second, but it is only required at a rate of 0.3 Megabits per second, giving up to three seconds of stored data. This is more than sufficient for the laser to regain its position, and on one demonstration it was just enough to allow the disc to be removed and reinserted without any interruption to the music.

In many cases the laser will not be continually jolted. When this happens a point is reached when the buffer memory is full. When this occurs data will cease to be read from the disc until the memory empties slightly and more data can be read in. As a result of this data is read in from the disc in bursts as it is required.

Data compression & ATRAC

The Minidisc relies on the small size of the disc to give it that all important portability. Unfortunately reducing the physical size of the disc also reduces the amount of data which can be stored. If the data was stored in the same way that it is on a CD then it would only be possible for a Minidisc to give about fifteen minutes playing time. Clearly this is not acceptable and as a result Sony have developed a system of reducing the amount of data which needs to be stored which was called ATRAC, Adaptive Transform Acoustic Coding.

A CD uses a 16 bit data sample every 0.02 milliseconds regardless of the content of the waveform. However such a large data word is only rarely needed, and with a degree of signal processing it is possible to identify only those portions of the signal which need to be stored.

During ATRAC encoding the data is divided into time segments of up to 11.6 milliseconds, but in 0.02 millisecond increments. Then using a system called a modified discrete cosine transform the waveform is analysed.

During the analysis two main features are used. The first is the threshold of hearing. It is found that the sensitivity of the ear varies considerably with frequency. It is at its most sensitive at about 4 kHz falling off above and below this. Any sounds which are below this threshold will not be heard and can consequently be ignored by the encoding system.

The other effect occurs when two sounds are very close together in frequency. When this occurs it is found that the larger signal tends to mask the weaker one out. The closer the frequency the greater the masking effect.

To utilise this effect ATRAC splits up the audio into sub™bands. It uses different width bands dependent upon where they are in the audio spectrum. The reason for this is that masking effect is not a direct function of the frequency difference in Hertz. As a result the lower frequency bands are narrower than the higher frequency ones. Below 500 Hertz the bandwidth is about 100 Hz. Above this frequency the bandwidth increases by about a fifth every band.

The system then analyses the frequency components in each band to see if any sounds are likely to be masked out. If so then the weaker sounds will not be encoded, saving valuable disc space. In total the saving is such that only about one fifth of the data needed by a CD is required by the Minidisc. This means that the Minidisc can store a full 74 minutes of music.

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