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INTRODUCTION
The MiniDisc system was introduced in the consumer audio market as a new digital audio playback and recording system. The introduction time was just ten years after the introduction of the Compact Disc (CD). As is known, CD has effectively replaced the vinyl LP records in the audio disc market. CD technology is based on 16-bit quantization and 44.1-kHz sampled digital audio recording. The CD sound quality was fairly improved compared to any consumer analog recording equipment.
Before starting the CD business, many engineers engaged in the development of the CD solely for its improvement in sound quality, but after the introduction of the CD player into the market, we found out that the consumer became aware of the quick random-access characteristic of the optical disc system. The next target of development was obviously to be the rewritable CD. Two different recordable CD systems were established. One is the write-once CD named CD-R and the other is the re-writable CD named CD-MO.
Sales of cassette tapes had been decreasing since 1989. Even if recordable CD were to be accepted by the consumer, it would still be difficult to break into the portable market. Here, portable compact cassette dominated because of its strong resistance to vibration and its compactness. Clear targets for a new disc system were to overcome these weaknesses. Sony was able to achieve this by introducing a disc system called MiniDisc (MD).
The name, MiniDisc (MD), comes from its size. MiniDisc was developed by as an audio media that combines the merits of both CD (supreme quality) and Tape (recordable). The disc, with a diameter of 64 mm and thickness of only 1.2 mm, is placed inside a cartridge of 72 X 68 X 5 mm. The cartridge protects the disc from exposures and withstand forces eliminating problems that connects with CD (scratches) or tape (tangles). The Minidisc is based on Magneto-Optical technology, which is essentially a method of recording information by using a laser to alter magnetic information on the disc. In order to alter the information, the disc has to be heated to a high temperature, meaning that if left on a desk near a magnet, it should remain unaffected, unless you heat the disc to the required 180°C.
THE MINIDISC SYSTEM
TYPES OF MINIDISCS
Premastered MiniDiscs are used most commonly for music and are sold in record stores just the same as compact cassettes and CDs are. Minidiscs, just like CDs, are manufactured in large volumes by high-speed injection molders, and the music signals are recorded during replication in the form of pits. Moreover, the discs are encased in a cartridge, so there is no worry about their being scratched. The design of the premastered Minidisc cartridges is special. Prerecorded music packages require a label, featuring the artist’s picture or other information. Therefore the top face of the cartridge is left completely free for the label. A window for the laser beam to read the disc is only necessary on the bottom face. Both a CD and a Minidisc can store the same amount of music. The difference is that a Minidisc uses a digital compression technique called ATRAC (Adaptive Transform Acoustic Coding) to compress audio data in 1:5 ratio by eliminating inaudible frequencies and faint background noises.
Recordable MiniDiscs, uses Magneto-optical (MO) technology forms the heart of the recordable Minidisc technology. Not only are MO discs durable enough to withstand being rerecorded about 1 million times, they also have a life expectancy approaching that of CDs. 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 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 in Fig. 4. This MO layer construction has been engineered to enable Magnetic Field Modulation overwriting. According to Sony, it has already been proven 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 record a MiniDisc so many times!
Hybrid MiniDiscs has both recorded pits and a recordable wide-groove sputtered magneto-optical recording layer. The wide groove is wobbled by a 22.05-kHz carrier modulated by address data. This type of pre-groove modulation helps the drive system to control not only the tracking servo but also supports the constant linear velocity (CLV) control and access control during record mode.
It is mandatory for the MD system to be able to play back these three types of discs. This means that the MD recorder/player is equipped with a dual-function optical pick-up for the read-out of recorded pit patterns and the magneto-optical disc.
The given figure shows the 3 types of Minidiscs with their groove/pit layout.
Fig 2. MiniDisc disc types and groove/pit layout.
MINIDISC PICK-UP TECHNOLOGY
The optical pick-up developed for the MD system has the remarkable ability to read both recordable and premastered Minidiscs. For recordable discs, the pick-up detects the magnetic structure along the track. For premastered optical discs, it detects the geometric structure along the track. The MD system is based on the design of the standard CD pick-up with the addition of a MO signal readout analyzer and two photodiodes. During playback of recordable discs a 0.5mW laser is focused onto the magnetic layer. The magnetic signal on the disc affects the polarization of the reflected beam. The direction of polarization is converted into light intensity by the MO signal readout analyzer. Depending on the direction of polarization one of the two photodiodes are subtracted and depending on whether the difference is positive or negative, a “1” or “0” signal is read.
The same 0.5mW laser is used for the playback of premastered optical discs. The amount of light reflected depends on whether or not a pit exists on the surface of the disc. If there is no pit, a high proportion of the light is reflected back through the beam splitter and analyzer into the photodiodes. If a pit does not exist, some of the light is diffracted and less light reaches the photodiodes. The electrical signals from the photodiodes are added up in this case and depending on the sum, a “1” or “0” is read.
OVERWRITE TECHNOLOGY
In order to meet the requirements of a compact, lightweight, recordable audio system for personal use, the MD system employs a newly developed magneto-optical (MO) disc that utilizes magnetic field modulation with direct “overwrite” capability. With MO disc technology, the recording of data requires the use of a laser and polarizing magnetic field. When the magnetic layer in the disc is heated by the laser to a temperature above the Curie point (approximately 200°C), it is temporarily looses its coercive force. As the disc rotates and the irradiated domain returns to normal temperature, its magnetic orientation is determined by an externally applied magnetic field. Polarities of “N” and “S” can thus be recorded, corresponding to digital data “1” and “0”.
The MD overwrite system positions a magnetic head directly across from the laser source on the opposite side of the disc. A magnetic field corresponding to the input signal is generated over the laser spot. The rotation of the disc then displaces the area to be recorded, allowing the temperature at the spot to drop back below the Curie point. At that point, the domain takes on the polarity of the applied magnetic field regardless of the polarity that previously existed