Sony's Minidisc and MD-Data

by Francis Pelletier - © copyright 1994 mosarca
Completely unofficially translated for the Minidisc community by Marc Herbert
This article was published in the magazine MOS 129 - november 1994

Sony's MD-Data reader/recorder(Photo: © Sony)

The Minidisc, 2.5'' diameter, is the only consumer medium sold in a plastic cartridge with a metallic shutter, just like magnetic floppy discs. You can find today on the market two types of minidisc.

The first one is the pre-recorded medium, recorded with sound when intended for audio market, or with data and files in the MD-Data-ROM version. The second type is recordable by the user, thanks to the ad-hoc recorder/reader. The technology is then magneto-optic, and offers the capability to erase and record a high number of times. It's designed both for consumer and professional uses, as confirmed by the announce of the MD-DATA in March, (MOS N°122) which reader/recorder would take place in Personal Computers.

Sony's MD-Data reader/recorder (Photo: © Sony)

From audio use to computers

The Minidisc requires a particular reader/recorder. In addition to its unusual size, it uses a logical format needing special decoding circuits. A 64 mm diameter Minidisc can hold up to 74 minutes of high quality sound (44.1 kHz), compressed with an algorithm from Sony. Called ATRAC (Adaptive TRansform Acoustic Coding), this algorithm uses psycho-acoustic rules to deal with frequency sub-bands, along with a treatement derived from the DCT, called MDCT (Modified Discrete Cosine Transform). Decoding is calculated in real-time by a simple Minidisc reader, thanks to processors specially designed by Sony.

In the same manner, readers/recorders include an real-time encoding device. In addition to space gain, data compression has an other advantage. Temporary storage of sound data provides stability when reading, up to 3 seconds even there's a shock during this treatment. Data reading occurs by shots of 1.4 megabit/s transfer rate, until filling of a 1 megabit memory buffer, used to temporarily store data before decompression. These datas are transmitted to the decoding circuit at a 0.3 megabit/s transfer rate, and then go to a Digital/Analog Converter at 1.4 megabit/s speed.

The case of the MD-DATA, designed for personal computing, is different. Today, a MD-Data disc has a 140MB capacity, without any data compression. A dedicated reader/recorder is needed, with a controler/interface allowing to drive it with a personal computer. Microsoft and Sony have designed a logical format, similar to ISO-9660 for the CD-ROM, in order to make the MD-Data readable in differents environnements.

The recordable version of the Minidisc has interesting advantages. For the audio version, users dispose of two disc types, of 60 and 74 minutes duration, which can be re-written directly (direct overwriting), i.e. without the preliminary step of erasing previous recording, thanks to a mechanism combining laser modulation and a magnetic spool. Minidisc's size makes it an easily transportable medium, cheap, and reliable in time because of magneto-optical type. Advantages are the same for the MD-Data version but quite disappear in face of the following flaws : 140MB capacity, 150 kB/s transfer rate, 300 ms access time.
The Minidisc has until now met a middle success on the consumer market ; despite of this, due to Compact-Disc stagnation and audio cassette decline, it seems to be the medium with the best growth rate (discs and drives mixed). Maybe this will defeat its major flaw : the prohibitive price of drives. A portable reader costs in France about 2900 FF ($500), and a portable reader/recorder a bit more than 3900 FF ($650). Besides, Sony is the only one to do a real promotion effort. Concerning the MD-Data, it's not available on the consumer market yet, because it's on testing by big computer vendors. According some informations, a new wave of manufacturers of consumer electronics should start in 1995 to support the Minidisc with more enthousiasm. There will be for instance firms like Aiwa, Sanyo, Hitachi, Mitsubishi, JVC, Kenwood, Samsung, Denon, Clarion, Onkyo, RCA/Thomson, Goldstar, Telefunken. This new product rush, along with the even more miniaturization, should induce rather big price falls in the next months on readers and recorders.

According to Sony, the Minidisc is aimed at replacing the audio cassette on the consumer market. The pre-recorded version comes also on the Compact Disc audio market, which is not officially aimed by Sony. The commercial goals of the MD-Data, the computer version of the Minidisc, are several. Designed for replacing the 3.5'' floppy disk (a Sony creation), it can also come on the CD-ROM market, and in some contexts, on the 3.5'' magneto-optical disk market. But its current performance doesn't allow it to attack directly these two media. Although its capacity should double during the next months (from 140 MB to more than 250 MB), its tranfert rate and its access time are still too low for competiting with the CD-ROM or the magneto-optical disk. But only until now ! On the contrary, the MD-Data is really a excellent candidate for the 3.5'' floppy disk replacement ; on the condition that its cost/performance ratio is competitive, including the drives.