::: Reportajes JM Serrano :::

MINIDISC:
TOP QUALITY IN PORTABLE SOUND

Text and Photos José Manuel Serrano Esparza

BACKGROUND

After a supremacy of nearly 40 years since its launching in 1963, the popular analogue
cassette had remained obsolete (because of its deficiencies with regard to sound quality, the scarce unspoiled duration of recordings, the rather slow access to themes, the easiness of damage, etc) compared to the new digital formats with much greater quality and features as DAT ("Digital Audio Tape") tapes, a technocrat product par excellence, considered the Rolls-Royce of recording systems with its fabulous fidelity) and the Compact Disc (designed by Philips and Sony in 1982 and that from its massive spreading from 1990, put an end to the almost 40 years of preponderance of the classic vinyl record).

Nevertheless, neither DAT equipments and tapes (with an astronomous price only at the reach of music professionals or very exacting music bugs) nor compact disc (due to its relatively big size) have until now been able to replace the mythical portable Walkman (perhaps the most inspired achievement by Akio Morita), though going on fed by the already out-of-date analogue tapes.

In fact, while practically in every home, the household musical equipments are already equipped with digital CD player, nearly everybody continues using the analogue Walkmans or MP3 players (both of them with sensibly inferior sound quality when compared to MiniDisc), since the portable CD players are twice its size, with the space and inconvenience disadvantages that it implies.

But also in 1992, Sony introduced the revolutionary MiniDisc, the little size musical standard with the best sound quality appeared in the world until now, featuring a digital sound of very high quality (very near Compact Disc) within a very small framework, made up by a metallic magneto-optical disc kept by a plastic square casing of 64 mm diameter (with a look and size very similar to a computer 1.44 Mb diskette).

FAIRLY UP-TO-DATE TECHNOLOGY

This little disc is recorded using the most advanced optical and magnetic technology, cause on the other hand, the recording device incorporates a magnetic head that records the information, in such a way that when we want to record a blank MiniDisc, the magnetic layer of the MD disc is heated by the laser beam of the MD player at approximately 204, 44º Celsius, with which the quoted area is magnetized, the information being recorded on getting through the MD disc a magnetic head on that hot zone. And on being the disc constantly spinning, the recorded area cools then, recovering its stability.

This way, the MiniDisc, with its format of very reduced dimensions and great convenience of transport, its reputed resistance and durability and specially because of its extraordinary quality (as a matter of fact up to audio CD) and outstanding storage capacity (up to 140 Mb of data, that´s to say, 75 minutes of digital stereo sound with a resolution of 20 bits and a dynamic range reaching 126 dB, with the option of 148 min. hi-fi in monoaural recording) has been and goes on being the diachronic portable referent of sound quality in a reduced size format, though it hasn´t attained to become into the universal portable standard, due to the massively widespread of MP3 players sporting fairly small proportions, great capacity of storage, very light weight and linkage to internet and all kinds of computers for the recording of thousands and thousands of songs and different themes.

Notwithstanding, almost since its introduction in the market in 1992 until presently, the MiniDisc has indeed had a massive diffusion in Japan at a popular level, due to the high levels of requirements regarding sound quality demanded by Japanese users, a lot of them being full-fledged melomaniacs and lovers of classical music, even in the portable domain, to the extent that currently a lot of multinational firms of audio have sold portable dual digital devices including both players and recorders of MiniDisc and Hi Minidisc together with MP3, hugely versatile fixtures, satisfying the most different needs and that are being remarkably successful in sales all over Japan, a country in which from the beginning of nineties a very high percentage of citizens has been acquiring both high end musical equipments with a built-in Minidisc player for specialized audiophiles - as the mythical system MJ-L1 by Sony- and specially millions and millions of MD Walkmans (the historical masterpiece of the fabled Akio Morita, President of Sony), in spite of its price relatively steep.

On the other hand, the MiniDisc contributes manifold advantages, with the possibility of adding written texts with the songs titles, names of groups or soloists, duration, etc, appearing on the screen. As a matter of fact, large numbers of audiophiles from all over the world, specially classical music Japanese enthusiasts who have been making copies to MiniDisc from legendary performances by mythical conductors like Wilhelm Furtwängler, Bruno Walter, Otto Klemperer, Arturo Toscanini, Erich Kleiber, Herbert von Karajan, etc, for a long time so as to listen to them while going to work, during their trips and so forth.

And the sampling frequencies conversors built-in both in MD Walkmans and MD players and recorders allow to directly import sound coming from other digital sources with different frequencies (as the DAT recorders or the digital tuners), for they usually have optical digital connections providing an unbeatable purity in the transmission of data, with the option of carrying out direct Hi-Fi digital recordings approaching the level of a professional recording studio, with the added bonus of a great durability in time, estimated around an average of 50 years and more than a million recording/erasing cycles, since MiniDiscs are not affected by magnetic fields and its special protective casing avoids all kind of scratches or damages due to ultraviolet light.

ATRAC AUDIO DATA COMPRESSION SYSTEM

From the beginning, two types of optical MiniDiscs were developed: one, the recordable magneto-optical MiniDisc for recordings and playings of music by the user and the other one an only playing MiniDisc for the publishing of all types of original music in the most diverse styles by the sound domain multinational firms.

The recording technology on a magneto-optical disc had been used for some years before 1992 and relying on it, Sony began to develop the new MiniDisc system with a recording density similar to that featured by Compact Disc.

On the other hand, a new antishock memory control was designed for portable use and a new digital audio compression system called ATRAC (Adaptive Transform Acoustic Coding) was applied, allowing to use a 64 mm diameter disc, which together with the technological improvements in semiconductors helped to become MiniDisc into a new musical format sporting high sound quality and little size.

It´s important to underlie that with its quoted 64 mm diameter, the MiniDisc is much smaller than a CD, in such a way that it can only include 1/5 of its data. Therefore, the MiniDisc needs a 5:1 compression ratio to offer a 74 minutes duration both in playback and recording.

So, the ATRAC encoding process starts with the 16 bit quantization and the audio stereo signal sampled at 44.1 kHz. The fundamental trait for the efficiency of the high quality ATRAC compression system is its unique method of analysis using a non uniform cut both in the frequency and time axes, reflecting psychoacoustic principles.

The encoding processes are divided into three phases:

a) The Quadrature Mirror Filters (QMF´s) block.

b) The Modified Discrete Cosine Transform (MDCT) block.
c) The Bit Allocation block.

The transform block size is adaptively chosen and there are two types: the long mode with 11.6 ms for all frequency bands and the short mode with 1.45 ms for the high frequency band and 2.9 ms for mid and low frequency bands. Usually, the long mode is chosen to provide optimum frequency resolution. However, problems may occur during some attack portions of the signal, specifically the quantization noise spread over the entire MDCT block time. Just before the attack of the sound, one may hear some noise. And in order to avoid this type of noise, ATRAC compression system automatically switches to the short mode.

Then, the MDCT spectral coefficients are grouped into a Block and the spectral values are quantized by using two parameters. One is the word length and the other one is the scale factor. The scale factor defines the full-scale range of the quantization and the word length defines the resolution of the scale. Each Block Floating Unit (BFU) has the same word length and scale factor, reflecting the psychoacoustic similarity of the grouped frequencies. The scale factor is chosen from a fixed table and reflects the magnitude of the spectral coefficients in each BFU. The word length is determined by the bit allocation algorithm. For each sound frame (512 data samples) the following information is stored on disc:

        1 MDCT block size mode (long or short).
        2 word length data for each BFU.
        3 scale factor code for each BFU.
        4 quantizied spectral coefficients.

The data size of each sound is fixed as 212 bytes. On the disc, 11 stereo sound frames are recorded every 2 sectors. In case of monoaural recording, twice the amount of time can be recorded on the disc.

The bit allocation algorithm divides the available data between the various BFU´s. ATRAC does not specify a bit allocation algorithm. The word length of each BFU is stored on the MiniDisc along with the quantized spectra, so that the decoder is independent from the allocation algorithm. This allows for an evolutionary improvement of the encoder without changing the MiniDisc format. The decoding process is divided into two steps. The decoder first reconstructs the MDCT spectral coefficients from the quantized values, by using the word length and scale factor parameters. The coefficients are transformed back into the time domain by inverse MDCT using either the long mode or the short mode as specified in the parameters. Finally, the three time domain signals are synthesized into the output signal by QMF synthesis filters.

It´s true that the MiniDisc ATRAC compression system ignores the sounds beyond our auditive capabilities, but the successive improvements which have been introduced in it, make the real differences in sound quality minimal, though logically, its sound quality doesn´t reach the exceptional levels of DVD-Audio, SACD or even an original vinyl record in good condition.

SHOCK RESISTANT MEMORY

One of the most outstanding features of MiniDisc is its Shock Resistant memory, avoiding all kind of shocks and vibrations usually bringing about reading errors in any conventional optical disc.

For a long time, it was thought that the magnetic tape was the best solution for external usage, though the CD players have used a mechanical suspension protecting them from shocks and external vibrations since the beginning of eighties.

But in the very widespread manual portable musical equipments such as Walkmans, it´s difficult to use a mechanical suspension, because it requires a lot of space.

Therefore, Sony audio pundits looked for another way to overcome this problem.

Starting from the fact that vibrations and shocks are not continuous, the tremendously knowledgeable Japanese sound engineers introduced a semiconductor memory (in the shape of an electronic data buffer placed between the optical reader and the ATRAC decoder) inside the MiniDisc recorders/players. And using a 4 bit memory, approximately 12 sec of compressed audio sound can be stored in the memory. When the optical reader begins to interpret the MiniDisc data, it only takes a few seconds to fill the memory, because there´s a quintuple difference in bits between the optical reading and the ATRAC decoder.

Thus, during a normal playing situation, the optical reader is interpreting the data in an intermittent way to keep the memory utterly full. If the optical reader loses its position due to a mechanical shock and to the interruption of data flow towards the memory, such data will go on flowing from the memory, allowing a continuous playing of around 12 sec. And once the laser beam recovers its original location, it will read again the memory data as usual.

Because of the information found in the MiniDisc at intervals of 13.3 ms, the laser beam can generally return to its original position in 1 sec.

CONCLUSION

The MiniDisc is undoubtedly the portable sound format in small size boasting the highest quality ever invented by man, an authentic technological tour de force accomplished by very skillful Japanese engineers from Sony as Tadao Yoshida, Kyoya Tsutsui, Hiroshi Suzuki, Osamu Shimoyoshi, Mito Sonohara, Kenzo Akagiri, etc, who achieved to create a full-fledged masterpiece getting ahead of its time and in which a very wide range of technological breakthroughs of the highest level both in conceptual originality and design were introduced.

Certainly, Sony, the most prestigious multinational audio domain firm worldwide, staked all and it has even been introducing year after year successive improved variants of the ATRAC compression system which have placed the sound quality of MiniDisc very near the CD.

Likewise, in addition to Sony, more than 20 different firms from the audio sector have manufactured all kind of player and recordable MiniDisc Walkmans in the most various designs, presentations and colours, all of them being quite beautiful, and even the capacity of the recordable MiniDiscs has progressively been increased up to 80 minutes with full stereo hi-fi quality.

Notwithstanding, due to the vigorous emergence of the PC sector within home market domain and the massive introduction all over the world of the hugely versatile and convenient digital MP3 players/recorders sporting several Gb storage capability (with its easiness of linkage to internet so as to directly record all sorts of songs), featuring a size and weight even lesser than MiniDisc and with a huge capacity for recording thousands and thousands of musical themes, the MiniDisc hasn´t been able to be massively introduced, its use being relegated (except in Japan and United States) to a narrow scope of purists and lovers of the maximum possible quality portable sound, since its very important to underscore that MiniDisc has a sound quality far superior to MP3 and the recordings made with MiniDisc from any original sound source exhibit a much better quality than those ones made with MP3 recorders, the latter being a system in which the compression ratio is very high, which results in a clear impairment of the sound quality, though most normal users content themselves with it, giving more importance to use convenience, little weight, transportability and internet synergy above all.

But above all, saying that MiniDisc has been a failure is something unrealistic, for it will go on being for many years the historical best top-notch portable sound quality device and the only alternative in little size format for the diehard audiophiles and authentic musical bugs of the hi-fi stereo sound, in the same way as the legendary valve amplifiers never were and never will be a flop, in spite of its reduced implantation, because top-notch stuff are rarely massively widespread. In fact, a lot of sound engineers claim that maybe the only error by Sony in its strenuous effort to become MiniDisc a daily reality for the masses was trying to turn such a high quality ´Rolls-Royce´ stuff as MiniDisc is into a very diffused format among people. And this is not a discriminatory or technocrat reasoning at all, but simply the reality. Can you imagine Gustav Mahler´s Ninth Symphony conducted by Bruno Walter or a violin concert by Jascha Heifetz recorded in MP3? Please!