Sony minidisc


Part One - The story


In engineering, as in biology, the central problem is too much diversity. Given a set of goals, designers often come up with six solutions rather than just one. Back in the ‘70s, for example a dozen formats were proposed for the digital audio disc. These were narrowed down to three compact designs (including one from Telefunken called Mini-disc) before the Philips/Sony Compact Disc emerged as the worldwide standard. That selection process was not an afterthought but a practical necessity. The music industry learned from the con-*test*-('") between the Edison cylinder and the flat Berliner gramophone disc a century ago, and has insisted on standards ever since.

In each era, uniform standards have made it practical to distribute music everywhere in not than two formats, often only one.
“The survival of the fit-*test*-('"),” Darwin’s phrase for the competition among biological species, may now be applied to the digital audio formats that have been designed to replace the analog tape cassette. Since each of these new formats were supposed to have the same relationship to the CD that the analog tape had to the LP. It must be user-recordable, compact, highly portable and supported by a raft of affordable pre-recorded software.

The R-DAT format, met all but the last of these goals, but in 1985 sixteen companies agreed on a single R-DAT format as the digital replacement for the analog cassette. But this choice was exclusively Japanese, with no participation by the European and American interests that eventually have to accept it.
They didn’t.

In Darwinian terms the DAT may prove to be the dinosaur of digital recording formats. Like some of the dinosaur species of old, which are thought to have survived by evolving into flying birds, the DAT were dying as a mass-market consumer format but has evolved into unexpectedly popular system for professional and high-end audiophile markets.

The road was, once again, wide open for a new format that would become the worldwide mass-market digital successor to the analog cassette. The Philips Digital Compact Cassette (DCC) seemed to be that product.
All of the potential obstacles to its success have been dealt with. The hardware will be produced by major manufacturers in Europe (Philips), Japan (Matsushita and Sony), and the US (Radio Shack). Ditto for blank tapes: BASF in Europe, Tandy/Memorex in US and several companies in Japan. Seagate, the world’s largest manufacturer of computer hard-disk drives using thin-film technology, joined with Philips to develop the thin-film heads required for the DCC.

DCC’s future seemed clear until Sony threw a brick into the works by introducing the Minidisc! Now the only thing that seems certain is that there will be another format war.

Why didn’t Sony cancel the MD? Why start another format war? One possible answer has to do with personal egos of corporate heads. It points to the long standing rivalry between Sony and Matsushita, exemplified first by the con-*test*-('") between Beta and VHS and between camcorder formats (8mm vs. VHS-C). Matsushita, Sony’s enemy, has a technology sharing alliance with Philips that goes back more than half a century, to before WWII. In this view the Sony/Philips partnership that produced the CD in the ‘80s was just a temporary marriage of convenience.

A better answer, perhaps, is that Sony genuinely believes in optical disc technology. Sony created a laser/optical digital audio disc in the late ‘70s, before Philips unveiled its prototype CD. Sony was/is a major CD/laser disc manufacturer, an important supplier of laserdisc players, the principal developer and supporter of the 3” mini-CD, a leading maker of computer based CD-ROM players, and at that time has been selling a pocket size Dataman “electronic book” that combined half-sized CD-ROM information discs with integrated computer acess-circuitry and a small LCD screen. It easy to make the case that the optical disc, in all its guises, were the technology of the future.

Tape, whether analog or digital, were the technology of the past. (Of course Sony won’t said that in public. As part of its campaign to become the leader in all things digital, Sony was the biggest producer of digital tape recorders for professional and consumer use.)

A third possible answer lies in the way Japanese executives always look at the future….

In the mid ‘70s the executives of Sony and Matsushita experienced an epiphany – a new view of the future of audiovisual entertainment. Hi-fi audio and video must move out of the living room, not only to the kitchen and bedroom, but also to the car and the great outdoors.

New products categories such as the boom-box, the Walkman, the 3” mini-CD, the 10-CD changer for the car trunk, the portable DAT recorder, the color-LCD pocket TV and the tiny 8mm camcorder for certain did not arise spontaneously from technology labs; they were part of a long-term plan to invest in the creation of audio and video products that could be used everywhere, becoming part of nearly every activity. Thirty years ago people listen to music or watched TV only a couple of hours a day, mainly in the living room. Nowadays we enjoy music and TV anywhere and everywhere, whenever we are not working or sleeping. And with all these new product categories, manufacturers make a lot more money than if they were selling only stereos and TV sets for the living room.

This world-view affected the design of the CD, for which must of the development work had already been completed by Philips before Japanese designers got their hands on it. Recall Akido Morita’s now legendary demand when Sony joined Philips as co-designer: The CD must have enough playing time to accommodate the longest performance of Beethoven’s Ninth Symphony, yet the disc must be smaller than 5” so that a player could fit within a DIN-size car-audio cutout.

The Minidisc was Sony response to the failure of both the CD-3 and the DAT as mass-market formats. Minidisc was/is two products in one. As a playback medium for mass-produced software, it is basically identical to the CD except for its small size and reduced bit rate, made possible by 5:1 digital data compression. But the Minidisc is also a recording medium. A recordable Minidisc has a magnetic coating on its “label” side, and an MD recorder contains a clever magneto-optical recording system. (See my next post about minidisc technology)




Works cited:
Revue du Son
Stereophile
Sonido Y Audio

nice write up.

i was under the impression that the minidisc was equal to DAT in terms of bit rate? i dont know where this idea came from, i probably just assumed it.

Tanx.

You were not far from the truth. The Philips DCC, not the DAT, have in fact almost the same compression bit rate as the Sony Minidisc. The DCC had a 4/1 data-compression and the Minidisc uses 5/1.

Sony Minidisc

The Technology - The ATRAC

In development since 1986, the Minidisc based on four technologies either invented by Sony or refined by them, is an incredible tour de force of technical innovation.

These technologies are:

1- A 5:1 data compression scheme called ATRAC (Adaptive Transform Acoustic Coding) still used by Sony in many of today devices like cell-phones or car-audio.


2- Magneto-optical (MO) record/playback with direct "overwrite" capability.

3 – A dual-function laser pickup that will play both pre-recorded polycarbonate discs and MO recordable discs.

4 – A "shock-proof memory" that provides skip-free playback even when shaken violently in portable applications.

The Minidisc was so packed with new technology that nearly 300 patents have been applied on it. I think that's almost a technology record. I don't know any other audio unit using such a number of new patents for its manufacturing.


ATRAC (Adaptive Transform Acoustic Coding) Data Compression

To provide 74 minutes' playing time on a 2,5" disc (and make the entire format possible), Sony developed the ATRAC data-compression scheme. Using more efficient coding techniques as well as throwing out musical information judged to be inaudible, ATRAC reduces the storage requirements of the medium by a factor of five.

Like all data compression systems, ATRAC relies on human hearing models to decide which information is audible and which isn't. With these techniques, a stereo digital signal that consumes 1.41 million bits per second with PCM encoding (as on a CD) requires only 256kb/s after ATRAC encoding (looking just at audio information; error correction and subcode bring the total up to 300kb/s), or one fifth the amount of data.

This compression rate is greater than the PASC encoding Philips developed for DCC.
ATRAC, however, differs from PASC in many ways, maybe because it takes advantages of the higher ambient noise levels present outdoors and cars, where Minidisc was most likely to be used. On its announcement, Sony representatives were hesitant to say anything disparaging about ATRAC, but did admit that in classical music some "harmonics will be lost" and by -*test*-('")s made by their engineers only 2% of people can hear the difference between a CD and a minidisc ATRAC 5/1 compressed data.

But, after all we are talking about a portable device where fidelity is less critical than in the home.

This leave us to the new portable audio devices or as we call it today, the mp3 players.
I had compared the same music (Michael Jackson's Black or White) with 256kb bit-rate in my iPod and in a Sony minidisc using the same earphones. The sound with the minidisc is incredible better/richer in terms of transparency, clarity and dynamics than with the iPod mp3 player. This is incredible because we're talking about a technology with more than 20 years of old….

Now, we can see how ATRAC really works by checking Sony's page at : http://www.sony.net/Products/ATRAC3/

And at minidisk.org here: http://www.minidisc.org/aes_atrac.html[B][/B]

Awesome post- I embraced the minidisc format when it came out, shunning the iPod for quite a bit of time.

A fun format to mess with, I still have my portable player and a stack of discs.

Tanx Paul.
I think i will ask my friend if he wants to sell me one of the two that he still has.
I still think that its the best portable player around, even by today standards.

Sony Minidisc - The Technology Part II

The Magneto-optical technology with overwrite capability

The record/erase aspects of the Minidisc are based on magneto-optical (MO) technology first used in the computer industry for data storage. Computer MO drives were large and expensive, Sony, however, has vastly refined MO to make it practical for such a small consumer product.

As it's name implies, magneto-optical is a combination of magnetic and optical techniques. The process is based in two phenomena:
1) the coercivity of a magnetic material (its resistance to its magnetic orientation being change) drops when heated, and 2) laser light's polarization is rotated when reflected from a magnetized material.

Here's how this incredible MO works:

A laser with a very small spot size is focused on a spinning disc coated with magnetic particles standing on end – like trees in a forest instead of flat like most magnetic media. The laser heats the magnetic particles to the "Curie point", the temperature threshold at which their resistance to being magnetized is minimal. A magnetic head on the opposite side of the disc from the laser source is driven by the signal we wish to record. The disc's rotation displaces the area to be recorded, at which point the magnetic material takes on the polarity of the applied magnetic force. By heating a small are of the disc with a laser, a very weak magnetic field can change the magnetic particles' orientation. The signal applied to the magnetic head is thus converted to a magnetic pattern on the disc.

During playback, the laser is reflected from the spinning disc where it is exposed to the magnetic fields recorded on disc. These magnetic fields rotate the laser beam's polarization, a phenomenon called the "Kerr Effect".
I don't want to be very technical here… but in resume this data passes through various processes of oriented light polarization, polarization filters and encoding/decoding patterns…Think of one magnetic orientation as a pit, the other orientation as land, with the information encoded in the lengths between transitions and you get the picture...

Another Sony breakthrough in MO technology is the very low coercivity magnetic material, allowing magnetization with a very weak field. This reduces the size and power requirements of the magnetic head, reducing weight, cost, and power consumption.
The magnetic material is called Terbium Ferrite Cobalt and has a coersivity of 80 Oersteds, about a third of the coersivity required at that time in conventional (computers) MO media.

Ah, one more thing. The magnetic head can perform polarity reversals at the rate of 100ns per reversal…..

To complete the Sony Minidisc extensive review , two more important tech innovations must be mention:

The Dual-function laser pickup

Perhaps the most remarkable aspect of the Minidisc is its ability to record and play recordable/erasable MO discs as well pre-recorded polycarbonate discs with the same 0,5mW laser.
Pre-recorded discs work exactly the same way as conventional CDs, with pits impressed in a polycarbonate substrate. Despite the many technological achievements in realizing a light-weight, portable and inexpensive MO device, the adaptation of MO technology to be compatible with an existing optical format is indeed, at that time, a remarkable achievement.

Without this capability of playing pre-recorded discs and home-recorded discs at the same player, the Minidisc would likely have been doomed at birth. Further, conventionally manufactured polycarbonate discs were a requirement because of their low cost and ability to be manufactured at existing CD plants.


Shockproof memory

A major stumbling block to widespread acceptance of portable CD, at the time, has been its susceptibility to mistracking when jostled. Sony has addressed this problem with a so-called "Shockproof Memory" - still used nowadays mostly in car-audio – that produced uninterrupted music even under the worst conditions.
The shockproof memory is based on the fact that data can be read from the disc at the rate of 1.4 million bits per second – like a simple CD – yet the decoder needs to be fed data at the rate of only 300kb/s.
This differential is exploited by putting a large buffer (1megabit) between the optical pickup and the ATRAC decoder. The buffer is fed data in chunks from the disc, just often enough to keep the buffer full. If the laser mistracks during a shock, the buffer continuous outputting uninterrupted data. Meanwhile, the laser pickup has plenty of time to start feeding data into the buffer again, ensuring uninterrupted playback.

A technologic achievement with great impact even in today's audio standards.

In simple terms this technique is analogous to water flowing slowly from a hole to a bucket, with the water replaced by periodically dumping water from another bucket into it. The water flows slowly and continuously from the hole despite the periodic bursts used to refill the bucket.

But, there's a trick in all this "simple design". When a laser skips on a CD it almost never returns to the spot where it left the track. How can the Minidisc player produce skip-free audio even when the laser pickup is shaken off the track?

This clever trick is based on CD-ROM (CD Read-Only Memory) technology. Like the CD-ROM format, the Minidisc block structure includes a unique address that identifies each block. When the laser is shaken off the track, the Minidisc knows which block address was read last and positions the pickup so that it begins reading the next block into buffer. Meanwhile, data is continuously clocked out of buffer, making laser mistracking completely transparent to the listener. Clever, hem?

Further more, the Minidisc also uses the same Cross Interleaved Reed-Solomon Coding (CIRC) error correction system used in the Compact Disc. By using the same error correction system and EFM encoding scheme in the Minidisc as the CD, both formats could share decoders and error correction chips, realizing a substantial cost savings.

Conclusion:

After making this study/research about the Sony Minidisc I came to the conclusion that this is one of the most incredible technologic units in the digital consumer audio. After all, Sony had registered almost 300 patents to manufacturer it. Just incredible for such a small unit.

Those were the days…