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The MiniDisc Explained


MiniDisc has been around for a while - but many people still are not sure what it's all about. So here is your beginners guide to this miniature marvel...

Everyone loves cassettes because they are small, cheap, convenient and recordable. You can have a cassette machine as part of your Hi-Fi, in your car and in your pocket, and you can take your favourite sounds with you wherever you go.

The only problem is that cassettes don't quite match up to the high quality we've come to expect from CD. They hiss, they distort, they stretch and they break - and that's not the sort of thing we want these days. We want crisp, reliable digital clarity, and that's what MiniDisc offers - great digital sound, discs that can be re-recorded over and over again, full editing facilities (on some recorders) and almost shock-proof playback.

THE DISCS

There are two different types of MiniDisc, both of which have a small (64mm) shiny disc inside a not-quite-square (68x72mm) plastic case, much in the manner of a 3.5" floppy disk. The disc is protected by a slide and this is where you see the most noticeable difference between the two types.

The first type is the pre-recorded disc. It has a slide on one side only, and the disc inside is produced almost exactly like a CD. It's read in the same way as one, too, with laser light from the read head being reflected towards or away from a light receptor by pits on the surface of the disc. Pre-recorded discs used to be scarce but are becoming increasingly available.

The second type is the more interesting for most of us - it's the recordable one. Here the slide opens on both sides of the case, giving the machinery access to both sides of the disc inside. It's a magneto-optical system, and in record mode the laser is turned up a bit and used to heat the surface of a spot to 180C while a magnetic record head on the other side re-aligns the particles at that spot. The magnetic alignment of the particles simulates the presence / absence of pits on the shiny side which is read, on playback, just like a normal CD. The system is called Magnetic-Field Modulation Overwrite (MMO) and manufacturers claim discs can be re-recorded at least a million times without failing.

THE MACHINES

So what are the options if you want to get into this new, Hi-Tech audio miracle? You could go for a MiniDisc component for your Hi-Fi system - it could take the place of your cassette deck for making compilations. You'd only be able to listen to your MiniDiscs at home, but they'd be great copies of the originals. On a similar tack, you could opt for one of the many systems that include a MiniDisc player / recorder.

If you want to make full use of MiniDisc, you might want to think about a personal system. You can get players (which are of only limited usefulness unless you've got a separate recorder) and player / recorders. The player / recorders have digital and analogue inputs and can all be connected to your main Hi-Fi. But when you've made your recordings you can take your player and discs and play them wherever you may chance to roam.

Or you might want to opt for an in-car MiniDisc player. There are plenty of in-car systems around and you can get car adaptor kits for the portable machines as well.

SO HOW DOES IT WORK, THEN?

A CD stores 74 minutes of music on its 12cm disc (74 minutes is an arbitrary length chosen by the boffins at Sony and Philips when they were designing the original CD's, because it's the approximate length of Beethoven's Ninth Symphony). When Sony came to devise the new MiniDisc system the eggheads decided they still wanted a playing time of 74 minutes, but wanted all the information to fit onto a much smaller disc. They thought about making the tracks smaller or compressing the music data (using the same sort of compression systems used for computer data) but then they found another trick.

They turned to the mysterious and arcane science of psychoacoustics, which tries to find out exactly what we hear and how we hear it. Never mind what the measuring instruments tell you is there, they said, let's find out what people can actually hear. It's all very well trying to get a near-perfect record of the performance, where every tiny sound is rendered with digital precision and clarity, but if only the machines can hear it, it's a bit of a wasted effort. It's going t be listened to by cloth-eared humans, they thought, so let's find out what they hear.

The results were surprising. It seems we're not all that good at listening to sound, and there's a great amount of information we just ignore - and some we don't take in at all.

The first thing they looked at was our "Threshold of Hearing". Although we supposedly have the ability to hear sounds at frequencies between 20Hz and 20kHz, our hearing threshold is different across the range and our ears are most sensitive to relatively high-pitched sounds between 2.5kHz and 4kHz. On either side of this frequency band our sensitivity trails off, so that very high or very low notes are more difficult to hear.

The second little failing that caught their attention is called Simultaneous Masking. Within a certain frequency range, a louder sound will mask a quieter sound of a similar frequency. If the frequencies are close enough, we don't even know that the quieter sound is there. So why record it?

ATRAC

Conventional CD-type sampling records every sound (and silence) regardless of whether actual, real live humans can hear it or not. But with the information gathered by the psychoacoustics people, Sony realised it could discard a great amount of the information and record only the sounds that people would actually be able to hear.

Accordingly, Sony's engineers developed a system called ATRAC (Adaptive TRansform Acoustic Coding) and the result was that only one fifth of the original data needed to be recorded. Once they'd got that far, the rest was easy. The fact that 80% of the information is just thrown away alarms a lot of music purists, but if the hardware and software work properly (and these days they work very well) the difference between a CD and an MD recording is becoming indistinguishable.

Using the principles of Threshold of Hearing and Simultaneous Masking, the ATRAC system works on both recording and playback, encoding the incoming music data so that it can be recorded on the disc and then turning it back into intelligible music when you want to play it back again. The result is a sound that under normal listening conditions is indistinguishable from a CD, but which fits onto a disc only 6.4cm in diameter.

ATRAC has been improving steadily since its release and is now up to Version 5. Version 1 was a bit duff, frankly (another reason for MiniDisc being slow to catch on), but by now the quality of the sound and levels of noise are easily on a par with CD and DAT.

SHOCK-PROOF

The sound data is so compressed by the ATRAC system that the player can read the information from the disc much faster than it's needed for playback. The developers quickly realised that this could be a great help in shock-proofing. The data comes off the disc so fast that at least ten seconds' worth (sometimes more) can be stored in a memory buffer, so if the system is knocked and the read head loses its place, there's plenty of time for it to find where it left off and begin reading the disc again before the buffer is emptied. The listener never notices anything went wrong. Compare that to a standard CD, where even the slightest jog can throw the system out and leave you with a load of jumps and stutters.

So, MiniDisc: it sounds great, it's recordable, and it's portable. Isn't it time you took one home with you?