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From Studio To Street - The Story Of DAT

With DAT poised to make the leap from the professional recording studio to the high street, David Mellor looks at the benefits of the format to home recording.


Casio DA7: A replacement for the DA2 featuring SCMS and digital i/o.


Several years ago I heard a rumour that Sony intended to make analogue tape obsolete, in all formats from studio multitrack down to personal stereo. Sony undoubtedly have an excellent track record in designing audio and video equipment that is technically way ahead of what other manufacturers are able to produce, but their marketing success hasn't been totally 100% — think of the Beta and Video 8 video cassette systems.

It was Sony who presented the Rotary Head Digital Audio Tape system — R-DAT for short — in 1983, in prototype form. By the end of 1990, however, in terms of sales it hasn't achieved anything like the success that you might expect from its outstanding convenience and sonic accuracy. Is DAT destined for universal acceptance, consigning Compact Cassette to the consumer dustbin, or will DAT stay solely in the studio domain as a mastering medium? One could say that time will tell, but of course time has nothing to do with it. It's the man or woman in the street who votes with his or her credit card. If they find it attractive, then DAT stands every chance of becoming as ubiquitous as the standard cassette. If DAT gets the thumbs down, then doubtless the manufacturers will go back to the drawing board and come up with another digital recording format which they hope will sweep the world.

In my opinion DAT deserves to succeed, and the Compact Cassette is more than ready for retirement after a damn good run. But let's look at DAT in more detail, to see how and why it came about, and to find out what advantages it offers to the people who create music, and to the people who listen.

HISTORY



To understand why we need DAT, we first of all have to consider why the Compact Cassette system came about many years ago. (Compact Cassette is the full title of the standard domestic cassette tapes that we all use in our hi-fi, Walkmans etc.) Philips, who invented the Compact Cassette back in the early 60s, only ever intended the medium as a low quality 'fun' system. It certainly was plenty of fun, as the sales figures show, but the sound quality of the early machines was absolutely dreadful.

Manufacturers then decided that they would attempt to find a place for the Compact Cassette in home hi-fi alongside the record player and radio tuner, both of which can achieve a far higher sound quality on a good day. The first attempts didn't meet with much success, but then along came Mr Ray Dolby with his famous switch. Activate the Dolby B circuitry on record and playback, and the hiss produced by the medium is reduced very nearly to a quarter of what it was before.

There have been many other developments since then, and these days the cassette can achieve a reasonably satisfying sound — when it is working at its best. However, the tape heads wear, guides shift out of alignment, the tape warps and stretches, and very soon what you have is a travesty of what the recording should be like. By the 1980s, the time was ripe to develop a system which would do away with all these problems, and digital technology offered a way forward.

Fostex D20: First professional 4-head DAT recorder with timecode, still going strong.


The big advantage of a digital recording system is that it has a very high tolerance of faults. As long as the strain on the system isn't too great, any problems will be eliminated completely and a recording will play back 100% perfectly. Many of the problems associated with analogue recording — principally noise, wow and flutter and distortion — are avoided.

Noise. In a 16-bit digital system the signal can be resolved so finely that the noise is at a level 96 decibels below the loudest signal. In a cassette, this figure is more like 55dB, even with noise reduction.

Frequency Response. The tape in a Compact Cassette travels very slowly past the heads. Since high frequencies have very short wavelengths it follows that it is difficult to squeeze them onto the tape in the space available, and difficult to play them back too. (In the DAT system, the frequency response depends on the sampling frequency in the same way that it does with a sampling keyboard, and this is not directly dependent on the tape speed. However, in order to record the large amount of data that 16-bit 48kHz encoding requires, a high tape speed is necessary. In fact, DAT moves even more slowly than analogue cassette tape, but the tape head rotates to increase the speed of the tape relative to the tape head. In a Compact Cassette recorder, it is 4.75 centimetres per second. In DAT it is over 3 metres per second.)

Distortion. In a digital system, noise and distortion go hand in hand, therefore if the noise is low, then the distortion is low too, all the way up to peak level. Compact Cassette suffers from the normal amounts of harmonic and intermodulation distortion at reasonable recording levels, but when high recording levels are used to bring the signal up above the noise floor, then distortion increases.

Wow And Flutter. If a Compact Cassette recorder is new or particularly well preserved, then wow and flutter can be kept to reasonable levels, but there seems to be no limit to how bad things can get, particular in car and personal stereos. In a digital system, the data may be read off the tape at an uneven rate, but it goes through a digital buffer where it can be read out at a constant speed, no matter what. That means no wow and flutter.

Sony PCM7050 & 7030: New top-end editable DAT machines.


Of course, the Compact Cassette must have some good points or it wouldn't have become so popular. The first and most obvious benefit is that it is so incredibly cheap. Cost will be a very big hurdle for any digital system to jump if it is to compete. Compact Cassettes are also very er... compact, for want of a better word. The players they go into can be little bigger than the cassette itself. Acceptance of DAT for home hi-fi would probably come relatively easily if domestic use was the only consideration, but since personal stereos are so important, DAT will have to compete in that market too. If DAT can't win the whole game, then I very much doubt that it will enter into the lives of many people at all.

DAT FORMAT



Since DAT was intended to be a consumer product right from the start, the cassette is very small, roughly three inches by two inches and just under half an inch thick. For professional users, this is rather too small, not just because it makes the cassette easier to lose, but because there will always be the thought that DAT could have been a better system if there had been a bit more elbow room.

The problem is not so much that DAT is unreliable, but that it has been found that a tape recorded on one machine will not always play back correctly on another. This is, I would imagine, a problem that will be solved as manufacturing tolerances tighten. Professional users want this problem solved quickly and absolutely, and no doubt domestic users will too.

The same is true of Compact Cassette, however. A cassette may play perfectly on the machine on which it was recorded, but try and play it on another machine and it will sound dreadful, because of the alignment of the tape heads. This is a problem for home recordists who are constantly giving tapes to other people, who will not be very impressed if the sound is mangled due to a tiny misadjustment.

Having said that DAT's size is a disadvantage for professional users, it really is amazing how it achieves what it does working at microscopic dimensions. As I said earlier, not only does the tape move, the tape heads move too. DAT's full title, R-DAT, indicates that the system uses a rotary head like a video recorder. Unlike analogue tape which records the signal along a track parallel to the edge of the tape, a rotary head recorder lays tracks diagonally across the width of the tape. So even though the tape speed is a mere 8.15 millimetres/second, the actual writing speed is a massive 3.133 metres/second. Figure 1 shows the pattern of the tracks recorded across the tape. The width of each track is 13.591 millionths of a metre. To give a comparison in imperial terms, you could fit 2000 tracks into just one inch.

Stelladat: Precision portable from Swiss company Stellavox.


Figure 1. DAT tape format.


Unlike on analogue tape, the tracks are recorded without any guard band between them. In fact, the tracks are recorded by heads which are around 50% wider than the final track width and each new track partially overlaps the one before, erasing the overlapping section. Since the same heads are used for recording and playback, this may seem to present a problem: if the head is centred on the track it is meant to be reading, then it will also see part of the preceding track and part of the next track. Won't this result in utter confusion? It doesn't, of course, because a system originally developed for video recording is used, known as azimuth recording. The 'azimuth' of a tape head refers to the angle between the head gap, where recording takes place, and the tape track itself.

In an analogue recorder the azimuth is always adjusted to 90 degrees, so that the head gap is at right angles to the track. In DAT, which uses two heads, one head is set at -20 degrees and the other to +20 degrees, and they lay down tracks alternately (see Figure 1.) So on playback, each head receives a strong signal from the tracks that it recorded, and the adjacent tracks, which are misaligned by 40 degrees, give such a weak signal that it can be rejected totally. Damn clever.

Mechanically, there is a strong similarity between a DAT recorder and a video cassette recorder. Both use a rotary head drum on which are mounted the record/playback heads. But there are differences. A video recorder uses a large head drum with the tape wrapped nearly all the way around. This is necessary so that there can always be a head in contact with the tape during the time that each video frame is built up on the screen. With digital audio, data can be read off the tape at any rate, and stored in a buffer before being read out at a constant speed and converted to a conventional audio signal. The head drum in a DAT machine is a mere 30mm in diameter (and spins at 2000 revolutions per minute). The tape is wrapped only a quarter of the way around the drum, which means that at times neither of the two heads is in contact with the tape, but of course the buffering means that playback is still continuous.

This 90 degree wrap has its advantages: there is only a short length of tape in contact with the drum, so high speed search can be performed with the tape still wrapped; tape tension is low, giving long head and tape life; if an extra pair of heads is mounted on the drum, simultaneous off-tape monitoring can be performed during recording just like a three-head analogue tape recorder.

The signal that is recorded on the tape is of course digital, and very dissimilar to either analogue audio or video signals. The standard DAT format uses 16-bit PCM (Pulse Code Modulation) sampling at a sampling frequency of 48kHz, the analogue audio signal being converted to a stream of binary numbers representing the changing level of the signal. Since the dimensions of the actual recording on the tape are so small, there is a lot of scope for errors to be made during the record/replay process, and if the wrong digit comes back from the tape it is likely to be very much more audible than a drop-out would be on analogue tape. Fortunately, error correction techniques exist to solve this problem, which pre-date DAT (they are useful for data storage in computers).

Inside a DAT machine: Sony's TCD-D10 Pro stripped bare.


There are various ways of checking whether data is correct or not. DAT, like Compact Disc, uses a technique called Double Reed-Solomon Encoding which duplicates much of the audio data (37.5% of it in fact) in such a way that errors can be detected, then either corrected completely or concealed so that they are not obvious to the ear. If there is a really huge drop-out on the tape, then the DAT machine will simply mute the output rather than replay digital gibberish. As an extra precaution against drop-outs, another technique called interleaving is employed, which scatters the data around the tape so that if one section of data is lost, there will be enough data elsewhere — hopefully beyond the site of tape damage — to reconstruct the signal.

The PCM audio data is recorded in the centre section of each diagonal track across the tape, but there is other data too. ATF signals allow Automatic Track Finding, which ensures that the heads are always precisely positioned over the centre of the track, even if the tape is slightly distorted and the track curved. More important at an operational level are the Sub Code areas of the track, which allow extra data to be recorded alongside the audio information. Not all of the capacity of the Sub Code areas is in use as yet, which leaves the way open for extra expansion of the DAT system. Those Sub Code facilities that are in use include: time codes (not SMPTE/EBU timecode, apart from some professional machines) which can log the total elapsed time and the time since the beginning of each item on the tape; the Start ID marks the beginning of each item; the Skip ID tells the machine to go directly to the next Start ID, thus performing an 'instant edit'. These codes make DAT easier to use.

DAT IN THE STUDIO, DAT AT HOME



In the professional recording studio, DAT is simply a fact of life. It wasn't intended to be a pro system, and Sony already have their 1630 system which records digital audio onto a U-Matic video cassette. But DAT is just so convenient — it's cheap enough for producers and artists to have their own DAT player at home, and the sound quality is good enough to make CDs from. In the home studio, or any studio where there are budget restrictions, there is bound to be the question of whether to use DAT or 1/4" analogue tape for mastering if you can only afford one or the other. It boils down to the question of whether or not you are going to have to do any editing. DAT is not as yet fully accepted as an editable medium, so it would have to be transferred to a 1630 tape or to a hard disk editing system. Many music studios should be able to manage without the chopping block, and DAT will certainly deliver the goods as far as sound quality goes.

Analogue tape isn't as accurate sonically, but if you are running a general purpose studio where you have no idea of what sort of work is going to come in from one day to the next, then you just have to have analogue tape. It is still the easiest and quickest medium as far as total turnaround time from job in to job out is concerned. For myself, I prefer to have both DAT and analogue to hand so I have the best of both worlds, and I can back up my DAT masters to analogue to give me an extra degree of certainty that my recordings are, at least physically, going to stand the test of time.

Sony's new domestic DAT player, the DTC55ES.


In the home hi-fi market, if the cost can come down sufficiently, DAT will be an ideal replacement for the Compact Cassette. The only possible snag might be that personal stereo DAT may not be possible at the degree of compactness we have become accustomed to. Apart from that, there are several advantages to DAT. The sound quality isn't merely good — in a good machine it is state-of-the-art, or at least state of the available art, bearing in mind manufacturers' research into 18 and 20-bit recording systems. Also, two hour DAT tapes are available, which puts the Compact Cassette's maximum of 60 minutes a side to shame.

Unfortunately, the time has come for me to tell you about the fly in the DAT ointment that is going to spoil a lot of our fun. And that fly's name is...

THE SERIAL COPY MANAGEMENT SYSTEM



For years now the record industry has been whinging that people are copying their products on to cassette instead of going out and buying them. They used to conduct surveys asking people how many records they had copied in the last month, and then claim that whatever the answer was, that was the number of sales they had lost, regardless of the fact that it costs a lot more to buy a record than to tape it.

My personal attitude to the problem is: yes, it is morally wrong to infringe someone's copyright. I don't like it much when people infringe mine so I would prefer not to do it to someone else. But I also think that people ought to be able to do what they want to do, as long as they pay for the privilege, and currently there is no legal way of copying a record or CD — even one that you have bought — on to a cassette to use in your car or Walkman. This is just plain stupid, and the record companies and government ought to work something out so that ordinary people don't have to feel like criminals every time they press the record button.

DAT has really put the wind up the record companies, because they see it as a way of producing an infinite number of exact digital clones of a Compact Disc recording, so they have done their very best to stifle the introduction of DAT onto the domestic market, with a good deal of success. On the other side of the fence, the manufacturers of equipment which many people are going to use to make illegal copies of copyright material have given way to pressure, and are fitting their domestic DAT machines with something called the Serial Copy Management System (SCMS).

To cut a long story short, what this does is to allow you to make one copy of any existing recording on to DAT, but you will then find that you are unable to make further digital copies of that DAT cassette. The idea is that this will prevent copies from reaching any further than the owner of the DAT machine in question. In practice, this isn't going to make any difference to the record companies, because it's not personal copying that is damaging sales. The people who do the copying are people who like music, and either buy the records themselves or at least support a musical culture that promotes record buying. The pirate record and cassette producers, who make illegal copies by the barrowload and sell them wherever they can, will still make their multiple copies, either with a bank of machines, or via the analogue inputs which can't detect the SCMS flag.

The record companies' argument is that the people who will become DAT owners are people who will damage their sales figures by copying their products. So where does the home recordist fit into all this? Not very well, I'm afraid, because SCMS will think that any music passing from the output of your mixing console into your DAT recorder is somebody else's copyright — not yours — and that 'somebody else' has not given permission to copy. You can make a recording onto DAT, but when you borrow your friend's machine to make a back-up digital clone, you will find that it will not record. SCMS may think that it is protecting some imaginary record company's rights, but it is infringing your right to make as many digital clones of your own music as you like.

DAT is good and there should be more of it about. It's streets ahead of analogue tape on sound quality, and once you have it, you'll wonder how you ever did without. But any of the forthcoming crop of domestic machines with the SCMS system are not as suitable for mastering as they should be. I would imagine that the choice will eventually be between an inexpensive domestic machine with SCMS, or an expensive professional machine without. Or perhaps SCMS will kill the idea of low cost DAT entirely. Is that what the manufacturers want? Perhaps we should be thinking of starting an anti-SCMS lobby right now. Are you ready to stand up and be counted?

WHAT COMES NEXT

These days, a product hardly has chance to come on to the market before it is superseded, and that might happen to DAT — at least DAT for domestic consumption. I'll never believe anything until I actually see it in the shops, so I think it best that I accord the following the status of rumour.

Sony have announced a new digital recording system which is ostensibly for dictation machines only. It's called the Micro DAT, and uses a cassette smaller than even a conventional micro-cassette. 'Large scale' DAT is already working to incredibly fine tolerances so Sony have come up with a new system where the rotary heads throw the data onto the tape in identifiable blocks. On playback, the heads don't attempt to gather the data in the correct order, but sweep up whatever information they come across in a more or less random fashion and store in a buffer from where it is reassembled back into the correct order.

The recording format is 12-bit rather than DATs' 16-bit and the sampling rate is only high enough to give a frequency response up to 15kHz (which is the same as FM radio). This level of sound quality would probably be more than a match for the typical personal stereo, and apparently machines could be produced very cheaply. Will this system see the light of day, or is it Sony's implied threat that if we don't accept DAT, they'll keep throwing new formats at us until we succumb?

Rather less substantial is the rumour of Philips' Digital Compact Cassette which will use a cassette either very similar or identical in form to a conventional cassette, but with a higher quality tape inside. Instead of a rotary head like DAT, it uses an 8-track stationary head to record a stereo signal at a rather lower level of quality. Will this format rise up and pull the digital rug from under Sony's feet, or should Philips stick to making lightbulbs (and CDs)? We'll have to wait and see.


DAT MODES OF OPERATION

The DAT system was designed with several operating modes for different purposes. The standard mode is 2-channel, 16-bit, 48kHz sampling which was designed to give good audio quality while being incompatible with the CD standard, to prevent digital copying. All machines support this mode for recording and playback, and will also play back tapes recorded at 44.1kHz, the DAT standard for pre-recorded cassettes. [On the subject of pre-recorded tapes, very little material is currently available on the format. The major record companies say that they have no plans to release pre-recorded DAT cassettes at present — even CBS, who are now owned by Sony. Factory seem to be the only UK label active in this area — Durutti Column, New Order and Joy Division albums have all been released on DAT, and from next year more of the label's new releases and back catalogue will appear on the format — Ed] Some machines will also record in this mode, which is essential if you intend to make a CD from your master without going through either an additional analogue stage or a sampling rate convertor.

Other modes are not so well publicised, nor so useful: a 32kHz mode which is compatible with satellite broadcasting; a long play mode which samples at 32kHz and cuts the format down to 12-bits; a 4-channel mode which is also 32kHz, 12-bit. The final mode of operation is one where prerecorded tapes are duplicated with a wider than normal track for increased reliability and speed of production at the expense of a reduced maximum playing time.

Although the 48kHz DAT format is perfectly good in itself, the 44.1kHz standard is necessary for full compatibility with CD. I would advise any studio owner to buy a machine that can record at both 44.1 and 48kHz, preferably without SCMS.


HHB DAT REPORT

HHB's pro version of the Aiwa HD-S1, the HHB1 Pro.

HHB Communications have produced a comprehensive report on DAT and its use by audio professionals. The report, containing user comments and an appraisal of the technology's service history, charts the growing use of the DAT format in broadcast and video post production applications. HHB report sales of more than 5,000 DAT recorders since December 1987, a figure that is estimated by Sony's Dr Roger Lagadec as representing around 25% of all DAT recorders in professional use worldwide.

The report acknowledges that the low cost of DAT has made some users more forgiving of its limitations, but equally it confirms that DAT is a hugely popular format, universally preferred to the EIAJ technology it has replaced in so many applications. Customers also prefer DAT to most other digital formats for reasons of quality and convenience as well as price. Concern about the longevity of DAT recordings has not been an issue for HHB or its customers, with one BBC respondent expressing greater concern about the production life of the format itself.

Both hardware and tape manufacturers come under fire in the report, from HHB and several of the company's customers. Incompatibility problems have been experienced between Sony and Panasonic machines, and there is a general call for tape heads to be made more accessible for operator cleaning maintenance. The tape media itself has frequently been of inconsistent quality, leading HHB to recommend specific brands to customers. Broadcasters are divided on the benefits of the small cassette size; some find it convenient for acquisition and interstation exchange while others show concern at the ease with which tapes can be mislaid. Both HHB and many of its customers believe that, as a professional digital format, DAT should also be backed by a range of support technology affording a more accurate means of error rate detection.

HHB are gently critical of DAT users themselves. Although the service return percentage of 11% within three years is low by professional recording equipment standards, the company believe that the figure could be lower still if users displayed better knowledge about the format's basic maintenance requirements. "The same engineers who are prepared to clean and realign analogue recording heads every session fail to realise that digital error rates will build up if DAT head cleaning tapes are not used on a weekly basis", says HHB Sales Manager Steve Angel.

While HHB's report acknowledges the need for a wide choice of 4-head, timecode-equipped DAT systems, the company challenges the broadcast community to overcome its reservations about DAT's original conception as a consumer format. "Major broadcasters and large manufacturers are used to working together over long periods to develop formats of mutual benefits. With DAT neither has been afforded that opportunity." concludes Steve Angel.

"The format was not designed by SMPTE, the AES or the EBU, but it has been chosen — and emphatically so — by thousands of professional end users. Perhaps this time the customer really does know best."

The report also notes that DAT's entry into the consumer market is good for professional users, bringing additional shared VLSI innovation to professional machines. With no costly licensing fee required for DAT manufacture, the company also looks forward to a wide range of hardware developments by an international cast of specialist OEM manufacturers.

FURTHER INFORMATION

HHB Communications Ltd, (Contact Details).


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EMU Proteus 2

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Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Nov 1990

Feature by David Mellor

Previous article in this issue:

> EMU Proteus 2

Next article in this issue:

> Keeping The Dream Alive


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