Digital Audio Tape Recorder
After months of waiting, speculation and peering at display models hidden behind bullet-proof perspex screens, R-DAT has finally appeared in Britain. Are we all ready for digital recording yet or should we stick to the tried and trusted Revox? David Mellor looks at the first product from Sony, the DTC-1000ES, and gives it a taste of professional abuse.
After months of waiting, speculation and peering at display models hidden behind bulletproof perspex screens, R-DAT has finally appeared in Britain. Are we all ready for digital recording yet or should we stick to the tried and trusted Revox? David Mellor looks at the first product from Sony and gives it a taste of professional abuse.
Congratulations! Whether this is your first copy of Sound On Sound or your twenty-fourth (if you have been reading from the first issue, as I have), you are in the company of thousands of other readers who have forsaken the warped world of the hi-fi comics and got hold of the real stuff! Never again will you have to make the agonising choice between the 'subtly fragrant sound-stage' of Monster Mega Cable and the 'dry, ascetic qualities' of Super Single Strand. We know that it is possible to make an infinitely greater sonic difference just by moving a microphone a couple of inches. Look at any professional sound engineer's home hi-fi and you will see simple, basic, good quality gear. After a hard day on the road or in the studio, you don't want to have to twiddle any more knobs and look at any more flashing LEDs than you have to.
There is, of course, a reason why I start my review in this way. The Sony DTC-1000ES is intended to be a digital version of the conventional cassette recorder, for domestic use only. Since this is not a hi-fi mag (the male equivalent of Mills and Boon?), I shall be looking at whether the Sony unit is suitable for studio use, be it home studio or fully professional outfit. I am not interested in its usefulness for copying CDs or otherwise interfering with the laws of copyright which were invented to help us make a living in the music and recording business. Indeed, the need to persuade the public that they shouldn't carry on this sort of activity may make this machine, or the version that is released onto the market, rather less suitable for our particular needs than it could be.
Before coming on to the machine itself, it would be useful to explain a bit about what R-DAT is and why it promises so much.
Most readers will be fairly familiar with the concepts of digital audio (especially if they read my article on the subject in the March '87 issue), so I don't propose to expand on the whys and wherefores of sampling rates and numbers of bits. It's worth going back, however, to look at Sony's first foray into the home digital market - the F1.
The Sony F1 was a two-piece system: a digital processor unit and a more-or-less conventional Betamax video recorder. You could have used any format of video recorder, but this particular Sony one had switchable video error concealment so that the digital processor could employ its own error-correction mechanisms more effectively. Although it was probably intended as a domestic system (the professional market was catered for by the Sony 1610 system, which used a U-Matic format video recorder), the F1 found a good deal of favour in pro circles and a lot of F1-mastered material found its way onto compact and vinyl disc.
Although this system had a lot going for it, it didn't sit quite right in either the public or the professional consciousness. There was a feeling that if digital recording was going to be the 'next big thing', then any possible standard format would have to be fully thought out. Two possible systems were developed by rival companies: Stationary-head Digital Audio Tape (S-DAT) and Rotary-head Digital Audio Tape (R-DAT). S-DAT turned out to be a non-starter, for reasons which will be explained in some future history of digital audio. R-DAT, probably because of Sony's professional clout, is the one which we are all going to have to consider very, very seriously.
As you probably know, video recorders have rotary heads - and so does R-DAT. Why should this be so? Why can't you just scrape a piece of tape past a good old fixed head like we have been doing on tape recorders for years?
It won't take more than a couple of sentences to describe how digital audio, sampled into 16-bit chunks at a rate of 48,000 times per second, means a data rate of over 1.5 MegaHertz (for a stereo signal). When you consider that your Revox can just manage 15,000 Hz on a good day (mine can manage 20,000 because I clean the heads!), you can see that there is a problem. If 20kHz needs a tape speed of 15 inches per second, then 1.5MHz should need a tape speed of nearly 100 feet per second. You can forget about moving tape at this speed unless you can think of how to cram 68 miles of tape into a cassette for an hour's recording! The answer is to make the head move so that you can obtain a greater relative speed between the tape and the head. This has exactly the same effect as moving the tape faster, and is known as the writing speed. Figure 1 shows the arrangement of the cylindrical head drum - on which the two heads are mounted - and the tape, and how the tape is written onto in diagonal stripes.
In a video recorder, which works in the same way, each stripe would correspond to one line of the TV picture. In an R-DAT machine, each stripe contains a fragment of digitally encoded audio together with control data segments which the recorder can use for different functions. It doesn't matter that audio data and control data are mixed up in this way because it is a simple matter for the machine's circuitry to sort it all out. As a point of interest, half the time neither of the two heads is in contact with the tape at all. Since the heads are spaced at exactly opposite sides of the drum, and the tape is only wrapped a quarter of the way around the drum (for ease of handling) every half-turn, the heads contact nothing but free air! It's amazing what those digits can do.
So you've seen the sketch, let's fill in a few details. The factors which are most important in any digital audio system are the sampling rate and the number of bits in each sample. These two parameters, respectively, define the frequency response and the signal-to-noise ratio. Compact Disc, as you are aware, uses 16-bit samples. This gives a maximum theoretical signal-to-noise ratio of 96dB (not counting any manufacturer's dodges to 'massage' this figure) and a sampling rate of 44.1kHz, which enables a frequency response up to 20kHz or thereabouts.
Unlike Compact Disc, R-DAT is not restricted to just one recording mode. There are several, but not every machine will support all of them. The modes which must be supported are these:
• 16-bit coding; 48kHz sampling rate; 2 channels
• 16-bit coding; 44.1kHz sampling rate; 2 channels (replay only)
Other optional modes are;
• 16-bit coding; 44.1 kHz sampling rate; 2 channels (record and replay)
• 16-bit coding; 32kHz sampling rate; 2 channels
• 12-bit non-linear coding; 32kHz sampling rate; 2 channels
• 12-bit non-linear coding; 32kHz sampling rate; 4 channels
This is getting a bit complicated. To make things more simple, the chances are that when you get your first R-DAT machine, you will be able to record from analogue and digital sources at 48kHz - and, of course, play back. You will not be allowed to record at 44.1kHz, only play back. The reasoning behind this is that pre-recorded DAT software will be sampled at 44.1 kHz, so you would need not only your mate's DAT machine but also an expensive 44.1 kHz to 48kHz sampling rate convertor to make illicit copies. Compact Disc is also sampled at 44.1kHz, so you will not be allowed to copy these digitally either. You would, of course, be able to copy either in the analogue domain but a certain amount of quality would be lost.
The optional 32kHz sampling rate is used for digital dubbing from satellite broadcasts. The first of the 12-bit modes allows for a doubling of the available recording time, from a maximum of two hours to four. The second 12-bit mode allows for two extra channels to be recorded, if you really feel you need them. The recording quality is obviously not as good as in the 16-bit modes. Not all R-DAT machines will incorporate the optional modes.
As far as quality of reproduction goes, a sampling rate of 48kHz is obviously better than 44.1kHz. Either the frequency response could go a little higher, or the necessary filtering of the output could be less harsh - that's the choice of the manufacturer. To be honest, I don't think it's going to make that much difference. Mitsubishi are talking about using a sampling rate of 96kHz for their next generation of professional digital recorders. That may make an audible difference, but I think that pundits who are saying that R-DAT will trash CD sonically are being just a little over-optimistic.
That's enough of the description of the format, let's get on to the practical stuff.
Although R-DAT is imminent - so imminent that it might be in the shops when you read this - I did not have a proper UK model to play with, although I am told that the 100 volt model liberated from Japan by HHB Hire & Sales is as near as dammit to what we will be getting.
The DTC-1000ES (they love these names) is basically a digital version of a domestic cassette recorder. It may outperform the Studers and Nagras of the analogue tape recording world by light years, but it is definitely intended for home rather than studio consumption. It's not rack-mounting - although it would fit nicely on a shelf in a 19" rack; it has phono rather than decent XLR connectors; it doesn't have output level controls or tweakers which any self-respecting pro or semi-pro machine should have; and it has about a million super-flash buttons which may make it convenient to select your favourite Madonna tracks but don't make life any easier when you're trying to get a job of work done. I'm not complaining, it is what it is. I'm just making sure that you know what you may be letting yourself in for.
The first thing I tried with it was copying some 15ips analogue tapes onto R-DAT cassette. Needless to say, it passed this test perfectly. I had a tape copying job to do and, being a one tape recorder household, it was easier to use the R-DAT machine as an intermediary than to hire in a second Revox. The result was totally satisfactory, as though the music had gone straight from one tape to another without digitisation. Chalk up one success.
The next trial was to dig out an old MIDI sequence, fire up all my synthesizers and mix straight on to R-DAT. This seemed to be a better test because if I decided to buy one, this would be its function in life, as a mastering machine. Once again, total satisfaction. Reel-to-reel always annoys me here because, basically, I'm into subtlety and it's frustrating when you find that half of what you were trying to do has got lost in tape hiss and modulation noise. What next?
One good test for any piece of equipment is to bung it in a box, send it half-way around the world and see if it still works when it gets there. (It's funny how you can ship something in a grotty cardboard box and it will arrive in perfect nick, yet when you use an expensive custom-built flightcase the components seem to want to jump off the circuit boards). I wasn't in a position to try this test, so I had to think up something else.
Although the Sony DTC-1000ES doesn't have the programming facilities you might find on a CD player, I was able to get it to go over and over the same three second bit of tape. Would you believe over 2000 times? [Nobody can say that Sound On Sound reviewers are not dedicated! - Ed.] I had expected some deterioration in sound quality due to tape wear, but no - the 2000th play was as good as the first (I didn't listen to every one in between!). R-DAT would seem to be a robust format.
What about the ultimate test? I took out the R-DAT cassette and opened the flap. Then I firmly pressed my sweaty thumb onto the playing surface of the tape, leaving a nice print in case the police ever want to know who the culprit was. I wasn't quite ready to believe that it would play perfectly but it did. Following this line of enquiry, I tried the crumple test. Minor creases played without difficulty, but more significant distress to the tape resulted in drop-outs and a very audible coarsening of the sound.
So far, I am impressed, but there are tests I can't perform. Like how will the mechanism cope when it is getting on a bit. Everyone has had a conventional cassette mangled by a rogue machine at some time - even the most expensive and sophisticated machines will occasionally do this. Is there any reason why R-DAT should be different? Time will tell. The point is, that it doesn't matter with a compact cassette, because it's only a copy. The master is safe, out of harm's way - or, at least, it should be. Conventional reel-to-reel tape is a very robust format. I have seen 25-year old tapes play very well, with just a little oxide shedding. I have also seen poorly stored tapes, which have actually gone mouldy, cleaned up and play just as well. I can't help but worry just a little about recording a master on a cassette which may or may not stand the test of time. The difference is, of course, that the digital recording could be copied from time to time onto a fresh R-DAT cassette without loss of quality. Responsible record companies - there are some - will probably develop archiving systems which will guard against tape deterioration by careful regular checking and copying. The rest of us will probably trust to luck.
Back to the Sony DTC-1000ES. Taking my torch with me, I climbed into the loft and dug out my trusty oscilloscope from under a pile of old hi-fi mags(!). When I had a Sony PCM 501 (F1 format) on loan a few months ago, I tried a little experiment. A digital tape recorder ought to be able to record tones all the way up to full level and play them back so that they are indistinguishable from the output of the oscillator - try that with a reel-to-reel! Did it work? Well, no, it didn't. It was better than analogue tape but there were definite glitches which increased as the level increased. Perhaps this was a rogue PCM 501 machine, but I thought the same test with the DTC-1000ES would, at least, be revealing.
Oh dear, it's almost getting boring. Perfect again. Mind you, I did detect some quantisation buzz if I lowered the input sine tone to just above the noise level and increased the monitor gain to compensate. It's usual to hear a little quantisation noise in digital systems, and with the monitors set to a normal level it was completely inaudible. This is something about digital audio that critics pick upon - that distortion at low levels is higher percentage-wise than in analogue systems. But most people agree that the advantages outweigh the disadvantages, so I for one am prepared to live with it.
I mentioned that there was space on the R-DAT cassette tape for control data as well as audio data. The DTC-1000ES has two facilities which make use of this space. One is the 'start of track' marker. It is possible to identify the start of each track with a unique number, making subsequent cueing simple. These numbers can be inserted automatically during recording or afterwards during playback. Either way, they can be changed at any subsequent time without erasing the recording. Skip markers can also be inserted. This makes it possible, with the 'Skip' button switched to 'on', for the machine to wind quickly to the next 'start of track' cue when it encounters a Skip command.
Although both these functions possibly have more use in a hi-fi context, I can see valid uses for them in a professional context - taking over the function of leader tape between tracks on conventional tape, for instance. Fast wind time, by the way, was pretty nippy at 47 seconds for a two-hour cassette.
So much for how wonderful it sounds and what facilities it has - I think we were all expecting it to be pretty good anyway - but there are other questions to consider. As I said, it's a domestic machine, so how might it fit into a professional set-up? For a start, it will not record at 44.1kHz, so there is no chance of making a CD direct from a master recorded on this machine. There are such things as sampling rate convertors, but will CD mastering facilities be prepared to handle 48kHz sampling rate recordings?
There will be such things as professional R-DAT machines. Two have already been announced by Sony, the PCM 2500 and the PCM-2000, priced at £3500 and £5200 respectively. These will be able to record at the CD sampling rate of 44.1kHz via both analogue and digital inputs. Presumably, if you can afford these prices then you can afford to buy your own CDs and will not be tempted to copy them. The PCM-2500, so I am told, is based on the DTC-1000ES, presumably with such niceties as balanced inputs and outputs, output level control and error-correction indicators. It's a big price differential, considering that it probably won't sound any better than the DTC-1000ES, but I am slightly concerned that 48kHz recordings will be left out on a very definite domestic limb and that only 44.1kHz recordings will be considered in professional circles.
Professional R-DAT machines (but possibly not all of them) will also support editing functions, so that with two machines and a suitable controller you will be able to edit in much the same way as video editing is done. There is no obvious way in which the DTC-1000ES could handle this. I can't speak for anyone else but editing is an important professional tool to me and I am reluctant to give it up. With two of these machines it would be possible to make compilations of takes - copying via the digital output and input - provided they had silence in between and you were not too fussy about the gap between being exact to the millisecond, but there is no way you could directly edit parts of two bad takes into one good one - or make a 12-inch mix from a three-minute multitrack master.
If £1000-ish is your price bracket and you are convinced that 48kHz sampling rate masters are going to be acceptable to the people you deal with, whether they are record companies, publishers or paying studio customers, then it's probably right for you to invest in R-DAT straight away. If you want to wait and see, then you might as well save up for a professional machine while you are waiting because I can see no possibility of anyone bringing out a cheap 44.1kHz recording machine.
As with any completely new product, it will take a little while for the market to settle down, both in professional and domestic circles. I like the DTC-1000ES but I think I shall wait six months and see what's happening. If I thought that I could get a return on my money then I would buy one now, but until I know that R-DAT masters are acceptable to the recorded music libraries I deal with, then there would be no economic point. I feel a little guilty that I'm not writing a 'Wow - this is wonderful' type of review, because the Sony DTC-1000ES is very good and deserves one. It's unfortunate for everyone that political arguments about R-DAT have muddied the water, but I feel that my duty is to give as complete a perspective as I can.
Thanks to Ian Jones of HHB Hire & Sales for the loan of the Sony DTC-1000ES.
Further details from: HHB Hire & Sales, (Contact Details).
Review by David Mellor
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