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Alesis ADAT

8 -Track Digital Recorder

Paul White finds his Holy Grail in a 3U rackmount package — Alesis' revolutionary ADAT, the first digital multitrack tape recorder to compete in the same price bracket as affordable analogue multitracks.



Ask any producer, engineer or recording musician what is the most talked about piece of equipment at the moment and the answer is almost certain to be 'ADAT'. The fact that it is months overdue in arriving seems only to have increased its mystique but, according to Alesis, the delay is simply to avoid using the customer as a quality control department, which is so often the case with leading-edge products. Understandably, everyone has been seduced by the idea of low-cost, high-quality digital multitrack recording and, quite apart from the sound quality benefits, ADAT's ability to form the basis of an expandable multitrack system must make it very attractive to the private studio owner who can't afford to make a single, large investment, but who might build up to two or three ADATs over the period of a year or two.

Confidence in important factors such as long-term reliability and tape interchangeability will only be achieved when the machine has established a track record in a variety of working environments, but our initial impression is that worries on these counts are largely unfounded. No problems in these areas came to light during the course of our review and, furthermore, it is unlikely that either head life or replacement costs will be worse than for comparable analogue machines — indeed, I would expect head life to be longer and replacement costs lower, though only time will tell if I'm right.

ADAT is revolutionary in many ways, and several factors combine to provide a high technical specification at a low cost. The first must be the transport, which is based around a conventional helical-scan, VHS video transport fitted with a four-head drum designed specially for the ADAT project. The transport speed is around three times that of a video recorder, providing 45-50 minutes of continuous recording from a standard S-VHS 180 minute video cassette. Extensive use is made of ASICs (Application Specific Integrated Circuits) designed by Alesis to keep the component count down, and a proprietary error correction system has been developed to maintain data integrity under arduous conditions. In addition, high quality convertors which are mass produced for the consumer audio market have enabled Alesis to fit the machine with convertors on all eight inputs, rather than specify these as optional extras. Similarly, the optical interface appears to utilise many components found in consumer products. Alesis seem to have taken a leaf out of Apple's book, in that the machine will work from any AC power source between 90 and 250V with no need to switch.

ADAT, like many of the current DAT and CD machines, works on a 16-bit linear sampling system with delta-sigma conversion and 64-times oversampling. The sampling rate is nominally 48kHz, though this may be adjusted between 40.4 and 50.8kHz, enabling varispeed techniques to be applied. In musical terms, this provides a varispeed range from -3 semitones to +1 semitone. The resulting audio bandwidth is essentially flat from 20Hz to 20kHz (+/- 0.5dB) with the unmeasurable wow and flutter that we have come to expect from digital systems. Using 16-bit linear sampling, the theoretical maximum dynamic range is 96dB and Alesis claim 92dB (A-weighted) for this machine with a THD figure of .009% at 0.5dB below the maximum output level. Like all digital systems, the distortion is lowest at the maximum signal level where more bits are used to represent the signal; at lower recording levels, the distortion increases accordingly. Crosstalk is quoted at 90dB at 1 kHz which means that the main source of crosstalk is likely to be the wiring harness and the mixing console.

THE PACKAGE



Physically, ADAT looks like a hybrid between a 3U front-loading, rack-mount video recorder and a multitrack cassette deck, complete with front panel metering, familiar transport controls and a large, legible numeric readout. To satisfy both professional and semi-pro requirements, both balanced ELCO and unbalanced jack inputs and outputs are provided, working at +4dBu and -10dBV levels respectively. The jack inputs are normalised — 1, 3, 5 & 7 and 2, 4, 6 & 8 — which means that a multitrack recording can be built up using a 2-output mixer with no repatching. You only need to feed the mixer into inputs 1 and 2, and then select which tracks are to be recorded using the record enable buttons. Few users are likely to work in such a simplistic way, but it's a useful option to have and it does mean that recordings can be made using a simple PA-type mixer if necessary. There's also an optical input and output which allows all eight tracks to be piped to another ADAT for backup purposes. It's always a good idea to back up digital data, and given ADAT's relatively low tape costs, there's no reason not to do it if you have access to more than one ADAT.

By way of external control, there are two rear-panel jacks, one for the LRC remote controller, included with ADAT, and one for hands-free punch-in/punch-out. The LRC socket can also be used with a footswitch to access the machine's locate and play functions. Three 9-way D-connectors are also fitted, two to handle sync in and out, and one to connect to the optional external meter bridge. Alesis claim sample-accurate sync from one ADAT to another, the machines being daisy-chained using 9-way D-type leads, and our tests suggest that this level of accuracy is indeed achieved.


Being an inquisitive sort of person, I couldn't resist the temptation to whip off the top cover, and I was impressed by the build quality. Internally, the machine appears to be very soundly constructed; all circuit boards are glass fibre rather than resin/paper, and the neat inter-board wiring is effected using plug connectors where necessary. The transport itself looks reassuringly heavy given that it is based on a Japanese VHS mechanism, and accessibility to the drum for cleaning is excellent once the top cover has been removed. Such transports tend to whir quite loudly and this is evident with the top cover removed, but a couple of thin foam damping panels have been applied to the top panel, which considerably improves matters. In normal use, the mechanical noise from a couple of units is quite unobtrusive.

The inclusive remote control unit (LRC) is a compact, plastic affair which duplicates the front panel transport/locate controls and plugs into the ADAT via a captive mono jack lead around 2m long. (That's a little short perhaps, but you can always use a standard guitar lead extender if you need to locate the remote any further from your ADAT.) It is fairly basic and has no indicator LEDs, but it provides convenient access to the transport controls, especially when two or more machines are being used in sync. A far more comprehensive remote, the BRC, is due later in the year, which provides more advanced synchronisation, overdubbing and editing functions and can be used to control multiple ADAT setups. An optional meter bridge can attach to the BRC remote, providing simultaneous visual monitoring of 32 tape tracks; it is possible to sync multiple ADATs without the BRC, but the BRC does include full SMPTE and MTC facilities and will apparently provide comprehensive offset, locate and editing facilities as well as full control over up to 16 machines, giving a maximum of 128 tape tracks. Full details are as yet unavailable, presumably as the software is still in development but, judging by such preliminary information as we have, expect a very powerful and physically imposing unit.

USER INTERFACE



In many ways, the ADAT is designed to feel much like an analogue recorder, the one main difference being that you need to format the tape before use. It is possible to format while recording, but as no tracks can be switched in or out of record during formatting it is preferable to do all the formatting first. To format an unused tape, wind it back to the start, then press the Format button, then Play and Record. ADAT writes some setup data at the head of the tape, and writes a real-time sub-code track for the length of the tape. It is also possible to extend the formatting of a tape that has been formatted only part way through but, again, the most sensible way of working is to format the whole length of the tape at the outset.

The only other major operational difference between analogue and rotary head digital machines is the concept of Engaged and Disengaged tape. In operation, the tape is partially wrapped around the head drum and is in contact with the moving tape heads. This is the engaged mode, which allows recording, play, and fast cueing to take place. In fast cue mode, the sound can be heard in disjointed segments in. much the same way as the fast cue on a DAT machine or CD player.

The disengaged mode is used to fast wind the tape without any contact with the heads. Going into play from stop is faster if the tape is engaged, but to safeguard the tape from excessive wear, the machine drops out of engage mode if left for more than four minutes. All these concepts will be quite familiar to anyone who has ever operated a domestic video recorder, and you can switch between engaged and disengaged modes by hitting the Stop button while the tape is stationary. In the disengaged mode the stop light flashes, while in engaged mode it remains solidly on.

RECORDING



Recording is accomplished in the usual way by selecting 'record ready' on the desired tracks, then holding down Play and pressing Record. It is not possible to drop in or out of record using the record status buttons as it is on most analogue machines, but it is possible to drop in and out using the transport buttons. Dropping in involves holding down Play and hitting Record while the tape is in motion, in which case recording starts on whatever tracks are in the record ready mode. The drop-in appears to be both instant and seamless, as does dropping out. Strangely, dropping out is also most easily accomplished by holding down Play then hitting Stop or Record, which isn't quite so reassuring as being able to do it with a single button push. However, it works well enough, and an optional footswitch can also be used to do the same. Either a normally open or normally closed momentary action footswitch will do the trick; the machine interrogates the switches at power up to find out what type it is dealing with. If an attempt is made to record over the first couple of minutes of tape where ADAT stores its header data, recording is inhibited.

Monitoring during recording is also refreshingly straightforward and is controlled by just two buttons. The Auto Input monitor button switches between two modes of operation; with the Auto Input LED off, all tracks set to record enable monitor the input signal, while the rest monitor the off-tape signal. When the LED is lit, all tracks monitor the taped signal until one or more tracks are put into record, in which case the tracks being recorded switch to input monitor. This is the normal way of working, but for setting up levels prior to a session or for monitoring input levels during a live recording, the All Input switch enables all eight inputs to be monitored regardless.

FACILITIES



Three locate points are provided, and these work very much like those on a Tascam or Fostex analogue machine. There is a locate to zero facility, and two further points which you can programme simply by holding down the Set Locate button and then pressing either Locate 1 or Locate 2 at the appropriate time. You can cycle between the two loop points but there is no auto punch-in/punch-out facility built into the machine — you need the BRC for this.

Pitch control is implemented by a pair of up/down buttons, and while these are being used the pitch change in musical cents is displayed in the tape counter window. The pitch change takes a second or two to happen, but is otherwise what you'd expect. To reset the pitch to normal, it is necessary only to press both buttons together. Both buttons have inbuilt LEDs and indicate whether the pitch is higher or lower than normal. Both LEDs are extinguished when the pitch is normal.

Using two ADATs, one tape may be cloned onto the other machine by setting the second machine to Digital In and using the fibre optic cable provided. The second machine must be set to record on all tracks and both transports must be started manually. In practice, this was quite straightforward, and I encountered no problems. I even stopped the tape part way through the backup, rewound a little and then went back into record to see if I could spot the join — I couldn't.

To use two or more ADATs in sync, you'll need to obtain or make up some 9-way D cables to link the machines. The sync out signal feeds from the master machine to the sync in socket of the slaves, and so on, up to a maximum of 16 machines. After connection, they automatically assume ID numbers according to the order of connection. When so linked, the transport buttons on the first machine control all the machines, though the record enable buttons remain independent, for obvious reasons. In this mode, the machines lock to single sample accuracy, though it is prudent to give the machines a few seconds of pre-run to get into sync, especially after fast winding where the tape is disengaged. If the tape is left engaged, sync is, to all intents and purposes, instantaneous. This degree of synchronisation accuracy means that the two (or more) machines really can be treated as one, in that it is safe to split a stereo pair between machines without the risk of the phasing effects that invariably occur with locked analogue recorders. On conducting a test with identical musical signals on our two review ADATs, no audible or measurable phase difference was evident between the machines.


PERFORMANCE



To cover the sound performance aspect first, the ADAT produces results that are subjectively identical to those from a DAT machine employing delta-sigma conversion — most listeners are unlikely to be able to distinguish the input signal from the output. The level of noise is far below that inherent in most sound sources and mixers, and so isn't going to be a significant factor. If there's a negative side to digital recording it is that no headroom exists once all 16 bits have been used up, so you have to avoid pushing the meters into the red. You can get away with brief flashes of red on snare drum peaks, but otherwise venture into the red at your peril.



"The very success of ADAT as an engineering exercise must sound the death knell for analogue tape machines in this sector of the market."


With any multitrack machine, the quality of the drop-ins and drop-outs is very important, and at no time did I actually detect a gap or any delay while doing so. Considering the complex interleaving of data involved in any digital recording system, this is remarkable. Indeed, so intrigued was I by this capability that I made a test recording of a continuous triangle wave tone and then dropped in a different pitch triangle wave, transferred the result to Sound Tools, and viewed the result. It would appear that a smooth digital crossfade is performed in something under 10mS which accounts for the total lack of gap or glitch.

Fast winding through a whole tape takes around two minutes, which is quite acceptable, though the wind rate is halved if you leave the tape engaged. In rough terms, the wind rate when the tape is disengaged is 20 times the play rate, and 10 times the play rate when engaged. Setting Auto Play will cause the machine to enter play as soon as any locate point has been reached, and the same effect can be achieved by pressing Play once the machine has started to search. However, this isn't quite as snappy as most analogue machines, where you can hit Locate directly followed by Play and expect it to happen — with ADAT you have to press Play after the tape has started to wind towards the search point, or you end up going directly into play.

At no time did I encounter any tape dropouts or compatibility problems when a recorded tape was moved from one machine to another, and cycling round a short section of tape a few dozen times to simulate the battering a tape gets during a typical session caused no ill effects.

SUMMARY



After all the waiting for ADAT to arrive, it seems rather an anti-climax that it does exactly what it was supposed to do. If you like the sound quality of the latest generation of DAT machines and CD players, then there's nothing about the sound of this machine you won't like, and the more you think about the potential of this type of system, the more it makes sense. For example, in a facility with two or more ADATs, the ability to make digital clones of the tapes for backup purposes is invaluable — the procedure works flawlessly and simply. It is also eminently practical that multiple machines can be accurately synced (with no loss of tape tracks to timecode) providing a practical means of building up the number of tracks in a system in multiples of eight when finances permit, rather than having to sell an existing machine at a loss and then trade up. A multi-machine setup runs very smoothly and is only marginally less convenient than a single machine.

The smaller facility might find it adequate to use six tracks of the ADAT for recording, and then mix these down onto the remaining two tracks via a mixing console. Certainly the gapless punch in and out makes this an attractive proposition for mixing songs in sections rather than all in one. This being the case, it's unfortunate that no direct connection is provided to take out, say, tracks 7 and 8 as a digital pair via an SPDIF socket. Such a provision would have made cloning the finished mix to a standard DAT machine possible rather than having to rely on the analogue output (which, realistically, is still quite clean enough). This is probably impractical due to the specialised error correction system, but the optional AI-1 ADAT to AES/EBU and S/PDIF Digital Interface (see 'ADAT Accessories' box) makes it possible to access track pairs in the AES/EBU digital format.

You need the BRC to fully exploit ADAT's more advanced features, but as this is not yet available it is impossible to comment on its performance. It should allow precise SMPTE and MTC operation without the need to sacrifice tape tracks to timecode, and promises to provide comprehensive offset facilities allowing, for example, a chorus of a song recorded on one ADAT to be bounced to any desired location on another ADAT, all in the digital domain. It will also provide full autolocation and remote control facilities for a multiple ADAT setup.

Despite the fact that, track for track, ADAT costs around twice as much as a semi-pro analogue machine, it is certain to attract the type of user who needs the extra quality and versatility this format offers. In some ways the lack of noise offered by a digital format is a bit of a red herring, in that an analogue machine running with a good noise reduction system can approach or even exceed the dynamic range of a digital machine — my dbx-equipped multitrack manages over 100dB — but even that is not the whole story. Analogue noise reduction is not without its audible side-effects, and there's considerably more distortion involved in analogue than digital recording. More noise comes from the sound sources than from the tape format, as my test recordings on ADAT proved, especially if there's a guitar in the mix somewhere. The real beauty of digital recording is that there's no wow and flutter, the phase relationship of the harmonics that make up the sounds are preserved, and bouncing is much cleaner, even in the analogue domain. On top of that, ADAT enables the user to build up a system without the anguish of selling older gear at a loss just to get more tracks. The sync features work beautifully, and although the BRC will offer far greater flexibility, the LRC provided is adequate for ail routine work.

Criticisms are both few and minor. The LRC could do with at least one LED to confirm whether the machine is in or out of record mode, and it would be nice to be able to drop out of record using a single button such as Play. Similarly, I can't see why it takes two keys to program an autolocate point where most recorders allow this to be accomplished with a single action. And talking of location points, there seems to be no easy way of interrogating the locators to find out at what time values they are set.

But these very minor whinges pale into insignificance compared with the advantages. Lockup and chasing when using two machines is quite transparent when the tape is in engaged mode, enabling a multiple machine setup to be treated exactly like one machine. When the tape is disengaged, it can take five seconds or so for the transports to get their act together, but this is comparable with analogue machine lockup times. However, there are absolutely none of the timing or stability problems associated with analogue machine lockup, and its drop-in and out performance is amazingly clean and vice free. Furthermore, the machine controls 'feel' OK, which is no mean feat given the differences between a video transport and a conventional analogue transport.

The very success of ADAT as an engineering exercise must surely sound the death knell for analogue tape machines in this sector of the market, and it is impressive that a relatively small American company should bring such a project to fruition before any of the Japanese multi-nationals. No doubt they won't be too long in following suit, but in the meantime Alesis deserve their success — they've certainly earned it. Finally, do I want one? No way, I want a six pack!

Further information

£3,499 inc VAT.

Sound Technology plc, (Contact Details).

SECOND OPINION

Once upon a time having a home recording studio meant bouncing back and forth from one track to another on a stereo reel-to-reel tape recorder. It was fun, but the generations of copying quickly built up and the quality of the end result was pretty dubious, to say the least. With the two review ADATs, I now have in my home studio a 16-track digital multitrack recorder and I would only need to up the standard of my acoustics and monitoring to have the technical capability of equalling the sound quality achievable in the world's best studios. Yes, this is progress, and it feels very good indeed.

The best way to find out whether a piece of equipment really works is to use it on a practical project. I spent two days using two ADATs in place of my analogue multitrack on a project which I eventually intend for CD. As far as sound quality goes, I was quite happy that it was the equal of my Sony DTC1OOOES DAT recorder which, although it isn't indistinguishable from the live sound, is a million times better in my opinion than any analogue recorder ever made. When you're setting levels with the ADAT you do have to bear in mind that when the meter shows red it really does mean that you have gone too far — a test with a sine wave tone produced some very interesting whooshes and wails. While I had my sine wave generator connector I also tried some drop in tests. Yes, drop in and out are both gapless and clickless, although you will hear a click in the monitor. Crossfading at the join is very smooth, and you can drop in and out undetectably almost anywhere in real music. I wondered whether repeated drop ins at the same point would be a problem but I didn't find any ill-effects.

Cynics may be thinking that there must at least be something wrong with the ADAT (apart from stiff transport controls and a couple of slow-to-react autolocate buttons) but really there isn't. Affordable digital multitrack is here and it works. OK, I did find one thing, but I doubt whether you are going to come to grief over it. A pair of ADATs have the facility to clone any or all the tracks on a tape digitally onto another tape (which is great for backups). The clone can be given the same sync reference as the original, so if it was part of a master-slave pair and you cloned all eight tracks, it should be totally interchangeable with the original tape. In fact the digital clone will play back approximately 20μs (20 millionths of a second) later than the original — I spotted this accidentally and did a rule of thumb measurement on my oscilloscope. If you did by chance split up a stereo pair of tracks by cloning one of them and then mixed the two — one from the original tape on one machine, the other from the cloned tape on the second machine — into mono, you might notice a loss of very high frequencies, but this would be a very unlikely chain of events. With all normal copying and track bouncing operations, everything else is perfect (and a 20μs discrepancy is in fact within Alesis' claims of single sample synchronisation accuracy, as a single sample at 48kHz lasts 20.83μs).

I feel confident in saying that future musicians and engineers will look upon ADAT as a milestone in audio development. It's not the first digital multitrack, and it certainly won't be the last, but it's the first one the ordinary person in the home or small studio can seriously consider owning, and high-end professional digital multitrack users will be keeping an eye on this development too. It's unlikely that anyone will be throwing away their DASH or PRODIGI reel-to-reel multitracks, because they are both proven formats with a very strong user base. But just as we now have an alternative to a Tascam or Fostex analogue multitrack in the small studio, ADAT is showing the Sonys, Mitsubishis and Otaris of this world that there is an alternative way of doing things. Well done Alesis.

David Mellor


ADAT ACCESSORIES

Besides the forthcoming BRC remote, there are several other accessories for ADAT which will be available shortly.

RMB REMOTE METER BRIDGE
Designed to mount above the BRC remote control. Provides 32 channels of LED meters.

AI-1 INTERFACE
The ADAT has only a proprietary 8-channel optical digital input and output. The AI-1 will be able to translate this to AES/EBU (professional) and S/PDIF (domestic) digital formats. A sample rate convertor is also provided.

AI-2 ESBUS INTERFACE
One of the unfulfilled dreams of professional audio is that all equipment should have a standard synchroniser interface. ESbus is just such a standard, defined by the EBU (European Broadcast Union) and SMPTE (Society of Motion Picture and Television Engineers), but it hasn't made as much progress with manufacturers as it should. Full marks to Alesis for supporting it.


ALESIS ADAT SPECIFICATIONS

Transport 4-head, helical scan
Tape Format S-VHS
Number of Audio Channels 8
A/D Conversion 16-bit, Delta-Sigma with 64 times oversampling, single convertor per channel
D/A 18-bit linear, single convertor per channel
Sample Rate 48kHz, variable from 40.4kHz to 50.8kHz
Frequency Response 20Hz to 20kHz
Dynamic Range 92dB from 20Hz to 20kHz (A wtd)
Distortion 0.009% THD plus noise at 1 kHz, 0.5dB below maximum output
Channel Crosstalk Better than -90dB ot 1 kHz
Wow ond Flutter Unmeasurable
Analogue Inputs -10dBv unbalonced or +4dBu balanced
Input Impedance 10kOhms balanced, 11kOhms unbalanced
Output Impedance 510 Ohms
Digital Connections EIAJ fibre optic jacks for input/output (Alesis 8-track format)
Sync 9-way D-connector sync in and out


Also featuring gear in this article



Previous Article in this issue

The Integration Game

Next article in this issue

Roger Eno


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 - Sep 1992

Gear in this article:

Digital Tape Deck (Multitrack) > Alesis > ADAT

Previous article in this issue:

> The Integration Game

Next article in this issue:

> Roger Eno


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