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Take Two: Alesis XTc

Dave Lockwood tastes the aural delights of the MIDIVERB's upmarket stablemate - the XTc.

Affordable digital reverb could be said to have taken another giant step forward with the launch of the Alesis XT:c. Some very worthwhile improvements in specification and performance have been made to the California-based company's original low-cost XT model, but even more significantly for most of us, a price decrease has brought a full-function, high quality stereo unit more within reach than ever before. Dave Lockwood details his impressions...

Comparison with the market leaders in the budget digital reverb field, in particular the much respected Yamaha REV-7, is to be expected, but the chosen areas of omission on the Alesis XT:c are, I feel, more than adequately offset by the significantly large price difference and, somewhat surprisingly, the less expensive unit actually outspecifies its rivals in some parameters.

The XT:c offers four basic reverb programs, with a second version of each one, very different sounding although still perceptibly derived from the first, available via the Mod switch. These eight basic starting points can then be fine-tuned, or indeed quite radically altered, by the front panel rotary controls, Pre-Delay and Decay, in addition to high and low frequency filtering and damping.

Although this model is unable to memorise whole sets of control functions for instant recall, in practice, there hardly seem enough user-variable parameters to have warranted it. The XT:c has been made very easy to operate, with most of the hard work of reverb sound creation having been done for you in the carefully varied selection of programs available, which seem to cover all the most useful possibilities. Thus finding the desired effect becomes completely instinctive almost straightaway. Repetition of a favoured effect is always possible simply by jotting down the very small number of control settings, or indeed by remembering just the basic program number and then using your ears until it sounds right. No program memory means, obviously, no MIDI interface, so this unit can't be incorporated into a MIDI sequencer-based effects switching system, but at this price, who's going to complain?


The Pre-Delay facility, which is adjustable over an almost excessive range (0-200ms) is essential in the creation of natural sounding reverb effects. Delaying the onset of the reverb simulates the time taken for the first reflections to travel to the nearest reflecting surfaces and then back to the listening point. Adjustment of the Pre-Delay control can be clearly perceived as determining the apparent size of the structure causing the reverberant field and therefore, when natural simulations are being attempted, it is important to keep the pre-delay in scale with the other program parameters ie. a small room program with 200ms of pre-delay sounds rather odd! I found the maximum setting to be rather too long, even for the largest programs, but I'm sure it does have its uses for some of the less subtle special effects possible with this type of device, for wholly unnatural, deliberately synthetic ambience sounds are becoming an increasingly important part of the reverb vocabulary.

The range of the Decay time control is to some extent governed by the program selected, with the maximum decay time naturally being considerably longer for the larger room structures. With a small room program, the minimum decay produces virtually no reverb tail, just a slight 'hard' colouration, that is in fact a very realistic simulation. Extending the decay time of the small programs beyond anything that would be likely to occur naturally is possible however, and as usual with this type of device, it does begin to introduce a 'grainy', harsh quality to the sound, but the impression is so artificial any way as to be useful only for deliberately odd effects (although it does manage a fair simulation of a poor quality spring!). On the larger space programs, the whole range of the decay time control is more usable, and on the largest program of all, the maximum setting can produce a decay time that exceeds 15 seconds at low frequencies.

High frequency damping can be applied via a filter that acts at the digital stage as part of the software, in order to gradually roll off some of the high frequency content during the decay period. This makes for a much more natural sounding reverb, particularly on the large space programs, by simulating the high frequency absorption properties of air. Reverberant sound travelling over long distances needs to be perceived as having its spectral content modified with time. This, of course, only applies when a simulation of natural properties is being attempted, for the extra brightness of the undamped sound can equally be used to advantage. The renowned vocal enhancing qualities of a good plate reverb, with its peculiar characteristics of a fast build-up, combined with a relatively long, bright decay, can be easily synthesized in this way, and some very big and aggressive, modern drum sounds can also be created with such treatments.

A low frequency filter, sloping gently (6dB per octave) down from 200Hz, can be used to advantage to roll off some of the bottom-end decay on the larger space programs. Although this always sounds less convincingly real in isolation, when long reverb times are used in a mix, this low frequency information cannot normally be heard and, in practice, its removal always produces a cleaner, tighter sound with no apparent loss of ambience.

A complimentary high frequency rolloff (12dB per octave), different from the HF damping in that it does not vary with time, can be adjusted from the full bandwidth setting down to 3kHz. Some types of signal benefit considerably from the removal of a degree of their high frequency energy before entering the reverb stage, either to avoid emphasising any HF noise (in cases where that is an unavoidable component of the source) or to produce a smoother, warmer reverb with more emphasis on a lush middle and low frequency decay. The bursts of FM synthesis noise that characterise some of the DX7 voices are unfortunately made more prominent by some of the more 'wide open' reverb settings, but careful tuning of the roll-off facility seems to enable an excellent compromise to be achieved.

The specified bandwidth of the reverb signal at the maximum setting is an excellent 16kHz, which actually out-performs some of the more up-market units in this field, and is responsible for the remarkably crisp, clean sounding quality that characterises most of the Alesis XT:c programs. The high sampling rate (39kHz) results in a response of 30Hz to 16kHz, and a dynamic range typically in excess of 80dB. This level of specification is quite staggering in what must be considered a budget-priced unit, but these are not merely meaningless figures, for the exceptionally transparent quality of the performance of this model is immediately evident to your ears!


One facility I have not yet mentioned on the Alesis XT:c is the Infinite Hold switch, which loops a section of the decay and captures what the manual rather aptly describes as 'sonic mush'. It is a nice idea which undoubtedly could have its uses in effects-orientated material, but in practice I found it very difficult to capture anything that was not ruined by a very obvious repeating 'glitch'. Nevertheless, if you were really keen on the effect, I suppose you could always pre-record a take of a perfect 'hold' and then edit it onto the end of your track. This device has more than enough range for experimentation, and the limits really are only in the imagination of the user.

The smooth decays of Program 2 and its variant gave the best results, with the decay time set to maximum, and waiting until the sound reached a steady state before pressing the Hold switch and 'freezing' it.

One total omission on the Alesis XT:c is any form of remote bypass switching, or indeed any form of bypass switching at all. Used with a mixer in conventional fashion this, of course, presents no problem. But the XT:c is otherwise so well-equipped for stand-alone use, and the absence of a footswitch bypass could be restrictive for live use.

Reverb performance has to be one of the most subjective areas of all, with specifications often able to tell you little about the practical effectiveness of a unit. Nevertheless, the exceptional bandwidth of this unit on paper is undoubtedly responsible for the unusually crisp transparency of sound that characterises the Alesis XT:c. No amount of external processing can significantly improve a reverb device that doesn't have sufficient fidelity on signals of a transient nature, and to ears more used to the efforts of plates and springs, the openness and refined quality of a digital unit of this calibre can be a revelation that can alter your entire outlook on the use of reverb.


Although the controls seem to have been reduced toa minimum, I don't think this has been the cause of any loss of flexibility, and it certainly does result in a device that is very easy to use. When you want to subtly alter a sound you don't need to stop and think which of a hundred parameters to go for - with so few controls it is invariably obvious and gives the instant gratification that aids instinctive use. There may be no memories and no MIDI on this unit, but there are no sounds missing either!

The perennial problem with versatile devices like these is that you can only use one sound at once. Thus at mixdown, you can't have gated reverb on the drums and a cathedral acoustic for the backing vocals, except by recording effects onto tape, which is restrictive and eats up tracks at an alarming rate. However, with the Alesis XT:c being, in digital reverb terms, so inexpensive, it would almost be possible to buy two of these for the price of some competing models, solving that problem very neatly for those whose budget can stand it!

The Alesis XT:c is a real winner, offering exceptional performance at a modest price. The inevitable areas of compromise have been very well chosen, for I am sure that for most users the quality of the sound is the prime consideration in this type of unit. Such remarkable value for money is, in my experience, very rare for a fully professional spec unit - all I've got to work out now is how I can avoid having to give this one back!


The eight programs fall roughly into four specific group types, with an appropriate rise-time, diffusion density and range of decays for increasing sizes of structure. The final program and its modification offer variations on the non-linear decay sounds that are now so popular, particularly with drums and drum machines.

Program 1 simulates a relatively small room with a very fast rise-time commensurate with the close proximity of the reflecting surfaces, and with little variation in the decay times at different frequency bands. Excessive pre-delay or decay immediately sounds artificial, but at short decay times a very convincing 'room sound' is achieved, often without any apparent reverb tail, but somehow adding an extra dimension to the sound source.

An effective stereo synthesis from a mono signal is possible with this type of program, although some of the extra spatial illusion seems to be retained even in mono - the sound is simply bigger and more real. A digital drum machine can be made to sound quite convincingly like a real drum kit played in a small room, or a close-miked guitar amp, which can so often sound very 'small', can be opened out into something that much more closely resembles what the player is used to hearing standing in front of the amp.

This 'ambient' setting can breathe life into some otherwise rather sterile synth voices, enhancing their realism beyond recognition - particularly useful in cases where an obvious reverb treatment needs to be avoided. This type of processing, although not recognisable as conventional reverb, has an immense range of uses and is one of the most exciting possibilities offered by the versatility of this type of sophisticated digital device.

The modified Version of Program 1 very obviously simulates a much larger space, in that longer decay times and more pre-delay can be used without sounding unnatural. The rise-time is still very fast however, and this program was certainly my favourite for plate simulation, with the full 16kHz employed for bright vocal sounds, or a degree of roll-off applied to give a warmer plate quality on more transient material.

Program 2 and its modification offer a more intense set of initial reflections with, in comparison with the other 'natural' programs, less development in the smooth decay that follows. The whole range of decay times are all readily usable and capable of enriching or adding perspective to virtually any source. The sound is reminiscent of a dedicated echo chamber where the reflecting surfaces have been deliberately arranged to optimise the density and diffusion of the reverberant field. Short decays offer an ambient enhancement that is almost chorus-like in quality, whilst at longer settings, Program Mod 2, with its high initial density also makes quite a good plate, if all the filters are left out.

Program 3 and its derivative are my favourites among the natural reverb simulations possible on the Alesis XT:c. The basic Program 3, with between 50 and 100ms of pre-delay, a longish decay, and HF damping applied, is one of the most realistic large hall simulations I have ever heard from a device of this type. The reverb has a long build-time and is very diffuse, with some low frequency elements hanging around a long time after the majority of the sound has disappeared. Application of the LF filter makes this sound readily usable in a mix, for just about anything - without the filter, the sound can tend towards muddiness on all but the most sparse of mixes.

Mod 3 is an excellent representation of really huge structures, offering aircraft hanger or cathedral reverb at its most epic! Slow to build, but vastly spatial, with a thunderous hangover in the bass which, at the maximum setting, can take over 15 seconds to die away. This program offers the possibility of some quite stunning enhancements of orchestral or choral material and I had a lot of fun successfully reprocessing some of my Sony PCM-F1 location recordings, often made in unsatisfactory halls, into something with a far more appealing 'acoustic'.

One interesting facet of developments in the digital reverb field is that the immense controllability now offered by such devices does sometimes facilitate the overall treatment of complete mixed programme material, something that I certainly have never been able to achieve with any other sort of reverb process. Use of the LF cut and careful balancing of the HF content helps to avoid the reverb becoming over-prominent and separate from the track, just adding a subtle, pleasing decay to ends of vocal and instrumental lines. This technique can occasionally work wonders for those finished tracks where you feel sure that the mix is not wrong, yet somehow the piece just doesn't seem to work as you know it should.

Program 4 offers a classic 'gated reverb' effect, with the Decay control setting the gate 'hold time'. Gated reverb gets its name from the established technique of patching a pair of noise gates across the returns from the reverb source, and then keying the gates from the initial signal, thus allowing very intense bursts of reverb which cut off sharply, tracking the envelope of the original sound. With the vagaries of the average live drummer, setting up the gates for a consistent effect can be a real headache, but of course the digital equivalent never fails to trigger accurately.

The pre-delay facility remains operative on this program and can determine whether the effect sounds like gated room mics (with pre-delay) or gated plate (no pre-delay). On percussion the effect is preferable without pre-delay, so the reverb sits directly on top of the original sound and almost becomes an integral part of it, rather than being discernible as a separate ambient source. Leaving all the filters out on this program gives the biggest, most intense sound you could possibly want, particularly suited to electronic drums. As the effect is so inherently unnatural anyway, the total artificiality of this sound is unimportant, if not an asset in some contexts.

Gated reverb need not be confined just to drums however, particularly when the effect is as precisely controllable as this.

I tried a variety of synth and guitar sounds with much success, particularly on those voices that can be played in a percussive, stabbing manner - the fact that the sound is so recognisably 'contemporary' probably means that we will all hate it by this time next year. Still, if you want that instant 'modern sound' for your tracks, almost regardless of what you put through it, this is it!

The modified version of Program 4 is very much confined to the occasional special effects department, being a simulation of reverse reverb, with the sound increasing in intensity to reach its peak at the cut-off point. The Pre-Delay and Decay controls still function, with Decay seeming to determine the length of sample that is reversed. This program is not very easy to use in a conventional musical context, but the effect can be very dramatic if used sparingly, and it's certainly a lot easier than mucking about turning tapes over.

I found that the best results were achieved by removing the dry sound from the mix, thus hearing only the reversed reverb. The problem then becomes one of the timing of the sound, for if something like a cymbal crash is to be treated, the tape reversing technique allows the cymbal to be placed at the musically appropriate spot, then when the reverse reverb is applied, the effect precedes, and builds up to the original sound. If you try this with a digital unit, with the original sound removed from the mix, the reverse reverb build-up effect is there, but it all happens at the wrong time, starting where it should have finished. The solution is, of course, to get the initial sound in early, so that it happens where the reverb build-up needs to start, with the Decay control then adjusted to place the finish of the sequence at the precise musical spot required.

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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 - Mar 1986

Donated & scanned by: Bill Blackledge

Gear in this article:

Studio FX > Alesis > XTc

Gear Tags:

Digital FX

Review by Dave Lockwood

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> Take Two: MIDIVERB

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