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...

CONTROL FUNCTIONS

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!

HOLD ON A SEC...

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.

APPRAISAL

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!

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