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US Audio Gatex

Article from Sound On Sound, May 1986

With the increased use of direct-injected sound sources like drum machines on modern recordings, the role of the humble noise gate has gradually shifted from a purely corrective to a creative one. Sound engineer Dave Lockwood discovered even more creative applications when he tested this American four-channel expander/gate from US Audio.

The Gatex is an expander/gate of some sophistication, with four independent processing channels housed in a single rack-unit. Hardly something to get excited about? Well, the simplicity of its controls to some extent conceals the versatility of this unit, which Dave Lockwood discovered to have many more potential uses than were at first apparent.

Back in the days when all engineers had to know how to record a real drum kit, the noise gate was one of the most important devices to be found in the outboard racks of most studios. Switching in a bank of well set-up gates could transform a rather average multi-mic kit sound, usually degraded principally by crosstalk and spill, into the clean, tight, satisfyingly 'produced' drum sound that perhaps more than anything else defined the difference between the 'pro' studios and the rest.

However, the ever-increasing use of drum machines, synths, samplers and other non-microphone sources, has made separation rather less of a problem than it once was, and consequently the emphasis in the use of expander/gate techniques has altered somewhat from simple isolation to a wider use in noise reduction and more sophisticated signal management and enhancement, and it is in this area, as well as conventional 'hard gating', that the US Audio Gatex is particularly well-equipped.


The stylish steel and aluminium chassis has all its connectors on the rear panel, and all the controls, including the power switch, on the front panel, making permanent rack installation no problem. Each channel offers input, output, and 'key' input connections via jacks, with both the signal and key inputs being balanced but compatible with unbalanced sources. The output is unbalanced and capable of driving +21dB (ref 0.775V) into 600ohms or more, so interfacing with high-level 'pro' equipment or low-level format gear is equally feasible.

Front panel controls are identical for each channel, consisting of three rotary controls for Threshold, Range, and Release in addition to two miniature three-way switches which govern both the selection of gating or expansion modes and the source of the control signal.

A three LED vertical display is fitted, with a different colour for each stage, providing a very effective aid to setting the controls. Unfortunately, this display is only activated when the device is switched in, so pre-setting the controls before committing the unit into the signal path, as is possible with most compressors etc, is unfeasible unless you have separate send and return jacks on your patchbay, in which case it is possible to feed the input with the switch set to 'In', completing the circuit only when the return line is patched.

The Threshold setting is continuously variable over a 60dB range, from -40 to +20dB, and in practice this seemed wide enough for any application. Range determines the amount by which a fully attenuated signal is reduced, and can be adjusted from 0dB (no audible effect) to -80dB which is effectively infinite attenuation, or fully 'off'. When gating a prominent signal, low attenuation settings can sometimes reduce the unnaturally abrupt cut-off effect that invariably exposes the presence of the gate in the signal chain, whilst often still producing a worthwhile effect.

Release time, or the time taken for the device to achieve maximum attenuation after the level of the triggering signal falls back below the threshold setting, is continuously variable over the range 0.05s to 5s/20dB, which is more than sufficient to cope with anything from fine-tuning the decay of a tightly gated drum, to allowing an extended, smooth release fade on longer envelope material such as guitars, vocals or keyboards.

Unusually, the Gatex does not have Ratio or Slope controls, but offers instead a choice of three fixed-slope settings, designated 'Gate', 'Exp 1', and 'Exp 2', giving nominal slopes of 1:10, 1:2 and 2:3 respectively. In use, this proved no limitation, perhaps indicating that an optimum ratio has been accurately selected for each mode of operation. The expander attack times are programme-modified, but nominal figures of 100μs, 5ms and 10ms are specified for the three slope settings.

The overt simplicity of this line-up of controls belies the sophistication of this unit, and whilst in no way limiting the flexibility of the processing available, succeeds in making it very easy to use - surely an ideal combination?


The Gate mode is naturally best suited to signals of high transient content with short decay times, which in terms of real sources effectively means drums and percussion. Gating a multi-miked kit can sometimes prove a frustrating exercise, trying to find a narrow window of acceptability between failure to trigger at all when the drummer plays more lightly than usual, and spurious triggering from other drums played with extra vigour. The Gatex, having both attack and release to some extent programme-dependent, seems particularly forgiving in this respect, allowing for the subtleties of nuance, and just plain error, inherent in any human performance. Although the average user never really needs to know how or why, such invisible attributes make all the difference between a gate that 'works' and one that you finish up switching out because it creates as many problems as it solves.

The classic gating 'tricks' - tightening up flabby bass drums, shortening the decay of an underdamped snare, keeping hi-hat and cymbals out of the tom-tom mics - can all be performed to a very high standard with this device. The action of the gates is predictable and repeatable, and therefore the Gatex rapidly becomes a unit that you can turn to with confidence, rather than with a speculative attitude of 'let's see if this can do any good'.

Room or ambience mics controlled by gates triggered via their external keying facility, is a favoured technique for producing 'big' drum sounds, enabling, say, a tight bass drum and hi-hat combination to accompany a hugely ambient snare and toms. Accurate tracking of the control signal and reliable triggering is vital in this application, and budget gates rarely perform adequately; the Gatex was no disappointment here, however, performing up to the required standard in all respects.

On any gate with an external keying facility, highly selective 'frequency-conscious gating' (a la Drawmer) can also be achieved, simply by feeding an equalised version of the input signal into the 'Key Input', either emphasising the principal frequencies of the main source, or de-emphasising the range of the sound spill to be discriminated against.

Of course gating can be, and often is, employed with less transient sources, such as noisy guitar amps or effects devices, with the principal aim being to achieve a reduction in the perceived level of background noise. However, as no such device can be expected to distinguish between wanted and unwanted input on any basis other than simply absolute level, the gate must be set to remain open during the presence of useful signal, and shut down only when it ceases, thus removing the noise solely in the absence of signal. In theory, the 'masking' effect of the signal should render simultaneous noise inaudible, but sadly the result is more often simply noise modulation which, paradoxically, is more obtrusive than the same noise in a steady-state condition. It is in this area that the dedicated steep-slope gate loses out to more sophisticated devices that offer highly-controllable dynamic range expansion facilities, and it is in this department that the Gatex displays some of its best qualities.


Expansion, being the opposite of compression, means that, at a ratio of 1:2, every 1dB increase in input level above the threshold results in a 2dB increase in output, thus making loud signals louder relative to quiet ones. But, however good the theory, in practice this would be of limited value, for the system would rapidly run into distortion through lack of headroom; consequently, in a device such as this, the principle is applied in terms of 'downward expansion' only. Signals at, or above, the threshold level are passed at unity gain, whilst those below are proportionally attenuated; thus a 1dB drop in level at the input appears as a 2dB drop in level at the output. System headroom is maintained, but the audible effect is one of proportionally increased difference between high and low-level signal elements.

The less obtrusive action of expansion can be used in situations where an abrupt cut-off of a gate might be unacceptable, such as in live recording or PA work, allowing low-level sounds such as amplifier hiss, pickup buzz in guitars and stage rumble in live performance, to be gently attenuated without the process being overtly evident.

At extreme settings, perhaps a high-ish Threshold level and a high Range setting combined with a fast release, the apparent sustain of an instrument may be reduced, as the expander hastens the natural decay of notes. However, if extreme settings are required, the technique is not really suitable anyway - expansion tends to make subtle improvements rather than dramatic transformations.

Heavily compressed vocals can suffer from unacceptably high levels of headphone spill, especially with vocalists who insist on using their favourite 'open' headphones which they only half wear, and then need the foldback turned up twice as loud as anybody else! However, gating vocals during recording, rather than off-tape, has always been a bit risky - if the gate 'clips' a quiet word-ending you have no option but to re-record, and in my experience singers tend not to be very understanding in this situation.

Nevertheless, even with the Gatex patched after a tight 8:1 compressor, the controllable nature of the smooth 1:2 expansion that can be achieved in the 'Exp 1' setting proved able to significantly lower the noise floor to a far less obtrusive level, whilst consistently exerting an apparently error-free control over the lower portion of the remaining dynamic range. Tailed-off words were not abruptly cut, as with a gate setting, but faded to a lower level, which, in context, tends to sound entirely natural. It certainly is some advantage to obtain 'clean' tracks on-tape in this way, for not only do you avoid the problem on mixdown of having to reset the gate threshold every time you tweak the EQ, but the gates are also left free for other uses at this stage, such as controlling sequenced 'live' sources, or effects returns.

An interesting potential use of the expander stage of this device is for artificially controlling the decay time of a reverb system. Cheap plates, and spring units in particular, often tend to sound rather better at longer decay times (with most springs and dedicated natural 'echo chambers' this parameter is not actually variable at all), but by patching expanders across the return lines, set to high attenuation (60dB or more), and with a high Threshold setting, the Release controls become, in effect, reverb decay time controls. Greater reverb density can thus be achieved, using higher return levels, without the decay going on forever! It won't turn your GBS into an AMS but it certainly does help, and contributes a significant amount of noise reduction at the same time. With four independent processing channels to play with on the Gatex, there ought to be a couple spare for a useful task such as this.

The 'Exp 2' setting of the Gatex offers a very subtle 2:3 expansion ratio, which is best used in the most demanding application of all for this type of device. This is the first expander I have encountered that really can usefully process mixed programme material without serious unwanted side-effects. Accepting that no single-ended process can eliminate noise simultaneous with music, the usual shortcomings are 'pumping' - with obvious unnatural level shifts occurring; 'hunting' - or repeated switching during a long decay; and artificially shortened reverb tails. The programme-adjusted characteristics of the Gatex system help avoid some of these problems, and in combination with the pleasing transparency of the audio electronics, certainly make it possible to achieve a worthwhile improvement in a noisy signal source, whilst successfully concealing that processing is taking place - only by switching the device out at an appropriate point could evidence of its operation be displayed.

Subtle expansion has other uses beyond noise reduction, for it can be used to enhance the playing dynamics of instruments with unspectacular dynamic capabilities such as velocity-sensitive synths, or indeed occasionally, to actually recover 'lost' dynamics in a previously recorded part. I recall occasions during mixing when I have wished that I could 'uncompress' a heavily sustained guitar part a little, in order to put a bit more expression into it. The Gatex set to 'Exp 1' or 'Exp 2' certainly proved smooth, transparent and controllable enough to perform such tasks most effectively.

Use of the external keying facility also makes it possible to use the expander stage of the Gatex as an 'envelope follower'. With a fast Release, a high attenuation setting, and the Threshold adjusted to achieve unity gain on peak levels, the dynamics of the keying source can be imposed on the input signal. This is a favourite technique in 'electronic music' and can sound a bit cliched when alien envelopes are imposed on familiar sounds, but it can also have a practical use in achieving exceptionally tight multitracking, using the dynamics of the original part to process subsequent layers.


The US Audio Gatex looks, feels and indeed operates like a high-class professional unit. The standard of internal construction and layout is exemplary and should ensure the years of trouble-free operation that the professional user has a right to expect. My sole reservation would concern a minor construction detail of the generally rugged rack-mount casing, the top and bottom panels of which seem inadequately supported at one end and able to deflect excessively under moderate pressure.

With so much professional equipment now finding its way into home studios, manufacturers seem to be making efforts to make the operation of sophisticated processors such as this more readily accessible to the average user, and in this respect US Audio have certainly succeeded. The Gatex is never confusing, and rarely fails to deliver precisely what's required after some thought and a little speculative control-tweaking. Owner's Manuals are generally getting better too I feel, and the manufacturers are to be particularly commended for their manual's explanation of the principles involved in the use of this product, which would be invaluable to a first-time user. Professional performance is no longer the province solely of the professional, and this capable, versatile unit will surely gain acceptance at all levels of the industry.

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The Alan Parsons Project

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Talking MIDI

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

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Sound On Sound - May 1986

Donated by: Gavin Livingstone

Gear in this article:

Studio/Rack FX > US Audio > Gatex

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Feature by Dave Lockwood

Previous article in this issue:

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