Despite the informative editorial content of previous issues of HSR, the functioning and use of the 'noise gate' is still widely misunderstood, and so it is necessary to shed some light on this murky subject before proceeding.

Gating Principles

In its simplest form, the noise gate may be considered as an electronic switch which shuts off the audio signal passing through it when it falls below a given volume level.

The purpose of such a device is to switch out low level, unwanted signals which may occur during gaps in the wanted programme material, and in a recording environment this could include background tape noise, crosstalk (sound leakage) and amplifier noise.

From this simplistic description, it can be appreciated that a noise gate only operates when no 'wanted' signal is present and can do absolutely nothing to remove noise that is present at the same time as the wanted signal.

A good example is the case of a vocal recording which may include a little spillover of sound from the vocalist's headphones, breath noises and perhaps the rustling of lyric sheets. The gate would, in this instance, be set to reject all of these low level sounds ensuring silence between words or phrases, but the second quality (including noise content) would be unaffected during the time the actual words are being sung.

Returning to our simplistic 'switch' model of a noise gate, it is soon apparent that there are serious limitations which can only be overcome by the addition of more controls. The gate has a 'threshold' and all sounds below this level are rejected; anything above it is allowed to pass through the circuitry unaffected.

If a sound such as a crash cymbal is processed by the noise gate, when struck, it will pass through, but as the cymbal sound dies away, the gate is likely to switch off, cutting short the natural decay of the cymbal and so producing an unnatural effect.

In order to overcome this serious shortcoming, most commercially available gates have a variable 'release' time, rather like the envelope shaper control on a synthesiser, which enables the gate to shut down smoothly and gradually, thus avoiding unpleasant sudden switching effects.

Designs with these two basic functions, Threshold and Release, are to be found in budget equipment (eg. Accessit) and foot-pedals, but the discerning studio engineer requires further sophistication.

Firstly, the time taken for the noise gate to open after the input signal has exceeded the threshold level is finite and does have some effect on the final perceived sound. If the gate opens too quickly, for example, it may cut in half way through an audio cycle, and this would produce harmonics which would be audible as a 'click'. In the other extreme, a long opening or 'Attack' time would rob the sound of its natural dynamics and so a suitable compromise must be sought.

There is no optimum attack time (also known as 'rise time') that will be correct for all types of material, and so the sound engineer will use his experience to set a suitable attack for the type of sound being processed, although many budget units compromise by having a preset attack time of around one millisecond.

Drawmer Dual Gate

The Drawmer DS201 is a sophisticated studio device which incorporates two independent gates in its 1U high (1¾") rack-mounting 19" case. In addition to the controls previously mentioned, the DS201 incorporates further features not found on its less elaborate counterparts.

A gate normally opens and closes depending on the loudness of the signal, but in some cases it is useful to use one signal to gate another, and this facility is provided in the form of an External Key Input.

The Key Input may be used to open the gate, or in its 'ducking' mode, used to reduce the level of one signal in the presence of another. This is obviously useful in the production of voice-overs where the background music can be made to drop in volume during sections of speech. (Much-used by Radio 1 DJs to ruin the introductions to most good records!)

Key Filter

Gates are often used to maintain separation between microphones when recording drums in a multi-mic situation and, because of the close proximity of the sound sources within a drum kit, spurious triggering of noise gates (from the hi-hat mic picking up the snare, for example) can become a real problem.

To improve performance under these adverse conditions, Drawmer have included a frequency selective filter which acts on the key source signal so that the gate may be made to respond only to a specific frequency band. This may be helpful for preventing a cymbal from triggering the gate on a tom-tom mic, for example.

Controls

The Key Source selects whether the gate is activated by the audio signal being processed or by an external key input. The Key Filter has low and high frequency controls so that the bandwidth of the sounds to which the gate will respond, may be precisely defined. A Key Listen position on the output switch enables the filtered key sound to be monitored for setting up purposes.

Threshold levels may be set such that the unit will trigger from signals of -54dB upwards, and a Function Select switch facilitates operation of the Gate or Duck mode, the attenuation in the 'gate off' mode being variable from 0 to -80dB.

In order to increase flexibility, the usual Attack (10 ms to 1 sec) and Decay (2 ms to 4 secs) time controls are augmented by a Hold time control which keeps the gate open for up to 2 seconds before the decay phase is initiated.

Linked operation may be selected whereby both gates respond to the control section on channel one, and this makes possible effects like stereo gating and ducking to be readily achieved.

A novel LED 'traffic light' display provides visual indication of operation, the red segment lighting when signal attenuation is taking place, green when the gate is open. The yellow LED indicates keying and follows the envelope of the audio switch control signal.

In Operation

A noise gate, however good, is only capable of providing useful results when used intelligently. Likewise, if used as a salvage tool to rescue an abysmally noisy track, the resulting noise pumping (continuous level changes) is liable to be less than aesthetically exquisite.

Used to improve the quality of well-recorded material or to improve separation in a well-thought out audio environment, a good noise gate is capable of excellent results.

Used conventionally, the Drawmer unit gave predictably fine results on voice and instruments, providing that the Threshold and time parameters were carefully optimised, and no side effects were then experienced.

The Key Filter controls do help the gate to ignore noise in a different frequency band to that of the signal being processed, and the Key Listen facility is a vast help in setting up suitable cut-off points, indeed these would be virtually impossible to adjust correctly without this feature.

The effectiveness of the Key Filter is such that the gate can be fine tuned to remove sibilance (excessive emphasis of 's' sounds), or the odd 'pop' from vocal recordings as well as the more conventional uses. The external key facility means that you can perform the old trick of gating the bass guitar with the bass drum to tighten up sloppy (or intoxicated) playing.

Construction

The electronic and mechanical assembly is conventional but extremely well-thought out, and all the components are common which is good news when it comes to servicing. PCBs are fibreglass whilst the front panel is firmly built from black anodised aluminium with tastefully screened lettering.

Conclusions

At a price of £275 (plus VAT), the Drawmer DS201 is not particularly cheap, but still very good value for money. The quoted noise performance of the device (-95 dBm) is such that it would not disgrace digital recordings and I could find no trace of background noise in normal use.

The Key Filter really does make the unit so much more versatile than a conventional (cheaper) noise gate, and Drawmer's ever-strengthening reputation in the field of professional audio is a testament to this. Pro studios such as Trevor Horn's Sarm West don't use these devices just because they are inexpensive, you know!

However, home studio users may find it hard to justify spending this amount of money on a pair of noise gates when they are struggling to repay the loan for their secondhand 4-track but, if you have any professional aspirations, or can afford to be choosy, you should really give the Drawmer DS201 a thorough audition. After all, they are British!

The jacksocketed DS201 sells for £316.25 and the XLR version from £345 (both including VAT). Further details from Drawmer Marketing & Sales, (Contact Details).