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LA Audio 4G

4-channel Frequency Conscious Noise Gate By Paul White
Published April 1994

Some ingenious filtering and noise reduction possibilities combine with four channels of frequency‑conscious noise gating to make this new studio processor more than meets the eye. Paul White investigates.

Once again, LA Audio have managed to add a new twist to an old idea; in the case of the 4G, they've expanded the standard gate concept with a filter section which can be deployed in three different ways — to provide frequency‑conscious gating, a straight gate plus a completely independent filter, or a single ended noise reduction system.

I've commented in previous articles that the side‑chain filters on Drawmer gates also make very powerful equalisers, due to their sharp cutoff characteristics. The designers at LA Audio obviously thought along the same lines, because they've included a switch that lets you separate the filter from the gate, allowing it to be used as an independent equaliser. This leaves the gate operating as a straight gate which, for many applications, is quite adequate. I find this kind of filter especially useful for cleaning up noisy electric guitar pads or for taking the digital hiss out of a mellow synth pad. They're also quite effective at removing hum and boom from the bottom end of the spectrum.

Single‑ended noise reduction is normally associated with dynamic filters which vary their characteristics according to the signal's level and high frequency content. In the case of the 4G, however, the task is performed by a simple but ingenious re‑deployment of the filter. Put simply, the filter works in tandem with the gate's Range control so that as the gate closes, the unprocessed sound crossfades into a filtered version of the sound at a rate set by the gate Release control. If the filter is set to remove frequencies below 100Hz and above 3kHz or thereabouts, hum and hiss will be attenuated whenever the signal level falls below the gate threshold, but without actually 'gating off' the mid‑range part of the signal. Though not as flexible as a top‑end, dedicated dynamic filtering system, the results are nevertheless quite dramatic.

The 4G

There's little that's unusual in either the appearance or construction of the 4G. The mains powered, 1U processor houses four sets of identical channel controls with Link buttons between adjacent channels to permit stereo, 3‑channel or 4‑channel linked operation. The inputs and outputs are on jacks, wired for either balanced or unbalanced operation, with sufficient range to accommodate both ‑10dBv and +4dBu operating levels without the need for switching. A Key input jack permits triggering from an external source for rhythmic chopping effects and similar processing tricks. Access to the filter is via a stereo jack wired Ring‑Filter In, Tip‑Filter Out. The front panel Filter switch separates the filter from the gate for independent use.


The gate has fewer controls than most dual‑channel units, the traditional attack control being replaced by a 'Fast/Auto Attack' switch. The Fast setting is used for all percussive sounds, while the Auto setting automatically copes with anything else. A conventional Release control is provided, with a range of 5mS to 4S, and the Range control is variable from 0 to 90dB. In normal operation, the Range control sets how much signal passes through the gate once it has closed, and in SNR mode, it simply sets how much filtered signal passes through once the gate has closed. A standard threshold control operates over the range ‑60dB to infinity, with a green status LED to indicate that the gate is open.

The filter section comprises two independent, variable‑frequency shelving filters, each with a 12dB per octave response. The Lo filter (25Hz‑4kHz) has a high‑pass response; the Hi filter (250Hz‑30kHz) has a low‑pass response which appears to be identical to the way in which Drawmer filters operate.

Moving onto the switches, 'Filter' moves the filter out of the gate side chain and connects it to the rear panel jack. This is a good feature; it would have been possible for the manufacturers to save money by using a switched jack socket and missing off the switch, but that would have made it very awkward for users who want to leave the Filter input connected to a patchbay. Full marks for thoughtfulness here.

When 'Ex Key' is selected and a different line‑level signal connected to the External Key input, the signal being fed into the channel may be gated by an external source, such as a drum machine, to produce rhythmic gating. 'Listen' routes the filter output to the gate's audio output so that the filter's effect can be monitored. On the review model, the Listen function only worked if the signal exceeded the gate threshold — which is not what you usually expect — as though the gate's normal output was being monitored through the filter. I don't know if this is a necessary compromise associated with the rather flexible filter switching, but as long as you're aware of it, it doesn't present too much of a problem.

'SNR' brings the single‑ended noise reduction into play; the Range control still sets the level of audio that passes when the gate is closed, but now the filter settings determine how much high and low frequency information is removed when the input signal falls below the threshold. With the Range control set fully clockwise, the gate behaves normally, but when it is set fully anti‑clockwise, the filter also processes the audio signal when the gate closes. By changing the Range control setting, you can mix conventional gating with SNR to achieve the best possible results. For example, setting the range control to around 6dB means that when the gate closes, the remaining signal is both filtered and attenuated by 6dB.

In Use

The 4G's gate, when used conventionally, works both positively and predictably — both hold and hysteresis are included to prevent the gate chattering when short release times are set. On the review sample, the hysteresis was quite noticeable, which means the gate triggers at one level but the signal has to fall several dBs below that level before the gate will think about turning off again. I'm told that this has been remedied for production models.

By changing the Range control setting, you can mix conventional gating with SNR to achieve the best possible results.

Selecting fast Attack proved ideal for drums and percussion — the attack is very fast indeed — but processing more 'relaxed' signals using the fast setting tends to result, as expected, in audible clicks as the gate opens. Switching to Auto mode completely cures this problem, and the programme‑dependent circuitry seems to cope with anything you throw at it.

The filters are very effective at helping 'focus' in on a sound, but equally, they make useful stand‑alone equalisers when used separately. Any instrument with a naturally limited bandwidth can be cleaned up simply by setting the two filters to 'bracket' its natural range. For example, a typical electric guitar produces little below 100Hz and little above 3.5kHz. By setting the filters to these values, much of the hiss and hum is removed without compromising the basic sound. Similarly, they are quite effective in de‑booming acoustic guitar recordings or mellowing out an over‑bright synth pad.

SNR mode is much more effective in cleaning up a sound than using the filters alone, since when the signal is above the threshold set by the user, no filtering takes place at all. As the signal falls below the threshold, the effect of the filtering effectively fades in at the rate set by the Release control. Because most signals are at their brightest when they're at their loudest, this method of noise reduction can be quite unobtrusive, and even the starts and fades of complete mixes can be tidied up with a high degree of success.


Despite making relatively few compromises, LA Audio have produced a quad gate at an attractive price; they've also added a couple of genuinely useful and novel features. As a straightforward gate, the 4G is both competent and simple to use, and though the idea of using a gate filter as an EQ isn't new, LA Audio is the first company I know of to have incorporated switching with that possibility specifically in mind.

The SNR mode, though conceptually simple, is surprisingly useful, easy to set up, and can be used both on separate tracks and on stereo mixes — though separate tracks are always easier to treat. Unlike most other types of single‑ended noise reduction, the approach taken here allows low‑frequency hum to be filtered out as well as high‑frequency hiss, which is a great bonus when dealing with troublesome sound sources. As ever, you can't expect SNR to perform miracles, but it will help turn a reasonable recording or sound source into noticeably quieter one.

Though there are any number of gates on the market competing for your money, the unique selling points of this particular model mean there is no direct competition. Given the sensible asking price, I have to concede that LA Audio have once again proved themselves very able jacks of all trades.

Frequency Conscious Gating

Most of you will be familiar with the concept of frequency‑conscious gating, where a side‑chain filter is used to make the triggering of the gate frequency selective. This facility, first introduced by Drawmer, is invaluable in helping to prevent false triggering where two sound sources are close in level but different in terms of their frequency content. For example, cymbals and drums are at the opposite end of the audio spectrum from each other, so a frequency‑conscious gate is useful in preventing a cymbal crash from opening a nearby drum mic.


  • Straightforward operation.
  • Useful SNR and Filter modes.


  • Unorthodox 'Listen' implementation.


A good all‑rounder which will be especially attractive to those who have limited outboard signal processing.