The Symetrix 544 houses four expander gates in one unit of rack space and so is economical both in terms of cost and space. Paul White explains the ins and outs of expanders and gates before tackling the model in question.

Tired of spec sheets? Baffled by jargon? Confused about the difference between an expander and a gate? Don't worry. Just take H&SR once a month after meals and feel the scales drop from your eyes. Just lie down on the couch please...

Expanders and gates, like compressors and limiters, belong to a family of signal processors who's function in life is to control dynamic range. Getting confused already? Let's take a look at dynamic range.

Any varying signal, such as music, must obviously have a maximum and a minimum level. In the case of music this manifests itself in terms of how loud the loudest bits sound relative to the quietest bits. A pop record for example might have a maximum level during a particularly loud drum beat and a minimum level tucked away between two inordinately quiet notes somewhere, and the difference between these two levels is termed the dynamic range. However, pieces of equipment have their own dynamic range too and this is the difference between the smallest signal they can handle without it getting swamped by the noise of the processors own workings, and the largest signal that can pass through without distorting. This difference is generally expressed in dBs which, being a measure of ratio, will bear no relationship to the actual input signal levels involved; it's only the ratio that counts. To take an example, if the highest signal level possible is 1000 times bigger than the lowest signal possible, then the ratio is 1000:1 which in fact translates as 60dB. It doesn't matter if the highest level is 1 volt or 20 volts... just so long as it's 1000 times bigger than the lowest level.

Signal processors that modify dynamic range can be very helpful in making the dynamic range of a piece of music fit into the dynamic range of the equipment used in the studio. For example, an unprocessed orchestra's dynamic range is somewhat greater than that of a conventional record pressing or recording tape (without noise reduction) and so a straight unprocessed recording would mean the quiet passages getting buried by hiss or the crescendos distorting. Some compression would help in this case.

Compression, Expansion and Limiting

The job of a compressor is to artificially reduce the difference between the low level parts of a signal and the high level parts, and it can do this by boosting only the quieter parts or by turning down the gain during loud sections (which amounts to the same in terms of the ratio between high and low). A limiter is just a high ratio compressor which can prevent a signal exceeding a specified level regardless of how big the input is.

Expanders and gates are the exact opposite of compressors and limiters. An expander takes an input signal and increases its dynamic range by making quiet sounds even quieter and loud ones even louder. As a limiter (high ratio compressor) absolutely prevents a signal from exceeding a threshold level, e gate (high ratio expander) completely shuts off a signal once it has fallen below a threshold level.

You may be a little mystified by this threshold level business, but don't be put off; it's quite straightforward. Most of the time it wouldn't be necessary to change the dynamic range of a signal. It's generally only necessary to engage the signal processor when the input gets dangerously high (distortion) or dangerously low (too much noise). This is where the threshold level control comes in. On a compressor, any signal higher than the threshold level is processed (compressed) and any signal below the threshold is passed through unchanged. Conversely, in the case of an expander, any signal falling below the threshold is treated, but signals above this level pass through unchanged. In practice, this means that because small signals below the threshold are subjected to gain reduction, the noise content of these low level signals will also be reduced by the same amount. The threshold is variable by the user via a front panel control, so that the processing can be made as unobtrusive as possible. An expander that only comes into play below a threshold is often known as a downward expander whereas one that operates over the whole dynamic range of the input is known as a linear expander.

So much then for clearing the mists of confusion. Now onto the piece of equipment in hand.

"The expander is quite excellent and its performance in the context of noise treatment is exceeded only by Symetrix's own 511..."

Overview

Four identical channels are neatly fitted into a solid but stylish 1U rack case and each channel may be switched to operate as either a gate or as an expander. Other switchable functions are In/Out (or bypass mode) and Int/Ext which permits the side-chain circuitry to be driven from an external source or via an equaliser rather than by the direct input signal itself. This gives the user the opportunity to use one signal to gate or expand another signal.

The essential controls of Attack, Release and Threshold are all present and operate similarly in either the gating or expanding mode. The Ratio/Range control sets the expansion ratio when the machine is in the expand mode and sets the attenuation level in gating mode. Expansion ratio is a measure of how much the dynamic range is adjusted once the signal has fallen below the threshold and, like a compressor, this is expressed as the ratio of the level change in the input signal to the level change in the output signal. A ratio of 1:1 would mean that a 1 dB change in the input signal would cause a 1 dB change in the output signal; so there would be no difference. An expander with a 1:10 range on the other hand would cause the output level to fall by 10dB for only 1 dB of input change (which is very severe). In practice a ratio of around 1:2 is usually appropriate for reducing noise during quiet passages.

In the gating mode, the Range control allows the user to attenuate a signal by a fixed amount once it has fallen below the threshold rather than simply turning it off altogether, and this is again handy both for corrective and creative use. A 5-section LED ladder display shows the amount of gain reduction taking place in each channel.

Attack and Release

Because expansion is not a simple on/off affair, we can't express the attack and release times in terms of seconds, as we would with a compressor or a simple gate, so we have to talk in terms of dBs per second. However, don't let this put you off because you would normally set up these controls by ear as you would with a compressor. The attack time (or more correctly, attack rate), is stated as variable between 120dB/mS, (which is very fast indeed) to 0.25dBs/mS which would be useful when treating individual tracks that have slow attack sounds such as synthesised strings.

Release is from 300dB/Sec to 5dB/Sec, the latter being fairly slow. This is vital with gently decaying sounds or sounds with reverb as it prevents the decay from being unnaturally cut off; instead it gives the effect of a gentle fade-out after each note which is generally quite unobtrusive.

Ins and Outs

All the connectors are to be found on the rear panel and are in the form of jacks: balanced inputs (45kΩ) and unbalanced outputs (100Ω); the inserts for the sidechain are on stereo jacks with the send being the ring and the return to the 544 being the tip. The mains lead is captive and conforms to British colour coding standards, unlike mains leads on some American equipment from a manufacturer who should remain nameless (though it is of course Ashly). If incidentally you do come across American colour coding, which is black, white and green, it's easy to remember which is which; green is earth like our system but live is black, which is the colour you go if you touch it. The remaining white wire is of course neutral.

Within the unit, a DBX1750 voltage controlled amplifier acts as the variable gain element and allows for very low noise, low distortion performance.

Tricks of the Trade

The most obvious and probably most popular use of an expander/gate is to reduce the effect of noise already present in a signal, whether it be electrical hiss, or unwanted leakage from other sounds. A gate or expander can't reduce the amount of noise present in a signal but what it can do is to turn that signal (and any noise it contains) down, or even completely off, during short gaps in the music. During louder passages, the music itself will mask the noise unless the problem is really acute. Drums are a good candidate for straight gating as they have easily recognisable 'on' and 'off' periods, but a complete mix isn't going to improve much when processed by a simple gate as there are unlikely to be any extremely quiet passages. In this case the gate will open at the beginning of the song and will stay open until the end. An expander is a bit more useful on a complex mix as it doesn't switch off low level signals but just reduces the level of any signal that falls below the threshold, and the lower the level falls below the threshold, the more the expander turns down the gain. If you overdo this treatment it is still intrusive, but a properly set expander can improve on the apparent noise content of a piece of music with minimal side effects. So in this context, the expander fulfils a similar role to that of a gate but its operation is less noticeable and can be beneficial even when there are no gaps in the music. The best way to set up the expander or gate is to set the threshold just above the background noise and then tailor the attack and release times to the type of input. For a complete mix, a fast attack and slowish delay is a good starting point but individual tape tracks need to be treated according to their content.

Another application is to kill unwanted ring in a sound, such as an underdamped drum. A gate would cut out the decay of the drum altogether or at best apply a fixed decay envelope to it, which is fine in some applications, but you may want to simply reduce the ring without losing it all together. An expander will do this far more effectively than a gate and can be considered in this context as the electronic equivalent of damping. It's also likely to be more tolerant of input level fluctuations.

By setting the threshold control to its maximum value, the expansion can be applied to signals of all levels (linear expansion) and, by choosing a fairly low ratio, it can be used to widen the dynamic range of a piece of music to enhance the contrast between high and low level sounds.

"...this one is almost cheap, especially when you consider that it has both gate and expander options."

Side-chain

You may remember that when we looked at compressors, we discussed the possibility of inserting equalisers into the side-chain, to cause the compressor to respond more to some frequencies than to others, for example for de-essing. Well, we can do the same with gates and expanders and the standard example of this is the drum kit, where we want to clean up the snare mic output, but get false triggering from the hi-hat. By putting an equaliser into the side chain, we can monitor the filter output and tune into the snare drum sound whilst doing our best to reject the hi-hat. This will then modify the expander/gate's action enabling it to ignore the hi-hat and only accept the snare. Of course the output from the expander/gate will not itself be affected by the equaliser.

The other popular use is to control one signal by another. Triggering the sidechain from a bass drum and feeding a bass guitar through the main input gives a bass guitar note only when there is a bass drum beat which gives a tight sounding result. Conversely you could set the gate attenuation to let some bass guitar through all the time but, when a bass drum beat arrived, the bass guitar would be accented. Using the same principle, a snare drum track on tape can be used to trigger the gate so that a suitable continuous sound from a synth can be chopped up to reinforce or replace the snare drum sound when mixing. There are endless variations on this theme such as chopping up a synth line to form a tight, percussive bass part, but once you understand how it works and gain a little experience, the possibilities are endless... just as this article will be if I don't get on with the story.

Performance

Symetrix have always been good at building expanders and this one is no exception. I'd use this device in its gate mode for cleaning up individual drum sounds, creating special chopping effects or for gating reverb, but for most applications including noise taming, I'd go straight to the expander mode. There's nothing at all at fault with the noise gate, though sadly there is no hold control which makes reverb gating a bit tricky, (but then many digital reverbs have gated effects built-in now); it works quite conventionally, the LED display giving a good indication of what is happening.

The expander is quite excellent and its performance in the context of noise treatment is exceeded only by Symetrix's own 511 which combines the action of a downward expander and a dynamic filter to give one of the most versatile, single-ended noise reduction systems currently available. It is however, easy to set too high a ratio and then the expander acts almost like a gate shutting things right down during quiet passages, but it doesn't take long to gain the experience needed to apply the minimum treatment necessary to obtain the desired result.

Conclusions

Symetrix equipment has always been good but the pound/dollar relationship has always made it on the expensive side in the U K. However, on a 'gates per pound' basis, this one is almost cheap, especially when you consider that it has both gate and expander options. With more analogue recordings being digitally mastered or even destined for CD release, it's common practice to gate or expand virtually every multitrack output when mixing, especially ones containing guitar solo's or vocals, and up-market desks like SSL give you a gate on every channel as standard. I don't suppose many of our readers are quite in this league, but with the 544, it's at least economically viable to gate the tracks that really need it.

The Symetrix 544 costs £488.25

Further details are available from: Sound Technology, (Contact Details).