A compressor-limiter is a very useful piece of equipment to have available in a studio or as part of a PA rig. Not only does it let you run at levels higher than normal — handy both for giving increased 'punch' to a particular instrument or mix and also increasing the effective loudness of a power amplifier — but it can also help to reduce the risk of overloading following equipment. This overload protection minimises both distortion in tape recordings and also the risk of blowing up expensive loudspeaker systems.

The unit under inspection this month has an unusual split personality. Though manufactured in the UK by Audio & Design, it has two model designations depending on whether you buy it in the US or Europe. In the States it is known as the Gemini Compact, and is available through A & D's normal agencies, while in Europe it goes under the name of The Compliment, marketed by ITAM. Both units have identical specifications, the only difference being the front-panel lettering and the name on your receipt.

The Compliment/Gemini Compact is a two-channel unit, each identical channel of which can be used independently for limiting and compression, or linked for stereo operation by means of a front-panel switch. Controls per channel comprise an input gain control; three pushbuttons for selecting compression or limiting modes, a compression ratio of 1:5 or 3:1, and slow or fast attack time; a release time control with a continuously-variable 25ms to 3s range, plus an 'automatic' setting; an in/out pushbutton switch; and two indicator lamps — one red and the other orange. Input and output connectors mounted on the rear panel are two-pole, unbalanced jack sockets — a total of four, two for each channel — with preset controls mounted next to the output sockets to adjust the level of the signal leaving the unit.

Setting up the compressor-limiter was simplicity itself. I was using it with a Teac Tascam Model 5 mixer (described in detail on page 25) and, for test purposes, connected the line output of groups 1 and 2 to the unit's inputs. The outputs were then returned to the mixer and routed to groups 3 and 4. Thus I could monitor the level and dynamic range of the signals both being fed to and coming from the compressor-limiter. (Normally, of course, the unit would be connected between the insertion points of a pair of input channels or group outputs; in the latter instance after the line amps but before the metering.) Initial adjustments were made in the limit mode according to the manufacturer's handbook. For this fast attack and release time are selected. Then, with high-level signals being fed into the unit, the input gain control turned up until the orange indicator was fully on, and the red just illuminating.

What are these indicator lamps supposed to be telling you? In essence they are allowing both you and the compressor-limiter to agree on what are peak signals, and hence need to be reduced (ie limited). In the limit mode for input levels up to 0dBm the output accurately follows the dynamics of the input signals. Above this level, however, limiting begins and the output level remains fixed, no matter how high the input may peak (see fig. 1) Thus a reference point is established with all parameters, including the compression threshold, being related to this. The rear panel controls are then adjusted so that the level of input signals matches the compressed or limited signals. This allows comparisons to be made, by means of the in/out switch, between compressed and non-compressed signals at a similar peak level.

Once this reference has been established, you have the choice of compression or limiting, with two slopes, two attack times and a continuously adjustable release time. The choice of slope, attack and release time will depend very much on what you want to do to the input signals, and is greatly determined by ear. In fact, operation is pretty foolproof. As long as the red lamp is just lighting the output will remain constant for any ratio or time setting, while in the compression mode it shows that the limiting function has just kicked in and to watch your input level.

The fast attack time was sufficiently quick to catch most peaks without being so tight as to cause a 'gritty' sound. The slower attack setting was more suited to drum tracks, for example, which can sound decidedly nasty if treated to abrupt limiting. In such cases it is often better to let some of the transient through and saturate tape rather than try and stop it all and squash it in the process. But as with most equipment it is better to trust your ears, and adjust to suit your needs.

Even more critical can be the choice of release time, since it determines the moment-to-moment changes in gain of the signals and hence loudness. Speed up the release and with a tight slope you get most of the signal brought up to peak-level and the louder it sounds. You can only go so far, however, because very fast changes of gain can result in 'breathing' and 'pumping', a rhythmic rise and fall of background hiss and ambience in time with the peak signals being limited. The best way to avoid this, of course, is to make sure the recording is done in as dead an acoustic as possible — so there is minimum extraneous noise to be affected by the gain changes — and also to carry out any compression or limiting before going on to tape rather than later. If this cannot be arranged, then very passable results can be obtained by setting the control to 'auto', which selects a release time according to the amount of gain reduction in action. For fast gain changes a fast release time is automatically selected (to reduce breathing and pumping) and this then reverts to a longer time as gain changes become slower.

Choice of compression slope is very much dependent on what you want the final sounds to be. Most instruments, including bass, piano, and bass drum, can benefit from a tight slope to add more 'punch'. But once again, trust your ears and try combinations of slope, attack and release times until it sounds right.

All in all, a very useful little compressor-limiter that would find application in many a small recording studio or PA set-up. If I had any criticism of The Compliment/Gemini Compact it would be that the indicator lamps are rather difficult to read (I would have preferred conventional LEDs) but could be mastered with a little practice. Otherwise, ease of operation couldn't have been better.

Mel Lambert is a member of the editorial staff of Studio Sound.