Serious Sidechain
Everyone knows how to direct a high-frequency boosted signal to the sidechain to perform a crude type of de-essing - now superseded by more sophisticated stand-alone de-essers such as the Drawmer MX50. But what about applying EQ to the sidechain in general, rather than this one specific application? If you have never done it, do it now. Parallel a signal so that it enters the normal input of the compressor, and at the same time is connected to the sidechain input via an equalizer. Now play some signals through this set-up. We all know that different compressors have different sounds, but this little trick allows the compressor that's in your rack right now to have an incredible range of sounds going far beyond the normal differences between models, when used in the standard configuration. You will find that the compressor becomes another type of EQ but, instead of simply cutting or boosting different frequencies, you allow different frequency bands to control the amount of compression applied. When you are in search of that elusive 'phat' sound and simple EQ and compression are not getting you there, EQ'ing the sidechain might just do it for you. In fact, I would go so far as to say that all serious compressors should have sidechain EQ built in. Once you have really tried it you won't want to do without it.
The sidechain can do more. Everyone knows that different compressors sound different, and that soft-knee types are more subtle than hard-knee, which go immediately from uncompressed to compressed at the exact threshold level rather than the gentle blending of the soft-knee type. The precise knee curve of a compressor is an important factor in its sound, but few compressors allow you to modify the knee curve in any way. So can it be done? Well of course it can, otherwise I wouldn't have mentioned it. Here's the deal: set up a sidechain configuration as above, but this time, instead of an equalizer, insert a distortion processor. A guitar effects unit such as the SansAmp GT2 would be fine (Figure 5). Remember that you are not going to hear any signal coming out of the sidechain, unless there is some internal crosstalk within the compressor, so the output signal isn't going to be distorted. What the GT2, or similar device will do is apply soft or hard clipping, which will bend the shape of the knee curve of your compressor. What effect this has depends on the compressor itself, on such factors as whether peak or RMS detection is used for example. However, the result will be that you will feel as though you have a totally different compressor in your rack. In fact, when different settings are used on the distortion box you will feel as though you have installed a whole rack full of different compressors.
Another option for the sidechain is to insert an advanced version of the signal to control the level of the signal itself. One of the enduring problems of compressors, and gates for that matter, is that they can only react to whatever information they receive, they can never anticipate what is going to happen and prepare for it. Well now they can. Using a digital tape or hard-disk multitrack it is commonly possible to delay individual tracks with respect to the others. Even if it isn't possible to advance a single track, you can always delay the rest, and perhaps make a delayed copy of the track you want to process. Armed with this you can connect the advanced version of the track to the sidechain - just 50 to 100 milliseconds should do - and the delayed version to the normal input. Now you will find that the compressor anticipates the amount of gain reduction required and transients in particular are rendered very much more realistically than doing things the normal way. In fact, you can do it the other way around too - delay the sidechain so that the compressor takes a moment to react. 'Why would you want to do this?', you might ask. The answer is that percussive sounds often benefit from a relatively slow attack, allowing the initial transient to come through unaltered before the 'body' of the sound is compressed. This is just a different way to do it, but this time with a little more control.
