Where To Use Processors & Why (6 part article series - sos)

[supernova777]

6 parts:
compressors, gates, equalisers
1 http://www.soundonsound.com/sos/1997_articles/oct97/learningprocess1.html

noise, sibilance, contrast, listening fatigue
2 http://www.soundonsound.com/sos/1997_articles/nov97/processors2.html

space:the final frontier, keep your distance, bright ideas
3 http://www.soundonsound.com/sos/1997_articles/dec97/processorsdec.html

chorus, variations on a theme, going through a phase, strange flange
4 http://www.soundonsound.com/sos/jan98/articles/learnprocessors.htm

delays, completely tapped, too pure for comfort?, more psycho-acoustics, delaying tactics, delayed gratification
5 http://www.soundonsound.com/sos/feb98/articles/processors.html

distortion, noise
6 http://www.soundonsound.com/sos/mar98/articles/procesor.html

Processors are basically tools designed to do a job; if the job doesn't need doing, there's no point in using the tool. The idea of this article is to look at the more common types of signal processor used in the studio and to see where and how they might best be employed.

COMPRESSORS

Compressors remain one of the least understood, most abused processors in the recordist's armoury. The main job of a compressor is to reduce the dynamic range of a signal -- low-level signals can be brought up and high-level signals brought down -- but under what circumstances is this a good idea, and what, if any, are the trade-offs? One of the side effects of compression is that the sound being processed changes: at the very least, its envelope is modified as the compressor goes through its attack and release phases. Perhaps the most benign side effect is that of added 'warmth', but go too far and you can introduce audible pumping, which doesn't usually sound that great unless you need it as an effect.

Most vocalists need compression to keep their levels even; bringing down the peaks means that you can make the average signal level higher, and this results in a tighter, more confident sound that sits well in a mix. However, don't just rush in and apply maximum compression -- listen to the singer run through the song, watch the record level meters, and try to decide for yourself whether the voice needs a lot of control or simply a light touch. In any event, it's better to under-compress during recording, because you can always apply more compression when you come to mix, whereas over-compression is virtually impossible to reverse.

It's when you're processing vocals that the subtle differences between compressors come to light. Some models, such as the Focusrite Green I tried recently, manage to even up the level without making the process in any way obvious -- it just sounds as though you have a beautifully controlled vocalist -- whereas Joemeek units, for example, introduce a deliberate 'character' to the sound, which makes the vocal sound slightly larger than life. I can't say which is best, because different jobs need a different approach: the ideal situation would be to have at least one fairly transparent model and one 'warm' compressor in your rack. Some units attempt to do both, but few are entirely successful.

As a rule, if you want to make a transparent compressor produce more of an obvious effect, you can increase the ratio and at the same time shorten the release time. The shorter the release, the more obvious the gain pumping effect of the compressor. As you make these adjustments, watch the gain reduction meter and adjust the threshold control to get the amount of gain reduction you need. When you're recording, 5 or 6dB of gain reduction may be enough; if you're compressing a track that's been recorded dry, you might want to use as much as 12dB of gain reduction, or even more if you're after a strong effect. A tip here -- if you're not too sure about adjusting the compressor attack and release times, use a model with an Auto setting. If you don't have one, set the attack to its fastest, and the release time to between 300ms and half a second. Soft-knee compression usually provides the smoothest compression, but hard-knee models give tighter control and may be better for creating hard compression effects.

 
"Acoustic guitars sound smoother and have better sustain when compressed."

 

Before moving onto other areas of application for compressors, I need to mention noise. Compressors are not inherently noisy devices, but because they reduce the dynamic range of the signal being treated, some make-up gain is needed to bring the peak signal level back to where it originally was. In other words, although compressors really just turn down loud sounds, once you've adjusted the make-up gain control, the loud sounds are back where they were and the quieter sounds are much louder. The quietest of quiet sounds is noise, so if you're compressing to achieve 10dB of noise reduction, any noise that happens to be part of the signal will also be increased by 10dB for all input levels that are below the threshold. Periods of silence between words or phrases are most vulnerable, as it's here that the compressor gain is highest. As well as starting off with the cleanest signal you can, it might be wise to gate the signal immediately before it enters the compressor. Some compressors have built-in expander gates for this purpose: used properly, they really can make a difference.

Other sound sources you might want to compress include acoustic guitar, clean electric guitar, and bass guitar. Bass in particular tends to fluctuate in level by a considerable amount, so don't be afraid to hit it hard with the compressor. As well as keeping those peaks from clipping your recorder, compression will fatten up the tone and save weakly played notes from getting completely lost. Acoustic guitars sound smoother and have better sustain when compressed; with clean electric guitars, you can increase the compressor attack time slightly to get a nice 'ping' at the start of each note.

Compression can give mildly distorted guitars more sustain, but I've never seen much benefit from compressing heavily distorted guitars -- the mechanism of an overdriven amp or pedal essentially produces a limited output level anyway. All that will happen is that, during any pauses, the background hum and hiss will rise even further than usual.

Acoustic instruments tend to reveal compressor artifacts more obviously than electronic instruments, so for 'invisible' gain control, use your most transparent compressor, set to soft-knee if there's a choice, and a fairly low ratio --probably 4:1 or even less. Set the threshold to give you just as much gain reduction as you really need, and if there's any sign of pumping, increase the release time until it stops. A fast attack time will retain the natural attack of an instrument or voice; if you need a more defined attack for a percussive instrument, though, the compressor attack time can be slowed slightly.

When it comes to electronic sound sources, you can compress these in exactly the same way as you would their acoustic counterparts, but be aware that many of the sounds used in drum machines and synths are heavily compressed already. By all means experiment by adding more compression, but don't be surprised if you can't add much more punch to a sound.

Compressors should be switched to stereo link mode when they're treating stereo material, and I know some engineers and producers who like to compress all their finished stereo mixes, to push the average energy level even higher. Whether this is a good thing to do is almost entirely a matter of taste, but I strongly suggest that if you plan to compress your entire mix, you actually monitor the compressor output while mixing, rather than simply creating a stereo DAT tape then compressing it later. The reason for this is that compressors can change the way the balance of the mix sounds, and vocal levels in particular can seem to vary after global compression. If you compress as you mix, and monitor post-compression, at least you'll know exactly what you're getting.