Audio Compression : The Basics
Compression is the process of reducing the dynamic range between the loudest and quietest parts of an audio signal. This is done by boosting the quieter signals and attenuating the louder signals. The standard compression controls you have on any given compressor is usually:
- Threshold – the loudness of the signal has to be before compression is applied.
- Ratio – how much compression is applied. Example, if the compression ratio is set for 6:1, the input signal will have to cross the threshold by 6 dB for the output level to increase by 1dB.
- Attack – how quickly the compressor starts to work.
- Release – how soon after the signal dips below the threshold the compressor stops.
- Knee – sets how the compressor reacts to signals once the threshold is passed. Hard Knee settings mean it clamps the signal straight away, and Soft Knee means the compression kicks in more gently as the signal goes further past the threshold.
- Make-Up Gain – allows you to boost the compressed signal. as compression often attenuates the signal significantly.
- Output – allows you to boost or attenuate the level of the signal output from the compressor.
Compressors come in a few different types. These are used for different tasks and some sound far better in certain situations than others.
Voltage Controlled Amplifier compressors use an integrated circuitry to give accurate control. They are less coloured and suffer from fewer side effects like distortion, which make them ideal for many tasks.
Opto, meaning optical, describes light sensitive circuits that control the compression amount in opto compressors. They often react slower than other compressor types, but this can be desirable. The famed Teletronix LA2A is an optical compressor that many producers swear by for vocals and mix bus compression. The LA2A is also a ‘leveling amplifier’ — which means it is working nearly all the time, not just when a threshold is reached.
Field Effect Compressors use transistors to emulate a valve sound with more reliability, but with a higher signal to noise ratio. They are popular for vocals and great for drum compression.
Valve compressors work in one of the three ways described above, but use valves in the amplifier circuit to get that ‘creamy’ sound. The LA2A, mentioned above, is an opto compressor that uses valves.
Don’t be phased about all this semi analog talk all of the compression types mentioned above have plug-in emulated friends. A quick search will reveal how many compressor plug-ins there are out there in the desired type.
How Set Up a Compressor
1. Hardware or a plug-in compressor, setting it up works the same way. Insert the compressor on the channel you want to compress.
2. Adjust the threshold until the peaks in the signal are pushing over the threshold and triggering the compressor. Unless, of course, you really want to clamp something—like a live bass maybe—in this case it can work to make it push over the threshold all the time.
3.Set the Ratio to suit. Bass guitars sound good at 4:1, drums at 2:1, vocals also at 2:1 and electric guitars anywhere from 2:1 to 6:1.
4. The Ratio and Threshold work together. Adjust them together and see how they affect the output.
5. The attack and release controls shape how the compressor reacts. A fast attack would be useful for a rapper or anything that has sudden peaks early in the signal. Slower attack times suit mastering uses and buss compression.
6. The release control can really affect the sound of the compressor. Short release times cause the compressor to sound like it’s working hard, but long release times sound more natural.
7. Use the make-up gain and output control to sit the signal back into the mix without adding any unnecessary noise.
8. Setting the hard/soft knee depends on the material. Hard knee works better drums, bass and percussive stuff. Soft knee is a little more transparent and is better for vocals and sometimes guitar parts.