How to use an audio compressor?

INTRODUCTION
This important device which is available as a separate hardware module and also embedded into the recording software like Sound Forge and Voyetra Orchestrator Plus, is an inescapable requirement to add the punch in the recorded audio data. An audio compressor alters the dynamics or difference between low and high volume within a audio signal. It is useful to take out the sharp peaks in the audio to maximise the volume going to the recorder without distortion at the time of recording. It can also be used as an effect to make voices sound bigger and fuller, increase loudness, and smoothen mixes. You hear ads on TV sound louder than the programs and a professional DJ's voices on the radio sound so loud. These are effects produced by compressors.

The first stage is an uncompressed audio sample, the second same sample compressed. It can be noticed that the high volumes in the sample decrease while the low volumes remain relatively unaltered.

In the last stage waveform is the compressed sample amplified using the output gain so the highest volume matches that of the uncompressed volume. This is achieved by normalizing. Low volumes are now much louder than the original.

COMMON FUNCTIONS

THRESHOLD
Volume level above which the compressor begins compressing. It is marked out in dB where 0dB is the distortion point and -6dB is half that volume level. Everything above the threshold will be brought down in volume.

RATIO
Ratio is how much the volume above the threshold will be brought down by. For example - at a setting of 1:1 ratio the signal will not be compressed, at 3:1 the volume will be brought partly down and at Infinity:1 the volume will be brought down to match the threshold.

ATTACK & RELEASE
Attack controls how fast the compressor reacts to signals passing over the threshold point. release is how quickly it stops reacting once the the signal passes under the threshold again. Some compressors have automatic Attack and Release detectors.

OUTPUT GAIN
Gain amplifies the signal after it has been compressed to compensate for the reduction in high volumes. The third waveform shown above has had the output gain turned up.

OVER EASY OR SOFT KNEE
This function smoothens out the threshold point and makes it around a particular threshold point rather than exactly at a particular point. it is best displayed on a compression graph to understand properly. Uncompressed signal would be displayed as a diagonal line through the graph (or 1:1 ratio). The first graph is Hard Knee compression, compression starts exactly at the Threshold point. The second graph is Soft Knee compression, compression starts gradually around the threshold point.

CONTOUR
With this function on the compressor is less sensitive to low frequencies. This is handy when compressing an entire mix.

LIMITER
This is another type of compressor with a infinity:1 ratio. The only control you have is over the threshold point. A limiter is built into many compressors to catch any extra volume peaks that might have escaped compression and to stop the signal from going over a particular volume and distorting.

GATE
The gate fades your audio down to silence if the volume passes under the gate threshold point. This is useful to remove hiss from the quiet sections of your audio.

STEREO COUPLE / DUAL MONO
With this button down the knobs on channel 1 control channel 2 as well. This is used when compressing a stereo signal.

SIDE CHAIN (SC)
This is used when the compressor is connected as a De-Esser. This would take a completely different set of notes to explain and does not effect normal operation.

Useful Compressor Settings

The compressor is a creative tool, there is no right or correct way to use it. The best way to work out what to do is to use your ears. Bearing this in mind here are some settings I have found useful.

Input from Microphone to Computer
Patch the compressor between the desk output and the computer input. Set the controls as follows :

GATE: OFF, THRESHOLD: -15dB, RATIO: 3:1, OVEREASY: ON, ATTACK: FAST, RELEASE: MEDIUM SLOW, LIMITER: +5DB, OUTPUT: +6dB.

Adjust the mic input volume on the desk so that gain reduction flashes between 4 and 8dB.

Big Radio Voice
GATE: OFF, THRESHOLD: -20dB, RATIO: 6:1, OVEREASY: ON, ATTACK: FAST, RELEASE:SLOW, LIMITER: +5dB, OUTPUT: +10dB.

Drums
GATE: -30dB, THRESHOLD: -15dB, RATIO: 4:1, OVEREASY: OFF, ATTACK: FAST, RELEASE:Medium, LIMITER: +5dB, OUTPUT: +10dB.

General Use
Those who use Sound Forge software , the compression ratio at 2:1, threshold at -10 db, attack 1ms and release 300 ms should give a considerable amount of punch.

by Major Venudhar Singh, Research Associate (SurAurSaaz Team)

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