By John Moxey

A compressor is a device for automatically controlling the level of an audio signal. Specifically, a compressor reduces the volume of an audio signal when the volume exceeds a threshold set by the user. A limiter is a specialised form of compressor that effectively limits the input signal to the threshold level.

The Controls

Typical controls you will find on a compressor are:

1. Threshold

2. Ratio

3. Attack

4. Hold

5. Release

6. Gain

7. Bypass

Threshold

The threshold control is used to set the threshold level. This level is the level, in dB, above which compression will be applied to the input signal.

Ratio

The amount that the signal is attenuated by is dependent on the selected compression ratio. For example, if the compression ratio is 4:1 and the input signal exceeds the threshold level by 4 dB the output signal will only exceed the threshold by 1 dB. When the signal falls below the threshold no compression will be applied. If the compression ratio is set to 20:1 or more the compressor is being used as what is known as a limiter. This is because the signal is effectively being limited to the threshold level. Most compressors have a broad enough range of compression ratios that they can operate both as compressors and limiters.

Attack, Hold and Release

To make a signal sound more natural at the moment of applying or removing compression, i.e. the signal crosses the threshold level, most compressors provide the ability for the user to adjust the attack and release times. In some compressors these controls are automated. Using the attack and release times, the application of the effect can be made less blatant and the transition smoother.

Attack time: How long after the signal exceeds the threshold level that compression will be applied.

Release: How long after the signal drops below the threshold level before the gain/attenuation returns to uncompressed levels.

Hold: A minimum time before the release phase of compression will be entered.

If Attack and especially Release times are very short, the resulting rapid changes in gain create an effect called either "Pumping" or "Breathing". This simply means that the compression is an audible effect, rather than a subtler enhancement. If the attack and release times are too short bass frequencies can be distorted as the gain rapidly changes. A hold time of about 50ms will prevent this happening to audible frequencies.

Gain

Primarily, compressors reduce gain. To address this most compressors have an output, or "make up", gain control. This basically allows the peak level of the input signal to be maintained. The overall process of compressing the signal above the threshold and then boosting the overall signal amplitude reduces the difference in amplitude between the loudest and softest sound.


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