The Controls of a compressor

The compressor is one of the most important tools in a studio engineer’s tool kit. Virtually every modern recording makes use of a compressor in some way. Compression is the reduction of the dynamic range. The dynamic range is the signal level difference between the loudest and quietest point of a signal.

Audio Signal before Compression:

Audio Signal After Compression:

Threshold

This sets the compression level, the compressor will only react once the signal level has passed this point. On the diagram this is represented by the vertical line

Threshold Diagram

Ratio

The amount of compression applied to the signal. A 5:1 ratio means that if a signal rises 5db above the threshold the output signal will be 1db giving a gain reduction of 4db. On the diagram this is represented by the blue, green and brown lines

Ratio Diagram

Attack

This controls how quickly the compressor reacts to signals that exceed the threshold

Attack Diagram

Release

This sets the time it takes for the compressor to stop applying gain reduction once the incoming signal has fallen back down below the threshold

ReleaseDiagram

Hard Knee / Soft Knee comparison

With hard knee compression any signal that reaches the threshold is immediately compressed. With soft knee compression, as the signal approaches the threshold the compressor begins to apply a small amount of gain reduction reaching its maximum level of compression after the threshold has been crossed.

Hard Knee / Soft Knee Diagram

Gain Reduction Metre

The amount of compression applied to the signal. A 5:1 ratio means that if a signal rises 5db above the threshold the output signal will be 1db giving a gain reduction of 4db. On the diagram this is represented by the blue, green and brown lines

Gain Reduction Metre Diagram

Make up Gain

This controls how quickly the compressor reacts to signals that exceed the threshold

Make up Gain Diagram

PPM / RMS

PPM stands for Peak Program Meter. This means the compressor responds to every single individual peak in level. This is important when the situation requires that not even the briefest of transient peak slips through uncompressed. RMS, which stands for Root Mean Square, acts more like the human ear in that it responds to more of an average level. Use RMS when you need a more gentle effect. On This particular compressor When the Analysis dial is left it is in PPM mode and full Right is RMS mode

PPM / RMS Diagram

Task / Experiment:

1. Download this vocal sample and load it into your sequencer (DAW – Digital Audio Workstation): Vocal Audio
2. Set up a Compressor as an Insert effect on the same channel as the audio sample
3. Set up the compressed to Peak Program Meter (PPM)
4. Set the Ratio controller to 5:1
5. Adjust the threshold until you have approximately -10db of gain reduction
6. Apply a Hard Knee
7. Increase the output gain to approximately 7db
8. set an Attack time of approximately 20ms
9. Set a short Release time approximately 5ms
10. Use the bypass button to compare the original with the compressed version