Compressor
Canonical Definition
A compressor is a dynamics processor that reduces gain according to an input-output relationship when its detector indicates that the signal has entered a defined level region. Real behavior also depends on detector type, attack, hold, release, knee, linking, sidechain, and makeup gain.
Threshold And Ratio
For a simple hard-knee compressor with threshold T and ratio R:1, a steady input Δ dB above threshold produces approximately Δ/R dB of output increase above threshold.
| Input above threshold | 2:1 output increase | 4:1 output increase | 10:1 output increase |
|---|---|---|---|
| 4 dB | 2 dB | 1 dB | 0.4 dB |
| 8 dB | 4 dB | 2 dB | 0.8 dB |
| 12 dB | 6 dB | 3 dB | 1.2 dB |
The threshold is not an output ceiling. It is a region where the input-output slope changes.
Hard Knee And Soft Knee
A hard knee changes slope abruptly at the threshold. A soft knee transitions gradually over a level region. Soft knee can make the onset less abrupt, but it also means gain reduction begins below the nominal threshold and the exact curve must be inspected rather than inferred from one number.
Peak, RMS, And Combined Detection
- Peak-oriented: responds quickly to waveform peaks and is useful when short excursions matter.
- RMS or mean-square-oriented: follows average energy over a defined time behavior and may allow brief peaks through.
- Combined: uses both detector paths so peaks and sustained energy can influence gain differently.
These labels do not guarantee identical timing or calibration across products. Record the actual algorithm and controls.
Attack, Hold, And Release
Attack controls how gain reduction develops after the detector rises. Hold delays recovery. Release or decay controls how gain returns as the detector falls. Manufacturers may define these values by different percentages, dB rates, envelope constants, or program-dependent behavior, so identical numbers in two processors may not produce identical results.
- Too-slow attack can allow excessive overshoot.
- Too-fast attack can reshape low-frequency waveforms or remove desired transients.
- Too-fast release can create ripple, distortion, or pumping.
- Too-slow release can suppress following notes and reduce impact.
Stereo Linking
Independent left and right compression can move the stereo image when one side triggers more gain reduction. Linked operation can use the maximum, average, sum, or another combination of detector signals. The link rule must be documented because it changes both image stability and the amount of gain reduction.
Makeup Gain Is Still Gain
Makeup gain raises the compressed output. It can restore average level, but it also raises noise, average power, downstream voltage, and clipping risk. Recheck the DSP output, DAC, line stage, amplifier input, amplifier output, and loudspeaker after adding it.
Verification
- Use stepped levels to map the static transfer curve.
- Use bursts to measure attack overshoot and release recovery.
- Compare peak, RMS, and combined detector behavior.
- Test asymmetric stereo signals with linking on and off.
- Measure absolute output and distortion after makeup gain.
- Check average power and temperature during sustained compression.