Sidechain And Dynamics Detector
Canonical Definition
A dynamics detector estimates a control level or envelope from a selected signal, while a sidechain is the signal path and processing that feeds that detector. The detector may monitor the controlled audio, an external signal, a filtered band, or a linked combination of channels.
Audio Path Versus Detector Path
audio input ───────────────→ time-varying gain → audio output
└→ sidechain filter → detector → gain computer ─┘A sidechain filter changes what the detector hears. It does not remove that frequency from the audible path unless the audio is also routed through the filter.
Detector Families
| Detector | What it follows | Typical consequence |
|---|---|---|
| Peak or absolute-value envelope | Short waveform excursions | Fast peak control, more sensitivity to waveform ripple |
| RMS or mean-square envelope | Energy averaged over an implementation-defined time | Smoother level control, possible transient pass-through |
| Peak plus RMS | Both fast peaks and sustained energy | Program-dependent compromise between the two |
| Filtered detector | Selected frequency weighting | Reduced response to excluded bands, but no removal from audio |
| External detector | Another signal or channel | Ducking, keyed control, or protection from a separate sensor |
RMS Is Not One Universal Algorithm
Products may use different windows, exponential smoothing, mean-square approximations, dB-domain time constants, hold and decay rules, or combined detectors. The label “RMS compressor” is not enough to predict attack or release. Measure the actual envelope and gain response.
Stereo And Multichannel Linking
- Independent: each channel controls its own gain; image can move.
- Linked maximum: the loudest detector controls the linked group; image is stable but one channel can dominate.
- Linked average or sum: shared gain follows a combined value; short single-channel peaks may be treated differently.
- Frequency or band link: selected bands or channels control a common gain rule.
Record whether the channels share the detector only, the gain reduction only, or both.
Sidechain Filtering
A high-pass sidechain can reduce broadband pumping from strong low-frequency energy. A band-pass detector can control one problem band. But energy outside the detector band can still clip or damage the audio path. Verify full-band output separately.
Tap Point Matters
- Pre-EQ detection ignores later EQ boost.
- Post-EQ detection responds to the final shaped spectrum.
- Pre-sum detection cannot see correlation-dependent sum growth.
- Post-sum detection can control the sum but may lose individual-channel context.
- External detector latency can make gain action early or late relative to the audio.
Verification Signals
- Use a transient and a sustained tone at the same nominal level.
- Test low-frequency and high-frequency signals separately.
- Test left-only, right-only, center-correlated and asymmetric stereo signals.
- Measure detector level, gain reduction and audio output together.
- Change the sidechain filter and confirm the audio spectrum itself did not change.
- For external detection, measure relative latency and polarity.