sidechain

Sidechain is a specific kind of signal-processing technique: it processes signals by routing one signal to control the processing of another. A competent engineer would call sidechaining a form of signal processing. The nearest kind is signal-processing — broader categories like 'audio technique' or 'method' would be leaps when signal-processing already exists and captures the technical essence.

Sidechain routing requires a dynamics processor with a sidechain input to operate; remove the compressor and the sidechain control path has no effector to modulate. The sidechain technique is operationally dependent on a compressor (or similar) to perform gain reduction in response to the external trigger.

De-esser operates by detecting sibilant frequencies in one signal and using that detection to trigger gain reduction via a sidechain path. Remove the sidechain mechanism and the de-esser loses its core architecture — it becomes just a dynamic range compressor, not a de-esser. The sidechain is the operational mechanism that distinguishes de-essing from ordinary compression.

Removal test (Laws 2b/8b): ducking's defining operation is reducing one signal's gain in RESPONSE to the presence of another. That 'another' must reach the detector through a sidechain feed — a control path routed from a source different from the processed signal. Remove the sidechain and ducking cannot trigger: it has no way to know the foreground signal is present, and collapses to ordinary static gain. The mechanism it depends on is the routing (sidechain), not the compressor it may modulate — consistent with sidechain standing as the routing technique and ducking as the dynamics purpose built on it (Law 8d territory, not SERVES: this is operational dependency, not for-whose-sake).