as_fbComp is a feedback compressor / limiter with side-chain capabilities.
It has two modes of operation:
- MONO – the Left input channel is used as audio input, while the Right input channel is used as SideChain input.
- STEREO – nothing exciting in this mode.
When in MONO mode, the SideChain “Source” (for the detector) can be balanced between the L and R channels (where L is the audio signal, and R is the SideChain input signal). When the Source knob is in the middle – both L and R are mixed equally.
Additionally, there is a simple LowShelf filter in the SideChain, which can be adjusted from -1 (where it will cancel and act as a HPF) to +4, and its frequency can be set from ~16 to ~160Hz.
- Output is saturated by a slightly non-symmetrical shaper which clips hard to 0dB.
- There is DC-Filtering on the input and output.
- Dry knob mixes (adds) the uncompressed signal to the output.
- Attack and Release times are calibrated at 6dB of gain reduction, but due to the nature of the feedback compression – the effective Attack and Release times will be about twice slower at 0dB and about twice faster at 12dB of gain reduction.
The compressor is 4x oversampled internally, and uses windowed-sinc upsampling and downsampling filters (so there is some latency, and it’s not reported).
MONO mode is lighter on the cpu because the Right input signal (which is used as SideChain input) is not upsampled, thus only half of the resampling FIRs are actually used. There is smoothing on the INPUT, OUTPUT, and DRY parameters to prevent steppy volume changes.