How to set ratio / knee on Dynamic Processor Audio Unit

Question

I struggle to understand all params of Apple's Dynamic Processor Audio Unit (kAudioUnitSubType_DynamicsProcessor).

In most Audio Editing apps (Garage Band etc), Compressor (aka Dynamic Processor) has a "ratio" and "knee" knob.

1. There is no 'knee' parameter in Apple's Dynamic Processor. There's a kDynamicsProcessorParam_ExpansionThreshold - is that what people are referring as knee?
2. There is a kDynamicsProcessorParam_ExpansionRatio but I noticed additional comment about it:

Note that the dynamics processor does not have fixed compression ratios. Instead, kDynamicsProcessorParam_HeadRoom adjusts the amount of compression. Lower kDynamicsProcessorParam_HeadRoom values results in higher compression. The compression ratio is automatically adjusted to not exceed kDynamicsProcessorParam_Threshold + kDynamicsProcessorParam_HeadRoom values.

I dont fully understand how to interpret headRoom. Is it ok if I just set expansionRatio or should I only set headRoom? Can you please help me understand which parameters should I adjust when the user is using a "Ratio" knob in UI?

Answer 1

For anyone struggling to understand kAudioUnitSubType_DynamicsProcessor or other kAudioUnitType_Effect types, you can play around with these in Garage Band by first enabling Audio Unit effects, then adding them to a track: