make SidechainCompressor class for sidechain compression

[DBraun]

https://en.wikipedia.org/wiki/Dynamic_range_compression#Side-chaining

https://github.com/DanielRudrich/SimpleCompressor

Check out DawDreamer’s AddProcessor.h class for how it sums multiple buffers together. The input buffer will have four channels. We need to calculate a sidechain amount from channels 2 and 3 to apply to channels 0 and 1.

sidechain = engine.make_sidechain_compressor_processor("my_bus")
sidechain.threshold = -6. # threshold in dB
sidechain.ratio = 2. # ratio
sidechain.attack = 2. # attack in milliseconds
sidechain.release = 50. # release in milliseconds

vocals = engine.make_playback_processor("vocals", vocals_data)
drums = engine.make_playback_processor("drums", drums_data)

graph = [
    (vocals, []),
    (drums, []),
    (sidechain, ["vocals", "drums"])
]

# the first input "vocals" would be the primary signal. Its volume would be attenuated
# more when "drums" is louder.

engine.load_graph(graph)
engine.render(10)

Update 2020-12-08: I've confirmed that certain VST effects plugins can do sidechain compression if you pass two inputs. The second input will determine how much to compress. You should also make sure the VST parameters are configured to enable the secondary bus.

Update 2021-05-09: The new FaustProcessor would be great for this.

[DBraun]

Closing because I think Faust does this reasonably well

DawDreamer/tests/test_faust_processor.py

Line 45 in 183f9e5

"""Have the volume of the drums attenuate the volume of the bass."""