what plugins do this in Cubase SX?
You may be confusing parallel compression with side chain compression. They are two differnent things – and you could even apply both ideas at once. Parallel compression is when you mult the track and mix a compressed version of the track with the uncompressed track. The idea is to add some beef to the sound, without losing the transients of the original track. The big thing in a DAW is to avoid phase shifts caused by plugin latency, or within the plugin algorithmn itself. You could use a compressor in an Effect group, and use Send and Return to apply the compressed sound to the original sound. Or, you can clone the track and apply the compressor as an Insert effect. In either case – listen for phase shifts, and compensate with track shifting or delays. If your plugin compressor has a Mix control, that’s the easiest/best way to apply parallel compression. Side chain compression is a problem in Cubase because I don’t think the VST format really allows for it. There are several plugins t