Transient handling algorithm?

[Danijel.Domazet]

Just a quick idea: it may be a good idea to use ratio of envelopes, not their difference. Produces slightly better rectification. But in analog world this is not practical, of course.

Thanks Aleksey. I think env difference will do it for now.

One thing is still puzzling me, the user control: user gets to set the desired dB value, but how do you go from the envelope difference, which depends on the input audio and attack/release times, to exact dB values that user wants??

I am more in favor of percentage, something like [-100%, +100%] instead of exact dB. Plus a make-up gain in case the sound becomes too loud or too quiet.

[Miles1981]

In my case, threshold is the difference in power between the two AR filters. That's what I decided to expose to the user. It may be trickier to understand at first, but transient are difficult to select. Of course, make-up gain and also dry/wet.
You could indeed use a ratio (-100% doesn't make sense, we are talking in power, so positive values and they can't get higher than 100% either as the fast AR will always have more power than the slower one).