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What does a release time of 'auto' really do - compressor?

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What does a release time of 'auto' really do - compressor?

PostPosted: Sat Jan 19, 2019 5:31 pm
by DC-Choppah
I was doing some last listening to a mix that I thought was almost done, and I heard the piano track having some annoying rapid volume change. I traced it down to the Waves V-Comp that I was using for upward compression on the piano buss. The release time was set to 'auto'. On some long sustained chords you could see the GR needle sometimes wiggle rapidly back and forth as the chord decayed. Setting the release instead to 1.5 secs solved the problem. Now the compressor is doing what I wanted it to do. I am going for transparent upward compression to make the piano sound upfront as the details are brought forward.

I usually don't use the 'auto' setting but for some reason had tried it and forgot about it.

The Waves V-Comp manual just says:

Auto adjusts the release time according to the input signal to achieve a “nominal” release.

What ever it does did not work for me on the piano.

So I was wondering if anyone knows what these kind of auto release functions actually do? What are they trying to accomplish? I have seen it on a few other compressors as well.

Re: What does a release time of 'auto' really do - compressor?

PostPosted: Sat Jan 19, 2019 5:53 pm
by The Elf
It essentially prevents a relatively loud sound that has crossed back under the threshold from 'leaping' back to full tilt quickly from a short release time, and prevents something with a faster fade-off from being over-attenuated due to a longer release's 'overhang'.

Re: What does a release time of 'auto' really do - compressor?

PostPosted: Sat Jan 19, 2019 6:05 pm
by Hugh Robjohns
The exact implementation varies between different manufactures' designs, but in its simplest form the Auto-Release (or auto-recovery) setting switches between two different release time constants -- one very fast, the other very slow -- depending on the amount of gain reduction being applied. The aim is to give the best of both worlds: fast recovery for brief transients, and slow recovery for gentle level-smoothing.

So the idea is that for large amounts of gain reduction -- which would typically be triggered by a brief transient -- the recovery is very fast. This rapid change of level maintains the impression of a loud dynamic sound while still being dynamically controlled.

However, for smaller amounts of gain-reduction caused by more modest changes of source level, the recovery is much slower so that there is no audible change of noise floor (an effect often called noise-pumping). This is working much like an AGC (automatic gain control) system.

With auto-recovery selected, if you look at the gain reduction meter when the compressor is hit by some loud transient,nyou'll see it swing over to the appropriate amount of gain reduction, then fall back rapidly to about 6-4dB of gain reduction, and then slow right down as the rest gently ebbs away.

Of course, the auto mode is intended for use in normal downward compression applications. In parallel compression you're inherently operating with large amounts of gain reduction all the time. Consequently in the auto mode it will be using a very fast recovery time, as you noticed, which is not what you want! Hence the need to manually select a slow recovery for your specific application.

Re: What does a release time of 'auto' really do - compressor?

PostPosted: Sat Jan 19, 2019 7:40 pm
by blinddrew
Thanks Hugh (and DCC), I've wondered about that myself. :)