Undoing Compressor

Hi all,

I have made some recordings of nature (Brazilian forest). Very beautiful, bat, alas, in my audio equipment I had selected the compressor feature
(ZOOM H2n, Comp3(drum)). Now, the recording sounds a bit awkward …

So, is there a possibility to ‘undo’ this compression, like with the help of an ‘anti-compressor’, which ‘compresses’ sound with negative compression values?
This should be possible, technically speaking, so probably this plug-in exists for Audacity?

If not in Audacity (my preferred audio tool), is an other audio tool known which has this feature?

Thanks a lot!!

The reverse of a compressor is an expander.
Strangely enough, “SC4” which the manual describes as a stereo compressor is in fact an expander (at least in my Audacity 2.0.6 alpha).
However, you must have the exact parameters that your device applied in the first place. Also, the Zoom could have applied automatic gain control, which is not quite the same.

Are you sure that you’re not confusing that with the “SE4”? Steve Harris' LADSPA Plugin Docs

Yes, I’m sure.
“SC4” is the only one appearing in the effect list.
Very strange.

Curious indeed.
Can your screen reader read the title bar of a window? If so, does the title bar of the effect GUI say “SC4”?

No, it says only “Dialog”, followed by a lot of “Static text” statements until it reaches the Rms/Peak slider.
However, it says “SC4 icon” if I change e.g. from Internet Explorer to the still opened dialog.
The only plug-in I have in the official folder is “sc4_1882.dll” (created 26.01.2014, modified 26.01.2010 ’ [?]) (104 kB).
Renaming it to *.dlb removes it from the effect list.
It really does expansion, which can easily be seen from the fact that the ratio is expressed as 1:N, instead of N:1.

I’ve just tested with audacity-win-r13103-2.0.6-alpha-20-may-14 on Windows XP (in Virtualbox).
The file name is: sc4_1882.dll
The ratio is expressed as 1:n but it is definitely a compressor.

You’re certainly right.
However, it does not act the way I thought it would.
I’ve tried to use it as a limiter, but it didn’t work.
I took the “15take1again.wav” from Search not working
The initial stats are:

Length of selection: 920.393 seconds.
40589312 samples at 44100 Hz.
Analysis of first 920.393 seconds:
(40589312 samples)

Mono Track.
Peak Level: 0.0 dBFS
Peak Positive: 0.0 dBFS
Peak Negative: -1.2 dBFS
DC offset: 0.0 %
RMS: -27.6 dBFS
RMS (A-weighted): -30.5 dBFS

The Sc4 settings are:
Rms/peak 1
Attack 10 ms, Release 40 ms
Threshold -10 dB, Ratio 1:20, Knee 3.0 MUG 0 dB.

The stats are now just the same as before.
I’ve probably used one or more settings that aren’t right.
The effect has surely done something because amplify shows “-0.0”.
(Update, with threshold -30 and ratio 1:10, the rms is suddenly 4 dB lower, whilst the peak is still 0.0)
There might bee another reason for this behaviour of course, such as a nan-value in the sound, the long pause at the beginning or something else.

However, I better leave this subject for now.

The SC4 does not have “lookahead”, so sudden peaks will often “slip through”.

Could you counteract that by reversing the audio and then using SC4, and re-reversing

Brian Davies uses a similar technique in ClickRepair to avoid false positives caused by sudden deleliberate peaks - cause by say a strike on a drum or a hand-clap.


Yes, but that somewhat defeats the point. If you want a compressor with lookahead, use a compressor with lookahead. Compressors without lookahead (or with lookahead turned off) can be used creatively to great effect (but not as a peak limiter). For example, if you have a bass guitar and you want to keep the attack, but compress the sustain, then you could use a compressor with a fairly slow attack and no lookahead, thus keeping the transient peak of the initial attack.