[music-dsp] computing compressor automatic makeup gain (?)

Hannes Löschke hannes_loeschke at yahoo.de
Tue Sep 1 10:18:39 EDT 2009


Isn't the general aim of compression to get a louder signal?

The naive approach would be to adjust the gain in a way that 0dB input comes out at 0dB output by apllying the maximum gain reduction. In that case all the attacks would be pushed over 0dB into your (hopefully existing) headroom an possibly cause clipping.

Applying half the maximum gain reduction sounds like an attemt to prevent attacks from clipping for a reasonable range of attack times and signals. I don't know any TASCAM compresors, but maintaining percveived loudness doesn't sound like a sensible goal for automatic gain adjustments in compressors to me.

Taking attack time into account depends on the signal you use. Send an impulse through the compressor and any setting apart from 0ms schould essentially pass the impulse unchanged. Any automatic gain makeup would cause clipping in for an impulse. 

 
Hannes



----- Ursprüngliche Mail ----
Von: Tom Duffy <tduffy at tascam.com>
An: A discussion list for music-related DSP <music-dsp at music.columbia.edu>
Gesendet: Montag, den 31. August 2009, 18:27:02 Uhr
Betreff: Re: [music-dsp] computing compressor automatic makeup gain (?)

The way it is implemented in TASCAM products:

Calculate the amount of gain reduction would be applied to a 0dBFS
signal.
Halve that.
Add it.

e.g. At Threshold = -20dB, with 2:1 compression, a 0dB signal would
be reduced by 10dB. Therefore the auto makeup gain applied would be
5dB.
This method was found empirically to maintain perceived loudness
over a range of thresholds and ratios.
In your example, making up the entire gain to bring 0dB back to
where it was results in an overly "loud" signal.

There are more complicated things you could do by including the
attack and release times into the equation. A faster attack means
the loudness will be reduced more for the same threshold and
ratio settings, therefore more automatic gain is OK.
at Attack = 0ms, maybe a 100% makeup would sound OK.

Tom

-------- Original Message --------
Subject: [music-dsp] computing compressor automatic makeup gain (?)
From: Ross Bencina <rossb-lists at audiomulch.com>
To: A discussion list for music-related DSP <music-dsp at music.columbia.edu>
Date: 8/23/2009 5:35 AM

Hi Everyone

This question was asked here a few years back but without a clear answer.

I'm wondering whether there's a standard (or preferred) way to calculate 
the
amount of automatic makeup gain required in a compressor, given a 
particular
threshold/ratio/compression curve. I realise that the concept of automatic
makeup is problematic, but enough compressors offer the feature that there
must at least be known approaches to implementing it.

I assume that its done by calculating the amount of gain required to 
make an
input of  XdB have the same output level. I'm not sure what X should be
though, 0db?

E.g. With a hard-knee compressor, threshold at -10dB, ratio of 2:1,
compressing a 0dB input would give a -5dB output, and therefore the
automatic makeup gain should be +5dB. Does that sound correct/incorrect to
anyone?

I'd be interested to hear about alternative approaches or whether anyone
knows the actual reference levels for auto makeup gain in hardware
compressors.

Thanks!

Ross.

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