Talk:Compressor

Steve, is this correct?

Compressor settings:

• Threshold -12 dB
• Ratio 4:1
• Noise Floor -42 dB

• Blue graph is IO characteristic for "compress based on peaks" OFF and "make-up gain" OFF
• Red graph is IO characteristic for "compress based on peaks" ON and "make-up gain" OFF or ON
• Green graph is IO characteristic for "compress based on peaks" OFF and "make-up gain" ON

-- Bill

The characteristics when "Make up gain" is on cannot really be shown on this type of diagram. Unlike (most) hardware compressors, the "make-up" gain is not a fixed amount to compensate for the lower output, but is a Normailize function. The amount of gain depends on the peak level of the audio after compressing. The solid red line is approximately the effect of normalizing (make-up gain to 0dB) if the uncompressed signal had a peak of 0dB.

In the diagram below there are several alternative "post-make-up gain" plots drawn. The lower the peak amplitude after compression, the more make up gain is applied. The make-up gain would be described better as "Normalize to 0dB after compression".

This also applies to "Make-up gain for 0 dB after compressing" when using "Compress based on Peaks".

When Compress based on Peaks is selected, the threshold will appear to be at a different level, because it is working with peak values rather than RMS values. It's a bit tricky to show in this kind of diagram because we're mixing peak and rms measurements.

When compression is based on RMS, a threshold setting of -12dB will begin compression at an RMS value of -12dB (roughly -9dB peak). When compression is based on Peak, a threhold setting of -12dB will begin compression at a PEAK value of -12dB.

I'll add some better diagrams for upward vs. downward compression shortly.--Steve the Fiddle 17:18, 20 November 2009 (UTC)

Then again, perhaps the above diagram is sufficient if we take out the (incorrect) noise floor part?

Perhaps a diagram indicating Peak level vs. RMS level?

--Steve the Fiddle 18:03, 21 November 2009 (UTC)

Bill: Steve, here's a graph of the actual IO characteristic using Compress based on peaks, threshold = -12 dB, no make-up gain, attack and decay at minimum, noise floor at -80 dB. The red curve is for a chirp from 0 to 1, the green curve for a chirp from 0 to -0.707 (-3 dB). Note that the knee for the first case is at -9 dB, and moves left for the second case. Other than that the curves are very similar.

---

Technical details and input-output graphs

When "Compress based on peaks" is not checked

The following image shows the input-output characteristic of the Compressor effect when "Compress based on peaks" is not checked.

• The light blue line shows the characteristic when "Make-up gain for 0 dB after compressing" is not checked.

In this mode the Compressor effect behaves like a traditional hardware compressor.

• The dark blue line shows the characteristic when "Make-up gain for 0 dB after compressing" is checked, and corresponds to the line shown on the graph in the Compressor dialog.

In this mode the Compressor effect behaves like a traditional hardware compressor with a "smart" output gain control.

Note that the line from the threshold point to the left of the graph always has a slope of 1.

When "Compress based on peaks" is checked

The following image shows the input-output characteristic of the Compressor effect when "Compress based on peaks" is checked.

• The red line shows the characteristic when "Make-up gain for 0 dB after compressing" is not checked.
• The orange line shows the characteristic when "Make-up gain for 0 dB after compressing" is checked and the input signal had a maximum peak value of less than 0 dB.

Note that the line from the threshold point to the left of the graph no longer has a slope of 1.

When "Compress based on peaks" is checked the Compressor effect applies "upward compression" to the input. This is very different from the behaviour of a traditional hardware compressor. Note that when the volume of the input is below the threshold the volume is "stretched" to fit the new post-compression range. Sounds above the threshold still receive downward compression. The result is that very quiet sounds are not significantly amplified.