How to Measure Active Compression?

How to Measure Active
Compression?
•  No clear consensus has been reached regarding how best
to specify and document the performance of active
compression units;
•  Of course, software-based compression might be thought
to be completely specified via the underlying code.
•  Nonetheless, what measurements are needed will depend
on the application, and the means for characterizing
performance is still under investigation…
Measurement of Compression Units
FOCUSRITE RED 3
•  Let’s look at the typical responses of three of the compressor
units popular in studio use:
•  FOCUSRITE RED 3
•  GML 8900
•  DRAWMER 1960
•  Response to a gated test signal were recorded for each with:
–  Fast Attack and Slow Release
–  Slow Attack and Fast Release
The Red 3 is a Class A VCA-based dual mono/stereo compressor/limiter.
The compressor/limiter features a single Class A VCA to achieve truly
high class compression and limiting free of the usual compromises.
GML 8900
Dynamic Range Controller developed to react to loudness (rather than
volts) like our ears do. The 8900 uses multiple true-RMS detectors,
remarkably accurate log converters, and fast peak detectors to respond
intelligently to real signal energy, not just level.
This unusual design employs unique controls, like Crest Factor and
Release Hysteresis.
DRAWMER 1960
The 1960 is a hybrid vacuum tube/semi-conductor,
dual-channel compressor
On Peak-Detecting and RMS Feedback
and Feedforward Compressors
Abel, J. S. and Berners, D. P.
(Preprint 5914; Convention 115; October 2003)
First: What’s the difference between
feedback and feedforward compressors?
Feedback and Feedforward Compressors?
Feedback and Feedforward Compressors?
B
B
B
B
On Peak-Detecting and RMS Feedback
and Feedforward Compressors
(Abel & Berners, 2003)
Feedback and feedforward gain reduction trajectories
were shown to be equivalent by transforming the
feedback gain reduction into a feedforward gain
reduction having a level-dependent time constant.
This time constant has the effect of slowing down the
transition into and out of compression, and accounts
for much of the difference in compression character
between the two architectures.
On Peak-Detecting and RMS Feedback
and Feedforward Compressors
(Abel & Berners, 2003)
This paper presents differential equations governing
the behavior of first-order peak-detecting and RMS
feedback and feedforward analog compressors.
Based on these equations, the relationship between
feedback and feedforward compressor behavior is
explored, and simple, accurate digital emulations are
provided.