dbx Detection and the Dynamics Duo, Detection, Dynamics Duo Review Manual

Detection and the Dynamics Duo
(Or Why Do Some Compressors Sound
Great While Others Just Plain Don’t?)

Roger Johnsen

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Detection and the Dynamics Duo (or Why Do Some Compressors Sound Great
While Others Just Plain Don’t?) A Discussion of the Benefits of True RMS Level
Detection and True RMS Power Summing

Detection? Dynamics Duo? Sounds like a confidential Gotham City document that Cat
Woman or the Riddler would like to get a hold of for blackmail purposes.Perhaps the terms
“sidechain”and “compression”sound a little more familiar.In this paper,we talk about these
terms and explain the important role that “detection” plays in a well-designed “dynamics
processor”. Although we brief ly mention VCA’s, the purpose of this paper is to highlight
various detection schemes used by manufacturers, as well as the methods used to link
channels for stereo operation. We describe our True RMS Level Detection and True RMS
Power Summing methods that we at dbx profess to be the correct and logical, not to
mention best sounding ways to do dynamics processing.

1. Viscerally Compressed Amplitude

No,that’s not what “VCA”stands for,as anyone familiar with compressors could tell you,
but we have a gut feeling you already knew that (pun intended)! VCA, of course, means
“Voltage Controlled Amplifier.” The guts of any dynamics processor (compressor, limiter,
expander,gate, ducker,dynamic filter, modulator, etc.) contain, among other things, a VCA.
As a pioneer in the field of VCAs,dbx created the patented industr y standard chip used by
the zillions by many major manufacturers of professional and consumer audio equipment.
You’ll find our chips in dbx noise reduction systems throughout the world.

But this paper is not about VCA’s.We know that it takes more than a great VCA to make
a great compressor.Much more.And that’s where the years of leadership and expertise in
dynamics processing design make all the difference.

2.Dynamics Duo

The real art in making a great compressor is not only in the design and implementation
of the VCA,but in the design of the other,more impor tant half of the

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compressor. Any dynamics processor consists of the “Dynamics Duo"” or two signal paths:
an audio path through the VCA that needs to be as clean as possible--low noise, low
distortion--and a detector path that contains the smarts to figure out what to tell the VCA to
do.

The detector path is often referred to as the “sidechain.”(Sounds like a wallet protection
device).The sidechain consists of an audio level detector that converts the audio signal from
which the compressor is "keying" to a control voltage.This control voltage is then modified
by the threshold, ratio, attack, release, (etc.) controls which follow the detector. This
modified voltage controls the gain of the VCA.

If it sounds like Mr. Sidechain is the real brains of the Dynamics Duo,while Mr. VCA is
just the brawn,you’re absolutely right. The particular sound of any dynamics processor is
due more to the way Mr.Sidechain handles his affairs than how well Mr.VCA carries out his
orders. Common problems such as the proverbial “pumping” and “breathing” in some
compressors results from sidechain circuitry design,not from VCA deficiencies.But,lest we
fail to give credit where credit is due, never underestimate the essential role that Mr. VCA
plays in providing a clean signal free of noise and distortion.

3.Detection:RMS —True or False?

The principal component of the sidechain is the detector, and its ability to accurately
detect audio levels in the proper way considerably affects the dynamics of the processed
signal.Common sense would tell you that,given a particular audio signal,the detector must
compute a result that corresponds to the result computed by the human ear,if that detector
is to truly function as an audio detector.

There are many ways to do audio level detection, and as you may surmise, different
detection methods yield different results. While some compressor/limiters use peak
detection and fast response characteristics,their action is frequently displeasing to the ear,
even at modest compression ratios, as rapid gain changes result in unnatural signal
dynamics.This is due to the psychoacoustic phenomenon that the human ear responds,not
to the peaks,but to the average power of the signal.In other words,the ear responds to the
sum of the energies of all frequency components present.

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Detection and the Dynamics Duo