dbx Detection and the Dynamics Duo User manual

Detection and the Dynamics Duo
(OrWhy Do Some Compressors Sound
GreatWhile 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 briefly 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 CompressedAmplitude
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 ofVCAs,dbx created the patented industry 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 aboutVCA’s.We know that it takes more than a greatVCA 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 theVCA,but in the design of the other,more important 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 theVCA 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 theVCA.
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 fromVCA 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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Other processors use averaging circuits for level detection, created by simple resistor-
capacitor (RC) filters which may approximate average power with some waveforms,but in
general will not coincide with the way the human ear and brain interpret sound.This is
because there is not a one-to-one correspondence between the average amplitude of a
waveform and the average power of that same waveform,resulting in a mismatch between
the detector’s calculated signal level and your ear’s perceived level. It’s as if the averaging
detector is looking at the signal world through“fun house”mirror lenses.
A related method that has gained some popularity of late,due mainly to the availability
of these particular detector chips to the general public, is the “log average” method. But,
again, using averaging techniques, these logarithmic detectors exhibit the same error in
calculating signal level as illustrated above.Given these explanations,perhaps we have shed
some light on reasons why most processors work great only in certain situations with
certain types of signals.
Obviously, this is not the dbx way. Our processors use True RMS Level Detection for
signal sensing to most closely emulate human hearing response. Without going into a
detailed technical explanation, the mathematical concept of RMS (Root Mean Square)
describes an effective value of voltage or current that results in average power.This effective
value is called RMS because we take the square root of the mean of the squared values.
Disregarding the technical mumbo jumbo, just remember that the RMS value inherently
corresponds to the average power of the signal,as does the response of the human ear.With
sophisticated chips developed at dbx, utilizing patented integrated circuit designs to
calculate the RMS value of the signal,we are able to create unrivaled dynamics processors
with natural sound with the most listenable results over the widest range of signals.Those
who have used dbx processors know this to be true.
4.Duo Dynamics
What about stereo dynamics processing? Is there a special way required to link two
channels of compressors for processing a stereo signal, or do we just run each channel
through its own independent compressor?Those familiar with compressors know that for
processing stereo signals, each channel must react identically--that is, their VCA’s must be

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locked together so that their gains change identically--otherwise,the stereo image will shift
to the side with less gain reduction occurring at that instant.
We agree that theVCA’s must react identically when linked,but which detector controls
both VCA’s? Do they react to the left channel detector,or to the right channel detector, or
both?And,if to both,how? Do they react to the greater of the two signals? Do they react to
the sum of the signals?At this point you’re probably asking,“Who the @%&!*% cares?!”Well,
for starters,your ears and brain care.Bottom line:the detectors of both channels need to be
linked in an intelligent way.Let’s take a look at some not-so-intelligent ways and,of course,
the dbx way.
5.The Sum OfThe Squares OfThe Average of the Hypotenuse Sounds Bad! (OrWhyWe Do
ItTheWayWe Do!)
One way to do stereo linking is to ignore one of the channels,either the left one or the
right one, and have both VCA’s react to the signal on that one channel only. So which
channel signal do you choose to pay attention to and which one do you ignore, allowing
damaging peaks,speaker overexcursions,or fatal jaunts into digital 0VU land? If this seems
like a pointless question because no manufacturer would be stupid enough to implement
this method on a stereo processor, guess again.A particular manufacturer with a German-
sounding name whose boxes are made in China has implemented this method on their
“stereo” box. Hmm? Incidentally, their chosen method of detection on this box is the log
average method.
Another way to link detectors is to sum the left and right channel signals together before
going into one detector as suggested in an application note of a particular manufacturer of
anotherVCA.On the surface,this may seem like an acceptable way and will sometimes give
acceptable results,especially if the left and right signals are identical.The problem is that the
signals are,by definition,stereo!There is no problem detecting mono signals since they add
together without phase cancellation,but we’re not processing mono signals when we’re in
stereo link mode!
Stereo signals are,of course,different to some degree and will combine with some phase
cancellation.In other words,peaks in phase add together while peaks out of phase cancel.
Therefore, using this method, the detector will not calculate the correct level of the
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combined power of the left and right signals.The more dissimilar the left and right signals
are, the more error in the detector’s calculated level. It’s as if someone’s continuously
messing with the threshold knob!A much better method,although still deficient,is to link
log average detectors.This results in the sum of the average signal amplitudes, but, as we
have stated before,does not correspond with average power and therefore does not sound
as good.
6.True RMS Power Summing
The dbx way, of course, is different.With our True RMS Power Summing, the linked
detectors respond to the sum of the energy (power) of each channel’s RMS signal.The
intelligence of these RMS detectors allows the signals to be converted to the power domain,
then summed together in the power domain.This produces a true sum of the energies.The
more total energy,the higher the detectors calculate the combined signal to be.This is how
your ears work--the more screaming children in one room (i.e.,the more total energy),the
more irritated you get!
This sum of energies will gain track accurately over a wide dynamic range providing
excellent response at low or high signal levels. No phase cancellation, no inaccurate
modeling of human hearing response,and heaven forbid,no ignoring one channel of signal
information.When detectors are linked intelligently and sum signals the same way that your
ears do,compression,limiting,or expansion will sound the way it should!
7.Conclusion
No need for a conclusion here.You make your own. Thousands upon thousands of
industry professionals have already made theirs, making dbx the most respected and most
used dynamics processing line on the planet.

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