AN OVERVIEW OF COMPRESSOR /
LIMITERS AND THEIR GUTS

Gain Control Devices, Detectors, Side Chains
& Audio "make-up" Amplifiers

ã 1999 by Eddie Ciletti
and David Hill with comments by Paul Wolff at
API

SEE VU PLAY

Every compressor / limiter review
reminds me that the subject of dynamics processing is very
four-dimensional and not-so-well understood. Conversations with a
manufacturer-designer can often be revealing, as was recently the case
with Greg Gualtieri of Pendulum Audio. His physicist background
literally shed some light on the subject of optical limiters,
specifically how he overcame the idiosyncrasies of the devices used
in his OCL-2.

I tend to be heavy-handed when
testing compressor / limiters, slamming the meters just to see how "bad"
things can sound when abused. (To me, the better boxes don’t sound
that bad under aggressive settings.) My approach did not make the
Pendulum 6386, Variable-Mu "sing" until taking into consideration
the VU meter’s slow response time. By adjusting for less than a full
dB of (displayed) Gain Reduction, the "6386" came alive, making me
realize that older products with VU meters (Fairchild, UREI,
Teletronics or Neve) might also benefit from a kinder and gentler
approach. (Translation: more is going on inside than meets the eye
outside.)

PW:I do the same thing. Being "heavy handed"
is an easy way to see what the features do at their extremes,
then you can easily imagine what they would sound like at a
lower level. I also make comparisons by summing "dry" and
"processed" sounds together, but out-of-phase. Once you identify
the "differences," you can then go back and listen for those
things within the sound. It has really increased my
ability to perceive the "little" things, making me feel like I
really am good…

For
any device with a mechanical VU meter — from analog tape machines
to signal processors and pre-amps — the meter’s response time must
be a consideration, especially when processing "transient rich
material." While my goal is to assist in the process of
user-education, my own process of evaluation was "adjusted" simply by the
good fortune of finding people who are patient enough to answer
questions.

I re-learned that it is important to
listen — the eyes, a stubborn brain and an ego can easily get in the
way of sonic perception — yet it is equally important for products
to accurately display the work that is being done. (My other soapbox
is ease of parameter access, especially with digital gear).

PW:Ego? What !@#$ing ego? Are you accusing me
of having a large, over-blown ego? Well !@!$ you. I don’t have a
$#@ing ego. Oh>>>sorry>>>errrrr back to
work…

With all
of that in mind, I welcomed an e-mail that came from Crane Song’s
David Hill (former designer of Summit gear) and now creator of the
STC-8 and Trakker (dynamics manipulators). As a
designer-manufacturer, David is not alone in feeling that educated
consumers make better customers and so with his permission, his
"detailed correspondence" was used as the foundation for this
column.

OOPS!

An investigation into our vintage
audio heritage is helpful both for the "happy accidents" that occurred
(and endured) as well as gaining a greater knowledge of what made
these great boxes tick. This article is an overview, with another
planned to cover "just" the supporting amplifier technology and its
affect on the "sound." If you’re new to compressor - limiters and
just wanna get a leg up, skip to the paragraph labeled "Questions."

DEARLY BELOVED

Surely the Teletronix LA-2A is the
most famous and beloved Optical Compressor / Limiter. A close second
is its transistorized successor, the LA-3A. The heart of both units
is the T4 "optical attenuator," consisting of an Electro-Luminescent
(EL) panel as light source and a light dependent resistor (a.k.a.
photo-resistor or photocell) as optical receiver (as shown in
Photo-1a and Photo-1b, respectively).

Electroluminescent "EL" Panels come in all shapes and
sizes.  Originally intended to back-light aircraft
instrumentation panels, these high impedance devices are
particularly well-suited to operation at 400Hz
(power).

Variations
on a Photo-Resistive
theme.

EL Panels are sold commercially as "Nite-Lites" under
several brand names.

The "other" Gain Control topologies
include a vacuum tube / circuit called "Variable-Mu," the Field
Effect Transistor (FET) and the Voltage Controlled Amplifier (VCA).
(See Table One for a quick comparison.) All will be discussed
in this article, "Optical" first, because it’s easiest to
understand.

TOPOLOGY

** Approximate
Range

of Gain Reduction
**

Variable-Mu

12~25
dB

Optical

25~30
dB

pwm-FET

30
dB

FET

40~50
dB

VCA

100
dB

Table One: Topology Gain
Reduction "Window"

** Note:
The "approximate" range of Gain Reduction is typical for what is
practical for each topology.**

PW:Notice that all
feedback type compressors have a max reduction of about
30 dB. That is because you can never
get a feedback-based device to compress with a ratio of more
than 5:1. If you take the FB circuit and turn it into an FF
circuit — optimize it for 20:1 ratio, then switch it back to FB
mode — it will never get over 5:1 because of the gain riding FB
does.

PW:
The units that go over 5:1 typically have gain in the control
voltage. If you were to take this control design and turn a 20:1
FB circuit into an FF circuit, the ratio would be over inf:1 and
would be doing negative reduction. (The dbx 160X had this
feature.) I discovered this while
designing the 225. As far as I know, I am the only person to
make it — FB and FF — a selectable option.

PW:
As an experiment, I added gain to achieve a 20:1 ratio, but
because of the speed, it was on the edge of becoming unstable.
The sonically strange thing is that a 5:1 ratio in FB mode
sounds more compressed than 5:1 should. That’s because you can’t
really accurately correlate ratios on FB compressors because
they act differently with tones as compared to program material.
It’s a "dynamic" thang!

SPEED
OF LIGHT

A photocell, responding to light,
decreases its resistance as the amount of light increases. The time it
takes for this resistance change to occur varies with optical device
design and typical production tolerances. Photocells have a built-in
"Attack" time-constant limiting their ability to quickly respond to
transient signals. When used as a "Limiter," optical devices are not
fast enough for overload protection — especially in digital-land —
where there’s nowhere to go beyond 0 dB Full-Scale
(0-dBFS).

In response to a transient, the
photocell’s recovery is non-linear (initially fast, then slow). When
constantly bombarded with light — as would be the case with heavy
compression — the Release time increases, developing a "memory" that in
essence contours the release curve to be "program dependent." Most
users would consider these anomalies "the happy accident," for it is
nearly impossible to make a classic optical compressor / limiter
sound bad.

PW:The API 525 FB compressor had a circuit
called "release time over-ride" that allowed transients to recover
quickly and overloads to recover slower. The reason was to
"prevent the unit from sounding bad with overuse". I think in
those days there were more "tone police," because the engineers
could actually design.

The
response-time limitations mostly work in the optical device’s favor,
though as a gain control device, the photo-resistor is much less
flexible than its competitors. It is nearly impossible to speed up
the response time of a photo-resistor short of testing and selecting
the fastest devices. Temperature and time affect these components
more than others, making it difficult to achieve and maintain
accurate stereo tracking. One exception is the Pendulum Audio OCL-2,
which uses a proprietary approach to speed up and "tame" the optical
device.

OPTICAL TRANSITION

Recent products — both analog and
digital — may have more signal-processing ability, but in many cases
the interface obfuscates the user’s ability to take advantage of the
available power. By contrast, the LA2-A and LA3-A are simple
two-knob devices that are, in many cases, just right for vocals and
bass guitar. They serve as a reminder that less truly is
more. Inside, the aforementioned "classics" are as basic as
they appear on the outside. Only minimal circuitry is needed, one
amplifier to drive the EL panel and another amplifier to "make-up"
(recover) the gain lost by processing. Oh yeah, let’s not forget the
power supply!

Later, in the neo-IC age, the
designers of the UREI LA-4 substituted an LED for the EL Panel. In order
to drive the "transmitting" LED, a "detector" circuit must be added.
The perfect lead-in to a more global concept…

THE BLACK BOX

All Gain-Control topologies can be
modeled as a three-terminal "black box" with input, output and control
connections. In order to establish "control," it is first necessary
to convert the AC signal (via rectification) into a DC Control
Voltage (CV) that corresponds to the variations in signal amplitude.
This circuit can be designed to "detect" RMS or "peak" information
plus the ability to manipulate parameters such as Attack, Release,
Ratio and Slope (HEY---MY WIFE IS ASIAN…). For more information, see
theSidebar:
Detection and Side-Chain.

VARIABLE-MU

The highly treasured Fairchild 670
is one vintage example of the "Variable-Mu" circuit. More recent
versions include the Manley Labs "Variable-Mu" and the Pendulum
Audio "6386." In the latter instance, the model number refers to the
vacuum tube used for the task. No longer in production, the "6386"
is a Five-Star military-grade dual-triode vacuum tube made only by
General Electric (GE). All products currently manufactured are using
"New Old Stock" (NOS) tubes, so the supply is obviously
limited.

The vacuum tube predecessor to the classic 1176,

this is the front-end of the Universal Audio 175-B
(
version 175-C had a ratio switch like the
1176).

Photo Two is a simplified
schematic of the front end of the Universal Audio 175 "Limiting
Amplifier," the vacuum tube predecessor to the "solid state" 1176 and a
Variable-Mu device. Changing the grid-to-cathode voltage results in a
corresponding change in "mu," (m ) the gain of the circuit. In this
case, the BLUE line indicates the path of the Control Voltage (CV) —
from the detector and side-chain through a voltage divider — to vary
the bias of each half of the tube.

For "good" performance over its
useable range, adjustments are provided so that each half of the
Variable-Mu tube can be balanced to minimize the amount of CV
"feed-through" into the audio path (the red arrow). From a purest point of
view, CV feed through is bad, though it can also be part of the
"sound" that is desired in some cases. One example would be putting
some "attack" back into a kick drum (something that also happens
with noise gates). Although the total amount of Gain Reduction is
limited for Variable-Mu when compared to the alternatives, it is
easier than optical to make a balanced stereo compressor.

FET CONTROL ELEMENTS

Like optical, the Field Effect
Transistor (FET) acts as a voltage dependent resistor. Although it might
sound like the perfect answer to a photo-resistor, it’s not. As a gain
control element, the FET is level sensitive. Very large signals can
modulate the device resistance causing a gain-modulated distortion
that is independent of the control voltage used to manipulate the
gain. (Translation: overload the input of a FET-based device and
expect some funky, most likely undesirable sound.)

PW:in the API 525, the
signal is padded down 30 dB and then boosted back up with the FET
going to ground on the "—" input of the input op-amp,
controlling the gain to minimize
the effect of gain-modulated distortion. Another reason that
that FB compressors were "limited" to 30 dB of Gain Reduction is
that they could only attenuate as much as they were set up to
lose. A reasonable point considering that 30 dB of gain did not
add a whole lot of "extra" noise.

1176:
A Very Good Year

The Universal Audio (later UREI)
1176 is a vintage FET compressor / limiter that is extremely popular
with vocalists. The most favored of versions include an input
transformer (eliminated in later production runs, both black- and
silver-faced), a discrete audio path and an output transformer. The
early Allison Research Gain Brain is also an FET compressor /
limiter that is both transformer-less and unbalanced.

REALITY CZECH

To make the FET work as a gain
control element, the signal level must be kept low requiring more than the
usual amount of gain from the "make-up amplifier." One example is
the Input control on a Universal Audio 1176 — a dual-pot configured
as a constant-impedance attenuator. To satisfy the input
transformer, the attenuator must be pre-transformer as well
as pre-FET. Extra circuits can help minimize, but not
eliminate, the inherent distortion of this topology. What remains is
part of the sound of any FET-based device. (Discussed in a future
article, more than one output amplifier design was used as the 1176
evolved, affecting yet another facet of its sound.)

For stereo operation, FETs must
first be matched and the circuit must include adjustments to "find the
threshold" of the device so that it operates within its linear region.
The Attack and Release times can be much faster than an
optical-based product.

PW:The 525 could match the channels quite
closely. They used a 5-kohm, feedback resistor (variable) with the
FET connected from the "minus" input of the op-amp (a "virtual"
ground) to ground. By adjusting the gain to match the unity
level, they were almost always gain matched. The only other
problem was distortion matching, which was few and far between,
requiring some FETS to be replaced.

VOLTAGE CONTROLLED
AMPLIFIERS

Voltage controlled amplifiers, or
VCAs, have been around for a while. They are the most common method of
gain control, followed by the FET. Early VCA devices did not sound
very good, two of the difficulties included matching a half-dozen or
so transistors as well as keeping them all at the same temperature.
Modern VCAs are much better because they are "monolithic," a.k.a, an
"integrated circuit," or IC. Since all of the transistors are grown
on the same piece of silicon, they match and stay at the same
temperature.

PW:
They used to be called "modulators", and the
originalBlackmer patent was never
challenged on this, as no one ever mixed dc with audio. The
original VCA was Class-B and had thermal relationships to
distortion. That is why everyone in this country got into the
habit of leaving everything on! They (THE SILICON) were only grown in the same county, and had to
be potted. You first had to match the transistors, then pot
them.

PW:
The Allison VCA was Class-A and thermal relationships
only effected gain. The early Allison (blue) was also comprised
of discrete devices, matched and potted.

There are two basic types of VCA, the log /
anti log amplifier and the trans-conductance amplifier. For the log /
anti-log type to work well requires well-matched transistors with a
"perfect" relationship between base-emitter voltage and emitter
current. Using this over a large control range can run into
problems. Increased gain reduction results in less current flow in
the circuit, which means hi-frequency response decreases as gain
reduction increases. Mis-matched transistors in the VCA can create a
strange character of distortion than may occur at one gain setting
and not another.

The trans-conductance amplifier
looks like a discrete differential amplifier — the gain is manipulated by
changing the emitter current. Like the FET, this type of circuit
does not tolerate large signals well — distortion sets in — and with
large amounts of gain reduction it can also lose high frequency
response. Its well-matched transistors must also have very low
noise.

PW:
Trans-conductance devices also were noisy, comparable
with a noise level of a good cassette deck.

Modern integrated circuit technology has
resulted in great improvement over the years, but the VCA can still
suffer from control feed through and have different types of
distortion at different signal levels. A major advantage of using
VCA devices is that they can have a very useable dynamic range,
which is particularly useful in building automation systems.

PW:
That corporation has pretty much licked the problems with
VCAs.

PULSE
WIDTH MODULATOR

Using Pulse-Width Modulation (PWM)
to control an FET is what Crane Song uses in both the STC-8 and
Trakker compressor / limiters. The technology has been around for
about 20 years, having been used by several companies, including
EMT. With modern technology, a PWM gain-control circuit can be very
fast — from 0 to full gain reduction in 500 nS (0.0005 mS or 0.5 uS)
and an audio frequency response to 60Khz or more.

PWM works as a gate, quickly
switching the FET from "off" to "more-on" by varying the pulse-width. With
careful selection of components and PC board layout it is possible
to have very low control feed-through, a very low noise floor and an
audio-path frequency response that does not vary with gain
reduction. This type of circuit requires costly high-speed
components and as a result, there is a fair amount of power
consumption (it runs hot). Distortion is very low and does not
change with gain reduction.

CRANE SONG: Trakker with 12 Classic
Settings

CRANE SONG: STC-8 Stereo Compressor with interactive
Peak Limiter

PW:Dave is real smart. PWM is the basis of
the "Class-D" amplifier. Only a real engineer would be able to
think that up. It’s from having transmitters and oscillators
hammered into our heads at school.

Questions?

If you are unsure about how to set
the various parameter options — Attack, Release, Ratio, for example —
it’s not a bad idea to play it safe. Consider that the optimum range
of some gain reduction devices is limited. Rather than over-use a
compressor / limiter in one pass, try less-aggressive settings.
Process the signal twice, once going to tape (for example) and once
coming from tape (or your storage medium of choice).

Combining the fastest Attack with
the fastest Release settings can create undesirable distortion. Most
Compressor / Limiters feature a range of Attack times that are faster
than the range of Release times making it easy to smash transients.
(These same transients are also responsible for "image
localization.") To keep the sound "alive" without destroying it, do
the reverse of what is more-often the default.

On the first pass, go for the
fastest Release time and start the Attack time at its slowest setting
using a ratio not higher than 2:1. It’s ok to increase the Attack speed
until it just starts to have a dramatic effect, then back off. This
will even out the dynamics so that, on the second pass, conservative
but more traditional settings — Fast Attack, Slow Release — will
have a more dramatic and consistent effect.

Being "musical" is not necessarily
the most desirable end-result. Sometimes what might be considered
undesirable artifacts could end up being a cool effect. The best example
of this is the vocal effect (of the pitch corrector) on Cher’s big
summer hit "Do You Believe."

e-mail
eddie

Sidebar-1:
Detector and Side Chain

The function of the Detector circuit
is to convert (rectify) the audio signal (AC) into a DC Control
Voltage (CV) that corresponds with the signal’s changing amplitude.
The CV is then used to manipulate the gain of the control device. If
this sounds like the workings of an old analog synthesizer, you are
correct. Integrated around the rectifier (Peak or RMS detection) is
a time-constant circuit to manipulate attack and release speed, some
type of threshold circuit (soft-knee, hard knee plus ratio) and
perhaps an analog computer circuit. The side-chain control-voltage
should correspond as closely as possible with the useable range of
the gain control element.

Sidebar-2: Feed Me!

There are two basic compressor /
limiter topologies: feed-back and forward-feed, each with their own
characteristics, each appropriate for specific types of signals. The
block diagram, Figure-Three, is the easiest way to understand the
difference. I was lucky enough to witness a demonstration by API
designer, Paul Wolff, who included both options on the API 225L
compressor / limiter. The difference is so obvious.

API 225L from the Legacy
Series

Forward-feed processors can have
attack, release, and slope controls that are labeled with fixed numbers
— so that dialing up and achieving, a slope of 4 to 1 and an Attack
time of 50-mS yields consistent and repeatable results. Two examples
of Forward-feed processors are the dbx 160 and the Gain Brain
2.

Flipping the switch at the input of
the Detector from forward-feed to feed-back now selects the audio
signal, post-processing, that is, after the gain control
device has done its job. Now the compressor limiter can "see" what
it has done and literally tailor the processing to the signal, now
more than ever, a moving target. The feed-back topology makes the
front panel settings — Attack and Release times as well as the
Ratio and Knee — more arbitrary, which to some users can be slightly
disconcerting.

PW: That's why the controls don’t work the
same in both positions.

With
the 2500, we have added a patented circuit of ATI’s. it is a inverse
pink noise response curve filter (3 dD/oct hi-pass) to tip the
detector response to equalize the energy/oct of the signal. We call
it THRUST, and it is quite cool. I have also applied for a patent on
using a filter in the linking circuit to reduce highs or lows or
both from the link, eliminating the cross-coupling pumping, which
sounds pretty cool.

The Fairchild 660 and 670, API 525
plus the Teletronix LA-2A and LA-3A and UREI 1176 are all examples of
the "Feed-back" approach to dynamics manipulation.

Side Chain

Up until this point, we have only
considered that the Detector’s "job" is to create a DC voltage that
corresponds to the signal to be processed and assumed that there will be
various tweak-able parameters such as Attack, Release and Ratio.
Most detector circuits are designed to treat all frequencies
equally, but sometimes it is desirable to insert EQ into the
side-chain so that the processor responds, for example, to high
frequencies only as would be the case for a de-sser, a hi-frequency
limiter.

Other possibilities include
inserting a high-pass filter so that the processor would ignore low
frequencies. Or, a routing a kick drum to the side chain input to make the
processor "duck" a bass guitar with each beat. Taking advantage of
the side-chain can be very useful when trying to either create more
sonic space or control problem sounds.

End of sidebars.

LINKS

API

Manley
Labs

UNIVERSAL
AUDIO

CRANE
SONG

Mercenary Audio
(Sales)

REVIEWS

Pendulum
Audio

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