DYNAMICS PROCESSOR TIPS

ã2002 by Eddie Ciletti

TIP-1: Judging Bassy

Bass is hard to judge by ear. That’s because room acoustics and
monitor idiosyncrasies can create "local" sonic peaks and dips
making it hard to judge note consistency. Simple Limiters like the
Teletronics LA-2A (vacuum tube), The LA-3A (solid-state) and the LA-4 (IC
opamp) all feature a mechanical VU and Optical Gain Reduction that is
perfectly suited for Bass, Guitars and Vocals.

First set the Meter Mode switch to read Input or Output — the VU
meter is great for judging consistency of Bass notes.  Then meter Gain
Reduction, adjusting the Threshold to "level" the bumps, use more
if there are "dips" are too deep. A gentle RATIO of 2:1 or 4:1 will
do the job unless the Bass is agressively played, such as the
"slap" style.  What could be easier?

TIP-2: Delicate Under-Things

Conversely, a mechanical VU meter is slow to respond to transients —
percussion instruments have a Fast Attack — so the recording engineer
must be conservative when setting levels for tambourine, hand claps and snare
especially when the destination is analog tape. For example, VU meter
response to any of these instruments may indicate "–5dB," but
the actual level could easily be 6-dB to 12-dB higher. (A clunky tambourine
sound with low frequency artifacts indicates that clipping has
occurred.)

When using a VU meter only to judge Gain Reduction, adjust
Threshold until the meter just begins to move, then
back off a hair. Be sure to listen. For a Compressor-Limiter equipped with
LEDs, adjust Attack and Release to their FASTEST settings. Set Threshold for
no more than 6dB of Gain Reduction, select the most aggressive RATIO
("infinite-to-one" or 20:1) to achieve the desired result —
such as getting more "ambience" from a drum kit  by peak
limiting a stereo sub mix.  Peak limiting a Stereo Program mix is a bit
more complex, you can start aggressive and then back
of the RATIO and threshold.  Again, GR meter movement might just barely
be detectable to get the job done.

TIP-3: Reading Tea Leaves

Tip-1 and Tip-2 are examples of RMS Compression and PEAK Limiting,
respectively.  In the case of  limiters like the LA-2 and LA-3, the
Ratio and Response times are determined by an Optical Device that has a
variable response - faster with conservative Threshold settings (not more
than 6dB), slower with aggressive amounts of GR (6dB or more). Used
aggressively the attack slows to a medium while the release becomes non
linear, initially fast then slow.

NOTE: These GR examples are based on actual amounts, NOT as displayed by
the meter.

For Dynamics Processing "Speed" and Depth of Gain Reduction are
inter-related.  Peak Limiting requires fast response times assuming
conservative GR (6dB max).  Otherwise one of the response times must be
slower to avoid annoying modulation distortion.  Typically the Attack is
fast (5mS to 10mS) while the release is much longer 100mS and higher).
Try this with Kick Drum, varying the Attack from Fastest to Slowest settings
to hear what seems like an EQ change.  Compression begins with low RATIOs, Medium-to-Slow Attack and Release times along with
a soft knee can make processing transparent.

IF unsure, always start with the slowest settings and a minimal amount of
meter movement.  Then speed up the Attack until it "digs" more
into the track.  Some VCA compressors provide a pair of LEDs that assist
the user in setting the Threshold as well as the Release Time so that the
processing activity is centered around the
"Knee" whether hard or soft.  When effective but transparent
processing is required, extending the Release time helps considerably.

At the risk of being repetitive, any dynamics processor that relies on a
mechanical VU meter can become even more versatile once taking into account
the meter’s slow dynamic response time. This is especially true for
transient instruments, where an almost undetectable deflection of
"one-half dB" on the VU meter (in Gain Reduction mode) might yield
6dB of Peak Limiting. This is the starting point when trying to extract more meat
and ambience from drum tracks.

TIP-4: Find the Window

Of the four primary dynamics processing topologies — optical
(photo-resistor), variable-mu (vacuum tube), FET (Field-Effect-Transistor) or
VCA (Voltage-Controlled Amplifier: transistor) — ALL have a
"window," the optimum range where the device performs best. ALL
also have a maximum amount of gain reduction as detailed in Table-1.
The temptation is to smash a track within an inch of its life, but it is
better to conservatively process twice — to and from a recording
device, for example — rather than aggressively squash once.

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-1:
Topology Gain Reduction
"Window"

TIP-5: Understanding The Black Box

All of these topologies can be considered a three-terminal black box
—Input, Output and Control. Most of the time audio (AC) is converted to
DC, then "shaped" via Resistor-Capacitor (RC) networks to create a
Control Voltage (CV) that can be dynamically manipulated by Attack and
Release pots (variable resistors). The exception are the classic optical
Limiters — the LA-2a, the LA-3a and to a lesser extent the LA-4. The
simple act of charging and discharging a capacitor is a "Time"
issue that has a great effect on the overall dynamic envelope — the
sound of the Compressor / Limiter processing. The emphasis here is
that Input and Output circuitry of many Vintage Classics (Amplifiers and
Transformers, for example) are equal contributors to the magic associated
with these boxes.

The Optical Transmitter in the LA-2 and LA-3 is an Electro-luminescent
panel directly driven by audio — the former from a tube and the latter
from a transformer-coupled transistor-pair, most respectively. In this case,
LIGHT is the Control Signal so it is not necessary to create a CV. In the
LA-4, there is a detector (to convert AC to DC) followed by an op-amp (to
manipulate Ratio and Threshold) feeding a transistor to drive the LED. While
there is a capacitor in this circuit, it has more to do with gain than to
create a charge and discharge characteristic.

ROAD MAP

The addition of Attack, Release, Ratio, Soft Knee and Hard Knee controls
are great when you know how to use them AND when enough metering is provided
to correlate what is heard to what is seen. While for years the trend has
been to add every possible feature to please every possible user, I think
simplicity is one of vintage gear’s "love-factors." Sure, the
VU meter helps, so does the vast real estate required to accommodate the
technology AND make front panels easier to see.