Subject: Dynamics Processor TIPS
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Date: Mon, 22 Dec 2014 15:23:22 -0800

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.

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