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.
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.