DYNAMICS PROCESSOR TIPS
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2002 ~ 2013 by Eddie Ciletti and Manhattan Sound Technicians, Inc. (MST)
edaudio@tangible-technology.com
651-554-0304
V2d
Engineers know that the classic optical-based Limiters
- like the Teletronics LA-2A (vacuum tube) and the LA-3A (solid-state)
- are perfect for tracking Bass, Guitars and Vocals. Simplicity is part
of the charm - just Gain and Gain Reduction controls to dial in a little
opto-magic. As beloved as vintage compressor / limiters are, however, their
controls are often not labeled or calibrated. The early digital plug-ins
supplied all the parameters, but without presets, the learning curve was
pretty steep. That said, there is a pot of audio gold at the end of the
learning curve…
At the heart of the Gain Reduction or ‘opto-coupler’
module, sound is converted to light by an Electro-Luminescent "EL" panel
(originally intended to illuminate aircraft control panels). A light-sensitive
‘photo-resistor’ determines the response time – Attack time: 6mS to 12ms
– Release time: approximately 500mS. Considered "slow" relative to modern,
dbx-style VCA processors, the photo-resistor has an ‘intelligent’ non-linear
response time that varies with program material and the amount of gain
reduction. Together, these two components require no side-chain – which
is where - in more sophisticated processors - Ratio, Knee, Attack and Release
parameters are generated.
Released in the mid-seventies, the LA-4 got a new
optical transmitter (a red LED) and an IC quad-opamp, the RC-4136. A side-chain
circuit drives the LED, "tickling" the photo-resistor into delivering five
Ratio options - from 2:1 thru 20:1 - making the LA-4 a bit more versatile
than its predecessors. However, the RC-4136 in the signal path (balanced
input, gain make-up and output amps) ‘slowed’ down the signal when driven
hard (regardless of the amount of Gain Reduction) making the sound ‘darker,’
smearing detail and adding non-euphonic harmonic distortion in the process.
The EC-4136 opamp upgrade for the LA-4 preserves
delicate signal details – like acoustic guitar strumming - no matter how
hard it is driven. The sonic character is now solely from the opto and
the output transformer.
If you are happy with the EC-4136 upgrade, know that
three additional in-house mods are also available. "Dynamic Threshold"
eliminates the ‘jump’ between 2:1 and 4:1 - each RATIO can be auditioned
without the need to re-tweak Threshold for each step. A three-position
Stereo Link switch adds a high-pass option to the side chain. On the
rear-panel, a mod to the High-Gain Switch makes the LA-4 compatible
with self-powered condenser and dynamic mics when capturing loud sources
like drums, guitar and bass amps.
TIP-1: Judging Bass (compression and gentle
RMS limiting @ 2:1, 4:1 and 8:1 ratios)
When recording Acoustic and Electric Bass, for example,
it may be difficult to localize the source of note-to-note inconsistencies.
The problem may be due to monitors, control room acoustics, the instrument
or the player. VU meters are the perfect analysis tool for ruling out acoustic
anomalies. So, before doing any processing, first set the Meter switch
to read Output and monitor the bass for consistency. If the meter ‘suggests’
that the bass notes are inconsistent, adjust the Gain Reduction (GR) Threshold
to "level" the bumps. Confirm with the Output Meter and use more GR if
the notes are more than 3dB up or down. An aggressively played "slap" style
bass may require higher ratios or be processed in two passes – with peak
limiting and compression.
TIP-2: Compression or Peak Limiting? (NOTE:
Response Times are guidelines.)
Choose ratios below 8:1 for compression (Attack 10mS
– Release 500mS) and ratios above 8:1 for limiting (Attack 1mS – Release
50mS). At 2:1 compression, GR well beyond 6dB can be applied. But at ratios
of 20:1 and higher (Attack: 100uS – Release 5 mS), the peak limiting sweet
spot is between 3dB and 4.5dB - the max GR is quite literally ‘limited’
to 6dB - beyond which lies the negative artifact zone. Sometimes it’s necessary
to do both limiting and compression.
TIP-3: METERING
Using a VU meter as intended - to judge perceived
Volume (VU stands for Volume Units) - its slow 300mS response time is designed
to average complex audio material so that an engineer can visually judge
level discrepancies. (0VU = 1.23volts RMS in a +4dBu world). The VU meter
is fine for assessing compression, but the most effective peak limiters
have such super-fast Attack times (in micro-Seconds or uS)
that a VU meter is useless. High resolution electronic bar-graph style
metering have no mechanical inertia so they can accurately report transients
(peak response).
TIP-4: Peak Limiting (with ‘magnetic’ insights)
Compression and Limiting are variations of a similar
theme. The LA-4 for example, responds more quickly at 20:1 than at 2:1,
but will never be so fast as to get ugly. The FET-based 1176 is significantly
faster, crossing the threshold into Peak Limiting Territory (featuring
an ultra fast attack time of 20 microseconds to 800 microseconds), but
is still remarkably forgiving when ‘abused.’ Other ‘fast’ topologies include
VCA (dbx, Distressor), Variable Mu (Altec, Fairchild, Manley, UA and Pendulum)
and PWM (Crane Song, Great River, EMT). All can serve as effective Peak
Limiters when understood and be most unforgiving if abused.
That said, when a Peak Limiter has only a VU meter
to display GR - as was the case when I reviewed the Pendulum 6386 - adjusting
Threshold until the VU needle is just being ‘tickled’ can easily result
in a magical amount of peak limiting (3dB to 6dB). Here, listening is more
important than meter watching. Once the meter starting indicating GR, the
magic was gone. (I had to remind myself that recording tambourine to analog
tape required a similar consciousness.) Crane Song’s STC-8 and the Empirical
Labs Distressor - with their high-resolution LED bar-graph displays - are
great teaching tools because the display accurately relates to the processing
being done – whether you hear it or not. You’ll be amazed at how effective
and transparent 3db to 6dB of true peak limiting can ‘not sound.’ (PS:
Audio Suite can serve as a digital confirmation.)
If you are struggling with dynamics processing when
mixing, start with automation (clip-gain is great for this). Then try less
aggressive processing on individual tracks along with simultaneously processing
a sub- or main-mix (about 3dB of GR for each layer). Tweak each parameter
one at a time (Attack, Release, Ratio, Knee) to see which yields the most
dramatic results, keeping in mind that they interact. For example, slower
Release times allow faster Attack times without artifacts, and vice versa.
Release times for bass or kick drum could be as slow as 2 seconds! Response
times for peak-limiting percussion will be fast. Fast attack times will
bite into the peaks more - especially with higher ratios - allowing the
Threshold to be desensitized for less GR.
Thanks for reading!
Eddie Ciletti