MOTU 24Ai, Руководство пользователя

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M I X E R E F F E C T S
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LEVELER
The Leveler™ (Figure 8-5) provides an accurate
model of the legendary Teletronix™ LA-2A®
optical compressor, known for its unique and
highly sought-after Automatic Gain Control
(AGC) characteristics.
Figure 8-5: The Leveler module.
The Leveler is available on the Main Mix bus and
all Group busses, including the Reverb bus.
A model of an optical compressor
An optical leveling amplifier works by shining a
light on a photoresistor. The intensity of the light
source is proportional to the audio signal, and the
resistance of the photoresistor is in turn inversely
proportional to the intensity of the light. Photore-
sistors respond quite quickly to increases in light
intensity, yet return to their dark resistance very
slowly. Thus, incorporation of the photoresistor
into an attenuator followed by an amplifier which
provides make-up gain produces a signal which
maintains a constant overall loudness.
Automatic gain control using light
The AGC circuit of the LA-2A uses a vintage opto-
coupler known by its model number T4. The T4
contains an electroluminescent-panel (ELP) and
photoresistor mounted so that the emission of the
panel modulates the resistance. An ELP consists of
a thin layer of phosphorescent material
sandwiched between two insulated electrodes to
form a capacitor. Making one of the electrodes
transparent allows the light to escape. These
devices are essentially glow-in-the-dark paint on a
piece of foil covered by metalized glass or plastic,
and are the same devices used in low-power night
lights. Unfortunately, these devices need high
voltages to operate, and are best driven by tube
circuits which can supply voltage swings of several
hundred volts.
Response characteristics
Once the light has faded away, the photoresistor
then decays back to its dark state. The shape of the
decay curve varies depending on how bright the
light was, and how long the light lasted. A general
rule of thumb is that the louder the program, the
slower the release. Typically, the release can take up
to and over one minute. One thing to keep in mind
when using these types of devices is that the typical
concepts of compression ratio, attack, release, and
threshold do not apply. The light intensity is
determined by the highly non-linear interactions
of the input signal, AGC circuit, and ELP, and thus
exhibit a strong program dependence that is
impossible to describe without the mind-numbing
mathematics of statistical mechanics. The actual
results, however, can be almost mystical: even
when you feed the same material (a loop perhaps)
through the Leveler twice, you’ll often see a new
response the second time through a loop, complete
with unique attack times, release times and
compression ratios. Furthermore, two different
input signals with the same RMS levels may be
leveled in a drastically different manner.
It is precisely this self-adjusting behavior that
makes optical compressors the tool of choice for
smoothing out vocals, bass guitar and full-
program mixes without destroying perceived
dynamics.