MOTU 828es, Руководство пользователя

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M I X E R E F F E C T S
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Compressor graph
The Compressor graph below the Compressor
section (Figure 9-5) provides a thumbnail visual
indication of the current compressor settings for
the input channel. It is for visual reference only
and cannot be edited directly. However, you can
click it to open the full-size Compressor graph in a
separate window (Figure 9-6), which provides
graphic editing of the Ratio and Threshold
controls.
LEVELER
The Leveler™ (Figure 9-7) 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 9-7: 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.