3
USB 7706
Compressor Dual Channel
1.POWER Switch:
This switches the AC power ON or OFF.
Note: In order to avoid possible undesired noise transients in a live sound reinforcement application, switch on the power
to your COMPRESSOR before switching on the amplification system.
Expander/Gate Section (Operation)
Audio, in general, is only as good as the source from which it was derived. The dynamic range of signals will often be res-
tricted by noise. Synthesizers, effects devices, guitar pickups, amplifiers, etc., generally produce a high level of noise, hum
or other ambient background hiss, which can disturb the quality of the program material. Normally these noises are inau-
dible if the level of the desired signal is significantly above the level of the noise. This perception by the ear is based on
the" masking" effect: noise will be masked and thus become inaudible as soon as considerably louder sound signals in
the same frequency band are added. Conversely, the further the level of the desired signal decreases, the more the noise
floor becomes a disturbing factor.
Expanders or noise-gates offer a solution for this problem by attenuating signals when their amplitudes drop, thereby
fading out the background noise. Expanders extend the dynamic range of a signal and are therefore the opposite of com-
pressors. Expansion over the entire dynamic range is not desirable. For example, with an expansion ratio of 5:1 and a
processed dynamic range of 30 dB, an output dynamic range of 150 dB would result, exceeding all subsequent signal pro-
cessors, as well as human hearing. Therefore the amplitude control is restricted to signals whose levels are below a cer-
tain threshold. Signals above this threshold pass through the unit unchanged. Due to continuous attenuation of the sig-
nals below this threshold, this kind of expansion is termed "downward" expansion.
The noise-gate is the simplest form of an expander. In contrast to an expander, which continuously attenuates a signal
below the threshold, the noise gate cuts off the signal abruptly. In most applications this method is not very useful, since
the on/off transition is too drastic. The onset of a simple gate function appears very obvious and unnatural. To achieve an
inaudible processing of the program material, it is necessary to be able to control the signal's envelope parameters.
The COMPRESSOR is equipped with an IRC (Interactive Ratio Control) Expander, so that its expansion ratio is automati-
cally adjusted dependent on the program material. Critical signals in the vicinity of the threshold level are processed with
a minute expansion ratio, whereas signals that reduce in level will be subjected to an increasingly higher ratio, which will
result in greater attenuation. The result is expansion, which is less critical to adjust and more tolerant of useable signals
whose level is only slightly above that of the noise floor. Expansion therefore occurs extremely "50ft" with low ratio set-
tings, while the known negative effects of
expansion are inaudible. The Attack time of the IRC expander is set automatically and program-dependent, i.e., extre-
mely short for quickly changing signals and slower for a more balanced program material. Since the expander/gate adapts
itself automatically to the program material, you will note that the IRC circuit produces considerably better results than
conventional expanders.
2. THRESHOLD Controls:
Use the THRESHOLD controls to determine the threshold point below which expansion occurs. The range of these con-
trols are from OFF to + 10dS.
3. INAUDIBLE Indicators:
For a signal above the threshold value, the "+" LED lights up. For a signal below the threshold, the "-" LED lights up, indi-
cating expansion mode.
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