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USB 7706
Compressor Dual Channel
2.1 The COMPRESSOR As A "De-Esser"
"De-essing" is a special application of frequency selective compression. The sibilant (Ssss) sound of the human voice can
often cause problems during recording. High frequency, sibilant sounds and "pops" can produce very high energy levels,
which can sometimes cause an otherwise normal and undistorted voice to sound very harsh, shrill and sometimes unintel-
ligible. The solution is frequency conscious compression or limiting. The unit responds only to selected frequencies and redu-
ces the level temporarily, as soon as sibilance or "pops" are detected. If the detector circuit registers an excessive amount
of high frequency information within the program material, as in a normal compressor, the VCA is activated and the over-
all level is reduced. As this type of compression affects the whole frequency range, this process is called broadband de-
essing. Please note that this type of frequency selective compression is very different from simple, fixed equalization using
notch filters, since de-essing has no effect on the signal except at the instant the sibilance occurs. The general frequency
response is principally not affected during this process.
When de-essing, simply insert an equalizer, not into the audio path, but into the sidechain path of the compressor. The
equalizer is inserted between the audio input and the DETECTOR LOOP input of the compressor. The equalizer is now inser-
ted into the sidechain loop and controls the unit. The center frequencies of the equalizer are then adjusted exactly to match
the frequencies of the sibilant sounds. All other frequencies are filtered out, so that with maximum attenuation of these fre-
quency bands, along with a correctly adjusted threshold point, the unit responds solely to the selected signal being produ-
ced by the equalizer. The level of the sibilant sounds can therefore be effectively limited.
De-essing using the COMPRESSOR
Initial Settings for the De-Esser Functions
Controls:
Settings:
EXP.THRESHOLD CONTROL
OFF
THRESHOLD CONTROL
+20 dBu
RATIO CONTROL
∞
AUTO SWITCH
OUT
ATTACK CONTROL
0.1 msec
RELEASE CONTROL
0.1 sec
OUTPUT CONTROL
0 dB
PROCESS CONTROL
OFF
1. Turn the THRESHOLD control counterclockwise until the GAIN REDUCTION meter shows an appropriate drop in level.
2. Now adjust the equalizer's corner frequencies (generally 6-10 kHz), until it is within the range of the sibilance.
3. Recalibrate the THRESHOLD control, so that the unit reacts only when the sibilant sound occurs.
Level compensation using the OUTPUT control is not necessary. Although the above recommended attack and release times
for this function are proven, the time parameters can be adjusted if necessary to achieve maximum results. The AUTO func-
tion should not be used.
2.2 Frequency Selective Filtering of Unwanted Signals
Based on the set-up described in the de-esser section, the unit may also be used to eliminate rumble, hum and equipment
noise (air conditioning systems, camera noise, etc.). Adjust the frequencies of the equalizer to match the unwanted fre-
quencies and use a peak filter with a high slope. Take care to decrease the amplitudes of the frequencies you are trying to
eliminate. Proceed now as described in the previous section E 2.1 "The compressor As A De-Esser". This will result in com-
pression of the selected frequencies and thus a decrease in the gain of these frequencies in the program mate
2.3 Suppressing Instruments During Recording
The compressor allows useful corrections of previously recorded material. If, for example, an excessively loud bass drum
needs to be suppressed, reduce all the equalizers frequency bands above 150 Hz. This setting causes frequency specific
compression, which reacts as soon as increased energy is detected in this band. By increasing the threshold level, the com-
pression can be made to react only to loud pedal and stick actions. Generally, it can be said that relatively high threshold
settings prevent the overall sound from being impaired and lead to the compression of solo instruments or very loud sounds.
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