15
USB 7706
Compressor Dual Channel
2.4 Emphasizing Musical Instruments During Recording
Conversely, you can use the compressor to bring out an instrument solo or a lead vocal in a cluttered mix. Match the fre-
quencies of the equalizer to the frequencies of the instruments to be emphasized (it is best to use a notch filter with a high
slope). (Note: In this application, make sure you only reduce the amplitude of the selected frequencies.) The compression
results in a subjective decrease in the volume of the overall program material. Only the selected frequencies coming from
the equalizer remain uncompressed and are therefore perceived as being louder. This inverse type of compression also helps
to emphasize instruments during low-level passages, so that they become more pronounced.
2.5 Reducing Feedback in PA Systems
A common procedure in sound system set-up is equalizing the acoustics to remove feedback. This is generally accomplis-
hed by turning up the system gain to purposely induce feedback, searching for the center frequency of the feedback and
then equalizing at that frequency to remove the feedback. Once this feedback has been attenuated, the system gain is
again increased to induce another feedback point and the whole procedure is repeated until the engineer is satisfied that
the relevant problem frequencies have been corrected. In spite of this equalizing process, feedback remains a difficult pro-
blem. Often enough, acoustic changes occur as the audience enters the room, which again leads to feedback problems. In
addition, the frequency response of the whole system is modified and thus affected by equalizer operation. Dynamic feed-
back control is a better solution. Similar to the previously mentioned de-esser application, an equalizer is not inserted into
the audio path but into the sidechain path of the compressor. To effectively suppress feedback, the center frequency of the
equalizer is correctly adjusted to match the rooms resonant feedback frequency. This selected frequency now controls the
compressor. The signal coming from the equalizer is applied to the DETECTOR LOOP input, while the audio signal is routed
through the compressor audio path. As soon as feedback occurs, the unit temporarily reduces the system gain and thus
effectively suppresses the feedback. In contrast to the technique mentioned above, the frequency response of the PA sys-
tem is not affected in any way at all. The use of the compressor in this application can eliminate the possibility of speaker
or ear damage.
Reducing feedback in audio systems using the compressor
3. ANTICIPATED COMPRESSION
If you feed the audio signal directly into the DETECTOR LOOP input and send the audio signal through a delay before the
audio input, the compressor can anticipate the need for gain change. With experimentation, the effect can create a "zero"
attack time at a given frequency. Additional delay beyond this "zero" attack time will produce a special sound effect, simi-
lar to the dynamic envelope inversion you may already be familiar with from reverse tape playback.
Anticipated compression using the COMPRESSOR
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