Ashly DPX-200 Manuel d'utilisation - Page 10

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1. Dimensions and Schematics

Operating Manual - DPX-200 Parametric Equalizer - Compressor/Limiter

Broadcasting

Compression has long been used as a tool

to make an audio signal appear louder. A

good example is in broadcasting, where

competing stations with identical transmit-

ters and power attempt to sound louder than

each other. Since they are all restricted with

respect to maximum audio level (modulation), their best

tactic is to squeeze the dynamic range of their programs

to just a few dB. The audio output level of the station

virtually never changes, and the listener perceives this

continuous high-level sound as being louder than the same

material in an uncompressed form. Although both com-

pressed and uncompressed programs reach the same peak

levels, the compressed signal stays near peak level more

of the time, and thus sounds louder. This technique makes

the broadcast more intelligible over ambient noise, and

increases the geographical area over which the broadcast

is audible to the listener. Additionally, this compression

technique is extremely useful for FM and infrared trans-

mission systems for the hearing impaired.

8.3 Special Effects

Compression For Feedback Control

A common ritual in sound system set-up is equal-

izing the room to remove feedback. This is generally

accomplished by turning up system gain to purposely in-

duce feedback, searching for the center frequency of the

feedback, and then equalizing at that frequency to remove

the feedback. Once this frequency has been cut, system

gain is again increased to induce another feedback point,

and the whole procedure is repeated until the engineer is

satisfied that the significant problem frequencies have

been corrected. The major problem with this approach is

that the feedback can easily get out control, and the engi-

neer ends up dashing back and forth between the mixer

volume controls and the equalizer controls, while every-

one in the room plugs their ears and prays it will end

soon. The Ashly DPX-200 can turn this procedure into a

fast, painless job, eliminating loud feedback levels and

the possibility of speaker or ear damage.

Procedure:

1. Set up the DPX-200 limiter controls

as follows:

a. Output level control to -20dB.

b. Input Gain control to 0dB.

c. Threshold control to -30dB.

d. Ratio control to infinity ( )

e. Attack time to 5mS.

f. Release time to 1 Sec.

g. Limit switch IN

2. Using a 1/3 octave (31 band) or parametric

equalizer, set the EQ controls to a flat setting, and if the

equalizer has an overall volume control, boost it by 10 to

15 dB.

3. Open up several microphone input channels

to a normal operating level, with typical EQ settings, and

turn the console master fader up to a louder than normal

setting. At this point, the system should be well into feed-

back, but the room volume will be constant due to the

action of the limiter. You can listen to the feedback at

any level you like by simply varying the limiter output

level control, although below a certain monitoring level,

the feedback will stop.

4. Try to determine the feedback frequency, and

then equalize it by adjusting the center frequency, band-

width, and boost/cut controls of your parametric equal-

izer. (Note: a graphic equalizer can also be used, although

with less accuracy.) After eliminating the problem fre-

quency, try to further define it by sharpening up the band-

width, reattacking the frequency control, and making the

cut shallower, if possible.

5. As soon as the first feedback frequency has

been removed, the compressor/limiter will automatically

bring up system gain until another feedback point is in-

duced. Repeat the equalization procedure until it becomes

impossible to distinguish individual, predominant feed-

back frequencies.

6. Write down EQ marks for safekeeping if nec-

essary, and return all mixer, EQ master gain, and com-

pressor/limiter gain controls to normal operational

settings.

Altering the Texture of Musical Instruments

It would be impossible to mention here all the

ways that compression is used to create new sounds with

familiar instruments. Some typical uses are:

1. Creating a "fatter" kick drum or snare sound.

2. "Thickening" acoustic guitars.

3. Adding punch and sustain to electric bass or

guitar.

In general, use a gentle compression ratio, say

4:1, with a 10 mS attack time, 0.1 Sec. release time, and

a low enough threshold to cause 6 to 10dB of Gain Re-

duction. Try using this effect to help bring out a lead

vocal or instrumental solo in a cluttered mix. The com-

pressor is also a great corrective tool when working with