Ashly DPX-100 Посібник з експлуатації - Сторінка 9

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Operating Manual - DPX-100 Graphic Equalizer - Compressor/Limiter

if this type of transient doesn't destroy a speaker outright,

it may damage the speaker surround in such a way as to

cause mechanical abrasion and future failure.

Alternatives For Sound Installations

To install a compressor/limiter in a sound sys-

tem using a passive crossover, insert it between your mix-

ing console output and the power amplifier input. For

systems using electronic crossovers, there are two ways

to use a compressor/limiter. It may be inserted between

the mixer output and the crossover input, in which case it

will act on the entire audio frequency spectrum. Alter-

nately, if the limiter is inserted between an output of the

electric crossover and the input of a power amp, it will

only affect a specific band of frequencies.

Recording

The Ashly limiter can be used to prevent tape

saturation in analog recording. Also, with modern trends

toward inexpensive digital recording, it remains neces-

sary to protect against input overload. With digital re-

cording, the information stored on tape, hard disk, optical

disk, etc., is either a 1 or 0, so actual signal level on the

tape is not the concern it is with analog recordings, in

fact it is not even a user controllable parameter. What is

of concern however, is the signal level applied to the A-D

(analog to digital) converters. If clipping occurs at the

converter input stage, the resulting distortion is most un-

pleasant, and will be recorded digitally as if they were

part of the original audio signal, forever mixed with the

audio. To prevent converter distortion while preserving

the extended dynamic range of digital recording, look up

the max input level of your recorder/converter and set up

the limiter as follows:

1. Set Gain to 0.

2. Set Threshold to 2-3 dB below max converter

input.

3. Set Ratio to 10.

4. Set Attack to 2 mS.

5. Set Release to .2 Sec.

6. Set Output level to 0.

If you are exceeding threshold frequently, your

input signal is probably too high and should be turned

down. Of course, every situation is different, so experi-

mentation before final recording is always a good idea,

but this is a good starting point.

To obtain a gentler limiting action at the expense

of some dynamic range, decrease the threshold to -15 and

the ratio to 3-5. This is also a good starting point for

analog recording.

Broadcasting

Compression has long been used as a

tool to make an audio signal appear louder. A

good example is in broadcasting, where com-

peting stations with identical transmitters and power at-

tempt to sound louder than each other. Since they are all

restricted with respect to maximum audio level (modula-

tion), 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 compressed and uncompressed programs reach the

same peak levels, the compressed signal stays near peak

level more of the time, and thus sounds louder. This tech-

nique makes the broadcast more intelligible over ambi-

ent 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 transmission 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-100 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-100 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 ( )