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Behringer XR4400 Руководство по эксплуатации онлайн [5/14] 776635

5 MULTIGATE PRO XR4400 User Manual

P/dB

Ear

Microphone Ampliﬁer

Power Ampliﬁer

Tape Radio

Radio

Cassette

Recorder

140

120

100

Fig. 1.1: The dynamic range capabilities of various devices

It is therefore useful to keep the operating level as high as possible without

risking signal distortion in order to achieve optimum transmission quality.

It is possible to further improve the transmission quality by constantly

monitoring the program material with the aid of a volume fader, which manually

levels the material. During low passages the gain is increased, during loud

passages the gain is reduced. Of course it is fairly obvious that this kind of

manual control is rather restrictive; it is dicult to detect signal peaks and it is

almost impossible to level them out. Manual control is simply not fast enough

tobesatisfactory.

Noise floor

Clipping

Operating level

Effective SNR

Headroom

P/dB

+20

-20

-40

-60

-80

Fig. 1.2: The interactive relationship between the operating level and the headroom

The need therefore arises for a fast acting automatic gain control system

which will constantly monitor the signals and which will always adjust the

gain to maximize the signal-to-noise ratio without incurring signal distortion.

Thisdevice is called a compressor or limiter.

1.1.3 Compressors/Limiters

By measuring the dynamic range of musical instruments in live recording

situations, you will nd that extreme amplitudes occur which often lead to

overload in subsequent signal processing equipment. Especially in broadcasting

and record cutting techniques, these signal peaks can lead to heavy distortion.

To avoid this kind of distortion or, for example, to avoid loudspeakers being

damaged by overload, Compressors or Limiters are used.

The principal function used in these devices is dependent on an automatic gain

control as mentioned in the previous section, which reduces the amplitude of

loud passages and therefore restricts the original dynamics to a desired range.

This application is particularly useful in microphone recording techniques,

tocompensate for level changes which are caused by varying microphone

distances. Although compressors and limiters perform similar tasks, oneessential

point makes them dierent: Limiters abruptly limit the signal above a certain

level, while compressors control the signal “gently” over a wider range.

Alimiter continuously monitors the signal and intervenes as soon as the level

exceeds a user-adjustable threshold. Any signal exceeding this threshold will be

immediately returned to the adjusted level.

A compressor also monitors the program material continuously and has a

certain threshold level. With compression, in contrast to the action of a limiter,

signalsare not reduced in level abruptly once the threshold has been exceeded,

but are returned to the threshold gradually. The signal is reduced in gain,

relativeto the amount the signal exceeds this point.

Generally, threshold levels for compressors are set below the normal operating

level to allow for the upper dynamics to be musically compressed. For limiters,

the threshold point is set above the normal operating level in order to provide

reliable signal limiting, to protect subsequent equipment from signal overload.

1.1.4 Expanders/Noise gates

Audio, in general, is only as good as the source from which it was

derived. Thedynamic range of signals will often be restricted by noise.

Synthesizers,eects devices, guitar pickups, ampliers 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 inaudible if the level of the desired signal lies

signicantly above the level of the noise. This perception by the ear is based on

the “masking” eect: noise will be masked and thus becomes inaudible as soon

as considerably louder sound signals in the same frequency band are added.

Nevertheless, the further the level that the desired signal decreases, the more

the noise oor becomes a disturbing factor. Expanders or noise gates oer a

solution for this problem: these devices attenuate signals when their amplitudes

drop, thereby fading out the background noise. Relying on this method,

gaincontrolling ampliers, like expanders, can extend the dynamic range of a

signal and are therefore the opposite of a compressor.

In practice, it is shown that an expansion over the entire dynamic range is not

desired. With an expansion ratio of 1:5 and a processed dynamic range of 30 dB,

an output dynamic range of 150 dB will be the result, exceeding all subsequent

signal processors, as well as human hearing. Therefore, the amplitude control is

restricted to signals whose levels are below a certain threshold. Signalsabove

this threshold pass through the unit unchanged. Due to the continuous

attenuation of the signals below this threshold, this kind of expansion is termed

“downward” expansion.

The noise gate is the simplest form of an expander: in contrast to the expander,

which continuously attenuates a signal below the threshold, the noise gate

cuts o the signal abruptly. In most applications this method is not very

useful, sincethe on/o transition is too drastic. The onset of a simple gate

function appears very obvious and unnatural. To achieve inaudible processing

of the program material, it is necessary to be able to control the signal’s

envelopeparameters.

Behringer XR4400 Руководство по эксплуатации онлайн [5/14] 776635

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