Manual Allen-Heath GL2800 (page 26 of 42) (English)

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26 GL2800 User Guide

Using the Oscillator / Noise Generator The GL2800

includes a useful signal generator able to produce a pure 1kHz sine

wave tone, or a pink noise test source. This is invaluable in testing the

system components and setting up correct gain structure. The

generator can be routed independently to the various console outputs.

Use the 1kHz tone to line up the connected equipment. For example,

a recorder, video camera or broadcast feed from the matrix. Route the

tone to the matrix. Set the matrix LEVEL control to its 3 o’clock ‘0’

position. Press the matrix AFL and adjust the oscillator level until the

console monitor meters read ‘0’. Next, adjust the recorder input trim

so that its meters also read ‘0’. The recorder is now correctly aligned

to the operating level of the console. Similarly, you could match the

console LR output to a DSP speaker processor by sending the tone to

LR, setting the faders for output meters reading ‘0’, and then trimming

the processor input for ‘0’ on its meters. To prevent a loud tone

through the speakers it is best to do this with the amplifiers turned off.

Use the Pink Noise generator to check the signal routing and

loudspeaker response. ‘Pink noise’ is a random signal that contains

all audio frequencies and is therefore a very good test source. Unlike

‘white noise’ which has equal energy per Hz and sounds like system

or inter-band tuner hiss, pink noise has equal energy per octave, a

response that matches the logarithmic way our ears perceive sound.

You can hear all the frequencies clearly from deep bass, through mid

to treble. This means you can route the noise to a speaker system

and quickly hear if one of the sub, mid or HF drivers is faulty. With the

noise sent to several speakers at the same time you can listen for

comb filtering effects, and the severe phasing effect which indicates

that one of the speakers may have its wires reversed. Pink noise is

also used with an RTA (real time analyser) when measuring room

frequency response. Testing a system with pink noise through the

speakers is often referred to as ‘pinking the room’.

The GL2800 signal generator is an invaluable tool for calibrating

and testing the system during setup. To prevent any unexpected

mishaps during the sound check or show, remember to disable it by

releasing the recessed ON switch once you have finished the testing.

We also advise that you always start the calibration with the

OSC/NOISE trim turned fully off so that you can bring it up gradually

without the risk of overloading the destination.

Mixing with faders or gain controls? There is a technique

used by some operators where they set all the faders to ‘0’ position

and balance the mix using the channel gain controls. We do not

advise this method as the signal to noise ratio and control resolution

can be severely degraded. In addition, it is impossible to mix monitors

from FOH in this way as changes to the gain settings affect the

monitors too.

The correct method is to use GAIN to match the source to the

operating level of the channel for optimum dynamic range, and then

use the FADERS to balance each source into the mix. With correct

system gain structure, prime sources such as vocals would have their

faders operated around ‘0’ while sources low in the mix such as

backing vocals and acoustically loud brass and drums would display

their true contribution with their lower fader positions. This is a much

more visual and accurate way of mixing.

60 40-14 6

GAIN

1-2

AUX

3-4

AUX

5-8

AUX

9-10

AUX

MTX3-4

MTX1-2

LATCH

L-R

MAXMIN

OSC/NOISE

TALK

1kHz OSC

PINK NOISE

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Allen-Heath GL2800 Manual

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