Allen&Heath GL2800 - Manual (Page 26)

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
30
40
50
0
20
GAIN
1-2
AUX
3-4
AUX
5-8
AUX
9-10
AUX
MTX3-4
MTX1-2
LATCH
L-R
MAX MIN
ON
OSC/NOISE
M
TALK
1kHz OSC
PINK NOISE