Bedienungsanleitung Allen-Heath GL2800 (Seite 27 von 42) (Englisch)

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GL2800 User Guide 27

Using PFL / AFL Allen & Heath are renowned for bringing you

the most comprehensive engineer’s monitoring system in consoles at

this price point. We recognise the importance of correct gain structure

and signal handling. The GL2800 includes LED meters for every

input channel and main output, as well as a pair of high resolution

meters dedicated to monitor duty only. In monitor mode the M fader

and associated controls become a full featured PFL/AFL monitor feed

for the engineer’s listen wedge.

The input channels provide PFL (pre-fade listen) so that each source

can be checked using the meters and headphones before you bring

the fader up. You can even use PFL while the channel is muted to

prevent the signal reaching the house and monitor speakers until you

have checked it and are ready.

All the main and aux outputs provide AFL (after-fade listen) so that you

can check the exact level leaving the console. Once again, the AFL

switch gets its source before the output mute switch so that you can

check the signal before you send it to its destination. This can be very

important when you are feeding remote destinations such as

broadcast and recording.

The Decibel the ‘Bell’ is the unit of sound level. Decibel is 1/10

Bell, a more conveniently sized unit. dB = 20 log (Vo / Vi) where Vi

and Vo are two signal voltages, in and out (ignoring the impedances).

The dB is used to express the relationship between two levels, chosen

because of the logarithmic way our ears respond to sound. The ‘dB’

relates one level to another. For example, a preamp with a gain of

40dB (100x) would produce an output of +10dBu for an input of -

30dBu. Several audio standards exist to relate a signal level to a

known reference. Audio engineers need to deal with a variety of

equipment standards, for example, a -10dBV CD player plugged into a

console with +4dBu outputs connected to a 0dBu input DSP speaker

manager, or an operator using an SPL (sound pressure level) meter to

measure sound intensity.

dBu Relative to 0.775Vrms – professional standard

0dBu = 0.775V +4dBu = 1.228V

dBV Relative to 1Vrms - consumer audio standard

0dBV = 1V -10dBV = 316mV = -8dBu

dB-A Sound pressure with a filter contour to approximate

the response of the human ear. Three curves A, B,

C exist for different loudness.

dBfs Relative to signal maximum before clip (full scale)

Using Inserts The GL2800 channel inserts operate at 0dBu,

the output inserts operate at -2dBu. In practice this makes little

difference as long as the inserted equipment is intended for line level

operation (-6 to +4dBu). Simply set the gain through the device to

unity (0dB) with the bypass switch pressed (if available). With the

effect switched in, use the console channel gain control to make any

further adjustments needed. This keeps the gain structure correct

through the channel signal path.

With nothing plugged into the insert, the channel signal is routed

through a switching (‘normalling’) contact in the socket. As soon as a

jack is plugged into the socket the contact is opened and the signal

path broken so that the external device can be patched in series with

the signal.

AFL

SEND

INSERT

RETURN

TIP

RING

☺ If you suspect the insert

socket to be faulty or

intermittent through excessive

wear or contamination, test for

this by plugging in a jack with

its tip shorted to its ring

contact. This bypasses the

contact in the socket. Clean

using suitable electrical

contact cleaner.

Tech talk…

For an audio circuit stage:

Pi = Input signal power

Po = Output signal power

The Decibel is defined as:

dB = 10 log (Po / Pi)

= 10 log (Vo

/Ro / Vi

/Ri)

If input and output impedances

Ri and R0 are the same, then:

dB = 20 log (Vo / Vi)

dBm is defined as 1mW into

600 ohms = 0.775V

dBu = 0.775V ignoring the

600 ohms as we assume high

input and low output

impedance and therefore

maximum voltage transfer

between stages, appropriate

for modern audio.

Note: The stereo Aux 9-10

and matrix 1-4 outputs can

be monitored using the

monitor select switch bank.

This provides true stereo

monitoring of these mixes.

PFL/AFL automatically

overrides any source

selected on this

switchbank.

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