Allen&Heath 21 Series - Manual (Page 14)

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clipped signal reading is the overload margin of the system.
Going the other way:
Reset the piano mic 20 feet back.
Back at the console reset the controls to get 0VU again on
loud phrases and reset the monitor level for a true volume
in the control room. It probably sounds OK.
Now ask the piano player to give you his favourite quiet
subtle number. Reset the mic gain for 0VU on the PFL meter.
Notice anything about the noise level behind the programme?
You are probably losing detail in the quietest phrases and also
are bugged by a continual background hiss.
Do not worry - we are exploring the worst that can happen.
Hard fact number two - noise added to the programme by the
preamp cannot be removed by hard work later on either.
The optimum mic gain setting for the particular programme and
mic placement produces 0VU on loudsection, now you know why
the console has meters and peak indicators on it and why
there is a PFL switch for every section. These take the
guesswork out of setting gain controls.
The PFL meter tells you the channel signal level as determined
by the input gain control. Meter indications of 0VU on loud
sections are the maximum that can be used and still retain
the overload margin to faithfully reproduce the unexpected
changes in loudness from a performer. Run hotter at your
peril. Run cooler and you will be adding noise unnecessarily
at the preamp stage.
The channel peak indicator is a permanent, eye catching
warning that the overload margin in the channel amplifiers
has nearly gone, the longer it stays on and the more often
it comes on the closer you are to adding clipping distortion
at the preamp (or equaliser) stage.
At the end of the day your ears are the best and final judge
of sound quality and you may decide to override what the meter
says. Experience has proved (you may test it yourself) that
certain acoustic sources are not reliably analysed for signal
level by meters. If in doubt trust your ears.
Get it right at the input preamp first.