Nagra Seven Operating Instructions Manual page 100

You have the choice whether to control this gain with the potentiometer or by specifying a fixed value.
When using as limiter, it is probably preferable to use the potentiometer for control. The control will act on
the microphone preamplifier gain as if the limiter were not switched on, except there will be the headroom
offset subtracted. Just be aware that with large attenuations the headroom can be smaller than specified.
When using as an ALC it is better to use larger headroom, letting the DSP control the gain. With headroom of
42 dB, you can only have an adjustment of 3 dB for the microphone preamplifier gain: an input attenuation of
0 dB will set the microphone preamplifier gain to -42 dB and an input attenuation of -3 dB will set the
preamplifier gain to -45 dB which is the maximum. So the input attenuation will be mostly limiting the digital
gain and act as a "threshold" avoiding the gain being too loud when the ALC is not limiting the gain (during a
silence).
When the input attenuation is controlled with the potentiometer, the potentiometer will mainly act as
threshold and allow fade IN/OUT, if a fixed input attenuation is set, the potentiometer is disconnected from
input gain control and no longer does anything.
Attack:
This parameter manages the speed with which the gain is limited. With the "immediate" parameter, also
known as a "brick wall", the gain limitation will be done immediately avoiding the target parameter to be
overshot. When specifying a time (from 125 microseconds to 2 milliseconds), the attack parameter will act as a
filter on high-pitched / frequency sounds (such as keys being shaken for example). This avoids closing the gain
too much on unwanted sounds, by the way the target level can be overshot on such sounds. Typically the
overshoot can be of 1-2 dB for 125 µS setting; 3-4 dB for 250 µS setting; 4-5 dB for 500 µS setting; 5-6 dB for 1
mS setting and 6-8 dB for 2 mS setting.
Delay:
This is the time period during which the ALC/LIM process will not increase the gain after having limited,
keeping the gain constant even if the sound is too low. This parameter avoids the gain "pumping", that is the
gain increase during short silences. This setting should be of the same order as the release parameter to be
consistent and give a pleasant operation of the ALC/lim.
Release:
This is the speed of gain increase when limiting gain is not necessary. The speed is set by specifying the time
that the ALC/LIM process takes to reach the maximum gain starting from minimum gain. This parameter
mainly determines whether the ALC/LIM process works as limiter (set to minimum) or as an ALC.
Ratio:
This parameter determines the target level function of the input level. With a flat ratio, the ALC/LIM will only
limit the gain when the signal would overshoot the target parameter, giving a signal level that is independent
of the input level. Specifying a ratio other than flat, allows having a more natural relationship between
microphone signal and signal level. A ratio of 6:1, for example, will "compress" 6 dB into 1 dB; That is the
ALC/LIM will start limiting when the signal is lower than the target level. With headroom of 24 dB and a ratio
of 6:1, the limiter start limiting when the signal is 4 dB (24 dB / 6) before the target level, if the target level is -2
dB the ALC/lim will start limiting at -6 dB (-2 dB – 4 dB); for ALC or limiter, usually a ratio of at least 6:1 is used,
lower ratios (3:1, 4:1, ...) are for those who want the limiter to work more like a compressor than a limiter.
Graphic tabs:
On the graphic tabs you can see the effect of headroom, target, input attenuation and slope parameters for
clarity. These graphics show the relationship between the microphone level and recorded level for a constant
input microphone level. Dynamic parameters (Attack, Delay and Release) cannot be represented on these
graphics and are not shown.
ANNEX I 99
September 2013