Biamp A4 - Manual (Page 188)

Audia-Manual-A4
Auto Mixer, Ducker, Leveler, and Comp/Limiter functions are triggered by input signals that
exceed a specified threshold. With Levelers and Comp/Limiters, signal levels below threshold are
not considered unusual (they simply are not affected by the component). However, Auto Mixers
will not pass signals that are below threshold, and Duckers will not automatically attenuate
program signal if the sensing input signal is below threshold. Furthermore, signals containing a
large amount of background noise can falsely trigger these components, if the level is set too high
and/or the threshold is set too low. It should also be noted that any real-time control of signal
levels should not occur before these types of components. For example, control of individual Auto
Mixer channels should not take place ahead of the Auto Mixer. Instead, the Auto Mixer Input
Level controls (which are post-threshold) may be used for this purpose.
Ambient Noise Compensation (ANC) relies on a continuous and accurate model of the program
signal level, to differentiate it from changes in the ambient noise level. So, real-time control of
levels should not occur after this type of component. Acoustic Echo Cancellation (AEC) relies on
a continuous and accurate model of the signal to be cancelled from the microphones. So, any
real-time control of signal levels at the loudspeaker output should be duplicated for the AEC
Reference. If these signals are different, a 2-channel ‘ganged’ Level control may be used.
What is proper gain structure? Generally speaking, proper gain structure is establishing and
maintaining good signal strength throughout the entire sound system. In most cases, this means
that the relative volume of loudspeakers should ultimately be determined by adjustment of the
power amplifiers (after prior system gain settings have been established). Other system outputs
(such as recording feeds) may require lower levels, which should be established by selecting an
appropriate reference level at the output itself. Other than real-time level control (as described
previously), signal attenuation within the system should be avoided.
To establish proper gain structure, the primary element is input gain. Each system input provides
adjustable Gain In (trim), with an associated Peak indicator. For best performance, increase gain
on a given input until the Peak indicator just begins to flash on normal signal content. The Peak
indicator first comes on with 6dB of headroom remaining (before clipping occurs). To provide
additional headroom (allowing for louder input signals), it is recommended that gain then be
reduced by 12dB (two 6dB decrements). To monitor system levels, Peak Meters should be
connected at strategic points in the signal path, including at the inputs. With gain settings as
described above, input meters should indicate peak levels between 6dB and 12dB on normal
signal content (12dB ~ 18dB of headroom). This will provide a nominal level of approximately
0dBu, with good signal-to-noise performance and a safety margin of available headroom.
Throughout the rest of the system, various components will include level adjustment capabilities
(faders). By default, these faders are typically at 0dB (unity gain). This is a very good setting for
most applications, and does not necessarily need to be changed. However, any of these faders
can be used for real-time level control (as described above). Of course, faders can be used to
‘mix’ multiple signals at differing levels (i.e. microphones at 0dB and music at -10dB). Faders can
also be used to compensate for gain reduction (Leveling, Comp/Limiting, etc.) which may have
occurred earlier in the signal path. Note: Floating Point DSPs allow this type of gain staging
without the danger of clipping distortion or the loss of data bits. As long as signal levels do not
exceed maximum at the inputs or outputs (A/D & D/A converters), extremely high and low level
signals can be tolerated without negative impact on quality. However, remember that some
system components do not function well without proper signal strength (as described previously).
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