Gain Structure; Gain / Level Optimization - Lab.gruppen PLM Series Operation Manual

12.3 Gain structure

The architecture of the PLM Series provides gain
adjustments at a number of points in the signal flow
path. Thus, there are many places in which one can
adjust the levels in the PLM. Each point serves a
different purpose. The following can be used as
guidelines for adjustment of the gain settings, from
input to output. The block signal flow diagram (figure
8-1 page 22-23: PLM Signal Flow Diagram) can also
be used as a reference to help in understanding the
signal path through the PLM's architecture.
Locations for setting gain in the following
paragraphs refer to the Dolby Lake Controller
PLM Edition software.
Input headroom (Analog Inputs). As a rule, this
should be set to 12 dBu if the source is (or can be)
limited to 12 dBu; otherwise it should be at 26 dBu.
This setting has no direct influence on the rest of
the gain stages in the PLM, or on the overall noise
floor. The PLM's variable input gain is strictly meant
to provide the appropriate headroom at the input
stage. (Set in I/O Config window, Input Configuration,
left side.)
Input mixer. This setting should be left at "0" or
most setups. If only one input channel is used, the
other can be set to "- inf". (Set in I/O Config, module
pane.)
Input gain. This is used to adjust the level between
different cabinets in the system. It should be left
at 0 dB unless you want to lower the level for the
cabinet(s) driven by this module (Set in I/O Config,
module pane).
Output gain (Levels). One adjustment is provided
per output channel in the module to balance the
mutual gain between frequency bands in a multi-way
set up. Generally, this setting is derived from the
LoadLibrary "fingerprint" data within the PLM preset.
As such, it should not need to be adjusted further.
(Set in I/O Config, module pane.)
Attenuator. One adjustment is provided per output
channel in the PLM. This control replaces the
traditional knob found on conventional amplifiers.
It should typically be left at 0 dB during use. (Set in
Lab.gruppen Info, Status tab).
APPLICATION GUIDE 12
Amp Gain. Corresponds to the gain that you would
have available in a conventional system if the amplifier
was outside the PLM. It should retain the same
settings as in the module file that is loaded: The
limiter and output gain settings in the output stage of
the module were configured with this gain setting and
will not be automatically compensated if changes are
made. This configuration scheme, though unusual,
allows for backwards compatibility with other Dolby
products. (Set in Lab.gruppen Info, Control tab.)
The following details can assist in setting an
appropriate gain structure within the PLM.

12.3.1 Gain / Level Optimization

Optimum setting of the PLM gain structure requires
particular attention to two matters:
Make sure it can get loud enough.
To do this, you need to ensure that you have sufficient
headroom in the signal path to avoid clipping before
you reach the limiters. At the same time, you need
to achieve enough gain through the PLM to engage
the limiters and realize a high average SPL as a
consequence. Try to allow for a headroom of 10
dB or more for all channels. The simplest way to
accomplish this is to increase the input gain on the
module.
Minimize the noise.
As a first step, use a digital input signal (AES or Dante)
when possible. If using analog inputs, make sure that
you don't introduce headroom on the input side that
is unused or unreasonably high. If you don't need full
power, or at least not a high average power, you may
also lower the gain on the input of the module.
Figures 12-3 through 12-6 demonstrate how
altering the gain structure within the PLM affects
performance.
PLM Series Operation Manual 63