BSS Audio FDS 355, User Manual

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12.0 Headroom Management System
Headroom Management System (H.M.S) is employed in the FDS-355 to make
maximum use of the dynamic range of the devices inside the unit. In systems
where limiter threshold levels are low, the output signal excursions are
restricted, and thus make poor use of the available dynamic range.
HMS makes use of a variable gain stage in the digital domain, and a variable
attenuation stage in the analogue signal path to change the gain structure.
Whenever the limiter threshold or output gain is adjusted, HMS calculates the
excess headroom and simultaneously increases both the digital gain, and the
analogue attenuation to match. Since the Digital to Analogue Converter
(DAC) and various other noise contributing components lay between these two
adjustment points, the noise appearing at the output will be reduced.
Note that HMS is not a dynamic process; it only makes adjustments to the
gain structure when the user makes adjustments. It is therefore free of side
effects.
13.0 Phase Compensation
A classic 2-Way crossover using just a high-pass and a low-pass filter will
always meet the criteria predicted for a given alignment. A 2-Way Linkwitz-
Riley crossover ,for example, will produce its two outputs 'everywhere in
phase', and they will acoustically combine to a flat amplitude response. The
main lobe of the polar response will also be on-axis.
When more filters are added to the crossover for more than two drivers,
however, the crossover begins to depart from the mathematical perfection of
the 2-Way case. In a 3-Way crossover, the high-pass characteristics of the mid
band will be disturbed by the phase response of the low-pass in the mid band,
and vice-versa. This results in drivers of adjacent bands being driven out of
phase, producing irregularities in the amplitude response, and pushing the
main lobe of the polar response off-axis, further aggravating amplitude
response problems in some listening positions. Although these effects may be
subtle when the crossover frequencies are well separated, 4 and 5-Way
systems can produce significant errors.
The phase compensation scheme employed in the FDS-355 analyses these
phase anomalies whenever adjustments are made, and introduces phase
adjustment into certain bands such that the phase difference between all
adjacent bands is always close to zero degrees. It will of course allow the user
to introduce intentional phase differences, using the phase and delay
parameters. The FDS-355 will not attempt to apply phase compensation if the
high and low frequencies or shapes of the adjacent bands do not match, on
the assumption that the user does not expect to produce a standard alignment.
Headroom Management System (HMS)
Phase Compensation