Phase Compensation - BSS Audio FDS366T OMNIDRIVE User Manual

FDS-366T
Crossover Shapes and Frequencies
Phase Phase compensation to active crossovers was introduced in the classic FDS-
355 Omnidrive Compact Loudspeaker Management System. The FDS-366T
Compensation
Omnidrive includes this feature, freeing the user of worry about phase
alignment between bands.
More info about Phase Compensation:
A properly designed 2 way crossover always exhibits the inter-band phase
relationships that are characteristic of the chosen crossover shape. For
example, the Linkwitz-Riley filter shape, with its low-pass filter for the low
band, and high-pass filter for the high band, will maintain zero phase
difference between these bands at all frequencies, which means that the
acoustic outputs from the drivers will sum to a flat response, free of any shifting
in polar response.
In 3 way (or more) systems, things can start to go wrong. The mid band in a 3
way system for example, has not one filter, but two (high-pass, and low-pass).
The low-pass filter, set at the mid-high crossover frequency will produce some
phase disturbance at the low-mid crossover frequency, causing the low-mid
crossover filter pair to be misaligned. The same is true of the high-pass filter,
which may affect the mid-high crossover filter pair. This misalignment causes
incorrect acoustic summing, which results in a non-flat response, and a polar
response which shifts with frequency, causing further response problems in
some listening positions. Although these effects may be subtle when the
crossover frequencies are well separated, 4 and 5 way systems particularly can
produce significant errors.
The phase compensation scheme employed in Omnidrive analyses these phase
anomalies whenever adjustments are made, and automatically introduces
phase adjustment into certain bands so that the phase difference between
adjacent bands is always close to zero degrees. Intentional phase differences
can be introduced using the Delay and Phase parameters.
Omnidrive 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 crossover alignment.
By default the FDS-366T applies phase compensation to crossover setups
where there are 3 bands, or more, in use on a channel and where the
crossover filter shapes are identical. In general the application of this feature
is desirable to maintain the correct phase relationships between bands but, in
certain circumstances phase compensation may need to be defeated to avoid
phase inconsistencies between separated speaker installations, such as side or
under fills and certain monitoring applications. The facility to do this is called
'Phase Comp' and is found in the Utilities.
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