BSS Audio FDS-334T Owner's Manual - Manual (Page 40)

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The FDS-334T/336T allows full control over each high and low pass filter of a crossover seg-
ment in shape, slope and frequency. Graphically these parameters are labelled as below.
Filter Edge Identification
Phase Compensation
Phase Compensation to active crossovers was introduced in the classic FDS-355 Omnidrive
Compact Loudspeaker Management System. Minidrive includes this feature, freeing the user of
worry about phase alignment between bands.
A properly designed 2 way crossover always exhibits the inter-band phase relationships that is
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 will cause the Mid-High crossover filter pair to be misaligned. 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 Minidrive analyses these phase anomalies when-
ever adjustments are made, and automatically introduces phase adjustment into certain bands
such that the phase difference between adjacent bands is always close to zero degrees. Inten-
tional phase differences can be introduced using the delay parameter. Minidrive will not at-
tempt 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.
Outputs Crossover Shapes and Frequencies