Ashly Protea DSP480 Operating Manual - Page 8

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Operating Manual - PROTEA™ DSP480 and DSP360 System Processors

4 .6d Crossover (Xover)

Crossover functions are avail-

able only on the output channels. Every

channel's crossover consists of a high

pass filter (HPF) and a low pass filter

(LPF), along with the frequencies and

filter types used. Each output's crossover section is essentially a bandpass filter, making it necessary for the user to map out ahead

of time which outputs will be used for the various frequency bands, and set the overlapping filter frequencies and types accord-

ingly. Note: The HPF determines the lower frequency limit of the signal, while the LPF determines the upper frequency limit.

The frequency range for the high pass filter (HPF) is from 19.7Hz to 21.9kHz, with an option to turn the filter off at

the low end of the frequency selection. The low pass filter (LPF) offers the same frequency range, with the "off" option at the

high end of the frequency selection.

There are 11 types of filters available in the crossover section, each suited to a specific preference or purpose. The slope

of each filter type is defined by the first characters in the filter type, 12dB, 18dB, 24dB, or 48dB per octave. The steeper the slope,

the more abruptly the "edges" of the pass band will drop off. There is no best filter slope for every application, so experiment

to see which one sounds most pleasing in a specific system. Ashly factory default presets use all 24dB/octave Linkwitz-Riley

filters in the crossover section, but of course they can be changed to suit the application.

In addition to the frequency and slope, crossover filters can be selected as having Butterworth, Bessel, or Linkwitz-Riley

response. These refer to the shape of a filter's slope at the cut-off frequency, affecting the way two adjacent pass bands interact

at the crossover point. 24db/octave Linkwitz-Riley filters produce a flat transition through the crossover region, assuming both

overlapping filters are set to the same frequency, slope, and response type. 24dB/oct Linkwitz-Riley filters are the industry

standard, the easiest to use, and the filter type recommended by Ashly. Other filter types are available, but may require polarity

switching or other adjustments for proper results. The following paragraphs offer a summary of the three filter types as used in

DSP480 and DSP360 crossovers:

Output channels have time delay as well. Output delay is best used to align discrete

drivers within a speaker cabinet or cluster, normally quite close together. For example, a

typical three way speaker cluster would have low end, midrange, and high frequency drivers

all located near one another. The different drivers for each frequency band are not necessarily

the same physical depth with respect to the front of

the loudspeaker cluster, so there exists the problem

of same signals (at the crossover points) arriving

at the cluster "front" at different times, creating

undesirable wave interaction and frequency can-

cellation. The solution, again, is to slightly delay

the signal to the drivers closest to the cluster front.

Using the location of the driver diaphragm farthest

back as a reference point, measure the distance to

other drivers in the cluster, and set the output delay

for each accordingly, with the driver diaphragm

closest to the front getting the longest delay and

the driver at the very back getting no output delay.

Note: Although delay is adjusted only by time, the

corresponding distance in both feet and meters is

always shown as well.