Inter-m CSP-4.8, Operation Manual

Page: 11 / 24

SIGNAL PROCESSOR
8
CSP-4.8
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. Inter-M 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 Inter-M. 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 CSP-4.8 crossovers:
- Butterworth
Butterworth filters individually are always -3dB at the displayed crossover frequency and are used because
they have a "maximally flat" passband and sharpest transition to the stopband. When a Butterworth HPF
and LPF of the same crossover frequency are summed, the combined response is 3dB. With 12dB
per octave Butterworth crossover filters, one of the outputs must be inverted or else the combined response
will result in a large notch at the crossover frequency.
- Bessel
These filters, as implemented on the CSP-4.8 processor, is always -3dB at the displayed crossover frequency.
Bessel filters are used because they have a maximally flat group delay. Stated another way, Bessel filters
have the most linear phase response. When a Bessel HPF and LPF of the same crossover frequency are
summed, the combined response is +3dB for 12dB/oct, 18dB/oct, and 48dB/oct Bessel filters, and -2dB for
24dB/oct Bessel filters. One of the outputs must be inverted when using either 12dB/oct or 18dB/oct Bessel
crossover filters or else the combined response will have a large notch.
- Linkwitz-Riley
The 12 dB/oct, 24dB/oct and 48dB/oct Linkwitz-Riley filters individually are always -6dB at the displayed
crossover frequency, however the 18dB/oct Linkwitz filters individually are always -3dB at the displayed
crossover frequency. The reason for this is that Linkwitz-Riley filters are defined in terms of performance
criterion on the summing of two adjacent crossover HPF and LPF filters, rather than defined in terms of the
pole-zero characteristics of individual filters. The 18dB/oct Linkwitz-Riley individually are 18dB/oct
Butterworth filters in that they have Butterworth polezero characteristics and also satisfy the criterion for
Linkwitz-Riley filters. When a Linkwitz-Riley HPF and LPF of the same displayed crossover frequency are
summed, the combined response is always flat. With 12dB/oct Linkwitz-Riley crossover filters, one of the
outputs must be inverted or else the combined response will have a large notch at the crossover frequency.