Telex Integrated Signal Processor ISP-100 - Manual (Page 82)

8-12
10 July 1998
The lowpass filter LP
1
and the highpass filter HP
1
may each be
independently configured as:
Bessel 6 dB/octave (1 st -order)
Bessel 12 dB/octave (2 nd -order)
Bessel 18 dB/octave (3 rd -order)
Bessel 24 dB/octave (4 th -order)
Butterworth 6 dB/octave (1 st -order)
Butterworth 12 dB/octave (2 nd -order)
Butterworth 18 dB/octave (3 rd -order)
Butterworth 24 dB/octave (4 th -order)
Linkwitz-Riley (Butterworth-Squared) 12 dB/octave (2 nd -order)
Linkwitz-Riley (Butterworth-Squared) 24 dB/octave (4 th -order)
In addition, the cutoff frequency, attenuation, and polarity in the low
band and the high band may be independently adjusted, and the
filters in either band may be bypassed entirely.
The cutoff frequency in each filter is continuously variable from 20
Hz to 20 kHz.
The attenuation in each band is continuously variable from -96 dB to
0 dB, plus mute.
The polarity in each band is selectable as + or - (in-phase or out-of-
phase).
Each band may be bypassed, in which case all of the crossover
filters in that band will be bypassed while the crossover filters in the
other band will not be affected.
Two-Way Bessel and Butterworth Crossovers
Bessel crossovers utilize Bessel filters. Bessel filters are
designed for maximally-flat group delay, meaning that they have
nearly linear phase and excellent transient response. Their
amplitude response is not as flat and their cutoff is not as sharp
as those of Butterworth filters.
Butterworth crossovers utilize Butterworth filters. Butterworth
filters are designed for maximally-flat amplitude response,
meaning that the amplitude response in the passband is as flat
as possible without ripple. Their phase response is not nearly as
linear and their transient response is not as good as those of
Bessel filters.
The Bessel filters used in the ISP-100 crossovers are
automatically scaled so that their amplitude response is -3 dB at
the selected cutoff frequency, regardless of the filter order.
The amplitude response of a Butterworth filter is naturally -3 dB
at the selected cutoff frequency, regardless of the filter order.