Telex Integrated Signal Processor ISP-100 - Manual (Page 85)

8-15
Processing Components - Crossover
In the ISP-100, 3-way Bessel and Butterworth crossovers are
implemented as shown in Figure 8-6.
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.
Three-way Linkwitz-Riley Crossovers
Figure 8-7 Three-way Linkwitz-Riley crossover implementation.
In the ISP-100, 3-way Linkwitz-Riley crossovers are
implemented as shown in Figure 8-7.
Linkwitz-Riley designs constitute a special, even-order in-phase
class of two-way sum-to-allpass response crossovers, and they
must be configured in accordance with very strict phase and
polarity requirements. Because the basic Linkwitz-Riley
crossover is a 2-way design, extra steps must be taken in 3-way
Linkwitz-Riley crossovers to ensure that these phase and
polarity requirements are met.
Note the presence of the unexpected
θ
1
allpass filter in the high