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MITCHELL
STEREO ANALOG ELECTRONIC CROSSOVER
REVISION P
RB
– DECEMBER 2020
Copyright © 2020, M2Tech Srl
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7.1. All-pass Filter
An all-pass filter is a filter which amplitude response is flat over the whole frequency range
(0dB), while its phase changes linearly with frequency. In the MITCHELL, the phase goes
from 0 degrees at very low frequencies to -180 degrees at very high frequencies. The
frequency at which the phase is -90 degrees can be set by configuration.
An interesting feature of the all-pass filter is the group delay which it introduces, which is
useful to time-align a driver emission at certain frequencies to optimize the emission beam.
7.2. 1-pole Low-pass Filter
This block implements a 6dB/Oct low-pass filter. Its cut-off frequency can be set by
configuration. A 1-pole filter is the simplest filter that can be used. Many 2-way
loudspeakers use 1-pole, 6dB/Oct filters in the crossover to obtain the highest emission
coherence throughout the frequency range as 6-dB/Oct filters sport linear phase.
7.3. 1-pole High-pass Filter
This block implements a 6dB/Oct high-pass filter. Its cut-off frequency can be set by
configuration. Its setting is totally independent from the 1-pole low-pass filter.
7.4. State Variables Filter (SVF)
A state variable filter is a smart circuit that can be set to operate as 2-pole, 12dB/Oct. low-
pass, as 2-pole, 12dB/Oct. high-pass or as notch filter.
Being it a 2-pole filter, it has two parameters: the cut-off frequency and the dumping factor
(Q) at cut-off. This is useful when a certain pulse response is desired with specified
overshoot and ringing. Also, when composing 1-pole filters with SVF to obtain higher slope
filters, it's possible to set the Q of the used SVF to obtain a specified overall response
(Butterworth, Bessel, Chebishev or Linkwitz-Riley).
Two SVF are available in each filter module, each one independently configurable.
Composing them with a 1-pole filter allows for obtaining slopes up to 30dB/Oct (5 poles).
Each SVF can be set to make a low-pass filter, a high-pass filter or a notch. The latter is a
special filter which only deletes one single frequency and a very narrow band around it. It
can be useful with drivers breakups which often affect the sound, even outside their
allowed frequency range. This is typical with large woofers: They are cut quite low (say
300Hz), yet their breakups in the middle-range can be easily heard. Placing a notch filter
at their main breakup frequency usually cleans up the sound.
