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MITCHELL
STEREO ANALOG ELECTRONIC CROSSOVER
REVISION P
RB
– DECEMBER 2020
Copyright © 2020, M2Tech Srl
20
7.5. Gain block
As speakers systems usually use drivers and/or power amplifiers with different sensitivity,
the crossover must be able to compensate sensitivity differences. The
MITCHELL
allows
each way gain to be set in a wide range (-111dB to +30dB) in 0.5dB steps.
7.4. Inversion blocks
The phase of each output can be set to non-inverting or inverting with regards to the
input's absolute phase. This is useful because certain filter configuration need a phase
inversion between adjacent drivers in order to obtain the correct overall response and
beaming.
7.5. Composing blocks to obtain a desired filter configuration
As already said, the various blocks in a filter module can be composed to obtain more
complicated filter than each block itself.
E.g. to obtain a band-pass filter with a 12dB low frequency high-pass and a 18dB high
frequency low-pass, two SVF and thee 1-pole can be cascaded. By purposely choosing
frequencies and Q's, desired filters can be obtained. If Butterworth filters are needed, then
the Q in the SVF used for high-pass should be set to 0.707, while the Q in the SVF used
for low-pass should be set to 1. This comes from filters theory, which we will not address in
detail as it's not in the scope of this manual and it's considered to be already known by the
user.
To obtain the desired configuration, the user must send the SP command with the related
4-digit hex number as argument. The number is composed by selecting and chaining the
right values for the various parameters shown in Figure 3: APSEL, H6INSEL,
IN1SEL[1..0], IN2SEL[1..0], OUTMUX[2..0] and OUTSEL[1..0]. The 4-digit number is
composed in binary format as follows:
|| OUTSEL1 | OUTSEL0 | IN1SEL1 | IN1SEL0 | 0 | 1 | APSEL | H6INSEL || +
+ || OUTMUX2 | OUTMUX1 | OUTMUX0 | 0 | 0 | 0 | IN2SEL1 | IN2SEL0 ||
As an example, let's compose the SP argument to set the path for way A, both channel, to
be an inverting 6dB/oct low pass with a notch filter. APSEL must be '0' because we don't
want the signal to go across the all-pass filter. As we want a notch filter, we'll use the state
variable filter 1 to this purpose, therefore we must set INSEL to accept the output of the
6dB/oct low-pass filter. Its value is therefore '10'. Then, we must set the output mux to
receive the signal from the notch output of the SVF1. Therefore, OUTMUX will be '111'.
We want the signal path to be inverting. Therefore the output selector will choose the
signal from the inverting block connected to the output mux. OUTSEL will be '10'.
H6INSEL and IN2SEL are don't care and must be set to '0' and '00' respectively.
Summarizing, the final result is b'1010010011100000' or h'A4E0'. The command
#SP,A,B,A4E0
will select the desired configuration.