Crossover 102 - Electronic
Crossovers - Page 4
Odd order filter networks will always introduce a situation
where the output of the network is either +/- 90 degrees out
of phase with the input signal. For instance the 3-rd order
filter will introduce 270 degrees of phase shift, which is
still 90 degrees short of 360. If the output of an electrical
circuit exhibits a 360-degree change in phase, you are essentially
back "In-phase." However the signal does spend a finite amount
of time passing through four high or low pass filter circuit
networks. This is something called group delay (in my day group
delay meant that the band was late). Even though it took a
small amount of time to pass through the filter stages, since
you ultimately made a 360-degree turn, you are now headed in
the same direction; i.e. the output is in-phase with its input.
Even order filter networks will always give you a multiple
of either a 180-degree or 360 degree shift in phase. It is
accepted practice in professional audio that it is desirable
to maintain a unity of phase through out the system, in other
words, the output should be in-phase or headed in the same
direction as the original input signal. When a 2nd order filter
network is introduced, the outputs are 180 degrees out-of-phase.
This is not really a problem, because we can switch or reverse
either the outputs of the crossover circuit itself, or switch
the leads to the loudspeaker itself (but not both), to restore
phase unity. If you try reversing the outputs of an odd order
crossover filter design, since they are going to introduce
a shift in output phase that is some odd number multiple of
90, you are then still dealing with +/- 90 degrees of phase
shift.
Even though Linkwitz and Riley were correct in the areas outlined
in their paper, we continued to use 3 rd order crossover filter
networks (for the most part) throughout the remainder of the
20th century, and into the 21st. The reason is that 2nd order
filters do not offer enough protection for the high frequency
compression driver, and variable fourth order filter circuitry
can be very difficult to accomplish precisely or accurately.
In the electronic technology, there are controls called potentiometers
that are used as volume or tone controls, as well as to change
a crossover filter frequency. Remember we said if you vary
the R with a fixed value of L or C, you will change the turnover
point or crossover frequency of the filter. To have multiple
poles or orders of filter networks, we repeat the value of
R, L, & C, in each subsequent filter stage or order. The precision
of the filter design depends on the tolerances of these component
values.
There is a special type of potentiometer called a ganged control.
You can have two, three, or even four variable resistors that
are stacked or staged so that you can change the resistance
with a common control shaft that simultaneously moves two,
three, or four wipers or variable resistor contacts. One of
the technological challenges involving variable ganged controls,
is the typical tolerances of each potentiometer stage is usually
no better than 20%.
As a result of the research done by Linkwitz and Riley, we
now have a filter circuit topology that bear their names. Now
you will essentially find identical component values in each
filter stage of both a Butterworth and a Linkwitz Riley filter
network.
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