of Microprocessors

VOLTAGE-CONTROL METHODS 89
two is rare in the smaller systems. Voltage-controlled filters of adequate
performance are a recent development, and there is still considerable room for
improvement.
The signal inputs and controls are similar to those of the VeA, although
there may be only one or two provided. There are usually several
signal outputs, however, each corresponding to a different filter mode. The
most popular filtering circuit simultaneously generates a low-pass, highpass)
and bandpass output from a single input. A band-reject output may
also be provided at very little additional cost. Sometimes the multiple outputs
are applied to a tapped potentiometer, and the user seleCts a mixture of
these filtering functions to be sent to a single output.
Although there are multiple outputs, there is only one frequency
parameter. Under the proper operating conditions (low Qfactor) the low-pass
and high-pass - 3-dB frequencies and the center frequencies of the bandpass
and band-reject outputs are all the same. At higher Qs, the low-pass and
high-pass filtering functions undergo a phenomenon called corner peaking,
which is illustrated in Fig. 3-5. At very high Qs, the corner peaking becomes
excessive, and the audible filtering effect of low-pass, bandpass, and
high-pass outputs becomes similar. The notch output under these conditions
becomes so narrow that its effect on the sound is hardly noticeable.
Since the Q factor has such an important effect, it must be variable. The
most common configuration is a single panel control for Q. It is desirable,
however, to have a voltage-controlled Q probably with an exponential relationship,
since the useful range for Q is from 0.5 to several hundred at least.
One common imperfection in existing filters is that high Q factors lead to
instability and may even cause the filter to break into intermittent oscillation.
If this was not such a problem, Qs up in the thousands would be useful
for generating slowly decaying ringing sounds in response to a short input
pulse.
Normally, the frequency control inputs are straightforward and essentially
identical to the frequency control inputs on the VCO. However, there
is an interaction between frequency and Q that must be conSIdered. Taking
the bandpass output as an example, there are two things that may happen
when the center frequency is varied. In a constant Q filter, the Q factor
remains constant as center frequency is changed. This is desirable if one is
filtering white noise to give it a sense of pitch, since the same degree of
"pitchedness" will be retained regardless of the center frequency. However, if
the filter is used to create ringing sounds, the ringing time will be much
longer at lower frequencies than it will be at high frequencies. This is because
Q is really related to the rate of ringing decay in terms of number of cycles
rather than seconds. A constant bandwidth filter actually has a Q that is
proportional to center frequency. Thus, if the bandwidth is set to 100 Hz
and the center frequency to 300 Hz, the effective Q is low, but if the center
frequency is 5 kHz, the laO-Hz bandwidth represents a relatively high Q.
Although there are applications for both types of frequency control, most