of Microprocessors

VOLTAGE-CONTROL METHODS 85
Typically one might expect to find three frequency conrrol inputs even
though only one oscillator is to be controlled. In operation, the voltages at
the control inputs are summed together algebraically into an effective control
voltage. Thus, if input A is at 5 V, B is at 3 V, and C is at -4 V, then the
effective control voltage is 5+3-4=4 V. An unused input can be considered
as a zero voltage contribution. Having multiple inputs often eliminates
the need for a separate mixer module. The use of separate inputs is also
preferable to output mixing by tying two outputs together because the
inputs are isolated from each other, thus allowing the mixed outputs to be
used elsewhere as well. Occasionally, a fourth input with either a 10-V/
octave or variable sensitivity is provided. When calculating the effective
control voltage, the level at this input must be divided by its sensitivity
before summing with the other conrrol inputs.
Usually, two mechanical inputs, each of which might have coarse and
fine adjustment knobs, are associated with the control inputs. One of these in
effect changes the basis frequency by adding an additional control voltage,
which may range from -10 V to + 10 V. Sometimes a range switch is
provided in order to extend the low-frequency range down to fractions of a
hertz. The other control is a volts per octave adjustment with the standard
value of 1 V/octave being the middle of the range. Besides fine tuning the
oscillator, this control can make an ordinary keyboard microtonal wi th
perhaps 31 notes to the octave. Finally, there may be a sensitivity control for
the fourth control input mentioned earlier.
A VCO typically has several different output waveforms. Usually they
are available simultaneously, all at the same frequency and with a fixed phase
relationship. Sometimes, however, a tapped panel control is used to adjust a
single output waveform as a variable mixture of two or three internal
waveforms. Figure 3-4 shows the four fundamental synthesizer waveforms
and their associated harmonic spectra.
A sawtooth wave is so named because of its appearance. It has both odd
and even order harmonics that fall off in amplitude as the reciprocal of the
harmonic number, which is 6 dB/octave. Its timbre is full-bodied and somewhat
bright. At low frequencies, it does indeed sound "sawey."
The triangle wave resembles the sawtooth in appearance but has a quite
different spectrum. Only odd order harmonics are present and their
amplitude falls off as the square of the harmonic number or 12 dB/octave.
The timbre of the triangle wave is subdued and mellow and somewhat
hollow. The mellowness is due to weak upper harmonics, and the hollowness
is due to the exclusively odd harmonic spectrum.
The exact spectral characteristics of the rectangular wave depends on its
duty cycle. The duty cycle is the ratio of the time spent in the high state
divded by the overall period. Spectra are shown for a 50% duty cycle and a
10% duty cycle. Note that in the 50% case, which is called a square wave,
only odd order harmonics are present and they decrease at 6 dB/octave. The