of Microprocessors

SOUND MODIFICATION METHODS 65
longer harmonically related. Sounds that normally have harmonic spectra
undergo a complete change in some aspects of their timbre when shifted a
few tens of hertz, while other aspects are relatively unaffected. In particular,
they tend to take on a sort of "watery" texture. Quality spectrum shifters that
have good low-frequency response and work properly even with small
amounts of shift are still relatively complex and expensive.
A related but much simpler device is the balanced modulator, which is
also known as a ring modulator. The·device has two signal inputs that are
identical and an output. In operation, an instantaneous output voltage equal
to the product of the two input voltages with all algebraic sign conventions
observed is generated. Figure 2-9 shows the output waveform with two
different frequency sine waves as inputs. Note that the output spectrum
contains neither of the input frequencies but does contain a sum frequency of
1,200 Hz and a difference frequency of 800 Hz.
In order to more fully understand the physical effect, consider an
experiment in which one of the inputs is connected to a complex waveform
with a lot of frequency components and the other input is connected to a
variable-frequency sine wave oscillator. With the oscillator set to zero frequency,
the output is the same as the other input. As the oscillator frequency
increases, a spectral plot of the output, as in Fig. 2-10, would show each
frequency component splitting into twO components, which then move away
from each other. Thus, one copy of the input spectrum shifts down in
FREQUENCY (Hz)
+
Or----------r--------1'-----------t--~--~-.
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Fig. 2-9. Action of the balanced modulator. (A) Input to A.