3. "Modulation" means that some aspect of one signal (the carrier) varies
according to an aspect of a second signal (the modulator). Tremolo, slow
amplitude variation and vibrato, slow frequency variation, are examples of
acoustic modulation. When the frequency is low, below 20 Hertz, you get
tremolo; when the frequency of the modulator rises into the audible range (i.e.
above 20 Hz), audible modulation products or "sidebands" begin to appear.
These new frequencies are added to the spectrum of the carrier (typically on
either side of the carrier).
4. The amplitude of the carrier wave will vary in accordance with a
modulator wave. Amplitude modulation generates a pair of sidebands
for every sinusoidal component in the carrier and the modulator. The
amplitude of the two sidebands increases in proportion to the amount
of modulation, but never exceeds half the level of the carrier.
5. "Pushing" a second wave shape, called the program signal, against this
constant amplitude forces it to change, but while it changes in direct
relationship to its own amplitude and the amplitude of the program
signal, it retains its original frequency, with an output varying in amplitude
in direct relationship to the frequency and amplitude of the modulating
waveshape, or program. It is the frequency of the program that determines
the speed or rate of amplitude modulation and the amplitude of the
program will define the amount or peak of modulation. If a program
frequency has a very low amplitude, the amplitude of the carrier will not be
displaced very far; the greater the amplitude of the program, the greater
the amplitude variation of the carrier signal.
6. By using a very low frequency modulator (0.1-7 Hz) it is possible to actually
hear the program wave shape. For example, using a 2 Hz sine wave to
modulate a constant carrier signal, one will hear a steady rise and fall in the
amplitude of the carrier. If a triangle wave is used as the modulating signal
(program), the amplitude of the carrier will rise gradually, then
instantaneously drop to silence and gradually rise again, etc. By using a
square wave as the program signal, the carrier will be instantaneously loud
and soft, with no audible rise or decay time in the amplitude. Amplitude
modulation with a 5 to 9 Hz sine or triangle wave will produce a very
pleasing tremolo.
7. Amplitude modulation is not limited to the production of various rates of
tremolo. As the frequency of the program signal approaches the audible
range, it is more and more difficult for the human ear to detect each individual
amplitude fluctuation in the carrier, and "sidebands" are produced. In
amplitude modulation, if the program signal is approaching the audio range, at
least three signals are produced. The most evident signal is the actual
modulated carrier. In addition to this carrier, toe modulation process produces
two entirely new frequencies known as "sideband" frequencies. One of those
frequencies is the sum, in Hertz, of the carrier and the program frequencies; it
is called the "upper sideband". The second new signal is
the difference between the program and the carrier, or the "lower sideband".
These sidebands are softer than the carrier signal and tend to sound like no
harmonic overtones and subzones; therefore, they are useful in tumbrel
constructions. When a program signal consists of more than one frequency (i.e.
a complex tone), two sidebands are produced for every frequency contained in
the program. Such a complex program may be the result of very high harmonic
content or a result of the mixing of two or more signals. In either case, the
number of sidebands produced by the process of amplitude modulation is in
direct proportion to the complexity of the program signal. The frequencies of
the sidebands are always equal to the sums and differences of the carrier and
program components.
8. As an example of amplitude modulation and sideband
production, consider modulating a 1,000 Hz sine wave (the carrier)
with a 100 Hz sine wave (the program). The result would be the
original 1,000 Hz signal plus signals at 1,100 Hz and 900 Hz. If the
program consisted of a 100 Hz and 300 Hz signal, the result would
then be frequencies of 1,000 Hz, 1,100 Hz, 1,300 Hz, 900 Hz, and 700
Hz.
9. A good example of composing with amplitude modulation
techniques is Karlheinz Stockhausen Hymen.
10. One of the first vacuum tube AM radio transmitters, built by Meissner in
1913 with an early triode tube by Robert von Lieben. He used it in a historic
36 km (24 mi) voice transmission from Berlin to Nauen, Germany. Compare
its small size with above transmitter.
