An Introduction to

Amplitude Modulation

"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).

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.

"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.

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.

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.

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.

A good example of composing with amplitude modulation

techniques is Karlheinz Stockhausen Hymen.

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.