A term originating in telecommunications usage describing the superimposition of characteristics of one signal (‘programme’) upon another (‘carrier’); it later entered the terminology of electronic music, where it is frequently used in a broader sense, sometimes as unspecific as ‘a process of change’. Many characteristics of signals may be modulated. In frequency modulation (FM) the frequency of the carrier is made to conform to the wave shape of the programme: for example, if the programme is a sine wave of frequency 6 Hz and low amplitude, the audible result of modulation will resemble the carrier in all respects except that a vibrato (small variation of pitch) will be superimposed upon it. Alterations in the wave form, frequency or amplitude of the programme will produce results more complex and less easily described; in particular, as its frequency enters the audio range (approximately 18Hz–22 kHz), distinct new ‘sideband’ frequencies will be produced. If the ratio between the frequencies if programme and carrier is simple (1:1, 2:1, 3:2 etc.) the sidebands generated will be in harmonic series, and the complex tones produced will resemble the overtone structures of real instruments; this is the fundamental sound-generation technique employed by FM synthesizers. In amplitude modulation (AM) it is the amplitude of the carrier that is made to conform to the wave shape of the programme: here a same sine wave of 6 Hz as programme will have the effect of superimposing a tremolo (small variation of dynamic) upon the carrier. Again, more complex results may be produced by changing the programme.
In contrast to FM and AM modulation, the distinction between programme and carrier is of less significance for ring modulation, the effect of which is symmetrical. The output from a Ring modulator consists of the sum and difference of the frequencies of the inputs: for example, the result of ring modulating two sine waves of 400 Hz and 500 Hz will be two sine waves of 100 Hz and 900 Hz. However, if either or both of the input signals is more complex than a sine wave, as is likely to be the case in a musical context, then the output will be even more complicated since each partial of the one input will be added to and subtracted from each partial of the other.
Frequency, amplitude and ring modulation are the oldest and most familiar modulation processes used in electronic music. However, with the development of voltage control systems the number of devices based on the programme–carrier principle has proliferated: all of these perform operations that may legitimately be described as modulations. Pulse modulation, for example, is the modification by control voltage of the length of individual pulses from a pulse generator. Phase modulation is produced by the superimposition of a signal upon itself after an extremely short but continually changing time delay regulated by a control voltage; with a slow rate of change in the time delay, the effect upon a complex signal will be of a band of noise sweeping through the signal.
This extension of applications has encouraged a looser use of the term. For instance, location modulation is a variation in the apparent spatial location of a sound (pitch and timbre may also be affected). The term has even been extended beyond the boundaries of electronic music to describe any continuous change in timbre, rhythm or other parameters.
DAVID ROBERTS