Modulation Synthesis

Amplitude and Ring Modulation

Modulation

In electronic and computer music terms, modulation means that some aspect of one signal (the carrier) varies according to an aspect of a second signal (the modulator) Although this definition would seem to apply to any

signal that makes use of an amplitude or pitch envelope, we usually exclude such instances.

When the modulating frequency rises into the audio domain (above 20 Hz), audible frequency components, called sidebands, appear.

Ring and Amplitude Modulation

Ring Modulation and Amplitude modulation both involve multiplying the amplitude of two audio signlas. Ring Modulation (RM): both signals are bipolar,

with their amplitudes fluctuating relatively equally in positive and negative directions.

Amplitude Modulation (AM): the modulating signal is unipolar, meaning it only fluctuates in one direction away from zero (positive amplitude usually).

Bipolar signals can be offset (amplitude addition) to become unipolar Direct Current (DC) offset

Ring Modulation

RMt = Ct x Mt (analog equation) RMn = Cn x Mn (digital)

Multiply signal outputs (amplitudes) Sidebands at Cfreq Mfreq

Carrier is suppressed (eliminated) What happens to negative frequencies?

A negative frequency is the same as a positive frequency with inverted amplitude.

Zero amplitude in either signal will cause modulated output to have zero amplitude.

Amplitude Modulation

Same equation as RM AMn = Cn x Mn Where M is a signal that varies between 0

and 1 in amplitude.

Sidebands at Cfreq + (Cfreq Mfreq). (carrier present)

Integer C:M ratios produce harmonic spectra.