Amplitude Modulation (AM) Synthesis is performed by combining two signals together. A source audio signal, the carrier, is multiplied by an unipolar modulation signal. A unipolar signal is simply a signal that contains only positive values (usually between 0 and 1). This process is typically used to alter the carrier signal in one of two ways.
The modulation signal can be used as an envelope which is applied to the carrier signal to determine the audio signals amplitude over time.
AM synthesis used to apply an amplitude envelope.
The modulation signal can be also be used to quickly cycle the carrier signal’s amplitude to form two additional frequencies known as sidebands, forming harmonic or non-harmonic sounds.