Frequency Modulation

Listening Example Keith Kothman In Time (1986) for clarinet and tape

Background Obvious relationship to radio transmission Also related to pitch vibrato, LFO modulation. Digital implementation was developed by researchers at Stanford’s CCRMA. Stanford licensed the technology to Yamaha, which led to the DX7 and other popular synth’s in the 1980’s. John Chowning has done extensive research into the simulation of acoustic instruments with FM synthesis. Obvious relationship to radio transmission Also related to pitch vibrato, LFO modulation. Digital implementation was developed by researchers at Stanford’s CCRMA. Stanford licensed the technology to Yamaha, which led to the DX7 and other popular synth’s in the 1980’s. John Chowning has done extensive research into the simulation of acoustic instruments with FM synthesis.

General Description Simple FM: carrier oscillator has its frequency modulated by the amplitude output of a modulating oscillator. Sidebands produced around carrier at multiples of modulating frequency. –Number generated depends on the amplitude of the modulator. Simple FM: carrier oscillator has its frequency modulated by the amplitude output of a modulating oscillator. Sidebands produced around carrier at multiples of modulating frequency. –Number generated depends on the amplitude of the modulator.

Modulator : Carrier Ratio Sidebands at C  (n * Modulator), where n is an integer determined by the strength of the modulating signal. Ratio of M:C determines whether spectrum is harmonic or not. –Simple integer ratio = harmonic –Non-integer ratio = inharmonic Sidebands at C  (n * Modulator), where n is an integer determined by the strength of the modulating signal. Ratio of M:C determines whether spectrum is harmonic or not. –Simple integer ratio = harmonic –Non-integer ratio = inharmonic

Modulation Index and Bandwidth The bandwidth of the FM spectrum is the number of sidebands present. The bandwidth is determined by the Modulation Index In simple terms, the index of modulation equals the number of sideband pairs present in the output signal. The bandwidth of the FM spectrum is the number of sidebands present. The bandwidth is determined by the Modulation Index In simple terms, the index of modulation equals the number of sideband pairs present in the output signal.

Sideband Amplitudes Energy for the sidebands is “stolen” from the carrier amplitude. Amplitude of sidebands varies according to Index. Highest amplitude at sidebands corresponding to index number. Energy for the sidebands is “stolen” from the carrier amplitude. Amplitude of sidebands varies according to Index. Highest amplitude at sidebands corresponding to index number.