Chapter Four: Synthesis
6. Principles of Audio-rate Frequency Modulation | page 2
Audio-rate frequency modulation
When the rate of the modulating oscillator is tuned above 20 Hz, or at an audio rate, very interesting things happen to the sound. Additional frequencies called sidebands appear symmetrically around the carrier frequency. Those above the carrier frequency are called upper sidebands and below, lower sidebands. As will be seen below, some of the energy of the carrier frequency is being stolen to create these additional frequencies.
Both the exact frequencies and the relative strength of the sidebands are predictable using digital technology, since all parameters can be precisely controlled. The bulk of our discussion will deal with classic or Chowning FM, named after its greatest proponent. Simple Chowning FM uses the most basic sine wave, which produces no other frequencies apart from its fundamental, as both the carrier and modulating waveform. Indeed, one of the beauties of Chowning FM is its ability to do so much from two very simple waves. The other qualification of Chowning FM is that the modulation be linear, whereby the carrier is pushed an equal number of cycles per second above and below its center frequency. Exponential FM, where the carrier is pushed up and down an equal musical interval (therefore more Hz up than down) drifts upwards in its pitch axis as the modulation depth is increased . Linear FM allows the strength of modulation to be increased without the perceived center frequency rising.