Tutorials and Topics - Peabody Computer Music

Tutorial 10Synthesis:Vibrato and FMaround a central frequency does not produce equal pitch variation above and below thecentral pitch. (In Tutorial 17 we demonstrate how to make a vibrato that is equal in pitchup and down.)• Set the modulation depth to 1000. Now begin dragging the “Modulator Frequency”number box upward slowly to hear a variety of effects.As the modulator frequency approaches the audio range, you no longer hear individualoscillations of the modulator. The modulation rate itself is heard as a low tone. As themodulation frequency gets well into the audio range (at about 50 Hz), you begin to hear acomplex combination of sidebands produced by the FM process. The precise frequenciesof these sidebands depend on the relationship between the carrier and modulatorfrequencies.• Drag the “Modulator Frequency” number box all the way up to 1000. Notice that theresult is a rich harmonic tone with fundamental frequency of 1000 Hz. Try typing inmodulator frequencies of 500, 250, and 125 and note the change in perceivedfundamental.In each of these cases, the perceived fundamental is the same as the modulator frequency.In fact, though, it is not determined just by the modulator frequency, but rather by therelationship between carrier frequency and modulator frequency. This will be examinedmore in the next chapter.• Type in 125 as the modulator frequency. Now drag up and down on the “ModulationDepth” number box, making drastic changes. Notice that the pitch stays the same butthe timbre changes.The timbre of an FM tone depends on the ratio of modulator amplitude to modulatorfrequency. This, too, will be discussed more in the next chapter.SummaryFrequency modulation (FM) is achieved by adding a time-varying signal to the constantfrequency of an oscillator. It is good for vibrato effects at sub-audio modulatingfrequencies, and can produce a wide variety of timbres at audio modulating frequencies.The rich complex tones created with FM contain many partials, even though only twooscillators are needed to make the sound. This is a great improvement over additivesynthesis, in terms of computational efficiency.113