Tutorials and Topics - Peabody Computer Music

Tutorial 10: Synthesis—Vibrato and FMBasic FM in MSPFrequency modulation (FM) is a change in the frequency of one signal caused bymodulating it with another signal. In the most common implementation, the frequency ofa sinusoidal carrier wave is varied continuously with the output of a sinusoidalmodulating oscillator. The modulator is added to the constant base frequency of thecarrier.Simple frequency modulationThe example above shows the basic configuration for FM. The frequency of themodulating oscillator determines the rate of modulation, and the amplitude of themodulator determines the “depth” (intensity) of the effect.• Click on the ezdac~ to turn audio on.The sinusoidal movement of the modulator causes the frequency of the carrier to go ashigh as 1015 Hz and as low as 885 Hz. This frequency variation completes six cycles persecond, so we hear a 6 Hz vibrato centered around 1000 Hz. (Note that this is distinctfrom tremolo, which is a fluctuation in amplitude, not frequency.)• Drag upward on the number box marked “Modulation Depth” to change the amplitudeof the modulator. The vibrato becomes wider and wider as the modulator amplitudeincreases. Set the modulation depth to 500.With such a drastic frequency modulation, one no longer really hears the carrierfrequency. The tone passes through 1000 Hz so fast that we don’t hear that as itsfrequency. Instead we hear the extremes—500 Hz and 1500 Hz—because the outputfrequency actually spends more time in those areas.Note that 500 Hz is an octave below 1000 Hz, while 1500 Hz is only a perfect fifth above1000 Hz. The interval between 500 Hz and 1500 Hz is thus a perfect 12th (as one wouldexpect, given their 1:3 ratio). So you can see that a vibrato of equal frequency variation112