Skidmore College



FM stands for: Frequency Modulation. How do you modulate the frequency of something? One way might be to slide your finger up and down the string of a violin while plucking or bowing the string. In this way, you would be changing, or MODULATING the pitch (and hence the frequency) of the sound yiou are hearing coming from the violin. In the “old” days of analog synthesizers (like the Moog for example), there were modules of the synthesizer that specialized in the output of some very basic and simple waveforms (sawtooth, square, triangle and sine). Each of these modules had at least two controls on them: one that controlled the level of the output (amplitude) and another that controlled the pitch or frequency of the waveform. To “MODULATE” the frequency of this waveform, let’s ay… a sine waveform for the sake of this example, all you had to do wasw turn a little knob up or down, thus “MODULATING”the frequency of the module up or down. You would hear the pitch go up and down in direct response to your moving this knob.

Vibrato used by singers, violinists, and many other musicians is quite similar. When these performer use vibrato, they “MODULATE” the frequency (or pitch) of their own sound. You have probably heard some not so good opera singers who have TOO MUCH vibrato! This is often comical!

If a performer could perform vibrato VERY fast, changing their frequency regularly back and forth, above and below a “center’ pitch, you would experience a new phenomenon. If the performer could achieve a vibrato rate a bit faster than 20 times per second, you would begin to hear a whole new tone color! In fact, depending on the amount of change above and below the center pitch (the change from the center being called “delta f”, for the difference from the center frequency and the center frequency itself being abbreviated as Fc, you might hear a DRAMATICALLY different sound! This is the basic idea behind FM synthesis.

Think of a superhuman singer who could control their center frequency (Fc) with tremendous accuracy, who could control how fast the change from the center frequency would occur (vibrato rate)… ranging from 0 times per second (0 Hertz) to… maybe….1,000 time or more per second (1 KHz)…this rate of change being called the frequency of modulation, or Fm, and finally, our singer could control the amount of change above above and below the center frequency (again, that change from the center being called "delta f".) As this "super singer" increased the vibrato rate beyond about 20 Hz AND the change from the center was more than 0 (any amount of change greater than zero), you would hear a WHOLE NEW WORLD OF TONE COLORS ! This is the secret of FM synthesis !

In our case, we use a computer as our "superhuman singer". The computer can precisely control the Center Frequency, which is really given the term "Carrier Frequency" in FM synthesis [this dates back to the terminology created by FM Radio engineers...which uses the same ideas but at MUCH higher frequencies !]. In any case, you can think of "Carrier Frequency" as "Center Frequency" or vice versa, it doesn't matter because they both have the abbreviation: Fc and you can think of "Modulator Frequency", or Fm as the "speed of the vibrato". Finally, you can think of the amount of change from the center or carrier frequency as "delta F", or the DEPTH of vibrato. Let's say, for example, our "computerized singer" was sustaining the pitch "A", at 440 Hz. As the computer begins using vibrato, the frequency would steadily rise and fall, say from a high of 445 Hz to a low of 435 Hz. In this case, the widest swing of frequency, from 435Hz - 445Hz shows that the total change in frequency is about 10 Hz, or 5 Hz above and 5 Hz below the sustaining, or center, frequency. In this particular instance our "delta F" or vibrato depth would be 5 Hz. In FM synthesis we would say 'delta f = +/- 5Hz.

You could try an experiment to see if you could change your vibrato faster than 20 times per second, but you'll probably land in the Guinness Book of World Records if you manage to do it ! Instead, let's let the computer control it for us....that's why "BESSIE"has all these sliders on it.


Skidmore College
815 North Broadway
Saratoga Springs, NY, 12866