When asked about FM synthesis, most typical synth users won’t have much to say. Many regard it as "rocket science", and even those that choose to use it still may not completely understand it. It most certainly requires a different approach to programming patches if one wants to get good sounds from it; however, to disregard it as some sort of voodoo magic that isn’t worth the time to invest in it is to keep one’s self from experiencing an entire world of tonal possibilities. Let’s take a look at what FM synthesis really is and what it is capable of producing.
What the FM?
In a nutshell, FM synthesis stands for “Frequency Modulation synthesis,” and the essence of how it works is to take one waveform and modulate its pitch by the level of another waveform. This works to create very harmonically rich tones, so much so that the basic building blocks of most FM synthesizers (which are typically called “operators” instead of “oscillators”) only use sine waves; any base waveform that’s more harmonically complex than a sine wave would be too unwieldy in terms of overtones created through modulation. Yamaha essentially brought FM synthesis to the public in their DX line of synthesizers in the mid-1980’s, although they employed a slightly different variation of synthesis called Phase Modulation, or PM, synthesis (which modulates the phase of the carrier wave instead of the frequency, creating a sound that’s about 95% similar but about 1000% more stable and predictable in practice.) These synths had anywhere from four to eight operators on-board, and these operators were routed into fixed modulation paths called algorithms. The DX7, for example, was the quintessential FM synthesizer of the age, and featured six operators and 32 algorithms. By varying the pitch of each operator relative to each other, adjusting the envelope for volume characteristics of each operator, and adjusting the modulation depth each modulator had on their respective carrier, one could achieve and incredible variety of both synthetic sounding and realistic sounding instruments. Most factory patches, in fact, were designed to sound like real acoustic instruments, including guitars, brass, and electric pianos, which are particular specialty sounds for FM synthesizers.
The Stumbling Block:
So where is it that everyone begins to lose grip with the FM synths? Surely, the fact that most FM synths treat pitch as a ratio rather than a fixed value like traditional synthesizers doesn’t help, nor does the fact that to get harmonically complex tones one must modulate one operator by another rather than use a filter to dial down the tone of a harmonically rich wave. But at its core, FM synthesis is far from being the "rocket science" many would believe it to be.
Here are a few tips for working with FM synths and getting to a point where you can get the sounds you want out of them rather than just experimenting and hoping for the best:
1. Take time to learn and get a feel for how adjusting the level of modulators being routed to carriers changes the tone, as well as changing the pitch of the modulators being routed to carriers.
2. Generally speaking, harmonic intervals between operators lend themselves to simpler sounds like plucked string tones and brass tones, while inharmonic intervals lend themselves to complex and sometimes unpleasant tones like bells and other natural-sounding metallic tones.
3. Envelope generators are one of your greatest assets with an FM synth, especially when trying to shape realistic-sounding instruments. Learn their nuances to get the most from your FM synth.
Audio Demo:
The demo here is of a few preset patches from a Yamaha TX81Z, which is a smaller rack-mount FM synth, most popularly known for being used for the bassline on Madonna’s song “Vogue”. Also there will be patches played from Native Instruments' FM7, a software replica of the Yamaha DX7 that expands greatly on the original instrument. The sounds from this instrument are some of the more non-traditional FM synth sounds that can be made. With this demo, one can see the huge variety of sounds possible from the FM synthesizers, even from the simplistic preset patches from the 1980’s.
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