In 1983, FM synthesis was the Next Big Thing. By 1993, it was as dead as last week's newspaper. FM synths were popping up in garage sales and at flea markets.
Fast-forward to 2006:
Why the change? Several reasons.
In the late 1980s, FM (frequency modulation) was supplanted by sample-playback keyboards. (A sample is a brief digital recording.) While these instruments were superior to FM at producing certain types of realistic sounds, such as string sections, after a few years musicians began to realize that sample playback is an imperfect technology as well. For one thing, a sample is pretty much set in stone, so it doesn't respond well to real-time control inputs.
Second, FM is a versatile, efficient way of producing a huge variety of sounds. It's superior to sample playback when it comes to responding to MIDI velocity (how hard you strike the keys), which is the most important factor in expressive playing. This is because velocity changes can alter an FM tone in many different ways, not just by making it louder and/or brighter.
The Yamaha DX7, released in 1983, became the most popular synthesizer in history thanks to its distinctive FM sound. Though stripped-down versions of its technology in early soundcards and cell phones gave FM a bad name, today's software FM synths are pushing the sonic frontier again.
FM is a general-purpose synthesis method, and can handle almost any musical task. But it's especially good at producing sounds with crisp high-end detail. That makes it an excellent choice for emulating mallet percussion (marimba and vibraphone), steel drums, and electric piano. The bell-like Rhodes sound in the Yamaha DX7 was so heavily used in 1980s pop recordings that it became a clich≥. Plucked sounds, such as jazz guitar and round-wound electric bass, are also easy to generate with FM.
Yamaha's PLG150-DX daughterboard brings the crisp, expressive DX7 sound to select host synthesizers.
Modern FM synths include resonant filters and a choice of waveforms, which allows them to produce many of the timbres associated with analog synthesis. However, oscillator sync and oscillator pulse width modulation, two of the popular techniques for tone color animation in modeled analog synths, are not possible with FM.
Third, now that computers are fast enough to do real-time synthesis, software developers search eagerly for ways to make good-sounding tones. FM is relatively easy to implement in software.
Fourth, while Yamaha owned the patent on hardware-based FM, which meant that until recently no other manufacturer could offer competing FM instruments, nobody owns FM when it's running on a computer. Each developer can put a fresh spin on the FM concept.
In this article, I'll explain how FM synthesis works, how can you program your own sounds, and how to choose from the four leading FM-based softsynths. For more background on what makes FM special, see the sidebar "That FM Sparkle."
Most real-world sounds are complex. That is, they contain numerous partials. (A partial is sound energy at a particular frequency.) We can identify sounds, such as an oboe or clarinet, because our ear and brain effortlessly detect and decode the frequencies and relative loudness of the partials.
Together, the partials in a sound make up its timbre, or tone color. Typically, the loudness balance of the partials in a sound changes over time, thus giving each note an expressive shape.
One way to produce complex timbres is to record and play back actual sounds. This technique is called sampling. Frequency modulation (FM) synthesis takes a very different approach. In FM, one simple sound--often a sine wave, which is the simplest sound of all--is used to modulate (alter) another equally simple sound. The results can be surprisingly colorful and varied. Figure 1A shows a sine wave.
In its simplest form, FM requires two oscillators. One oscillator, called the carrier, is the one whose output we hear. The other, called the modulator, is not heard directly. Instead, its output is used to modulate the frequency of the carrier.
Again, "modulation" is just a fancy word for "change." In frequency modulation, it's the frequency of the carrier that increases and decreases. The output of the modulator is a waveform, which means it has peaks and troughs. When the modulator's waveform is at a peak, the carrier's frequency will rise. When the modulator's waveform is at a trough, the carrier's frequency will fall.
This effect is most clearly visible in Figure 1B. In this figure, we're looking at the carrier's waveform, not the modulator's. Where the modulator's wave is in a trough, the carrier's wave spreads out. (In other words, its frequency drops.) Where the modulator's wave is at a peak, the carrier's wave bunches up. (Its frequency rises.)
If the modulator is in the sub-audio range (below about 20Hz), we can actually hear the frequency of the carrier rise and fall. This type of FM is well known--it's called vibrato. But when the frequency of the modulator is faster, we can no longer hear the fluctuations in pitch. Instead, what we hear is a change in the tone color of the carrier. (Listen to Example 1.) This phenomenon is perhaps a bit like looking at stills from a film: if one image replaces one another slowly enough, we see them as separate, but when the images replace one another quickly, we have the illusion that we're seeing continuous movement.
The tone color produced by a carrier/modulator pair depends on three things: the amplitude of the modulator (that is, how much modulation is being applied), the ratio between the frequencies of the two oscillators, and the waveforms selected for them. The relationships among these factors are complicated, and involve college-level math. For everyday musical purposes, though, it's enough to know that as the amplitude of the modulator increases, the waveform produced by the carrier will acquire more and stronger partials. In other words, cranking up the amplitude of the modulator increases the brightness and complexity of the resulting waveform.
Figure 1. As the frequency of the modulator increases, the waveform of the carrier changes. All four of these images show the same sine-wave carrier, and all are at the same zoom magnification. The only change is in the frequency of the modulator, which increases progressively in B, C, and D. This waveform, captured in Steinberg Cubase SX 3, is heard in Example 1.