There are several different methods for synthesizing a sound. The most common method is called subtractive synthesis. Subtractive synthesizers, such as the one shown in Figure 6.19, use one or more oscillators to generate a sound with lots of harmonic content. Typically this is a sawtooth, triangle, or square wave. The idea here is that the sound you’re looking for is hiding somewhere in all those harmonics. All you need to do is subtract the harmonics you don’t want, and you’ll expose the properties of the sound you’re trying to synthesize. The actual subtraction is done using a filter. Further shaping of the sound is accomplished by modifying the filter parameters over time using envelopes or low frequency oscillators. If you can learn all the components of a subtractive synthesizer you’re well on your way to understanding the other synthesis methods because they all use similar components.
The opposite of subtractive synthesis is additive synthesis. This method involves building the sound you’re looking for using multiple sine waves. The theory here is that all sounds are made of individual sine waves that come together to make a complex tone. While you can theoretically create any sound you want using additive synthesis, this is a very cumbersome method of synthesis and is not commonly used.
Another common synthesis method is called frequency modulation (FM) synthesis. This method of synthesis works by using two oscillators with one oscillator modulating the signal from the other. These two oscillators are called the modulator and the carrier. Some really interesting sounds can be created with this synthesis method that would be difficult to achieve with subtractive synthesis. The Yamaha DX7 synthesizer is probably the most popular FM synthesizer and also holds the title of the first commercially available digital synthesizer. Figure 6.20 shows an example of an FM synthesizer from Logic Pro.
Wavetable synthesis is a synthesis method where several different single-cycle waveforms are strung together in what’s called a wavetable. When you play a note on the keyboard, you’re triggering a predetermined sequence of waves that transition smoothly between each other. This synthesis method is not very good at mimicking acoustic instruments, but it’s very good at creating artificial sounds that are constantly in motion.
Other synthesis methods include granular synthesis, physical modeling synthesis, and phase distortion synthesis. If you’re just starting out with synthesizers, begin with a simple subtractive synthesizer and then move on to a FM synthesizer. Once you’ve run out of sounds you can create using those two synthesis methods, you’ll be ready to start experimenting with some of these other synthesis methods.