The EM Poll

Making Waves

Oct 1, 2005 12:00 PM, By Jim Aikin

           

The sound emitted from almost every synthesizer and sampler comes from a module called an oscillator. The output of the oscillator is usually processed by a filter and an amplifier, and perhaps by other modules. Without an oscillator to produce a signal, the filter would have nothing to filter, and the amplifier would have nothing to amplify. In this column, I'll discuss the sound-shaping parameters that are most often found on oscillators, beginning with the most important parameter of all — the waveform.

Synths do have other sound sources. Most have a noise source, and on some instruments the filter can be induced to “squeal” (self-oscillate) even when the oscillators are silent. We'll look at those, however, in a future column. We'll also hold off on any discussion of low-frequency oscillators (LFOs), which are slowly moving oscillators that are typically used as modulation sources. (Their signal is not present in the audio output.)

FIG. 1: This waveform diagram can -represent either the movement of a -loudspeaker cone toward and away from the listener, or the regions of higher and lower air pressure the cone creates as it moves.

One final exception: on synths that use FM (frequency modulation) synthesis, you may not see anything called an oscillator. Instead, FM synths use modules called operators. An operator is an oscillator coupled with an amplitude envelope. The oscillators on modern FM synths operate much like those on any other synth, so there's no need to single them out for special attention.

Now Hear This

You probably listen to music through loudspeakers at least some of the time. If you've ever removed the front grill cloth from a speaker enclosure, you've seen that the speaker cone moves rapidly in and out while the music plays. As it moves toward you, it pushes the air, creating a zone of increased air pressure that travels rapidly outward toward your ears. As the speaker cone moves away from you, it pulls air back into the space it has vacated, creating a zone of decreased air pressure. Sound consists of such changes in air pressure.

If I were to draw a diagram of the in-and-out travel of the speaker cone or of the increases and decreases in air pressure that the speaker produces, the diagram might look something like Fig. 1. In fact, the relationship between speaker movement and the changes in air pressure in front of the speaker is not as simple as this description would suggest. But for now, we'll skip the technical details.

When you choose a waveform for an oscillator, you're choosing a shape or a contour that will cause a loudspeaker to move in and out in a predictable pattern. (How often the speaker moves in and out is a function of the waveform's frequency, and how much it moves is related to the waveform's amplitude.) If you look at a diagram of the waveform, you'll see the pattern. The waveform may be simple or complex, smooth or jagged. The resulting sound will be determined in a very precise way by the shape of the waveform. Change the waveform in some way, and the resulting sound will change. (There are some exceptions to this rule, but they're highly technical.)

In general, smooth waveforms have fewer high-frequency partials. Those that have sharp, jagged edges have more highs. Waveforms whose peaks and valleys are close together are also higher in pitch, and those whose peaks and valleys are further apart are lower in pitch. It's usually impossible, however, to guess what a waveform will sound like by just looking at it.

FIG. 2: Analog synthesizers typically offer a few standard waveforms, including (top to bottom) sine, triangle, sawtooth, square, and pulse waves.

Early analog synthesizers had a small set of waveforms that were chosen because they were musically useful and because they were easy to generate electrically (see Fig. 2 and Web Clip 1). Today, many synths come equipped with dozens or hundreds of waveforms. These can include traditional analog types, which are only one cycle in length, and digital recordings of actual sounds, which are longer and more complex sampled waveforms.

Sampled waveforms can emulate acoustic instruments more realistically than analog waveforms can, but their strong character makes them less adaptable and all-purpose with regard to creative sound design. Analog-type waveforms are static (unchanging) and can become boring unless processed with a filter or effects. But their pure sound has a strong appeal for certain types of music.

Sonic Checklist

Every type of synthesizer has a slightly different set of sound-design parameters. But your synth probably has most or all of the parameters listed below. In this discussion, I've used the word “knob” to refer to any parameter control that can be adjusted over a range of values. Your synth may have sliders instead of knobs, or some combination of the two.

Octave tuning is often indicated using a type of terminology borrowed from the world of pipe organs. A longer organ pipe creates a lower pitch, and cutting the length in half raises the pitch by one octave. So synth octaves, selected with a multiposition switch, are often indicated by the numbers 32' (the lowest), 16', 8', 4', 2', and 1' (the highest).

Semitone tuning in half-steps is sometimes combined with octave tuning on a single knob that covers a multi-octave range. On synths that have a separate semitone tuning knob, the knob will usually have settings from -12 to +12, which covers the range from one octave above to one octave below the default pitch.

Fine tuning, also called detune, is used for adjusting the pitch of the oscillator in increments that are smaller than a half-step. On many synths that have two or more oscillators per voice, only the second and subsequent oscillators have detune knobs. That allows them to be tuned up or down relative to the first oscillator.

The pitch envelope amount knob applies an envelope that changes the pitch of the oscillator during the course of each note. The oscillator may have its own dedicated pitch envelope, or the pitch envelope amount knob may apply modulation from one of the general-purpose envelopes elsewhere in the instrument. Pitch envelopes are often used to create a quick upward or downward blip at the start of each note, which creates a percussive effect.

Oscillator sync, a switch found on only analog-type synths, causes the synchronized oscillator to retrigger a new waveform every time the master oscillator to which it is synced starts a new waveform. The result is that the pitch of the master oscillator determines the perceived pitch of the synced oscillator. The tuning controls, pitch envelope, and any other pitch modulation of the synced oscillator are transformed into waveform-shaping controls. Sweeping the pitch of a synced oscillator is useful for razor-sharp lead-synth tones.

FIG. 3: By using the pulse-width knob, a square wave (left) can be progressively turned into a narrow pulse shape (right). A narrow pulse has a thin, crisp sound and is good for synthesizing Clavinet-type sounds.

The pulse-width knob is found, like oscillator sync, on only analog-type synths. The pulse-width knob may be active only when the square wave (also known as a pulse wave) is selected, and allows you to adjust the square wave from a square shape to a narrow pulse, as shown in Fig. 3. Alternatively, this knob might be able to adjust the shape of all of the waveforms. For instance, you may be able to morph a triangle wave into a sawtooth by adjusting the pulse-width parameter.

Pulse-width modulation (sometimes abbreviated as “PWM” on the front panel) introduces a modulation signal that, in effect, turns the pulse-width knob for you. Pulse-width modulation is used to add animation to the relatively static tone of a pulse wave. Most often, pulse width is modulated by an LFO, but an envelope generator, a modulation wheel or lever, or Channel Pressure (Aftertouch) might also be available as a source for pulse-width modulation.

The Perfect Wave

Getting to know the waveforms in your synthesizer is the first step to becoming an ace sound designer. Open up the filter, shut off the effects, and use your ears. You might be surprised at what you hear.

Jim Aikin writes about music technology for various magazines and Web sites. He is the author of Power Tools for Synthesizer Programming (Backbeat Books, 2004). You can visit him online at www.musicwords.net.