The vocoder has been a novelty staple (if that isn't an oxymoron) of pop music for more than 30 years. Even if you've never heard the word vocoder, you've heard the smoothly sexy yet oddly alien sounds that emanate from this device. In the '70s and '80s, vocoder was used on songs by Kraftwerk, Stevie Wonder, Bon Jovi, Donna Summer, Phil Collins, Laurie Anderson, Pink Floyd, Styx, and many other artists. (See the "Famous Vocoder Songs" sidebar.) In more recent years, Foo Fighters, Metallica, Beck, Britney Spears, Madonna, Marilyn Manson, and other pop icons have dialed up the sound of the vocoder.

A vocoder (short for "voice encoder") is a device that makes ordinary sounds, such as chords played on a synthesizer, sing or speak recognizable words. In this tutorial we'll look at how vocoders perform this magic, explain how to set up your software vocoder, and suggest some unexpected musical uses for vocoding.

The best way to grasp the creative power of vocoding is to try it yourself, so I've prepared some downloadable tutorial files (2.2MB ZIP file) for the advanced vocoder in Propellerhead Reason. If you don't have Reason, you can listen to the audio clips or download the 30-day demo version of the software to experiment with vocoding for yourself.

The Korg MS2000B analog modeling synth comes with a microphone to feed its onboard vocoder. Here's an excerpt of a song I recorded with the MS2000B's predecessor, the MS2000 (same synth, different color). Listen for the vocoding at the end.

• "Have You Seen My Website?" (1MB MP3; ©2006 Jim Aikin. All rights reserved, including the right of electronic redistribution.)

Named Sources

A vocoder works by imprinting the constantly changing frequency spectrum of one signal onto the sound energy in another signal. Thus a vocoder always has two separate inputs—the speech and carrier inputs. The speech (also called modulator) input receives, not surprisingly, a signal containing spoken words and phrases. The carrier input receives the signal that will be "vocoded" by having the frequency characteristics of the speech signal imprinted on it.

Other sound sources can be used instead for the speech input, but spoken words are the most common sources. Note also that it's not necessary to sing into a vocoder; the pitch information in the output comes from the carrier signal, so speaking in a normal tone works fine.

The carrier signal will be processed and then routed to the vocoder's output. The speech signal will do its job and then be discarded.

To understand how a vocoder works, you need to know that almost all sounds contain energy at a number of different frequencies. If I bow a note on my cello, for instance, the note itself may have a frequency of 100Hz (100 cycles per second). But the tone will also contain vibrations at 200Hz, 300Hz, 400Hz, and so on. These higher-frequency vibrations are called overtones or partials.

What we perceive as the particular tone color of a real-world sound is the combination of these partials—their relative loudness, their precise frequencies, and the way they change over time. If a trumpet, a violin, and a soprano play or sing exactly the same note, normal listeners have no trouble distinguishing one from another. We can do this because each sound source produces different partials. Our ears can very rapidly decode the mix of partials in each tone, usually without the slightest conscious effort.

A basic vocoder contains several elements, as shown in Figure 1. It has two banks of bandpass filters, a bank of envelope followers, a bank of amplifiers, and a mixer. Let's look at each component in turn, and then see how they work together.

Fig. 1. This basic block diagram of a vocoder shows just six frequency bands; some software vocoders have hundreds. Most vocoders also have additional processing to enhance the sound, as shown in this more elaborate design.

Related Reading Digital Audio Essentials A comprehensive guide to creating, recording, editing, and sharing music and other audio By Bruce Fries, Marty Fries