Nuts & Volts - January 2006

over three decades ago on analog machines far less sophisticated than what you can build yourself today.

If this all sounds exciting to you, then let’s tuck in and see how to learn about analog music synthesizers.

THE THREE ASPEC TS OF SOUND

■ FIGURE 1. Three parameters of sound.

The emphasis here is on learning about analog music synthesizers. Not only are their circuits perhaps more accessible to beginners, but they’re also imbued with a certain warmth of sound that is unequaled. And lest you think analog technology is too feeble to be useful in this digital day and age,

keep in mind that some amazing music, like Walter Carlos’ famous Switched On Bach, was created

Analog music synthesizers depend upon the rather simple concept that any sound (musical or otherwise) can be described by three parameters only: frequency, amplitude, and harmonic content. The basic notion is that if we can analyze a sound into its three constituents, then by reversing the steps we should be able to synthesize it from scratch, as well. Let’s look at these three aspects in more detail.

Frequency is a measure of how often a waveform repeats in a given interval. Refer to Figure 1. Comparing the waveform in Part (a) with the reference waveform, we see that it is twice the frequency. In loose terms, the frequency of an audio signal corresponds to what a musician calls pitch. For the two triangle waves illustrated, the second one is an octave higher (twice the frequency) than the reference.

The second parameter of sound is that of amplitude. Again, refer to Figure 1. Here we note that the height of the

waveform in Part (b) is about half that of the reference waveform. If this were an audio signal we were listening to, we’d say that the sound is softer or less loud. Notice in this case that the frequencies of the reference waveform and Part (b) are identical; it’s only the amplitudes which differ.

Finally, the third aspect of sound can be referred to as harmonic content. An easy way to understand this is by trying a simple experiment. Sing the following vowel sounds in one long, drawn out phrase, “ooooh ... ahhhh ... eeeee.” Slur the sounds together, but don’t alter the pitch or volume at which you sing them. Even though you’ve held two of the parameters fixed, something is obviously changing. This is the harmonic content, called timbre by musicians.

Electronically, it corresponds to an oscillator generating different wave shapes or perhaps a filter modifying those shapes somehow. Refer one final time to Figure 1 and notice how the signal in Part (c) has the same frequency and amplitude as the reference, but the shape is clearly different. By the way, an oscillator that puts out a triangle wave will sound rather pure and flute-like compared to the raspier, violin-like timbre of a ramp wave.

If we can come up with the means to manipulate the frequency, amplitude, and harmonic content of an electronic waveform, then in theory we should be able to synthesize just about any sound we desire. A traditional musical instrument is pretty much stuck in a rut; its ability to change all three parameters is somewhat limited in scope. But with a synthesizer, many more combinations are possible including those never before heard!

WHAT’S INSIDE AN ANALOG SYNTHESIZER?

While the fundamental concept is quite simple, coming up with a complete circuit to usefully manipulate all three parameters of sound can be daunting. The only reasonable way to proceed is by divide and conquer. That is, rather than thinking of a synthesizer