Pulse Wave and PWM

The square wave is one of the previously mentioned fundamental waveform. Though it also belongs to another class of waveforms: The pulse wave. The two defining characteristics of a pulse wave are its shape and its width. The shape refers to the fact that it has a flat top and flat bottom. The width of a pulse wave refers to the difference in size between the high level and the low level. A width of 50% means the upper and lower sides are equal, and is known as the square wave. Anything other than 50%, and it is no longer a square wave, but still qualifies as a pulse wave.

Different pulse widths produce difference timbres. A square wave produces a big round sound. As the pulse width moves towards 0% or 100%, it takes on a nasal-like quality. It personally reminds me of picking a string on a guitar; You get a bigger, rounder sound if you pick the middle of the string, and a much lighter, mid-rangy sound near the bridge. In the first part of the listening example, the width starts at 50% and is decremented 5% for each new note until it reaches a width of 5%.

Furthermore, the width can be modulated in time just as we’ve done with amplitude and frequency. This is known as Pulse Width Modulation. At the end of the listening example, the width is modulated by a triangle LFO, producing a timbre that changes over time.

Download pulse_wave.csd here.

We’ve also encountered a special variant of the pulse wave in the 2600 Synthesis example from week two; The polypulse wave.

Note. The vco2 opcode used to generate the pulse wave in this examples produces a band-limited version of the pulse wave. If you we’re to open up the example in an audio editor, the tops and and bottoms would not be flat. We’ll get into the concept of band-limited later.

Synthesis Fall 2010

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