Tutorials and Topics - Peabody Computer Music

Tutorial 24Analysis:Oscilloscopethe second that has just passed. We have stretched the scope~ (using its grow handle) tobe 256 pixels wide—twice its default width—in order to provide a better view.On the next page we will describe the different waveforms created by the oscillators.• One by one, click on the different presets to see different waveforms displayed in thescope~. The first eight waves are at the sub-audio frequency of 1 Hz to allow you to seea single cycle of the waveform, so the signal is not sent to the dac~ until the ninthpreset is recalled.Preset 1. A 1 Hz cosine wave.Preset 2. A 1 Hz sine wave. (A cosine wave with a phase offset of 3 /4 cycle.)Preset 3. A 1 Hz cosine wave plus a 2 Hz cosine wave (i.e. octaves).Preset 4. Four octaves: cosine waves of equal amplitude at 1, 2, 4, and 8 Hz.Preset 5. A band-limited square wave. The four oscillators produce four sine waves withthe correct frequencies and amplitudes to represent the first four partials of a squarewave. (Although the amplitudes of the oscillators are only shown to two decimal places,they are actually stored in the preset with six decimal place precision.)Preset 6. A band-limited sawtooth wave. The four oscillators produce four sine waveswith the correct frequencies and amplitudes to represent the first four partials of asawtooth wave.Preset 7. A band-limited triangle wave. The four oscillators produce four sine waves withthe correct frequencies and amplitudes to represent the first four partials of a trianglewave (which, it appears, is actually not very triangular without its upper partials).Preset 8. This wave has the same frequencies and amplitudes as the band-limited squarewave, but has arbitrarily chosen phase offsets for the four components. This shows what aprofound effect the phase of components can have on the appearance of a waveform, eventhough its effect on the sound of a waveform is usually very slight.Preset 9. A 32 Hz sinusoid plus a 36 Hz sinusoid (one-half cycle out of phase for the sakeof the appearance in the scope~). The result is interference causing beating at thedifference frequency of 4 Hz.Preset 10. Combined sinusoids at 200, 201, and 204 Hz, producing beats at 1, 3, and 4 Hz.Preset 11. Although the frequencies are all displayed as 200 Hz, they are actually 200,200.25, 200.667, and 200.8. This produces a complicated interference pattern of six193