ARP2600 - Fundamentals of Music Technology - Cyborgstudio.com

020 - SECTION THREE: VCO-2of one waveform occurs at the lowest point of another. Thephase relationships of the other outputs can be easily summedup: outputs which are perpendicular to each other (i.e. abovebelow, or to one side) are 180 degrees out of phase. Thosediagonal to each other are in phase. Figure 3-5 illustrates thisrelationship. Red arrows represent out of phase relationshipswhile blue and green arrows represent in phase relationships.out ofphaseTriangleout ofphasein phaseSawtoothout ofphasePULSE WAVESVCO-2 doesn’t really have a square wave output, but has apulse wave output instead. This is a very important featuresince a pulse wave is like a square wave, but is much moreflexible. Pulse waves, like a square wave, are partially “on”and partially “off.” In terms of voltage, a pulse wave consistsof ten volts for a moment, then zero volts for a moment. For a square wave, these two ‘moments’must be equal. In a pulse wave, they can be equal, but do not have to be equal. Thus, a square wave isreally just a kind of pulse wave.PULSE WIDTHOne important aspect of a pulse wave is that it is another parameter to change. That parameter is thepulse width. Of course, in Section 2, the reader learned that there are only two parameters of anoscillator one can change: frequency and timbre. One might think of pulse width as a sort ofsubheading under the ‘timbre’ parameter heading.Sineout ofphasePulseFigure 3-5: The phaserelationships of VCO-2’s outputstime30% duty cycle50% duty cycletimeFigure 3-6: Pulse waves withdifferent duty cuclesPulse width is the name of the parameter that controls how much of thewave is ‘on’ and how much of the wave is ‘off.’ Another name for pulsewidth is duty cycle. The duty cycle of a pulse wave is expressed as a percentage.The percentage tells how much of the duty cycle is ‘on.’ For instance,a pulse wave that has a pulse width of 20% would be a pulse wavewhich is ‘on’ 20% on the time, and ‘off’ 80% of the time. Figure 3-6 illustratesa pulse width of 30%, and a pulse width of 50% (a square wave).Making changes in the pulse width changes the waveshape and thus thetimbre. This is why it is difficult to describe the pulse wave’s timbre: thetimbre changes significantly when the pulse width is changed. The timbrecan range from hollow when the duty cycle is near 50% to nasal when it isnearer 0% or 100%.On the 2600, changing VCO-2’s pulse width is simple. The pulse width ischanged by moving the PULSE WIDTH slider, right under the FINE TUNEslider. This control can be seen, along with VCO-2’s outputs and initialfrequency controls in Figure 3-7 on page 21. When the pulse width is set to50%, the VCO will produce a square wave (if it is properly calibrated).