Lesson Summary 

Identify harmonic variations in various duty cycles. 


Pulse wave - A periodic wave that oscilates between a fixed minimum value and fixed maximum value, its amplitude.  Also known as a rectangular wave.  A pulse wave's harmonic content is contingent on its duty cycle. 

Square wave - A special pulse wave signal with a 50% duty cycle. Contains only odd harmonics, producing a similar timbre to saxophones and clarinets. Highlighted in the diagram below. 



Duty Cycle - The percentage of one period in which a signal is active. 



Pulse Width Modulation - A variation of a sounds duty cycle to achieve subtle timbral changes. The animation below shows PWM varying the duty cycle of a pulse wave. 




The pulse wave may have any duty cycle and its harmonic spectrum is related to its duty cycle. For example, if a pulse wave has a duty cycle of 25%, or 1/4, every fourth harmonic is missing. If the duty cycle is 20%, or 1/5, every fifth harmonic would be missing. Given a duty cycle of 12.5%, or 1/8, then every eighth harmonic would be missing, and so on and so forth.  This relationship is most aparent in a square wave, which is a special pulse wave with a 50% duty cycle. In a square wave every other harmonic is missing.  

For our exercise we will hear the differences in a pulse wave as we change the duty cycle, and then apply a PWM using our LFO to automate our duty cycle changes.  First to hear a only a square wave set your Werkstatt to match the settings in Figure 1. 


Figure 1. Square wave settings.



As we turn the PWM knob we will be able to hear the duty cycle timbral changes in the sound.  Now lets automate this process using the LFO.  Match your Werkstatt to the settings in Figure 2. 


Figure 2. LFO to VCO MOD for PWM. 


Now we can hear the LFO automating the PWM changes.  As we turn the LFO RATE knob we will hear a faster or slower modulation rate. Experiment with VCO MOD Amount, LFO RATE, and LFO WAVE to hear a wide variety of timbre's.


Jumper Cables