e>Denom. Slope</code> is greater than 1, it acts like
  lowpass filter. At less than 1, it acts like highpass filter. At
  exactly 1, it generates amplitude of sawtooth wave spectrum.</p>
  <p><code>Interval</code> creates 0 in specrum. For example, when
  <code>Interval</code> is 3, amplitude of index 1, 4, 7, 10, 13, …
  becomes non 0, and amplitude of other indices becomes 0. When
  <code>Denom. Slope</code> is 1 and <code>Interval</code> is 2,
  resulting waveform becomes square wave.</p>
  <p>Followi