es too high or too low.</p>
  <p><code>A4 [Hz]</code> is frequency of note A4.</p>
  </dd>
  </dl>
  <h4 id="gain"><a href="#gain" class="header-anchor"
  aria-hidden="true">Gain</a></h4>
  <dl>
  <dt>A, D, S, R</dt>
  <dd>
  <p>Gain envelope parameters.</p>
  <ul>
  <li><code>A</code> : Attack time which is the length from note-on to
  reaching peak value.</li>
  <li><code>D</code> : Decay time which is the length from peak value to
  reaching sustain level.</li>
  <li><code>S</code> : Sustain level which 