Add links to CV analog circuits in documentation

gui/index.html

@@ -538,6 +538,10 @@ span.mainfunction {color: #993300; font-weight: bolder}
 	<p>Signal range is 0 to 1.2V</p>
 	<p>With a <a href="https://forum.pjrc.com/threads/40468-Help-with-Basic-Audio-Lib-results?p=126317&viewfull=1#post126317">small modification</a> Adafruit's <a href="https://www.adafruit.com/products/1063">MAX4466 microphone</a> can be used</p>
 	<p align=center><a href="https://forum.pjrc.com/threads/40468-Help-with-Basic-Audio-Lib-results?p=126317&viewfull=1#post126317"><img src="img/adccircuitmic.jpg" border=0></a></p>
+	<p>Control Voltage (CV) from +5V to -5V can be adapted with a
+		<a href="https://www.dorkbotpdx.org/blog/paul/control_voltage_cv_to_analog_input_pin">simple circuit</a>
+		or better <a href="https://forum.pjrc.com/threads/28423?p=71257&viewfull=1#post71257">opamp-based circuity</a>
+		to Teensy's 0-1.2V analog input range.</p>
 	<h3>Examples</h3>
 	<p class=exam>File &gt; Examples &gt; Audio &gt; HardwareTesting &gt; PassThroughMono
 	</p>
@@ -593,6 +597,10 @@ span.mainfunction {color: #993300; font-weight: bolder}
 	<p>By default, pins A2 & A3 are used for audio input.  This circuitry is recommended.</p>
 	<p align=center><img src="img/adccircuit2.png"></p>
 	<p>Signal range is 0 to 1.2V</p>
+	<p>Control Voltage (CV) from +5V to -5V can be adapted with a
+		<a href="https://www.dorkbotpdx.org/blog/paul/control_voltage_cv_to_analog_input_pin">simple circuit</a>
+		or better <a href="https://forum.pjrc.com/threads/28423?p=71257&viewfull=1#post71257">opamp-based circuity</a>
+		to Teensy's 0-1.2V analog input range.</p>
 	<h3>Examples</h3>
 	<!--
 	<p class=exam>File &gt; Examples &gt; Audio &gt; HardwareTesting &gt; PassThroughMono