Add AudioInputPDM documentation

gui/index.html

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 		{"type":"AudioInputAnalogStereo","data":{"defaults":{"name":{"value":"new"}},"shortName":"adcs","inputs":0,"outputs":2,"category":"input-function","color":"#E6E0F8","icon":"arrow-in.png"}},
 		{"type":"AudioInputI2Sslave","data":{"defaults":{"name":{"value":"new"}},"shortName":"i2sslave","inputs":0,"outputs":2,"category":"input-function","color":"#E6E0F8","icon":"arrow-in.png"}},
+		{"type":"AudioInputPDM","data":{"defaults":{"name":{"value":"new"}},"shortName":"pdm","inputs":0,"outputs":1,"category":"input-function","color":"#E6E0F8","icon":"arrow-in.png"}},
 		{"type":"AudioInputTDM","data":{"defaults":{"name":{"value":"new"}},"shortName":"tdm","inputs":0,"outputs":16,"category":"input-function","color":"#E6E0F8","icon":"arrow-in.png"}},
 		{"type":"AudioInputUSB","data":{"defaults":{"name":{"value":"new"}},"shortName":"usb","inputs":0,"outputs":2,"category":"input-function","color":"#E6E0F8","icon":"arrow-in.png"}},
 		{"type":"AudioOutputI2S","data":{"defaults":{"name":{"value":"new"}},"shortName":"i2s","inputs":2,"outputs":0,"category":"output-function","color":"#E6E0F8","icon":"arrow-in.png"}},
 	</div>
 </script>
 
+<script type="text/x-red" data-help-name="AudioInputPDM">
+	<h3>Summary</h3>
+	<div class=tooltipinfo>
+	<p>Receive (and filter) a Pulse Density Modulated bitstream.
+		</p>
+	<p align=center><img src="img/pdmmic.jpg"><br><small>PDM MEMS Mic</small></p>
+	</div>
+	<h3>Audio Connections</h3>
+	<table class=doc align=center cellpadding=3>
+		<tr class=top><th>Port</th><th>Purpose</th></tr>
+		<tr class=odd><td align=center>Out 0</td><td>Filtered Audio Output</td></tr>
+	</table>
+	<h3>Functions</h3>
+	<p>This object has no functions to call from the Arduino sketch.  It
+		simply streams data from the PDM data, filters out the high frequency
+		noise and gives you the audio signal.</p>
+	<h3>Hardware</h3>
+	<p>PDM has been tested with this <a href="https://www.adafruit.com/product/3492">
+		Adafruit MP34DT01-M Microphone Board</a>.
+		</p>
+	<table class=doc align=center cellpadding=3>
+		<tr class=top><th>Pin</th><th>Signal</th><th>Direction</th></tr>
+		<tr class=odd><td align=center>9</td><td>CLK</td><td>Output, 2.8235 MHz</td></tr>
+		<tr class=odd><td align=center>13</td><td>DATA</td><td>Input, Data on rising edge</td></tr>
+	</table>
+	<p>Data is input on the rising edge.  The SEL pin on MP34DT01-M should be
+		connected LOW for proper data capture.</p>
+	<!--<h3>Examples</h3>-->
+	<h3>Notes</h3>
+	<p>Filtering consumes approximately 39% of the CPU when running at
+		96 MHz.  The code currently consumes this time inside a high
+		priority interrupt, blocking other libraries.  Perhaps future
+		versions will perform filtering at lower priority.
+		</p>
+	<p>The filter used is a 512 tap FIR with approx &plusmn;1.1 dB gain
+		flatness to 10 kHz.  While far from audiophile grade, this should
+		perform far better than the rapid rolloff of Cascaded Integrator
+		Comb (CIC) or simple moving average filters commonly used on
+		other microcontrollers.  The filter also consumes 2104 bytes of
+		RAM for buffering and 32K of Flash for a lookup table to optimized
+		the filter computation.
+		</p>
+</script>
+<script type="text/x-red" data-template-name="AudioInputPDM">
+	<div class="form-row">
+		<label for="node-input-name"><i class="fa fa-tag"></i> Name</label>
+		<input type="text" id="node-input-name" placeholder="Name">
+	</div>
+</script>
+
 <script type="text/x-red" data-help-name="AudioInputUSB">
 	<h3>Summary</h3>
 	<div class=tooltipinfo>