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@@ -94,6 +94,12 @@ Options:
   -v VERBOSE, --verbose=VERBOSE
                         additional output
 </pre>
+<h3>MULTI-CHANNEL TRANSMITTER</h3>
+The <tt>multi_tx.py</tt> demonstration app transmits four RF channels, one for each of the supported protocols.  It requires SDR hardware (such as the HackRF); any device supported by the gr-osmosdr driver and libraries should work.  Example command line:<p>
+<tt>python multi_tx.py  --args hackrf --gains rf:0,if:0 -q -19.5 -Q 442187500   -f ~/rand3.wav -R</tt><p>
+The "-Q" (frequency) option gives the frequency of the first of the four channels; the remaining channel frequencies appear at regular intervals (default spacing is 100 KHz).  The ysf channel format may be set to "wide", using "-R" (default=narrow).  The "-q" option is needed for PPM correction (unless a time base such as GPSDO is used).
+
+The input file ("-f" option) requires WAV format, 8000 samples/sec, S16_LE. Audio from the WAV file is transmitted on all frequencies. However if the "-A" (audio) option is given, program content for the second DMR time slot is instead taken live from the sound card audio input port ("-I" option).
 <h3>GENERAL</h3>
 Input audio may come either from WAV files (8000/S16_LE/mono) or from the PC microphone / line input connector.  If <code>-f <em>filename</em></code> (or <code>-F <em>filename</em></code> in DMR mode) is selected the input is read from file(s); otherwise the GR audio source is selected.  Likewise output is sent to the soundcard unless <code>-o</code> is given.  When using files the <code>-r</code> option can be specified to replay the input file(s) in a continuous loop.  Without this option the program exits at EOF.  The <code>-p <em>protocol</em></code> option is used to select which of the four DV standards (dmr, dstar, p25 or ysf) is to be used.  The dmr and ysf modes also utilize a configuration file, specified using the <code>-c <em>filename</em></code> parameter.  See the specific section (below) for more details on the configuration file.
 <p>