some basic instructions for the fsk modem

git-svn-id: https://svn.code.sf.net/p/freetel/code@2664 01035d8c-6547-0410-b346-abe4f91aad63

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+README_fsk.txt
+David Rowe
+Created Jan 2016
+
+A FSK modem with a non-coherent demodulator.  Performance is within a
+fraction of a dB of ideal.  The demodulator can estimate and track
+frequency offsets.
+
+Credits
+-------
+
+The Octave version of the modem was developed by David Rowe.  Brady
+O'Brien ported the modem to C, and wrote the C/Octave tests.
+
+Quickstart
+----------
+
+Built as part of codec2-dev, see README for build instructions.
+
+1. Using a random bit stream input generate 100 bit/s FSK and play out
+   speaker:
+    
+    $ cd build_linux/src
+    $ /fsk_get_test_bits - 10 | ./fsk_mod 8000 100 1200 2400 - - | play -t raw -r 8000 -s -2 
+
+2. Check the bit error rate of 100 frames of 1200 bit/s FSK:
+
+    $ ./fsk_get_test_bits - 100 | ./fsk_mod 9600 1200 1200 2400 - - | ./fsk_demod 9600 1200 - - | ./fsk_put_test_bits -
+      FSK BER 0.000000, bits tested 9499, bit errors 0
+
+3. Automatically check C implementation against Octave using a variety of test conditions and 
+   channel impairments.
+
+    $ cd octave
+    $ octave
+    octave:1> tfsk
+