A new multi-mode USRP receive demonstration program in CVS

A new program in gnuradio-examples has been checked in that implements a
selectable AM, FM, or WFM (mono) receiver using the USRP.

Below is from usrp_rx_nogui.py:

This example application demostrates receiving and demodulating
different types of signals using the USRP.

A receive chain is built up of the following signal processing

USRP - Daughterboard source generating complex baseband signal.
CHAN - Low pass filter to select channel bandwidth
RFSQL - RF squelch zeroing output when input power below threshold
AGC - Automatic gain control leveling signal at [-1.0, +1.0]
DEMOD - Demodulation block appropriate to selected signal type.
This converts the complex baseband to real audio frequencies,
and applies an appropriate low pass decimating filter.
CTCSS - Optional tone squelch zeroing output when tone is not present.
RSAMP - Resampler block to convert audio sample rate to user specified
sound card output rate.
AUDIO - Audio sink for playing final output to speakers.

The following are required command line parameters:

-f FREQ USRP receive frequency
-m MOD Modulation type, select from AM, FM, or WFM

The following are optional command line parameters:

-R SUBDEV Daughterboard specification, defaults to first found
-c FREQ Calibration offset. Gets added to receive frequency.
Defaults to 0.0 Hz.
-g GAIN Daughterboard gain setting. Defaults to mid-range.
-o RATE Soundcard output rate. Defaults to 32000. Useful if
your soundcard only accepts particular sample rates.
-r RFSQL RF squelch in db. Defaults to -50.0.
-p FREQ CTCSS frequency. Opens squelch when tone is present.

Once the program is running, ctrl-break (Ctrl-C) stops operation.

Please see fm_demod.py and am_demod.py for details of the demodulation

For example, to receive a NOAA (NBFM) station with all the defaults:

./usrp_rx_nogui.py -f 162.55M -m FM

To receive an aviation band (AM) station with all the defaults:

./usrp_rx_nogui.py -f 120.1M -m AM

To receive an FM broadcast band station (WFM) with all the defaults:

./usrp_rx_nogui.py -f 98.5M -m WFM

To receive a ham radio repeater with a CTCSS tone of 151.4 Hz:

./usrp_rx_nogui.py -f 145.23M -M FM -p 151.4


-Johnathan, AE6HO