AM Transmitter gui





Here is a short script to assist in transmitting AM signals created from a Signed Word format audio file. The gui allows use of sliders to control modulation level as well as rf output level. The file to transmit is selected from an "open" command under the "file" menu. Also, while playing, the audio file can be stepped backward or forwards a bit, rewind and fast-forward.

The sw format audio file can be easily created from mp3 files if your 'sox' utility supports mp3. Odds are it does not (certainly not in Red Hat Fedora due to copyright, etc), but to get sox with mp3 support just download the source from sourceforge and get the mad and lame libraries (download those in source form and compile). Once you get sox with mp3 support just use: 
	$ sox -V audiofile.mp3 -r 32000 -c 1 audiofile.sw
being sure to use a .sw extension on the destination as that is what tells sox to encode in signed word (or 'short' in gnuradio) format. It doesn't hurt to specify one channel (-c 1) in case the source audio happens to be stereo.

Once you have the audio prepared, fire up this script with the usrp hardware connected:
amtx_3_gui.py
and open the audio file with 'open' under 'menu', then hit run. Adjust modulation and rf for best results.

Here is a screenshot of the AM transmitter, with a coax loopback on the usrp, being received with HF Explorer:





Notes: Stop and Run do not work as well as I wish they would. Once a running script is "Stopped" the next "Run" produces noise. Exit and start all over.

The carrier frequency is set in the script to 1080Khz plus or minus the command line argument, default 10Khz. So starting with no frequency arg puts it on 1090Khz. Use -f -20e3 for 1060Khz, -f 20e3 for 1100Khz etc.

I have used this with a 29db amp and a resistor load placed very close to a regular AM receiver, and also with another amp and a loop antenna for "about the house" reception. Just be careful to stay within FCC part 15 regs (1/10 watt and 3 meter antenna between 510 - 1705Khz).