In a really good NBFM receiver, where you are attempting to use it for voice communications, and hand helds are involved (say), it is important to have threshold effect improvement. This will improve intelligibility on important links such as official/government communications. This is done by using a PLL based demodulator. If you use a PLL based demodulator, it is better to use carrier detect for squelch than the high freq/low freq filter based squelch or in addition to it. I usually do primary/secondary tests such
if (signal strength> threshold) should_do_squelch = secondary_test(signal); In addition, most commercial NBFM transmitter have a fair amount of preemphasis and it is important to do deemphasis. Here is a good discussion of the preemphasis/deemphasis http://cnyack.homestead.com/files/modulation/pre_emp.htm This will provide much relief from listening to high frequency noise which can contribute greatly to fatigue. Several discussions of FM demodulators such as http://www.radio-electronics.com/info/receivers/fm_demod/fm_demodulation.php Here is a little gold mine as well. http://www.ee.byu.edu/ee/class/ee444/ComBook/ComBook/ComBook.html The piece de resistance for discussion of FM threshold effects based on the analysis of the noise and signal distributions when the communications signals are considered to be stochastic processes is presented in this treatise on noise in CW systems. http://www.mit.bme.hu/research/chaos/kolumban/eie331/handouts/pdf/eie331_07n oiseCW.pdf Have fun, Bob _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
