RE: [Discuss-gnuradio] NBFM

[Robert McGwier]

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