NR within AM/SAM mode

While I don't know the reason NR is not available for AM I can hazard a guess, and (maybe) offer a little insight into the workings of NR. Based on my NCAA tournament picks, however, my guess might be completely wrong.

Based on the way it sounds I suspect that the algorithm for NR is an adaptive FIR filter, and the method used to adapt the coefficients is based on the Least Mean Squares (LMS) algorithm. At the risk of oversimplifying things, the adaptive filter looks for correlated information and builds filters around it. A CW signal (a single tone) is an example of a correlated signal where adaptive filters work well. A speech signal is not highly correlated, but it's more correlated than noise, so with the proper choice of the adaptation constant and the choice of decay constants (to "unbuild" the filter when the signal changes) we can use an adaptive filter for speech signals as well.

My experience, however, has been that an adaptive filter never improves the fidelity of a speech signal, and I would even argue that it also does not improve the intelligibility of a speech signal. It can improve the "listenability" of a single sideband signal by suppressing the noise between words or syllables, although one could argue that proper choice of AGC-T and AGC settings can also do that.

When I choose AM I do so because it usually offers higher fidelity (due to greater bandwidth) and because the carrier keeps the channel quiet between words and syllables. If an AM signal is sufficiently noisy that you need NR then you have probably lost the benefit of AM, and maybe even the benefit of synchronous AM. So I would never likely use NR with AM, and didn't even know it wasn't available. But then I don't use it on SSB either for the reasons listed above.

My 2 cents worth. I have no idea if the above method is what Flex implemented, but that's what it sounds like to me. Won't hurt my feelings to discover that I am wrong because even in that case I can learn something new, and anyway I'm used to it :-)

73,
Doug K4DSP