Hello Dave, > Once the fundamental pitch is identified, one needs > to determine which of the present tones is the > "lowest transmitted". When I looked at the problem several years ago I failed on this first step. Surely many stations are easy and give a nice "comb spectrum", but there are also signals that look irregular and noisy on the waterfall. The fundamental I could extract was uncertain and when I accumulated the extended comb spectra outside the passband there was no common origin. Surely most stations would give a good tune, but I was looking for something that would not fail.
> The spectrum of the voice will tend to be stronger in > odd or even harmonics, so a further subdivision may be > performed, and this can help delineate whether the > lowest received pitch is 2nd, 3rd or 4th harmonic. When I look at real signals from HF bands this is not what I observe. I can not see any systematic behaviour in the overtone amplitudes. > Finally, I have not observed speech processing to alter > any of this. I think speech processing does not have to change the the comb-like nature of the SSB spectrum. Sometimes it does however. Only a few overtones survive and noise is filled in between. I did give it a try, but I was unable to find the carrier on the basis of power spectra in some cases. Stations that I could tune manually with no problem. My conclusion was that one should look at the phase also. I think I know how to do that now. The phase shift from transform to transform on each peak gives a better estimate of the frequency than a simple peak search. (Linrad uses that in its FM detector now.) Maybe I will return to this problem some day.... 73 Leif / SM5BSZ
