In article , W7TI
writes:

On Wed, 22 Oct 2003 06:52:37 -0700, "Joel Kolstad"
wrote:

After all... in the presense of some AM on regular double side band FM, most
receivers still perform just fine, don't they?
_________________________________________________ ________

In the USA, the FCC used to prohibit simultaneous amplitude and
frequency modulation. I did a search of Part 97 rules and I don't see
that exact wording now, but I would still tread lightly in this area.
Provided all the sidebands are confined to a band no wider than
conventional AM, you probably won't be bothered by Uncle Charlie but
caution is advised.
--
Bill, W7TI

Bill, I just dug out the 1977 issues of HR from storage and looked
the article over. Author Richard Slater (W3EJD) said almost the
same thing at the end of the article on page 15 under "closing
comments." The nomenclatures for different modulations were
formalized by the ITU-R since then but the FCC still doesn't have
anything covering this "single-sideband FM" modulation type for
U. S. amateur radio.

A general problem with understanding the concept is the simplicity
of the explanations of AM in today's amateur radio. The mathematical
representations of all modulations have been known and distributed in
text books for decades...my introduction to that was "Electronic
Designer's Handbook by Landee, Davis, Albrecht, McGraw-Hill 1957,
Section 5. Those who can follow the series expressions in a
summation formula, study it, will understand how a phasing-type SSB
modulator and demodulator can work. It is much harder to look at the
expressions and "see" FM or PM; Hewlett-Packard's Agilent site has
a neat little animated Java display that may help some on that.

Filter-type SSB from AM is almost intuitive when the AM spectrum is
shown. That is easy to comprehend...once all accept that the content
of each AM sideband has the same information. (there are still some
long-timers who refuse to accept that the carrier RF energy doesn't
change in AM at less than 100% modulation, heh heh) FM and PM
sidebands are definitely NOT easy to visualize since their individual
amplitudes and phases change depending on modulation index and
modulating frequency. There isn't any corresponding similarity of
FM and PM to AM for the repetition of sidebands' information when
looking at the spectral content.

What Slater was discussing in that January 1977 HR article was what
a group of researchers had already been doing in the early 1970s to
see if there were alternatives to SSB-like frequency multiplexing in
multi-channel circuits. Part of that investigation was to get around
some of the patents still existing on frequency multiplexing via single
sideband techniques (pioneered first on long-distance telephony, by
the way). Another part was to simplify (if possible) the circuitry
involved when carrying a LOT of channels. Equipent of 3 to 4 decades
ago was a lot bulkier than it is now for non-digital multiplexing. The
"narrowband" necessities of working in small-bandspace amateur bands
was not a prime criteria for that research.

Slater explained much of the above in that article and didn't claim any
exciting narrowband results of previous art. The (mislabeled in my
opinion) "single-sideband FM" technique of combining FM and AM
is simply a DIFFERENT way to communicate information.

A truly different way of modulation exists in everyone's telephone line
modem that can send/receive up to 56 Kilobits/Sec in a bandwidth of
only 3 KHz. That is a combination of AM and PM. That isn't intuitive
to AM-oriented minds and there still exist arguments in newsgroups
that such high rates "aren't possible!" :-) Yet most of us POTS users
with computers regularly get 33 to 56 KBPS rates over 2.5 to 3.0 KHz
bandwidth telephone circuits.

I've not seen much on that "single-sideband FM" stuff in the professional
literature after 1980. Based on what was published in the 1970s, it was
an interesting technique but did not come up with any advantages for
commercial or military adoption or much further work. I think it does
show that old paradigms aren't always worth four nickels and that, truly,
thinking outside the box might come up with something new and useful.

Just some comments from
Len Anderson
retired (from regular hours) electronic engineer person