Here's something that I believe is not well known

\bibitem[Davey3]{davey3} J. R. Davey and A. L. Matte {\it Frequency Shift
Telegraphy -- Radio and Wire Applications.} AIEE Transactions \bf 66\rm,
1947, p. 479.  Also in Bell System Technical Journal \bf 27\rm, April
1948, p. 265.

the FSK tones might be phase or
amplitude modulated at a frequency of 200 Hz or so.  The purpose of
this is to widen the spectrum of each tone, in hopes of avoiding
the effects of frequency-selective fading.  The experts seem to agree
that this technique is inferior to dual diversity reception,
but may be helpful where diversity reception is not possible.
(perhaps on shipboard where there is no space for widely-spaced
antennas)

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