As others have said, originally we used 2125/2975 and that was what
the commercial and military people used.  And originally the FCC
required the shift be about 850 Hz.

There was a standard series of filters for voice frequency carrier
TTY systems, in which the tone frequencies were 85 Hz apart, so I
guess the 2125/2975 got chosen because the filters were regularly
available off the shelf.  There was a time when MARS allowed narrow
shift but reqired it to be 85 Hz, like a single channel of a VF
carrier system.  This may have been Navy MARS only  I don't remember.

There was a lot of complaint about amateur use of 850 Hz shift
because it took up so much bandwidth in the CW portions of the
amateur bands.  Old hands at ARRL more or less opposed HF RTTY for
that reason.  And there was interference, some casual and some no
doubt intentional from hams using CW and trying to discourage RTTY.

Well then the hams started thinking about using narrower shifts, and
after a lot of things were proposed 170 Hz was settled on.  Irv Hoff
noted a reduction in interference from other stations when we used
170 Hz.  But I don't think 170 was ever a standard anywhere except
in the amateur service.  I guess it was chosen just because it was
a multiple of 85.

You and I argued about 850 Hz and frequency drift.  I'll admit that more
recent equipment was pretty stable, but older gear such as the BC-312
BC-342 receivers was subject to drift and in more fixed-stations
applications receivers like the SP-600 were popular and they drifted.
The discriminator in the AN/FGC-1 used a pair of filters that were
fairly flat centered on the mark and space frequencies, so the output
would not be affected much by drift until it drifted pretty far.
And the companion receiver AN/FRR-3 could use AFC to compensate for
drift.

Then the other change was the advent of good amateur SSB equipment that
considerably reduced frequency drift.  And used things like mechanical
filters for selectivity.  A lot of this gear had a filter bandwidth, and
hence an audio bandwidth of 2.4 KHz for SSB voice.  So 2125 Hz was in
and 2975 Hz was out.  One solution was to keep 2125 and substitute
2295 for the other frequency.  Another solution in some radios was to
have an RTTY mode that moved the BFO and hence the audio passband high
enough to permit 2975 to pass.  And another solution was to use lower
frequencies for both tones so that even if the receiver had a 1.8 KHz
audio bandwidth the tones would make it through.

I think a lot of military gear like the AN/URA-8 was intended to receive
any shift up to 1 KHz.  Using a linear discriminator.  A matter that was
widely understood was that the discriminator needed to be linear far
beyond the mark and space frequencies.  Most amateur designs put the
discriminator peaks at the mark and space frequencies.

A shortcoming of many receiving systems was that they depended on the
receiver bandwidth to restrict bandwidth ahead of the limiter.  Which
meant that the pre-limiter bandwidth was far in excess of the bandwidth
of the signal to be received.

So we hams went from drifty equipment and home made demodulators, to
better SSB grade radios and commercially manufactured demodulators
became available.  You could still build your own demodulator and put
the receiving frequencies anywhere you wanted them.  Two-tone detectors
replaced discriminators and ideally used pretty narrow filters.
Commercial demodulators typically offered the buyers a choice of mark
and space frequencies, perhaps with plug-in filters.

Then there was a generation of commercial demodulators, for example
Dovetron and HAL ST-8 and Frederick and Electrocom which offered
tunable mark and space frequencies.  It hs been suggested these were
intended for monitoring a single channel of a VF telegraph carrier
system.  And operational military gear became more exotic with the
MD-522/GRC and others.

And then we arrived at the present DSP demodulators, beginning with
K6STI RITTY and now fldigi and multipsk and mmtty, where all the settings
are user-settable and perhaps automatically adjusted during reception.

---

     "Ya can argue all ya wanna, but it's dif'rent than it was."
    "No it ain't! No it ain't!  But ya gotta know the territory."
        Meredith Willson, The Music Man
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