On Nov 24, 2011, at 5:27 AM, Russell Blair wrote:
> This mode can be used on 60m because it so narrow 23hz.
> When we can get on 60m digital.
Please note that a "23 Hz RTTY" signal that is not waveshaped can have
moderately wide keying sidebands.
There is a mode called Minimum Shift Keyed that is interoperable with RTTY (one
can copy the other moderately well, though not perfectly) that is narrow
enough. The baud rate of MSK is precisely twice the shift frequency. I.e.,
for 45.45 baud, the shift is exactly 22.725 Hz.
A number of software modems support 45.45 baud MSK. And if they don't, it is
moderately trivial to modify a PSK modem (yep, PSK, not FSK) to transmit an
45.45 baud MSK signal with Baudot encoding. PSK and MSK are closely related
mathematically. (Note that the FCC 60m rules expressively permitted various
encodings to encourage experimentation, so Baudot should be fine with them.).
That being said... I am not sure if a 45.45 baud MSK signal would even fit
inside a 60 Hz bandwidth.
A PSK31 signal _barely_ fits into a 60 Hz bandwidth (thus the 60H0J2B
designator) and its symbol rate is lower, at 31.25 baud. A 45.45 baud PSK
signal would not fit into 60 Hz, for example.
Thus my current doubt that a 45.45 baud RTTY or MSK signal could fit into 60 Hz.
BTW, if you are going to use PSK31 on 60m, it is interesting to note that the
FCC did not mention the center frequency of a PSK31 signal (as far as I could
understand the legalese). Each 60m channel is 2.8 kHz wide. That means
potentially more than one PSK31 (and CW) signal can sit within each 60m channel
if one very carefully calibrates the transmitter's dial frequency.
Perhaps there will eventually be an ad-hoc band plan where all PSK31 signal
stay inside one 60m channel, Pactor III uses one 60m channel, and voice mode
uses the remaining channels.
Many of us have tried 23 Hz RTTY before this. Remember the time when the ARRL
wanted to propose band plans that are bandwidth dependent which would drive 170
Hz RTTY away from the Extra class portions at the lower parts of the Amateur
bands? I made a 23 Hz RTTY QSO with W6WRT, I believe back then.
23 Hz RTTY has many advantages if your modem takes advantage of them. For one,
there is a 5 dB or so SNR advantage if you use good Nyquist filters.
But I have doubts that 23 Hz RTTY complies with the new FCC 60m rules under the
60H0J2B/60H0J2D modes.
Perhaps we can use RTTY as a subset of the 60m 2K80J2D emission mode (2.8 kHz
wide) that is earmarked for Pactor III. But that is a question for lawyers to
answer, not for a lowly retired engineer. If RTTY is covered under 2K80J2D, we
could even get away with 850 Hz shifts, HI HI.
73
Chen, W7AY
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