On Jan 16, 2011, at 1/16 6:47 AM, Bill wrote:
> The fellow who was in North Korea, and allowed to operate, used 23
> HZ for rtty.
No, Ed P5/4L4FN had a preference for 50 baud, not 23 Hz shift.
23 Hz shift at 45.45 baud data rate is more precisely a class of
modulation that is called "Minimum Shift Keying," MSK. Note that MSK
has nothing to do with MFSK (which is multiple tone FSK, stuff like
MFSK16, DominoEX and Olivia).
The precise MSK shift for 45.45 baud is strictly speaking 22.725 Hz.
I.e., the shift for MSK is always precisely one half the baud rate.
MSK is actually related to QPSK mathematically and ideal demodulators
can be constructed accordingly. Basically, the I and Q constellation
for the modulating signal are positioned at 4 corners of a square, and
the transition is always between constellation points that are closest
to one another (unlike QPSK, where you you can move any 4 different
constellation points). I.e., you are still transmitting one bit per
modulating symbol in MSK.
MSK can be demodulated using FSK demodulators whose filters are tuned
closely to 23 Hz together, but not optimally. Optimal demodulation
of MSK involves mixing down with a quadrature oscillator to a baseband
I/Q signal and demodulating from that baseband signal.
Similarly, MSK receivers can copy a narrow shift FSK signal, but not
optimally.
There is a common MSK mode, called Gaussian MSK (GMSK) where the
modulating waveform is first passed through a Gaussian filter.
73
Chen, W7AY
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