[RTTY] Question: 23 Hz RTTY

[Kok Chen]

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