[RTTY] Need to lawyer up for 60 meters

[Kok Chen]

On Nov 26, 2013, at 10:17 AM, Kai wrote:

> When done correctly, MSK is phase continuous and has nice spectral response. With Baudot 7.5 bit coding like we use, I'm not so sure. That 1.5 bit long stop bit can really screw things up.

I had said earlier that 22.725 shift FSK is substantially the same as MSK.  Or words to that effect.

And 1.5 stop bits will *definitely* widen the spectrum of a 22.725 Hz shift FSK signal -- MSK does not even allow you to use anything but integer bit durations.

Once you apply a Raised Cosine filter to a continuous phase AFSK signal, the only difference left between FSK and MSK is that MSK effectively forces the bit transitions to be aligned to when the tone crosses zero.  This makes 1.5 bits impossible to generate in MSK.

With MSK, the tone itself is perfectly in phase with the data transitions (thus you see it often described as "coherent" FSK with an appropriate amount of phase shift keying -- but it is much easier to visualize from the I/Q domain than talking about frequency and phase shifts :-).  

When you generate a 22.725 shift FSK or AFSK signal, this coherent property is not ensured.  Indeed, it is guaranteed to be false if you use 45.45 baud at 23 Hz shift, instead of 45.45 baud at 22.725 Hz shift.

When done correctly, MSK therefore has the error rate advantage over asynchronous FSK, somewhat akin to how much better coherent FSK is better than non-coherent FSK.

So, if the software supports MSK, use it instead of dialing AFSK shift down to 23 Hz.  It will have lower error rates.

I know that fldigi supports MSK in Hellschreiber, but I don't know if it supports MSK in standalone RTTY.  If Dave W1HKJ is reading this, perhaps he can tell us much more.

Anyhow, after writing all of this, I don't recommend such a narrow shift for the lower HF region, where selective fading and Doppler spreading are what determines errors (hey, same problem with going to higher symbol rates under Rayleigh Doppler spreading, too :-).

You really need FSK shifts of more than 100 Hz to start benefitting from applications of good ATC circuits/algorithms.

The places where MSK shine are when there is no ionosphere in between -- that is why variants of it have been used in GSM cell phones.  

73
Chen, W7AY