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
_______________________________________________
RTTY mailing list
RTTY@contesting.com
http://lists.contesting.com/mailman/listinfo/rtty
|