[RTTY] uh, what about spectrum?

[Kok Chen]

On Jan 16, 2006, at 10:13 AM, Peter Laws wrote:

>> Section 2.202 of the FCC rules specifies that the necessary bandwidth
>> of an FSK signal is equal to 1.2 x Shift + BaudRate.

Neccessary bandwidth is that is required for you to transmit and  
receive an RTTY signal.

How much must a signal be down at the bandwidth limits under the new  
proposed rules?

For today's 170 Hz, 45 baud signal, I suspect that most FSK rigs may  
not be down enough at the 250 Hz limits.  AFSK setups can apply  
brickwalled FIR filters to impose a 250 Hz limit if needed.

However, 85 Hz shift and 45 baud may be feasible inside 200 Hz for  
AFSK stations.  And perhaps even some FSK rigs.

(W6WRT and I have exchanged some off-line email about the following...)

When you reduce the shift, you will lose a lot of the frequency  
diversity properties (which make RTTY copyable when PSK is not) to  
counter selective fading.  RTTY with ATC can copy a signal (albeit at  
higher BERR) when one tone has faded completely away.  When the two  
tones are too close to one another, when one fades, so will the other.

Reducing the shift will also increase the problem where mark and  
space bits overlaps one another in multipathed conditions (for the  
curious who has the schematics, the ST-8000 has some circuitry to  
help alleviate this problem).

Does anyone have the precise rules of what baud rate and what shifts  
are allowed under current regulations?

I know hams have use shifts of 170, approx 185, 200 and 850 Hz (and  
other shifts when their rigs are out of calibration :-).  We have  
also use baud rates of 45, 45.45, 50, 75.

Are we allowed to arbitrary change baud rate and shifts?  If so,  
someone in California (he usually don't read this reflector) and I  
both run the same software that can alter both baud rate and shifts  
arbitrarily and can run some tests.

We can of course run numbers through FFTs and whatever without  
getting on the air -- but the key (sic) is going to be how well FSK  
rigs waveshapes their keying signal to reduce the sidebands.  And  
also if enough of the FSK rigs are capable of handling arbitrary shifts.

73
Chen, W7AY