[RTTY] 300hz or 500hz IF filter?

[Kok Chen]

On Aug 25, 2013, at 9:33 AM, Bill Turner wrote:

> Chen, we'll have to agree to disagree on that point, but that was not the
> reason for my original reply to Kai's post. Kai said "The theoretical
> bandwidth of 170 Hz shift 45.45 baud RTTY is just under 250 Hz."

As an "emission designator," the ITU uses the formula 

Baud + Shift*1.2 

for specifying channel spacings ("neccessary bandwidths") of an FSK signal.  See here if you won't take my word for it:

http://life.itu.ch/radioclub/rr/ap01.htm

Scroll down to Part B in the above link, look for the class of "direct printing telegraphy" as the representative formula.

The 1.2 factor is purely empirical.  But if you follow the ITU formula religiously, steam RTTY comes out as having a bandwidth of 249.45 Hertz. 

You can quibble that Kai's number is off by a quarter percent from the official ITU number, but Kai obviously rounded that ITU number up to 250 Hz.  Indeed, you often see Amateur RTTY specified with a "250H" prefix in its Emission Designator.

Anyhow, you now know where Kai got the 250 Hz from, and it is not from his backside, or I would have already called him out on it. 

73
Chen, W7AY