[RTTY] Re: external DSP audio filters

[Kok Chen <chen@mac.com>]

On Oct 2, 2004, at 10:25 AM, Bill Turner wrote:

> How does this work on those occasional stations using 200 Hz (or more)
> shift?  Seems too narrow to include the sidebands if the skirts are
> really steep.

I cascade a pair of INRAD 250 Hz filters on my 1000MP and can still 
copy 200 Hz shift signals.  You have to tune carefully, but that is 
what crossed banana scopes are for, and they can spot the skewed 200 Hz 
TNCs a mile away.

45 baud RTTY data's fundamental frequency is only 22.7 Hz maximum. 
i.e., the fastest a data bit can change is at each 22ms and it takes 
two changes to make a cycle of waveform.

With a brick wall 250 Hz filter and a 200 Hz shift signal, you will be 
still be able to reform the data, though not optimally from the S/N 
ratio standpoint.

There are at least two other factors at play.  One, when the band is 
really, really crowded, a 250 Hz filter may indeed give better 
signal-to-QRM ratio for a 200 Hz shift than a more ideal 340 Hz filter. 
 We are not talking about AWGN channels during a contest.

Two, as steep a skirt as the INRADs provide, their skirts are not brick 
walls.  I would not use a 250 Hz 1000-tap FIR filter on an RTTY signal 
for example -- my DSP filters are usually set to 350 Hz.  And I open 
_those_ up on occasions (heavy multipath).

73
Chen, W7AY

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