[RTTY] MMTTY RXlmsNotch setting?

[Ian White GM3SEK]

Kok Chen wrote:
>On Feb 11, 2009, at 11:53 AM, Ian White GM3SEK wrote:
>
>> he only ones that seem to make sense are
>> mid-way between the two tones at 2125 + 85 = 2210Hz, so I'm presuming
>> that with 1275Hz mark all the notches should be positioned at 1360Hz?
>
>I'm not so sure that ad-hoc notching of the center of the mark and 
>space tones helps in general -- depending on the Q of the notch, it 
>could actually hurt.
>
>If the notch is too wide, it will start reducing the important keying 
>sidebands of the mark and space signals themselves.  And that will hurt 
>the ultimate SNR of the demodulated signal.
>
>The proper way to demodulate RTTY is not through using an arbitrary 
>notch but to design a proper Matched Filter for the Mark and Space 
>tones to start with.  For relatively wide shifts like 170 Hz shift 
>RTTY, the match filters usually depresses the response in the region 
>between Mark and Space tones -- but not in an arbitrary manner. The 
>response between the mark and space tones should maximize the SNR. See 
>the explanation here:
>
>http://en.wikipedia.org/wiki/Matched_filter
>

Granted... but this discussion was specifically about making the best 
use of the facilities available within the MMTTY engine. There are many 
full-featured RTTY programs to choose from, but MMTTY is one of the very 
few that provides a minimalist plugin that can be integrated with the 
user's existing choice of logging software. For many of us, MMTTY is 
"the only game in town", so we're trying to sweat it for the maximum 
possible performance within its known limitations.



-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek