[RTTY] MMTTY RXlmsNotch setting?

[Kok Chen]

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

I.e., for an AWGN channel, the impulse response of the matched filter  
is the time reversal of the received pulse -- this is so that  
mathematically, the filter's convolution is just a simple cross- 
correlation and therefore maximizes the SNR.  The received pulse need  
not be a rectangle -- but will depend on the propagation conditions.

An arbitrary notch will almost for certain not give you the response  
described above.

I believe RITTY was the first Amateur modem to use a Matched Filter  
which adapts to a 45.45 baud pulse under different propagation  
conditions.

The output of RTTY Matched Filters for AWGN channels typically have  
responses that look like triangular waves, rather than sinusoidal  
profiles.  You can see this triangular shape alluded to in the figures  
in the above wikipedia page, and an actual Matched Filter output can  
be seen in Figure 1 here (about a quarter way down in the "RTTY  
Monitor" section):

http://homepage.mac.com/chen/w7ay/cocoaModem/More/Contents/part2.html

The slightly rounded corners in Figure 1 are due to a band pass filter  
that is placed around the RTTY signal -- you can tell from this figure  
that when you use a filter that is too tight, when there is no QRM, is  
detrimental to weak signal copy since the peaks of the triangle will  
be more rounded and you therefore lose SNR since the noise is not  
proportionally reduced.

73
Chen, W7AY