[RTTY] Dual Peak Filter question

[Dave Hachadorian]

I've never found the K3's dual peak filter to be an advantage 
under any conditions.  The slight ringing at the mark & space 
frequencies always makes me think that a weak signal is in there. 
I use only the 250 Hz filter for RTTY.  I told the K3 that is was 
a 400, so it switches in at DSP BW = 400.  I usually run it at 
DSP BW = 350.  With that setup, the passband looks super clean on 
the MMTTY display.


Dave Hachadorian, K6LL
Yuma, AZ




-----Original Message----- 
From: RLVZ at aol.com
Sent: Saturday, April 09, 2011 7:44 AM
To: rtty at contesting.com
Subject: [RTTY] Dual Peak Filter question

I appreciate the discussions on this reflector as I still  have a 
lot to
learn about RTTY.

Based upon the information below that basically contends that 
"software
based dual peak filters outperform the a rigs dual peak DSP 
filter" can I
assume then that I'd do better during RTTY contests by  turning 
my K-3's dual
peak filter off" and turning on the MMTTY dual peak  filter?  (I 
have never
tried MMTTY's dual peak filter)  In case it matters, I have 250hz 
and 400 hz
roofing  filters in the K-3.

Thanks & 73,

Dick- K9OM


As such,  optimal filtering for digital modes is better achieved 
in the
software modem  than by using approximations in the DSP stages of 
the rig.  And
software  UI can be simplified to anything that you wish.

With the processors in  today's computers, there are not that may 
things
DSP processors can do in real  time that the computers cannot 
also in real
time, where latency is not a  problem (the exception is the 
decimation process
of the direct SDR that starts  at 70 MHz).

A dual peak filter (usually done in the DSP stages  of a rig) is 
just an
approximation of two narrow bandpass filters around each  of the 
two RTTY
tones.  This is what produces the "depression" between  the two 
tones.  It
approximates what a pair of matched filters does in  software 
that supports
matched filtering.  In fact, one thing to note is  that if the 
software already
implements matched filtering, engaging a dual  peak filter will 
make copy
worse, not better.  If copy gets better, it  means that the 
software filter is
not optimal.

A simple dual  peak filter cannot track the baud rate of the RTTY 
signal
(at least I have not  seen one that does).  I think RITTY's 
matched filter is
also fixed at  45.45 baud, but there are other software that 
implements
matched filtering  which tracks the baud rate when you change it. 
In the case
of RTTY, a  matched filter has an impulse response that 
corresponds to the
rectangular  pulse of the demodulated RTTY signal, i.e., a pair 
of sin(x)/x
shapes in the  frequency domain.

73
Chen,  W7AY


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