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@aol.com
Sent: Saturday, April 09, 2011 7:44 AM
To: rtty@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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