Re: CW/Morse keying detection revisited

On Aug 27, 12:51 am, "John Barrett" <john.ae...@xxxxxxxxxxx> wrote:
I'm working on a CW/Morse demodulator/detector/decoder that takes 8khz 16bit
audio from any SSB reciever, and attempts to detect the state of the signal
(key down, key up) at 700hz. The current "circuit" (all software DSP on a PC
in C#) consists of a sin/cos converter to IQ running with 700hz sin/cos
signals, followed by a 300 tap 50hz LP FIR on I and Q, with integrated
decimation to a 500hz sample rate.

The FIR outputs are squared, summed, and the final output square rooted :)
Everything to this point appears to be working, excepting that out of band
(<650hz, >750hz) signals have a strong effect on the output, and I am more
or less unhappy with the size of the "noise window" (difference between the
peaks and troughs when no signal is present) given that many signals that I
can hear clearly enough to possibly copy show very little change in the
outputs from the DSP code, barely raising the signal out of the noise
window.

This is my 1st cut at doing DSP, and I have no formal training, so some of
the questions are liable to be pretty fundamental :) The code described
above is based on the recommendations of the developer of a BPSK
demodulator.

I need to achieve 2 goals in the next iteration of the code:
1. Fix any problems with the "demodulator" design (IQ conversion, filtering,
and summing) to clean up the noise issues.
2. convert the design to be frequency agile -- able to detect CW at any
frequency in the range 200-2500hz

After reading a number of articles on DSP SSB demodulators, I'm a little
confused because some show a Hilbert Transform after the LPF on the I
channel and a matching delay on the Q channel. I need to know if the lack of
this step in the process could be the cause of the excessive (in my opinion)
noise at the summing output.

I'm also confused about the LPFs after tthe IQ conversion and what exactly
they are intended to accomplish, as I've seen designs with both LP and BP
filters in that location. When using LP filters, is that essetially setting
the width of the window around the center frequency defined by the feed to
the IQ converter ?? or is it something else ??

Any help to straighten this mess out would be greatly appreciated !!

Getting an algorithm to do what the brain can do very well is very
very hard. The on/off nature of CW puts it at an extreme disadvantage
for automated decoding compared to, say, PSK or BPSK, so you will very
definitely need tricks that are not in the PSK/BPSK books.

Most experienced CW operators, and all EME operators, can easily hear
signals that are under the technical Minimum Discernable Signal (MDS)
limit you'd guess by simply using S/N and noise in bandwidth.

For the human ear, reducing bandwidth is not always the solution. In
particular, narrow bandwidth leads to ringing, which not only is
extremely fatiguing, but also makes out of band impulses show up as an
apparent signal.

The 100Hz bandwidth you are trying is way too narrow for optimal human
brain/ear copy.

One thing to look at, I don't know how they do it inside: CW Skimmer.
I believe (but have no evidence) that it is looking not for simple on/
off signal presence/no presence, but for coherent signals that happen
to match morse characters or even patterns of entire callsigns,
probably using something like PRML techniques, and it does it across a
spectrum.

I've seen CW Skimmer in action during a contest. It is amazing! I
could imagine hooking it up to a mouse and when you see (not hear)
someone calling TEST you just click on the waterfall and work them.
I'm not sure this is the way I'd enjoy CW, but I'm sure others will be
doing it soon.

Tim N3QE
.

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