CW detection on DSP

Hi all,

I've heared about particular algorithms to detect CW on a FFT.
Anyone can help me ? 

On Aug 1, 10:37 am, "Bruno DAJIN" <brunodajin_nosp...@yahoo.fr> wrote:
> I've heared about particular algorithms to detect CW on a FFT.
> Anyone can help me ?

Depends on the nature of what you need to detect.

Is the carrier wave frequency fixed or modulated?
Is the carrier wave frequency known or bounded?
What is the signal-to-noise ratio with which you
are working?
Do you need to detect just the carrier, or are you
interested in decoding Morse Code encoded CW?
Do you know the WPM of the Morse Code?
Is the WPM precise (machine generated), or is there
a noticeable "fist" or other timing variations?
Is the Morse Code dot clock synchronous or async
to your fft apertures/windows?
If the signal-to-noise ratio is very low, do you
have some statistics on the textual content?

A plain FFT might be good at finding the presence or
absence of a relatively strong CW carrier, given reasonable
assumptions about the frequencies, sample rates, frame
size and noise levels; but other techniques may be needed
to find the beginning and ends of the dots and dashes,
or the likely encoded content of any contained message,
depending on the exact nature and constraints of your
problem per the answers to some of the questions above.



IMHO. YMMV.
--
rhn A.T nicholson d.0.t C-o-M

Bruno DAJIN wrote:
> Hi all,
> 
> I've heared about particular algorithms to detect CW on a FFT.
> Anyone can help me ? 

You know it's CW if it has no sidebands: i.e., it has a line spectrum.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

On 2007-08-01, Bruno DAJIN <brunodajin_nospam_@yahoo.fr> wrote:
>
> I've heared about particular algorithms to detect CW on a FFT.

If you display an FFT as a waterfall (one example here:
http://www.nitehawk.com/sm5bsz/linuxdsp/usage/sdr14-w/sdr14-vw.htm  )
it's pretty easy to see.

-- 
Ben Jackson AD7GD
<ben@ben.com>
http://www.ben.com/

On Aug 1, 9:16 pm, Jerry Avins <j...@ieee.org> wrote:
> Bruno DAJIN wrote:
> > Hi all,
>
> > I've heared about particular algorithms to detect CW on a FFT.
> > Anyone can help me ?
>
> You know it's CW if it has no sidebands: i.e., it has a line spectrum.

Morse code modulated CW does have sidebands.  The
bandwidth required for typical human demodulation is
around 2 to 4 times the WPM, although wider anti-click
filtering is commonly used, which makes it easier to
copy by ear (less ringy sounding).

Synchronous DSP demodulated Morse code can be done
using less bandwidth than the WPM, limited of course by
the information capacity given the S/N ratio of the channel.

IMHO. YMMV.
--
rhn A.T nicholson d.0.t C-o-M  (amateur advanced class)

Ron N. wrote:
> On Aug 1, 9:16 pm, Jerry Avins <j...@ieee.org> wrote:
>> Bruno DAJIN wrote:
>>> Hi all,
>>> I've heared about particular algorithms to detect CW on a FFT.
>>> Anyone can help me ?
>> You know it's CW if it has no sidebands: i.e., it has a line spectrum.
> 
> Morse code modulated CW does have sidebands.  The
> bandwidth required for typical human demodulation is
> around 2 to 4 times the WPM, although wider anti-click
> filtering is commonly used, which makes it easier to
> copy by ear (less ringy sounding).

If its modulated, its not a pure carrier wave. Its an extreme form of 
AM, if it is pure OOK.

Steve

Ron N. wrote:

> On Aug 1, 9:16 pm, Jerry Avins <j...@ieee.org> wrote:
(snip)
>>You know it's CW if it has no sidebands: i.e., it has a line spectrum.

> Morse code modulated CW does have sidebands.  The
> bandwidth required for typical human demodulation is
> around 2 to 4 times the WPM, although wider anti-click
> filtering is commonly used, which makes it easier to
> copy by ear (less ringy sounding).

Note that this is exactly the problem that Nyquist was working on:

Quoting from Wikipedia:

   "In 1927 Nyquist determined that the number of independent pulses
   that could be put through a telegraph channel per unit time is
   limited to twice the bandwidth of the channel. Nyquist published
   his results in the paper Certain topics in Telegraph Transmission
   Theory (1928)."

http://en.wikipedia.org/wiki/Harry_Nyquist

I am not sure how you are converting WPM to bandwidth.

http://en.wikipedia.org/wiki/Morse_code

indicates that T=1200/W, where T is the dit time and W is WPM.

-- glen

On Wed, 1 Aug 2007 19:37:36 +0200, "Bruno DAJIN"
<brunodajin_nospam_@yahoo.fr> wrote:

>Hi all,
>
>I've heared about particular algorithms to detect CW on a FFT.
>Anyone can help me ? 

Hi Bruno,
  Can you tell us EXACTLY what you mean 
by "CW", giving us every characteristic 
that you can think of regarding this 
thing you call "CW"?

I'll bet the guys here can help you if 
you're able to clearly explain what it 
is that you're trying to detect.

[-Rick-]

R.Lyons@_BOGUS_ieee.org (Rick Lyons) writes:

> On Wed, 1 Aug 2007 19:37:36 +0200, "Bruno DAJIN"
> <brunodajin_nospam_@yahoo.fr> wrote:
>
>>Hi all,
>>
>>I've heared about particular algorithms to detect CW on a FFT.
>>Anyone can help me ? 
>
> Hi Bruno,
>   Can you tell us EXACTLY what you mean 
> by "CW", giving us every characteristic 
> that you can think of regarding this 
> thing you call "CW"?
>
> I'll bet the guys here can help you if 
> you're able to clearly explain what it 
> is that you're trying to detect.

Perhaps he's speaking of the basic detection problem
in which an observation has two hypotheses: H0 == there
is no signal present, H1 = a signal is present. By
"signal" he means a single sinusoid at a known frequency
but unknown amplitude. 

But you're right - it's a good idea for the OP to clarify
what he means.
-- 
%  Randy Yates                  % "The dreamer, the unwoken fool - 
%% Fuquay-Varina, NC            %  in dreams, no pain will kiss the brow..."
%%% 919-577-9882                %  
%%%% <yates@ieee.org>           % 'Eldorado Overture', *Eldorado*, ELO
http://home.earthlink.net/~yatescr

On Aug 3, 7:32 pm, Randy Yates <ya...@ieee.org> wrote:
> R.Lyons@_BOGUS_ieee.org (Rick Lyons) writes:
> > On Wed, 1 Aug 2007 19:37:36 +0200, "Bruno DAJIN"
> > <brunodajin_nosp...@yahoo.fr> wrote:
>
> >>Hi all,
>
> >>I've heared about particular algorithms to detect CW on a FFT.
> >>Anyone can help me ?
>
> > Hi Bruno,
> >   Can you tell us EXACTLY what you mean
> > by "CW", giving us every characteristic
> > that you can think of regarding this
> > thing you call "CW"?
>
> > I'll bet the guys here can help you if
> > you're able to clearly explain what it
> > is that you're trying to detect.
>
> Perhaps he's speaking of the basic detection problem
> in which an observation has two hypotheses: H0 == there
> is no signal present, H1 = a signal is present. By
> "signal" he means a single sinusoid at a known frequency
> but unknown amplitude.
>
> But you're right - it's a good idea for the OP to clarify
> what he means.

Clarification would be good, given that I've heard the term
"CW" used more often as an abbreviation for Morse Code
than for pure carrier waves, even though the letters don't
match... (or maybe one letter is upside-down :^)

For audio Morse Code, one poster mentioned a waterfall
display of fft's; but a scrolling spectrogram using fft's
would allow one to read off any dots and dashes left-to-right,
which might be easier than top-to-bottom (with the appropriate
band separations and WPMs for the fft frame sizes, of course).


IMHO. YMMV.
--
rhn A.T nicholson d.0.t C-o-M