Newbie: CW demodulation

Hi guys, I'm newbie in those groups. I tried to make a CW demodulator
(Telegraphy). I would like to make this because it's the simplest signal.
I have somes algorithms about USB/LSB/AM/FM demodulators, with theis I/Q
signals, but what do I have to use to try it? or are there any simple
solution?. In tha analog world, it's only IF-LO= CW Audio, but in the
digital world, is is simpler than analog?.

Can anybody help me? Maybe it's a simple firmware, so I think I'll do it
on a dsPIC30F2010(4K/20MIPS) to start experiments.

Thanks


		
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A USB or LSB demodulator should work for CW as well for starters.

After that we can work on reducing the bandwidth.

Mark

EA7FNJ wrote:

> Hi guys, I'm newbie in those groups. I tried to make a CW demodulator
> (Telegraphy). I would like to make this because it's the simplest signal.
> I have somes algorithms about USB/LSB/AM/FM demodulators, with theis I/Q
> signals, but what do I have to use to try it? or are there any simple
> solution?. In tha analog world, it's only IF-LO= CW Audio, but in the
> digital world, is is simpler than analog?.
> 
> Can anybody help me? Maybe it's a simple firmware, so I think I'll do it
> on a dsPIC30F2010(4K/20MIPS) to start experiments.
> 
> Thanks
> 

Are you trying to emulate a CW radio receiver?  If so, then Mark's 
suggestion is exactly correct -- a CW radio receiver intended to be used 
by a human is just an SSB receiver with a narrow filter.

If, on the other hand, you are trying to get CW into a computer to be 
demodulated into text then the problem can range from the simple to the 
complex, with a wide range of performance points.

Are you starting from RF, intending to sample directly into the DSP and 
get audio or data out?  If not, what conversion scheme do you want to 
use to get from RF down to something slow enough for the DSP?

With the right front-end processing the dsPIC should be a fine 
processor, but with the wrong front-end processing you won't get there 
from here.

So what are you trying to do?

-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

>
>Are you trying to emulate a CW radio receiver?  If so, then Mark's 
>suggestion is exactly correct -- a CW radio receiver intended to be used

>by a human is just an SSB receiver with a narrow filter.
>
>If, on the other hand, you are trying to get CW into a computer to be 
>demodulated into text then the problem can range from the simple to the 
>complex, with a wide range of performance points.
>
>Are you starting from RF, intending to sample directly into the DSP and 
>get audio or data out?  If not, what conversion scheme do you want to 
>use to get from RF down to something slow enough for the DSP?
>
>With the right front-end processing the dsPIC should be a fine 
>processor, but with the wrong front-end processing you won't get there 
>from here.
>
>So what are you trying to do?
>
>-------------------------------------------

The main idea is to make a IF-DSP, put a 10KHz IF to a DSP system and
demodulate the CW. So, first sampling tha IF, filter and after... does it
demodulate as a LSB or USB?. The CW is easier than others kind of
modulations, it's only open and close a unmodulate carrier to make dot or
dash. 

		
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EA7FNJ wrote:

>>Are you trying to emulate a CW radio receiver?  If so, then Mark's 
>>suggestion is exactly correct -- a CW radio receiver intended to be used
> 
> 
>>by a human is just an SSB receiver with a narrow filter.
>>
>>If, on the other hand, you are trying to get CW into a computer to be 
>>demodulated into text then the problem can range from the simple to the 
>>complex, with a wide range of performance points.
>>
>>Are you starting from RF, intending to sample directly into the DSP and 
>>get audio or data out?  If not, what conversion scheme do you want to 
>>use to get from RF down to something slow enough for the DSP?
>>
>>With the right front-end processing the dsPIC should be a fine 
>>processor, but with the wrong front-end processing you won't get there 
> 
>>from here.
> 
>>So what are you trying to do?
>>
>>-------------------------------------------
> 
> 
> The main idea is to make a IF-DSP, put a 10KHz IF to a DSP system and
> demodulate the CW. So, first sampling tha IF, filter and after... does it
> demodulate as a LSB or USB?. The CW is easier than others kind of
> modulations, it's only open and close a unmodulate carrier to make dot or
> dash. 
> 

OK, you still haven't clarified.  You've fallen in with a bunch of folks 
who have had salespeople demand that they implement things that you have 
never even heard of.

Do you want a receiver that does CW reception just like my Kenwood 
TS520, i.e. when the other guy transmits you hear "beep", the frequency 
of the beep depends on tuning and you hear all the static, 
over-the-horizon radar, and all that fun stuff?  Or do you want a 
_demodulator_ that you hook to an antenna and you see text start 
silently scrolling on your computer screen?

If it's the former then you want to implement a single-sideband 
demodulator.  Either LSB or USB will work, the only difference is the 
direction the CW note will change with changes in relative frequency 
between you and the transmitter.  Frankly it's probably marginally 
easier to implement a SSB receiver, if only because the filters are 
wider -- and a SSB receiver works pretty good for CW.

In either case I think you'll find that a 10kHz IF is pretty low -- if 
you can get down to 10kHz cleanly why not just go all the way down to audio?

-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

>
>OK, you still haven't clarified.  You've fallen in with a bunch of folks

>who have had salespeople demand that they implement things that you have

>never even heard of.
>
>Do you want a receiver that does CW reception just like my Kenwood 
>TS520, i.e. when the other guy transmits you hear "beep", the frequency 
>of the beep depends on tuning and you hear all the static, 
>over-the-horizon radar, and all that fun stuff?  Or do you want a 
>_demodulator_ that you hook to an antenna and you see text start 
>silently scrolling on your computer screen?
>
>If it's the former then you want to implement a single-sideband 
>demodulator.  Either LSB or USB will work, the only difference is the 
>direction the CW note will change with changes in relative frequency 
>between you and the transmitter.  Frankly it's probably marginally 
>easier to implement a SSB receiver, if only because the filters are 
>wider -- and a SSB receiver works pretty good for CW.
>
>In either case I think you'll find that a 10kHz IF is pretty low -- if 
>you can get down to 10kHz cleanly why not just go all the way down to
audio?
>

Weel, I only need to heard the dot or dash, my ear can decode the MORSE
code. I put a IF about 10KHz but maybe I use 100KHz, and not on AF because
I can to demodulate with a good filter or with a noise reduction on IF
better than on AF. In the IF I can clear the desire signal better than on
the AF. There are some transceiver that use IF-DSP: Kenwood TS870, TS2000
and the new ICOM IC7000 (not yet on sale). Some receivers, like YAESU
VR5000, have a IF output behind them. It's a good idea to practise hte
IF-DSP with it. Also it can use to demodulate APT signals from weather
satellites like NOAA that it needs a badwith about 50KHz. But this is a
future project, first, demodulate CW and step by step to others kind of
modulation.


		
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>>
>>OK, you still haven't clarified.  You've fallen in with a bunch of
folks
>
>>who have had salespeople demand that they implement things that you
have
>
>>never even heard of.
>>
>>Do you want a receiver that does CW reception just like my Kenwood 
>>TS520, i.e. when the other guy transmits you hear "beep", the frequency

>>of the beep depends on tuning and you hear all the static, 
>>over-the-horizon radar, and all that fun stuff?  Or do you want a 
>>_demodulator_ that you hook to an antenna and you see text start 
>>silently scrolling on your computer screen?
>>
>>If it's the former then you want to implement a single-sideband 
>>demodulator.  Either LSB or USB will work, the only difference is the 
>>direction the CW note will change with changes in relative frequency 
>>between you and the transmitter.  Frankly it's probably marginally 
>>easier to implement a SSB receiver, if only because the filters are 
>>wider -- and a SSB receiver works pretty good for CW.
>>
>>In either case I think you'll find that a 10kHz IF is pretty low -- if 
>>you can get down to 10kHz cleanly why not just go all the way down to
>audio?
>>
>
>Weel, I only need to heard the dot or dash, my ear can decode the MORSE
>code. I put a IF about 10KHz but maybe I use 100KHz, and not on AF
because
>I can to demodulate with a good filter or with a noise reduction on IF
>better than on AF. In the IF I can clear the desire signal better than
on
>the AF. There are some transceiver that use IF-DSP: Kenwood TS870,
TS2000
>and the new ICOM IC7000 (not yet on sale). Some receivers, like YAESU
>VR5000, have a IF output behind them. It's a good idea to practise hte
>IF-DSP with it. Also it can use to demodulate APT signals from weather
>satellites like NOAA that it needs a badwith about 50KHz. But this is a
>future project, first, demodulate CW and step by step to others kind of
>modulation.
>
>
>		
>This message was sent using the Comp.DSP web interface on
>www.DSPRelated.com
>



And, if I use FFT to get the carrier? in this case, I don't have to use
the SSB demodulator, do I? but, I do the FFT to th IF directly or to the
IQ signals?
		
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An FFT can be used to create  a spectrum display that can display the
various signals in the passband on a screen.   What you need is  a BFO
to change the on/off  IF carrier into an on/off audio tone.  Think
about a BFO, it's an oscillator that  beats against the carrier to
create an audio tone.  That is (part of)  what a SSB demodulator does.


Mark

I guess the very simplest starting point would be to just chop (switch 
on and off) the IF with a frequency which is away by 1 kHz (or what you 
want to hear as audio). Low-pass the result and put to the speaker.


Mark wrote:

> An FFT can be used to create  a spectrum display that can display the
> various signals in the passband on a screen.   What you need is  a BFO
> to change the on/off  IF carrier into an on/off audio tone.  Think
> about a BFO, it's an oscillator that  beats against the carrier to
> create an audio tone.  That is (part of)  what a SSB demodulator does.
> 
> 
> Mark
> 


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