Message-ID: <B14F838735DB426DB6581310573DDE37@p4>
From: "Mark" <markhughes1@blueyonder.co.uk>
To: <rsgb_lf_group@blacksheep.org>
References: <008c01c9652f$5e5d4950$0301a8c0@mal769a60aa920> <4951C1F1.3040101@telus.net> <008601c965ba$cc55fe80$0301a8c0@mal769a60aa920>
Date: Wed, 24 Dec 2008 13:20:30 -0000
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Subject: Re: LF: wspr
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Mal
The answer is simple in many cases.
Why not?
I will explain.
You can use a narrower filter if you like, it would have to be 200Hz wide to 
allow the reception of all WSPR signals in the range of the software. CW 
filters tend to be too narrow for that, SSB  filters are standard on most 
rigs, so it is easy to use this mode. It is also mandatory to use SSB mode 
for transmitting, and on some rigs it is difficult to transmit on SSB and 
have the CW filters in. A good CW 50Hz filter would probably work OK, given 
the very narrow bandwidth of the individual WSPR signals, with a small 
number of WSPR transmissions on the band you could use it. It simply isn't 
needed though, and it is desirable to allow the software to see the entire 
200Hz segment, even if it is just to allow for drift or decoding other 
signals.
For some modes the use of narrow conventional filters is bad. They 
intrinsically have poor group delay response and cause distortions to the 
signal not often directly audible, but that can degrade data is a way that 
reduces decoding margins.  This effect is worst when the wanted signal is 
relatively wideband (occupying the whole bandwidth of the filter) or near 
the edge of the passband. DRM, DAB and DTT signals that are OFDM based are 
vulnerable.

All the WSPR operation that the MF band would need, could be squeezed in a 
segment of the band less than 200Hz wide, probably 100Hz would be more than 
ever needed.  WSPR also time division multiplexes, so more than one station 
can occupy the same frequency. No one is proposing WSPR signals all over the 
band, that is self defeating as the software cannot analyse more than a 
200Hz chunk of bandwidth.

The key point is that just because the receivers have a wider passband than 
actually needed does not usually render the mode ineffective in the presence 
of signals that are outside the decode passband.

Like any system, reducing the bandwidth of any part of the system, means 
that subsequent stages are less affected by adjacent signals.
The antenna itself acts as a filter because it is tuned, your low pass 
filter removes many big broadcast signals, but no one is advocating a 
tuneable 50Hz wide bandpass filter implemented at 500KHz, or anything 
approaching this, for CW or any other mode. It is simply not necessary in 
most cases, and a disadvantage in most cases.
Almost all modern conventional receivers have roofing filters that are quite 
wide in early stages, followed by narrow mode specific filters. Just because 
the filter before is wider does not unduly degrade performance of the 
receiver after the more narrowband filtering.
Interestingly, my grabber receiver has only very simple roofing filters, 
many hundreds of KHz wide and very low Q, they stop almost nothing 'out of 
band'
All the rest of the filtering is done in software after an A to D converter 
(soundcard) which has a remarkably high dynamic range. There is no AGC, it 
is not needed, and the software filters which operate on the same principals 
as the WSPR decoding system, allow me to listen to a weak signal with an 
unwanted signal over 70dB higher, less than 100Hz away. I can actually still 
read the stronger stations on the band whilst transmitting low power on the 
main antenna, only a few tens of feet away, and a few hundred Hz away. 
Ironically I get more problem when I transmit on 6m, though the RX antenna 
is only about 2 feet away from the 6m beam.
I hope this helps your understanding of the principals behind WSPR and other 
FFT based systems. They are not so different. If that is not what you are 
looking for then please clarify.

I have to say that the immunity of WSPR to interference is not as good as I 
would expected, if an unwanted signal comes up in the 6Hz bandwidth even for 
a short time, it can corrupt the decode. It is pretty good at ignoring 
strong stable signals that are reasonably spaced away.  last night and this 
morning a small number of decodes of Jim's beacon were lost due to my 
'wandering sprog'.  The presence of the big CW and QRSS signals in my 
receive passband (pre WSPR processing) does not affect decode. BHZ and DI2AM 
are often 40dB above noise, and they are only a few hundred Hz away from 
Jims WSPR signal, and presented to the WSPR decoder unattenuated by any 
filtering.

I find the levels required to decode WSPR to screen about the same as my 
ability to read QRSS comfortably, and significantly better than my ability 
to read weak CW.
as an EME operator, I can read weak CW quite well.  The slow QSB on 600m is 
a problem for both WSPR and QRSS. and indeed there may be instances where a 
quick burst of normal CW would be better than the slow modes. WSPRs big 
advantage is that it automatically gathers propagation data. The ability to 
observe and accurately quantify the varying signal strength over time is 
extremely useful.
Mark GM4ISM


----- Original Message ----- 
From: "mal hamilton" <g3kevmal@talktalk.net>
To: <rsgb_lf_group@blacksheep.org>
Sent: Wednesday, December 24, 2008 11:28 AM
Subject: Re: LF: wspr


>I did not ask a question about filtering !!!!!!!!!!!!!!!!!!
> What I said was why use SSB mode which is 3 khz wide for a 6hz/200 hz wide
> signal when one could take advantage of the receiver narrow CW filter, and
> of course this is filtered further by manipulation of the soundcard by
> software. This is old hat technology and not new.
> WSPR was engineered for VHF/UHF with plenty of frequency spectrum 
> available
> and not MF/LF squeezed into a 3 khz slot along with other more robust 
> modes
> I am not opposed to any particular transmisson MODE but merely pointing 
> out
> that the advantage claimed by some for WSPR is not justified in some cases
> and my recent observations indicate that I could have read the transmitted
> signal had it been ON/OFF CW, instead I had to wait ages for the signal to
> improve before text printed. This was the case last night with WE2XGR 
> where
> the 2 minute interval trace was good enough to be read in on/off mode CW 
> but
> not strong enough to print most of the time due to slow fade(QSB)
> I might even research WSPR further for comparison purposes but I cannot
> imagine that I will get a print out first before I see a trace.
>
>
> G3KEV
>
> ----- Original Message ----- 
> From: "Scott Tilley" <sthed475@telus.net>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Wednesday, December 24, 2008 5:00 AM
> Subject: Re: LF: wspr
>
>
>> If you could read you would do some research at K1JT's wonderful website:
>> http://www.physics.princeton.edu/pulsar/K1JT/
>>
>> Joe has laboured for many a year on similar projects and has written
>> much about his application of the art.  What you will find is that
>> research into communication theory that started with CW has taken us
>> here...
>>
>> To answer your specific question the filtering is done in software. The
>> DSP is done in your PC, thereby making filtering in the radio somewhat
>> redundant unless you have strong neighbours in the passband.  So using a
>> wide SSB filter and the radio in USB makes for easy math in ones head.
>> Yes, we digital types use our heads from time to time.
>>
>> Often with modes like JT65 used on EME and now quite popular on HF one
>> wants as much bandwidth as possible in the receiver so you can monitor
>> up to the entire band in real time.  So lots of raw bandwidth into the
>> computer is a good thing...
>>
>> All BS aside, you may find the technology very interesting to study and
>> you may find that what you discover is that the spirit of the CW
>> operator of old is alive and well just evolving with the times.
>>
>> CW will never die as it has a rich history but it shouldn't be allowed
>> to impede the growth of new modes and technology.
>>
>> You should build yourself a Softrock SDR receiver or even a small
>> transceiver kit and witness a true revolution in radio technology.  My
>> little 40/30m rig allows me to watch the entire band of either in real
>> time.  With some new software you can monitor all of the CW QSOs at
>> once...  Pretty cool.
>>
>> Not to mention you can plug a key into the little box and do what you
>> love most and work'm.
>>
>> 73 Scott
>> VE7TIL
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> mal hamilton wrote:
>>> If recent published info is correct, this specifies a bandwidth of 6hz
>>> why is USB with a bandwidth of 3 khz necessary to receive this
>>> transmission.
>>> Surely it would be obvious that CW mode was more appropriate where
>>> one could use a narrow filter and dsp processor of a few hertz.
>>> I have asked this question before but never got an answer.
>>> g3kev
>>>
>>
>
>
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