Jim take NOTE

Impulsive noise generates a random output that looks like a valid callsign so this is also inventing a signal that you imagine to exist.

An immediate positive ID, don't depend on it !!

An appliance communications operator is easily hoodwinked.

Bletchley Park CW  expert Radio Operators managed to piece messages together and achieved the object., and no doubt used imagination.

No expertise is need to send/receive wspr and for that matter a lot of other appliance operator modes, know in the services today as IDIOT PROOF comms black boxes.

For a commercial operator that needs to shift thousands of messages worldwide then automated digital modes are necessary and useful, but for a radio hobbyist its like using a sledge hammer to crack the proverbial nut

 

g3kev

 

 

 

 

 Original Message -----

From: Andy Talbot

To: [email protected]

Sent: Wednesday, August 24, 2011 6:06 PM

Subject: Re: LF: WSPR or QRSS: which is better?

I know what you're referring to.  In cases of impulsive noise, there is a finite probability of something eventually  getting through the decoder and being flagged as valid.  The very nature of heavy source coding, means that the resulting random output will look like a valid callsign.   However, WE then apply the next level of error detection, by knowing the combination must be rubbish.

 

'jnt   [and there is another example of source coding]

 

On 24 August 2011 17:47, Graham <[email protected]> wrote:

>>  and gives absolutely guaranteed error free decoding, or nothing at all.  <<

 

Are you sure ? 

 

G..

From: Andy Talbot

Sent: Wednesday, August 24, 2011 5:12 PM

To: [email protected]

Subject: Re: LF: WSPR or QRSS: which is better?

WSPR works in a 1.46Hz signal bandwidth and because of its very high level of error correction and soft-decision decoding, means that it will work at a S/N of about 3dB in this bandwidth, and sometimes a bit lower still  (Normally, FSK with no correction at all needs about 10 - 12dB S/N for near error-free performance)

 

QRSS has to show about 6 - 10dB in its signal bandwidth to be able to discern fully what is sent, although a slightly lower S/N may be useable when you 'know' what you should be receiving.  (A form of forward error correction is now in use here as well perhaps :-)  So lets say 5dB S/N is a working value..

 

So take 3dB in 1.46Hz as a starting point and derive the bandwidth for QRSS needed to get 5dB S/N with the same signal.  This will have to be narrower to get a 2dB higher S/N and works out as 1.46 / 10^(2/10) = 0.92Hz

 

So QRSS used with a 0.9Hz bandwidth - which I think means about a 2 - 3s dot period ought to be decoded at the same S/N as a WSPR signal.   Which is probably the info you wanted.

 

But now compare source coding efficiencies.   WSPR fits a callsign, locator and power level into a 110 second transmission - and gives absolutely guaranteed error free decoding, or nothing at all.  About 12 characters in actuality, but that is being a bit unfair as the coding forces certain callsign and locator formatting.   So in all probablility, more like 7 or 8 effective characters (I'm being a bit empirical here)

 

Assuming standard QRSS - not DFCW - , which if like standard Morse, then 5 characters takes about 50 dot symbols to send (12WPM = 60 chars in 1 minute, = 1 char / second, or about 10 dot periods / second.    Dot speed = WPM / 1.2)   If we have 2s dots, that is 5 characters can be sent in the time for a WSPR transmission.

 

So as a quick estimate, WSPR wins by roughly 2dB in S/N terms for a given dot period / noise bandwidth.  And at similar S/N values, WSPR is about 1.5 times faster

Andy

www.g4jnt.com

 

 

 

 

On 24 August 2011 16:42, Roger Lapthorn <[email protected]> wrote:

A question for the coding experts here: WSPR is an excellent weak signal beaconing mode, but at what QRSS speed is QRSS "better" ?

73s
Roger G3XBM

--
http://g3xbm-qrp.blogspot.com/
http://www.g3xbm.co.uk
http://www.youtube.com/user/g3xbm
https://sites.google.com/site/sub9khz/