Thanks Jim for this observation and others for their feedback.

When I first set up WSPR, in the days when the s/n button did not exist I ran a 
series of tests where I changed the level into the sound card and looked at the 
S/n of a single station as reported by WSPR. I found some non linearity and 
settled for an area on the linear part of the curve. When the s/n button 
appeared. I found it was red and reading -27dB. I have been running it at this 
level for several months.  I have been getting decoded reports of s/n between 
-27 and -9  but usually about 6 -9dB worse than others were reporting.
Last week as you know I tried your sigs on 137kHz and picked up 3 signals each 
time. slot After the dialogue of the last few days I have  turned my audio down 
and am setting to  0 dB on the button. Plenty of decodes last night. Now I need 
to find time to redo my linearity measurements and also compare my results with 
others reports on the database.

regards
peter G8AFN

Dear Dave, LF Group,

There seem to be some misunderstandings here...

The little box with the receiver noise level is there so you can set the 
gain controls to a reasonable level where the incoming audio is well above 
the noise level of the sound card, but well below overload level. 0dB is 
what K1JT has settled on as an optimum level, but quite wide deviations from 
that (say +/- 10dB or more) will not have much effect on the decoding 
process, or the reported SNR of the signal. In fact, even if the overload 
level is reached on noise peaks, it still makes little difference provided 
it only occurs a reasonably small proportion of the time.

The reported SNR is obtained by measuring the level of each WSPR signal 
during the 2 minute period, measuring the associated noise level during the 
same period, and calculating the dB ratio. Thus it is largely independent of 
the actual receiver or sound card gain settings, and gain can be varied 
through many dB (see above) without significantly changing the SNR reports.

So there is no direct connection between the receiver output level and the 
SNR figures.

Cheers, Jim Moritz
73 de M0BMU


----- Original Message ----- 
From: "Dave Sergeant" 
To: 
Sent: Thursday, September 24, 2009 9:39 AM
Subject: LF: Re: s/n shown by wspr .... what is it actually showing ?


> In the ideal case of course you have just receiver noise as your
> background noise level. So you adjust your sound card input to give you
> the magic 0dB on that. During the night the static crash level will
> rise and push the S-meter to s9 at times. Every 15 minutes G4JNT will
> do its one minute of transmission on 503.7 at various power levels and
> push the little thing in the box well into the red. So one has to ask
> oneself what exactly the dB figures in reports really mean....