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" <[email protected]>
To: <[email protected]>
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....
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