Joe K1JT on setting audio level, filters and AGC

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Joe K1JT on setting audio level, filters and AGC

Hi. I asked Joe about some of the common questions that come up a lot on audio level, filters and AGC.

> (1) Most people accept that for best performance the WSPR sound level
> should be set to show 0dB for just background noise with a receiver
> bandwidth of 2500Hz - correct?

Rx noise level "0 dB" is a reasonable target, whatever the receiver bandwidth may be. It's not very critical, though.
Even +/- 10 dB from this level should not change the decoding capability by a significant amount.

The idea is to have the baseline noise level high enough for quantization noise to be negligible, but low enough to give adequate "headroom" in dynamic range.

> (2) One person claimed that if the background noise sound level isn't
> set to show 0dB, then the dB figures displayed in WSPR spots are
> invalid - is this true?

No. The reported S/N is just that: a measured ratio of signal power to average noise power. Within broad limits, this ratio is independent of the Rx noise level.

> (3) Some people say that the sound level is unimportant provided it
> doesn't go into the "red" positive and negative values - does that make sense?

Yes. Even if it does go red occasionally (say, on lightning crashes, or during occasional QRM) there will usually be little impact on the decoding of WSPR signals.

> (4) If I use my receiver's 200Hz filter, then I'm not sure how to set
> the sound level up correctly ... should I still aim for 0dB on
> background noise with the 200Hz filter in, or should I set the sound
> level up initially using 2500Hz then switch in the 200Hz filter afterwards and leave the level alone?

As described above, 0 dB is still a reasonable target.

NB: unless you have serious problems with very strong, close-in QRM, little or nothing is gained by using a bandwidth narrower than 2 kHz or so. WSPR does all necessary filtering, itself.

> (5) Finally, another area is AGC. Conventional wisdom is to turn AGC
> off for digital modes, but at least one person thinks that WSPR
> doesn't mind at all whether AGC is on or off. Which is best - on or off?

Under most conditions it's not critical whether AGC is on or off. I leave it on, and set my audio level by backing 'way
down on the RF gain control. In effect this means the AGC takes effect only when an extremely strong signal is present in the 2 kHz passband.

Please feel free to share these comments as you see fit.

-- 73, Joe, K1JT