Axis Video Server User Manual page 28

Axis video server - Audio (AXIS 241QA/SA only)
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When the incoming sound is louder than the threshold, it will pass without any changes.
When lower than the threshold, the incoming sound will be reduced by a certain
attenuation factor. The threshold value should be set higher than the background noise,
but lower than the useful audio.
The noise canceller threshold value can be set to High, Medium High, Medium low and
Low. A lower threshold will accept most of the audio to pass. Only the weakest
background noise will be reduced. A higher threshold will make the noise canceller act on
even stronger background noise. At the maximum level High, there is a risk of reducing
useful audio as well.
There is a trade-off between noise cancelling and sound quality. In other words, increasing
the noise canceller attenuation deteriorates sound quality. Adjust the threshold and the
attenuation to an optimum by listening and changing the levels.