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Date: Tue, 13 May 2014 20:31:18 +0200
From: Hans Petter Selasky <hps@selasky.org>
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Subject: OSS mains HUM filtering 50Hz / 60Hz ?
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Hi,

Some of my USB audio headsets seems to be picking up mains HUM even if 
powered from battery. At first I thought the mains HUM came from the 
computer, but then I did some work and I verified my findings by 
sampling the AC network using a non-switching power supply. Maybe it 
does not belong in the FreeBSD audio stack, but I think it would be very 
clever to have a configurable band-reject filter in the FreeBSD DSP 
framework for 50 and 60Hz? Do we have such a filter mechanism already?

Not sure if this is a so-called "feature" or not ... :-)

http://en.wikipedia.org/wiki/Mains_hum

http://www.selasky.org/hans_petter/hum.png

--HPS