Date: Wed, 14 May 2014 10:07:01 +0200 From: Hans Petter Selasky <hps@selasky.org> To: freebsd-multimedia@FreeBSD.org Subject: Re: OSS mains HUM filtering 50Hz / 60Hz ? Message-ID: <53732425.7070600@selasky.org> In-Reply-To: <537264F6.5060808@selasky.org> References: <537264F6.5060808@selasky.org>
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On 05/13/14 20:31, Hans Petter Selasky wrote: > 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 Hi, I tried the following patch with some success: === pcm/feeder_chain.c ================================================================== --- pcm/feeder_chain.c (revision 265912) +++ pcm/feeder_chain.c (local) @@ -721,7 +721,7 @@ /* Soft EQ only applicable for PLAY. */ if (cdesc.dummy == 0 && - c->direction == PCMDIR_PLAY && (d->flags & SD_F_EQ) && + (d->flags & SD_F_EQ) && (((d->flags & SD_F_EQ_PC) && !(c->flags & CHN_F_HAS_VCHAN)) || (!(d->flags & SD_F_EQ_PC) && !(c->flags & CHN_F_VIRTUAL)))) And setting: hint.pcm.0.eq=1 hint.pcm.1.eq=1 hint.pcm.2.eq=1 hint.pcm.3.eq=1 --HPS
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