[alsa-devel] Verifying mixer dB data/Invalid dB data from USB cards, especially Aureon 5.1 MkII
perex at perex.cz
Tue Feb 16 17:29:48 CET 2010
On Tue, 16 Feb 2010, Lennart Poettering wrote:
> On Tue, 16.02.10 14:40, Mark Brown (broonie at opensource.wolfsonmicro.com) wrote:
>> On Tue, Feb 16, 2010 at 03:32:01PM +0100, Lennart Poettering wrote:
>>> On Tue, 16.02.10 09:24, Jaroslav Kysela (perex at perex.cz) wrote:
>>>> Line: 0dB..47.81dB # seems correct -47.81dB..0dB
>>>> Line Capture: -0.18dB..23.81dB # seems correct
>>> Hmm, how did you choose where to put 0dB?
>> The Line one looks like someone got attenuation and gain confused, which
>> is a fairly common mistake for people to make when copying information
>> from one place to another. Possibly a similar thing going on with the
>> Line Capture control, plus a fixed 6dB gain (which may well be a
>> separate amp...).
> I remember that a while back we had a discussion on alsa-devel
> regarding the fact that ALSA does not define relative to what the dB
> values reported by the mixer elements actually are measured. Some
> mixer controls export dB values relative to max amplification (e.g. my
> integrated USB speakers here), others relative to the output of some
> other mixer element, and others relative to different standard or
> not-so-standard output voltages. And there is no API to figure out
> what the base is.
> Now, what Jaroslav listed above suggests that he chose carefully where
> he put 0dB. I was just wondering about that. Jaroslav, how did you
> choose where to put 0dB?
Basically, if I remember correctly the discussion, 0dB on all elements
should main unchanged volume, <0dB attenuation and >0dB gain. We cannot
measure precisely other components on path like analog volume wheels
(like SB MP3+ has to change line-out volume), analog amplifiers and other
analog signal modificators.
So, if you use one headphones and speakers with defined and fixed volume
settings, you can measure differences for the card with broken dB volume
ranges. I just used another HDA card in notebook with 0dB settings on all
playback controls on path, captured stream from this card on my USB card
and played this stream back on my USB card watching and relating the
volume levels according dB values. It's not 100% correct, but at least
it's something to start with.
Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.
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