[alsa-devel] Verifying mixer dB data/Invalid dB data from USB cards, especially Aureon 5.1 MkII

[Lennart Poettering]

On Wed, 17.02.10 11:30, Raymond Yau (superquad.vortex2 at gmail.com) wrote:

> 
> 2010/2/16 Lennart Poettering <mznyfn at 0pointer.de>
> 
> > On Tue, 16.02.10 09:08, Clemens Ladisch (clemens at ladisch.de) wrote:
> >
> > >
> > > Lennart Poettering wrote:
> > > > Getting back to the invalid dB data from the USB cards: the question
> > > > is whether the USB descriptor data is badly parsed, and the dB
> > > > mismatch hence systematic for USB cards,
> > >
> > > USB descriptors use units of 1/256th dB, while ALSA uses 1/100th dB.
> > > When the level is computed from the minimum plus a multiple of the step
> > > size, rounding errors add up considerately.
> > >
> > > ALSA 1.0.21 (kernel 2.6.32) added a different TLV type that reports the
> > > range with the min/max values to minimize those rounding errors.
> >
> > I am really sure that the discrepancy I measure with the Aureon is way
> > beyond rounding errors.
> >
> > Lennart
> >
> >
> if the program use a format FLOAT_LE  which is not supported by the hardware
> and need to use plughw , you have already added error ( resampling and
> format conversion ) to the result

That numeric noise is neglible in contrast to the big (20dB or so?)
discrepancy I am seeing here.

Lennart

-- 
Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net
http://0pointer.net/lennart/           GnuPG 0x1A015CC4