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

[Raymond Yau]

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