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

Clemens Ladisch clemens at ladisch.de
Tue Feb 16 09:08:08 CET 2010

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.


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