[alsa-devel] Verifying mixer dB data/Invalid dB data from USB cards, especially Aureon 5.1 MkII
perex at perex.cz
Tue Feb 16 09:24:11 CET 2010
On Tue, 16 Feb 2010, Clemens Ladisch 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.
Unfortunately, it's not the case of a rounding problem. The USB
specification is too much vague and vendors don't use the dB range
correctly. For example, I tested SB MP3+ USB device and reported ranges
Master: -47.87dB..0.06dB # correct
Line: 0dB..47.81dB # seems correct -47.81dB..0dB
Line Capture: -0.18dB..23.81dB # seems correct -17.81dB..6.18dB
I just did tests using another soundcard to generate sounds and
played back the captured result to determine the gain in the capture
I put my results to:
The SB MP3+ uses AC97 codec AD1995 with some own ASIC from Creative. So I
think that even the 47.87 values are not correct and the real ranges are
from -48dB..0dB and -18dB..6dB.
Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.
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