17 Feb
2010
17 Feb
'10
8:37 a.m.
2010/2/17 Eliot Blennerhassett linux@audioscience.com
There are two separate issues here.
- Incorrect dB<->linear mapping
ALSA Volume/Gain controls returning incorrect information about mapping from linear values to dB. E.g. adding "6dB" to the Volume controls while subtracting 6dB in software does not result in 0dB overall gain.
For volume controls the convention is 0dB == multiply by 1.0 == no change. Any sane digital volume control should obey this. For analog volumes, the 0dB position is entirely arbtrary, as I discuss below.
For the alsa db values are in step of 1.5 dB or 3.dB ,
but the software shift one bit is 10 log 2 = 3.0102996* dB
* That 's why adding "6dB" to the Volume controls while subtracting 6dB in software does not result in 0dB overall gain.