[alsa-devel] ASoC: How to declare volume TLV dB scale?

[Takashi Iwai]

At Fri, 12 Jun 2015 21:47:46 +0200,
Clemens Ladisch wrote:
> 
> Sergej Sawazki wrote:
> > On Fri, 12 Jun 2015 18:28:54 +0200, Clemens Ladisch wrote:
> >> Sergej Sawazki wrote:
> >>> could someone please help me to understand the mixer controls TLV scale?
> >>> What would be the DECLARE_TLV_DB_SCALE values for the following example?
> >>>
> >>> reg. value:  0    ..  0x3ff
> >>> phys. value: 0dB  .. -128dB
> >>> volume:      100% ..  0%(mute)
> >>
> >> 0% volume would be -∞ dB.
> >>
> >> And how are the register values mapped between the min/max values?
> >> Does 0x200 correspond to -64 dB, or 50 % of the sample value, or
> >> 50 % of the volume?
> >
> > The output of the DAC can be attenuated in 0.125dB steps, so 0x200
> > would be -64dB.
> >
> > register value | attenuation level (dB) | volume (%)
> > ---------------+------------------------+-----------
> >   0x000        |     0.0                |  100
> >   0x001        |    -0.125              |
> >    ...         |    ...                 |  ...
> >   0x200        |   -64.0                |
> >    ...         |    ...                 |  ...
> >   0x3fe        |  -127.75               |
> >   0x3ff        |    -∞ (mute)           |    0
> 
> 0.125 dB is too small for the 0.01 dB resolution of DECLARE_TLV_DB_SCALE,
> so you have to use DB_MINMAX instead:  TLV_DB_MINMAX_MUTE_ITEM(-128, 0)

And you need to implement the ctl get/put callback to revert the value
as (0x3ff - raw value).  An decrement value isn't supposed to work.


Takashi