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

[Clemens Ladisch]

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)


Regards,
Clemens