[alsa-devel] Mixer volume/decibel mapping

[James Courtier-Dutton]

On 20 May 2010 10:41, Sebastian H. <vand2 at gmx.de> wrote:
>
> That's clear.
> I imagined that maybe the dB scale could be the "real thing" and the volume
> steps would be a convenient but possibly coarse mapping into a denser
> stepped dB space provided by the hardware/driver.
> This was unlikely but I wanted to ask anyway ;-).
>

I believe it could be extended to support a continuous range.
For example, some hardware uses a 32bit value to control the gain.
The mix operation is simply "the_sample" * "the_control_value".
So in this case the hardware has a linear control that can be
considered continuous for our purposes.
The conversion from the control value to a usable value would have to
include a user land log operation.
It was never implemented due to lack of demand.
For the cards that do have this 32bit linear value, a simple lookup
table is used in the kernel code to convert it to 256 step values.
If implemented, one could then introduce professional gain controls
that do not produce "clicks" in the output when adjusted.
gain controls with steps in them produce clicks when the gain control
is adjusted.