hi,
I am trying to implement simple volume control using the ALSA library,
and I must say I am totally confused about the many different mixer
interfaces that exist in the ALSA library and of which none seems to be
properly documented. The only functions I found that seem to be supposed
to do what I want are
snd_mixer_selem_set_playback_volume_all
snd_mixer_selem_set_playback_dB_all
I have successfully opened the mixer interface and found a suitable
mixer element. I am even able to set the output volume using
snd_mixer_selem_set_playback_volume_all. unfortunately, the behaviour of
this function seems to be non-linear, and the documentation says nothing
about the meaning of the "value" I pass to the function:
> *Parameters:*
>
> /elem/ Mixer simple element handle
>
> /value/ control value
>
(from
http://www.alsa-project.org/alsa-doc/alsa-lib/mixer_2simple_8c.html#a21)
If I control the volume in alsamixer or kmix, the slider also seems to
behave non-linear. If I set it to 50% of the maximum position, I can
hardly hear any sound. Also, if I turn the value on my output device to
50%, the result is much louder than what I get by setting the value to
"50%" in ALSA.
It is a requirement in my application that the behaviour of the volume
control is well-defined. I want both linear volume control and
logarithmic volume vontrol in decibels. So I tried to use the
snd_mixer_selem_set_playback_dB_all function. Unfortunately, this
function doesn't seem to be defined in the ALSA headers. Instead I found
a function with a similar name which is not documented, but whose name
and signature suggests that it is what I need:
int snd_mixer_selem_set_playback_volume_dB(snd_mixer_elem_t *elem, long
value, int dir);
However, if I use this function, I get an undefined symbol error at
link-time.
(i.e. the function is not even defined in the ALSA library?!)
I also tried to install the newest ALSA headers and libraries from
source (version 1.0.12rc1). My application compiled and linked fine, but
setting the volume using snd_mixer_selem_set_playback_dB_all failed with
snd_strerror returning "No such device or address", probably because the
driver doesn't support the new interface
fw@magmell:~$ cat /proc/asound/version
Advanced Linux Sound Architecture Driver Version 1.0.9 (Sun May 29
07:31:02 2005 UTC).
So my question is:
Is there a function to set the playback volume of a mixer element in a
well-defined way using version 1.0.9 of the ALSA library?
By well-defined, I mean: The behaviour of the volume control must be the
same on all cards. I must be able to set the volume to, say, "50% of the
maximum volume" or "-6 dB". Whether the interface accepts linear values
or decibels doesn't matter. Accuracy also is not an issue. But the scale
and units should be documented and unified (i.e. independent of the
hardware and driver).
Is there such a function in ALSA 1.0.9? What is the name of this
function and where is it documented? Otherwise, are there functions that
I can use to determine what kind of values
snd_mixer_selem_set_playback_volume_all accepts? (i.e.
logarithmic/linear, minimum and maximum value, scale?)
Thanks in advance.
--
Florian Winter
Software developer
Motama GmbH
http://www.motama.com
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