http://cgit.freedesktop.org/pulseaudio/pulseaudio/tree/src/modules/alsa
/alsa-mixer.c
seem pulseaudio only control left channel when it only use the virtual master
playback volume control
in function element_set_volume, it check the element has left and right channel
static int element_set_volume(pa_alsa_element *e, snd_mixer_t *m, const
pa_channel_map *cm, pa_cvolume *v, bool deferred_volume, bool write_to_hw) {
snd_mixer_selem_id_t *sid;
pa_cvolume rv;
snd_mixer_elem_t *me;
snd_mixer_selem_channel_id_t c;
pa_channel_position_mask_t mask = 0;
unsigned k;
pa_assert(m);
pa_assert(e);
pa_assert(cm);
pa_assert(v);
pa_assert(pa_cvolume_compatible_with_channel_map(v, cm));
SELEM_INIT(sid, e->alsa_name);
if (!(me = snd_mixer_find_selem(m, sid))) {
pa_log_warn("Element %s seems to have disappeared.", e->alsa_name);
return -1;
}
pa_cvolume_mute(&rv, cm->channels);
for (c = 0; c <= SND_MIXER_SCHN_LAST; c++) {
int r;
pa_volume_t f = PA_VOLUME_MUTED;
bool found = false;
for (k = 0; k < cm->channels; k++)
if (e->masks[c][e->n_channels-1] &
PA_CHANNEL_POSITION_MASK(cm->map[k])) {
found = true;
if (v->values[k] > f)
f = v->values[k];
}
if (!found) {
/* Hmm, so this channel does not exist in the volume
* struct, so let's bind it to the overall max of the
* volume. */
f = pa_cvolume_max(v);
}
if (e->has_dB) {
long value = to_alsa_dB(f);
int rounding;
if (e->volume_limit >= 0 && value > (e->max_dB * 100))
value = e->max_dB * 100;
if (e->direction == PA_ALSA_DIRECTION_OUTPUT) {
/* If we call set_playback_volume() without checking first
* if the channel is available, ALSA behaves very
* strangely and doesn't fail the call */
if (snd_mixer_selem_has_playback_channel(me, c)) {
** Changed in: alsa-driver (Ubuntu)
Status: New => Incomplete
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https://bugs.launchpad.net/bugs/1497666
Title:
No right channel sound on Creative Xtreme Audio (CA0106) in Ubuntu
14.04-15.10
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