[pulseaudio-discuss] [PATCH] alsa-mixer: round, not truncate, in to_alsa_dB

[Ian Ray]

to_alsa_dB() returns a result rounded to two decimal places (instead of
using integer truncation) to avoid small errors when converting between
dB and volume.

Consider playback at -22 dB (which is supported by ALSA) but results in
the higher level of -21 dB plus software attenuation.

    pa_sw_volume_from_dB(-22) = 28172
    pa_sw_volume_to_dB(28172) = -21.9997351
    to_alsa_dB(-21.9997351)   = -2199

    ALSA value 106 = -2200
    ALSA value 107 = -2100
    ...

    rounding = +1  /* "accurate or first above" */
    snd_mixer_selem_ask_playback_dB_vol(me, -2199, rounding, &alsa_val)
    alsa_val = -2100

Signed-off-by: Ian Ray <ian.ray at ge.com>
---
 src/modules/alsa/alsa-mixer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/modules/alsa/alsa-mixer.c b/src/modules/alsa/alsa-mixer.c
index f59cad3..aeaf12c 100644
--- a/src/modules/alsa/alsa-mixer.c
+++ b/src/modules/alsa/alsa-mixer.c
@@ -700,7 +700,7 @@ void pa_alsa_path_set_free(pa_alsa_path_set *ps) {
 }
 
 static long to_alsa_dB(pa_volume_t v) {
-    return (long) (pa_sw_volume_to_dB(v) * 100.0);
+    return lround(pa_sw_volume_to_dB(v) * 100.0);
 }
 
 static pa_volume_t from_alsa_dB(long v) {
-- 
2.10.1