[pulseaudio-discuss] Please help to explain the algorithm in pa_sw_volume_to_dB

Tue Aug 9 05:44:07 UTC 2022

On 2022/8/8 下午9:29, zhaochengyi at uniontech.com wrote:
> Hi guys,
> 
> The dB range of Playback Master of Sound Card is between -65.25 and 0.
> 
> I changed the sink volume to 8%, but the dB volume calculated by 
> function pa_sw_volume_to_dB is -65.82 dB, this will result in
> no sound playing when the volume is between %0 and 8%.
> 
> I think this is an issue, but I don't understand the algorithm
> principle of function pa_sw_volume_to_dB.
> 
> Would you please help to explain it, thanks a lot!
> 
> logs:
> ------------------------------------
> 8月 08 21:06:32.535263 zhao-PC pulseaudio[5173]: D: [pulseaudio] 
> protocol-native.c: Client dde-session-daemon changes volume of sink 
> alsa_output.pci-0000_03_00.6.analog-stereo.
> 8月 08 21:06:32.535263 zhao-PC pulseaudio[5173]: D: [pulseaudio] sink.c: 
> The reference volume of sink alsa_output.pci-0000_03_00.6.analog-stereo 
> changed from front-left: 4587 /   7% / -69.30 dB,   front-right: 4587 / 
>   7% / -69.30 dB to front-left: 5898 /   9% / -62.75 dB,   front-right: 
> 5898 /   9% / -62.75 dB.
> 8月 08 21:06:32.535263 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] alsa-sink.c: Requested volume: front-left: 5898 /   9% / -62.75 
> dB,   front-right: 5898 /   9% / -62.75 dB
> 8月 08 21:06:32.535263 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] alsa-sink.c: Got hardware volume: front-left: 5919 /   9% / 
> -62.65 dB,   front-right: 5919 /   9% / -62.65 dB
> 8月 08 21:06:32.536525 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] alsa-sink.c: Calculated software volume: front-left: 65303 / 
> 100% / -0.09 dB,   front-right: 65303 / 100% / -0.09 dB 
> (accurate-enough=yes)
> 8月 08 21:06:32.536525 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] sink.c: Volume going up to 5898 at 6577292163
> 8月 08 21:06:32.536525 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] ratelimit.c: 745 events suppressed
> 8月 08 21:06:32.536525 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] sink.c: Next volume change in 7992 usec
> 8月 08 21:06:32.536525 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] sink.c: Next volume change in 7981 usec
> 8月 08 21:06:32.536525 zhao-PC pulseaudio[5173]: I: [pulseaudio] 
> module-device-restore.c: Storing volume/mute for device+port 
> sink:alsa_output.pci-0000_03_00.6.analog-stereo:analog-output-speaker.
> 8月 08 21:06:32.543465 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] sink.c: Volume change to 5898 at 6577292163 was written 104 usec 
> late
> 8月 08 21:06:40.507006 zhao-PC pulseaudio[5173]: D: [pulseaudio] 
> protocol-native.c: Client dde-session-daemon changes volume of sink 
> alsa_output.pci-0000_03_00.6.analog-stereo.
> 8月 08 21:06:40.507006 zhao-PC pulseaudio[5173]: D: [pulseaudio] sink.c: 
> The reference volume of sink alsa_output.pci-0000_03_00.6.analog-stereo 
> changed from front-left: 5898 /   9% / -62.75 dB,   front-right: 5898 / 
>   9% / -62.75 dB to front-left: 4587 /   7% / -69.30 dB,   front-right: 
> 4587 /   7% / -69.30 dB.
> 8月 08 21:06:40.507593 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] alsa-sink.c: Requested volume: front-left: 4587 /   7% / -69.30 
> dB,   front-right: 4587 /   7% / -69.30 dB
> 8月 08 21:06:40.507593 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] alsa-sink.c: Got hardware volume: front-left: 4605 /   7% / 
> -69.20 dB,   front-right: 4605 /   7% / -69.20 dB
> 8月 08 21:06:40.507593 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] alsa-sink.c: Calculated software volume: front-left: 65280 / 
> 100% / -0.10 dB,   front-right: 65280 / 100% / -0.10 dB 
> (accurate-enough=yes)
> 8月 08 21:06:40.507593 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] sink.c: Volume going down to 4587 at 6585248016
> 8月 08 21:06:40.507593 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] sink.c: Volume change to 4587 at 6585248016 was written 8010 
> usec late
> 8月 08 21:06:40.507593 zhao-PC pulseaudio[5173]: I: [pulseaudio] 
> module-device-restore.c: Storing volume/mute for device+port 
> sink:alsa_output.pci-0000_03_00.6.analog-stereo:analog-output-speaker.
> 8月 08 21:06:42.535546 zhao-PC pulseaudio[5173]: I: [pulseaudio] 
> module-device-restore.c: Synced.
> 8月 08 21:06:48.928087 zhao-PC pulseaudio[5173]: D: [pulseaudio] 
> protocol-native.c: Client dde-session-daemon changes volume of sink 
> alsa_output.pci-0000_03_00.6.analog-stereo.
> 8月 08 21:06:48.928087 zhao-PC pulseaudio[5173]: D: [pulseaudio] sink.c: 
> The reference volume of sink alsa_output.pci-0000_03_00.6.analog-stereo 
> changed from front-left: 4587 /   7% / -69.30 dB,   front-right: 4587 / 
>   7% / -69.30 dB to front-left: 5242 /   8% / -65.82 dB,   front-right: 
> 5242 /   8% / -65.82 dB.
> 8月 08 21:06:48.928812 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] alsa-sink.c: Requested volume: front-left: 5242 /   8% / -65.82 
> dB,   front-right: 5242 /   8% / -65.82 dB
> 8月 08 21:06:48.928812 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] alsa-sink.c: Got hardware volume: front-left: 5276 /   8% / 
> -65.65 dB,   front-right: 5276 /   8% / -65.65 dB
> 8月 08 21:06:48.928812 zhao-PC pulseaudio[5173]: D: [alsa-sink-ALC256 
> Analog] alsa-sink.c: Calculated software volume: front-left: 65114 / 99% 
> / -0.17 dB,   front-right: 65114 /  99% / -0.17 dB (accurate-enough=yes)
> --------------------------------------------

The relevant functions are as follows:

static double linear_to_dB(double v) {
     return 20.0 * log10(v);
}

double pa_sw_volume_to_linear(pa_volume_t v) {
     double f;

     pa_return_val_if_fail(PA_VOLUME_IS_VALID(v), 0.0);

     if (v <= PA_VOLUME_MUTED)
         return 0.0;

     if (v == PA_VOLUME_NORM)
         return 1.0;

     f = ((double) v / PA_VOLUME_NORM);

     return f*f*f;
}

double pa_sw_volume_to_dB(pa_volume_t v) {

     pa_return_val_if_fail(PA_VOLUME_IS_VALID(v), PA_DECIBEL_MININFTY);

     if (v <= PA_VOLUME_MUTED)
         return PA_DECIBEL_MININFTY;

     return linear_to_dB(pa_sw_volume_to_linear(v));
}