What logarithmic function do windows use for the audio volume slider? - c++

Question

I am currently struggling with the implementation of my audio volume slider in my c++ app.
The app is able to control the windows mixer volume level and the slider has the range 0.0f to 1.0f.
The problem I am facing is that my DB values aren't equal to the DB values windows is using.
Here are some values I set with my volume slider with the resulting DB values and the ones windows is using.
Below is the function I use for calculating the audio DB level. What am I doing wrong here?
Thank you in advance!
if (this->m_useAudioEndpointVolume)
{
const float slider_min = 0.0f;
const float slider_max = 1.0f;
const float logBase = 10;
m_ignoreAudioValue = TRUE;
if (volume >= 1.0f) {
volume = 1.0f;
}
if (volume <= 0.0f) {
volume = 0.0f;
}
float pfLevelMinDB = 0;
float pfLevelMaxDB = 0;
float pfVolumeIncrementDB = 0;
m_pEndpointVolume->GetVolumeRange(&pfLevelMinDB, &pfLevelMaxDB, &pfVolumeIncrementDB);
// Logarithmic formula for audio volume
// Volume = log(((Slider.Value-Slider.Min)*(B-1))/(Slider.Max-Slider.Min) + 1)/log(B) * (Volume.Max - Volume.Min) + Volume.Min
float calcVolume = log(((volume - slider_min)*(logBase - 1)) / (slider_max - slider_min) + 1) / log(logBase) * (pfLevelMaxDB - pfLevelMinDB) + pfLevelMinDB;
if (volume == 0.0f) {
m_pEndpointVolume->SetMute(TRUE, NULL);
}
else
{
m_pEndpointVolume->SetMute(FALSE, NULL);
}
float currentValue = 0.0f;
m_pEndpointVolume->GetMasterVolumeLevel(&currentValue);
// Todo: The calculation has to be logarithmic
m_pEndpointVolume->SetMasterVolumeLevel(calcVolume, NULL);
}

Answer 1

Assume the following:
volumeMaxDB = +5
volumeMinDB = -10
incrementDB = 5
To me this suggests a slider that would look something like the ascii art below. I've also shown my presumed mapping to your slider UI's scale.
dB Slider
| +5 <=> 1.0
| 0
- -5
| -10 <=> 0.0
First, calculate the total volume range in dB (e.g. -10 to +5 is 15 dB)
dBRange = abs(volumeMaxDB) + abs(volumeMinDB);
Second, scale the current slider position dB value of 0 to dBRange. This gives the following mappings
* 0.0 -> 0
* 1.0 -> 15
* 0.5 -> 7.5
dB = dBRange * slider;
Third, shift the range up or down so that +15 becomes +5 and 0 becomes -10.
dB = dB - (dbRange - volumeMaxDB);
Finally, you may want to round to the nearest dB increment.
Extra credit: If you have control over your slider's range, you could make your life way simpler by just setting the min and max value the same as minDB and maxDB and be done with it.

Answer 2

I found the solution.
The IAudioEndpointVolume has the function SetMasterVolumeLevelScalar. This function uses the range from 0.0 to 1.0 regarding to the MSDN documentation so you don't need to implement a logarithmic function yourself.
Seems like I overlooked this one.
Here's the current code sample I am using in case someone will need it in the future.
float pLevel = 0.0f;
m_pEndpointVolume->GetMasterVolumeLevelScalar(&pLevel);
// We have to set this first to TRUE to an avoid unnecessary callback
m_ignoreAudioValue = TRUE;
// Set the scalar value
// https://msdn.microsoft.com/de-de/library/windows/desktop/dd368062(v=vs.85).aspx
m_pEndpointVolume->SetMasterVolumeLevelScalar(sliderValue, NULL);
// We have to set this again to TRUE to avoid an unnecessary callback
// because the SetMasterVolumeLevelScalar triggers the OnNotify event
// and this causes the m_ignoreAudioValue to be FALSE again.
m_ignoreAudioValue = TRUE;
// If the value is higher the 0.0 unmute the master volume.
m_pEndpointVolume->SetMute(sliderValue > 0.0f ? FALSE : TRUE, NULL);
m_pEndpointVolume->GetMasterVolumeLevelScalar(&pLevel);
Edit:
It seems like Windows is using a linear volume slider. Thats the reason why 2% in Windows feels still too loud and everything above 50% isn't much of an increase anymore.
Here's a really good article about it why you should avoid it.
Volume Controls