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Need equation for converting decibels (dB) to standard scale 0 to 10000

Posted on 2001-09-11
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Last Modified: 2012-06-21
I need to fade volume of a DirectSound buffer but IDirectSoundBuffer:SetVolume wants the volume level specified in in hundredths of decibels (dB). Allowable values are between DSBVOLUME_MAX (no attenuation) and DSBVOLUME_MIN (silence). These values are currently defined in Dsound.h as 0 and -10,000 respectively. The value DSBVOLUME_MAX represents the original, unadjusted volume of the stream. The value DSBVOLUME_MIN indicates an audio volume attenuated by 100 dB, which, for all practical purposes, is silence.

Ramping SetVolume between -10000 and 0 (to fade in) results in a audibility between about -4000 and 0 and gives me a quick fade at the end of my ramp. I need a steady fade throughough. How do I convert say 0 to 10000 to necessary SetVolume argument that gives me a steady ramp? Thanks!!
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Question by:mortonsw
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by:mortonsw
ID: 6475808
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Accepted Solution

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c_i_wood earned 400 total points
ID: 6750723
Try this function (I've just written, not tested) to turn a fractional loudness (% volume) into 1/100ths of dB suitable for the SetVolume function:

int soundVolIn_mB(int percentage_volume)
{
  if (percentage_volume<0 || percentage_volume>100)
    return(0);
  if (percentage_volume==0)
    return(DSBVOLUME_MIN);
  if (percentage_volume==100)
    return(DBSVOLUME_MAX);
  return(
    (int)(  2000.0*log10( ((double)
              (percentage_volume))/100.0 )  )
       );
}


I think the log10 function is in math.h so you'll need to include it. It takes a double and returns the same so you need to do some casts. I think the code above should work OK.

Now you can ramp the percentage, run it through the function and insert the answer into the SetVolume function.

Let me know if it doesn't work!
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