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Does anybody know the math required to convert a logarithmic decibel scale to a linear value between 0 and 1, for an automation lane?

In this particular case - the linear value 1 = +15dB, the linear value 0 = -∞dB.

  • 'Decibel' is a ratio, not an absolute value. To answer this we'd need to know the range of decibel values and the scale reference. – Jim Mack Apr 8 '16 at 19:47
  • Ok so the unit delivering these values actually gives -32768 for the fader right down, but 0 when it is right up. So when I have this value (call it N) it aligns exactly to the physical fader positions with this function (N / 10) + 10 - so that effectively gives the same value as shown on the fader which runs from +10dB down to - infinity with -60 the last number. The scale seems logarithmic to me on the fader. – James Harcourt Apr 8 '16 at 20:37
  • What is the automation lane to be used for? – Marc W Apr 9 '16 at 13:28
  • @MarcW Volume on an audio track in a multi-track editor. The scale runs from +15db to -15dB in side the multitrack editor. In the session file which the multitrack editor loads, the value is stored as a linear point from 0 to 1 where 0.65 = 0dB, 0=-infinity dB and 1 = +15dB. – James Harcourt Apr 9 '16 at 15:18
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Practical version

  • dB → gain-multiplier:

    g = 2d / 6

  • gain-multiplier → dB:

    d = 6 · log2(g)

I find these definitions far more handy than the ones below: changing the amplitude by a factor of two is quite an intuitively relevant change. But, alas, in pre-computer times people couldn't seem to like logarithms of bases other than ten, so...

Official definition

  • dB → gain-multiplier:

    g = 10d / 20

  • gain-multiplier → dB:

    d = 20 · log10(g)

IMO base-10 is silly, but if you need to do exact calibrations, better use the official version. (Alternatively, use the base-2 version, but replace 6 with the factor 20 · log102 ≈ 6.020599913; this is then exactly equivalent to the base-10 definition.)

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So if we set that 0 dB gain is 1.0 factor, and -∞ db gain is 0.0 factor, it means that (if we are considering voltage gain as in a mixing desk fader) :

gain = 20.0*log10(factor)

therefore :

factor = 10^(gain/20.0)

If, as described in a comment, the 0 dB gain is at 0.65 factor, it means the ref is 0.65.

gain in db = 20*log10(factor/0.65)

factor = 0.65*10^(gain/20.0)

  • How does this change if +15 dB gain is 1.0 factor and -∞ db gain is 0.0 factor? – James Harcourt Apr 8 '16 at 21:04
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    @JamesHarcourt: just add / remove 15 from the dB scale. – leftaroundabout Apr 8 '16 at 21:47
  • @leftaroundabout that definitely doesn't work, the values that come out aren't anywhere near the linear values produced, i.e. 0dB = 0.65. – James Harcourt Apr 9 '16 at 15:18
  • @JamesHarcourt er, no! 10⁽⁰⁻¹⁵⁾’²⁰ ≈ 0.1778. What did you calculate? – leftaroundabout Apr 9 '16 at 15:24
  • The application in question is Adobe Audition. Their volume automation is stored in XML, so when there is an automation point of 0dB, it is shown thus: <parameterKeyframe sampleOffset="95176" type="linear" value="0.65" />. So I actually want 0.65! – James Harcourt Apr 9 '16 at 15:32

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