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I wonder why several RMS meters I'm using for music production show RMS values above 0.

For example, when using an input signal with a constant DC of 1 (0 dbFS), I get these results, although Wikipedia says that in such cases, the RMS is equal to the amplitude of the direct current.

Samplitude Pro X (RMS: 2.9)

enter image description here

TT Dynamic Range Meter (RMS 3.0)

enter image description here

Whereas other meters show the values I expect:

Voxengo SPAN (RMS: 0.0)

enter image description here

Why is it that meters come to such different results?

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0 dBFS may refer to a full-scale sine wave or more rarely a full-scale square wave. Full scale DC has the same RMS value as a full-scale square wave, but is 3.010299956 dBFS RMS if the reference is a full-scale sine wave. Looks like the different programs use a different reference (square or sine). Samplitude Pro X might use some approximation of RMS that gives in this case a 0.1 dB unit error.

The calculations:

$$\int_0^{2\pi}\frac{\sin(x)^2dx}{2\pi} = \frac{1}{2}$$ $$10\log_{10}\left(\frac{1}{2}\right) \approx -3.010299956$$

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    $\begingroup$ All the standards I've read agree that FS sine is 0 dBFS, and that FS square is +3 dBFS, so it seems it's always an RMS measurement (never peak) and there's no ambiguity. dBov is similar but 3 dB lower, so FS sine is -3 dBov and FS square is 0 dBov. $\endgroup$ – endolith Aug 1 '18 at 16:29
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    $\begingroup$ @endolith Adobe Audition 3 has RMS Settings: 0 dB = FS Sine Wave or 0 dB = FS Square Wave. They don't directly call the measure dBFS. $\endgroup$ – Olli Niemitalo Aug 4 '18 at 7:37
  • $\begingroup$ Yes, Audition was my first introduction to this concept $\endgroup$ – endolith Aug 4 '18 at 13:53

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