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Help needed for Gain calculation for audio signal for target loudness, Moved from Movie/Multichannel Audio
garfield85
post Jan 18 2012, 09:43
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ITU standard talks about loudness calculation of audio signal. EBU mainly talks about programme loudness, LRA and true peak calculation and also gives standard acceptable value for programme loudness (-23LUFS).
It also says if the programme loudness varies from this value, the signal must be amplified/attenuated to achieve target value.

My question is how that gain is calculated to be applied to audio signal?
Ex. If programme loudness comes out to be -18LUFS, this means I should give 5LU attenuation to my audio signal.
So, what this "5LU" transforms to in time domain so that I can multiply this to my audio signal?

How this is calculated and if there is any document/spec etc explaining this would be helpful.
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[JAZ]
post Jan 18 2012, 20:03
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The simple answer is that LU is like dB. I.e. it is a logarithmic scale.

There are people here more knowledgeable in this area, which I hope they will give the correct values, but this is how to calculate it based on dBs:


This is C code for conversion of dBs:

/// linear -> deciBell
/// amplitude normalized to 1.0f.
float inline dB(float amplitude)
{
return 20.0f * std::log10(amplitude);
}

/// deciBell -> linear
float inline dB2Amp(float db)
{
return std::pow(10.0f, db / 20.0f);
}
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chi
post Jan 18 2012, 20:35
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For a more formal specification, see EBU Tech 3343 (Practical guidelines for production and implementation in accordance with EBU R 128), especially section 6.2 scenario b. It also says in section 2: “For relative measurements, Loudness Units (LU) are used, where 1 LU is equivalent to 1 dB”, which is essentially what [JAZ] wrote above.
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garfield85
post Jan 19 2012, 06:23
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Thanks for the reply. I have gone through EBU tech 3343 and was thinking of the same but the loudness calculation is quite different than simple logarithmic calculation.
According to ITU BS.1770-2, the loudness calculation uses K-filtering, mean square, then weighted gain to each channel and then its summation.
My doubt is, its conversion back to time domain could not be simple anti-logarithmic scale.

Ex. if my signal needs +5LU gain then i doubt that simply multiplying each channel with "pow(10.0, 5 / 20.0)" will be enough.
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pbelkner
post Jan 19 2012, 07:48
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QUOTE (garfield85 @ Jan 18 2012, 09:43) *
How this is calculated and if there is any document/spec etc explaining this would be helpful.

Cf. e.g. this thread:
http://www.hydrogenaudio.org/forums/index....showtopic=85978
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