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LUFS and True Peak Max

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Re: LUFS and True Peak Max

Postby The Elf » Wed Jul 31, 2019 10:07 am

+1 for GClip, but use it as a last resort, not a first.
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Re: LUFS and True Peak Max

Postby hobbyist » Wed Jul 31, 2019 4:58 pm

Hugh Robjohns wrote:
hobbyist wrote:TP is just their guess for the converted peak at the sample rate they assume.

No, the True Peak value is not 'a guess'. It's a well-defined calculation of the reconstructed waveform's true amplitude, with an equally well-defined error-margin. The higher the oversampling rate used in the calculation, the smaller the error margin.

There is absolutely no guesswork involved!

That might just have been an unfortunate turn of phrase... but this is the way misleading and damaging myths build! ;)

Most True Peak metering algorithms employ a 4x oversampling rate which results in a maximum possible error margin of 0.688dB. That's why the BS.1770 specs set the maximum acceptable True Peak value at -1dBFS -- to ensure there is absolutely no possibility of digital clipping.

A TP metering system using 8x oversampling would have a maximum error of 0.169dB, but the additional processing overhead to achieve 8x oversampling is often considered excessive for the relatively small benefit in precision that it provides. Higher oversample rates obviously result in even smaller error margins.

The whole story (and the nature of the TP algorithm calculations) are described in Annex 2 of ITU-R BS.1770-4: https://www.itu.int/rec/R-REC-BS.1770-4-201510-I/en

H


If there is any uncertainty or error margin then that is effectively the same as a guess.

The 0.688 may be close enough for govt work though:)

And using the 'TP' at -1 would allow for it completely. What you can not do is compute exactly the actual TP after conversion.

The key point that most people do not realise is that if you do not go over 0dBFS in the digital domain then you do not have a problem in the analog domain *IF* you lower the signal so your A/D can handle it without conversion problems. And most A/D need a little more as they are less than perfect when right up near their max digital value that they can handle well.
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Re: LUFS and True Peak Max

Postby Hugh Robjohns » Wed Jul 31, 2019 5:29 pm

hobbyist wrote:If there is any uncertainty or error margin then that is effectively the same as a guess.

Again, we'll need to agree to differ on that one. To me a guess is a conclusion arrived at with insufficient information to be sure of being correct. A guess can be completely wrong.

The TP value is a calculation, not a guess, and it is guaranteed to be correct within a known error margin -- which is a perfectly legitimate way of expressing a scientific result. Technically, the TP value is an approximation to with 0.688dB (for 4x over-sampling)... Which isn't a guess! ;-)

If it was desirable or necessary, the true peak value -- ie the absolute waveform amplitude -- could be calculated with total precision since the amplitude of the waveform at every point in time is known absolutely from the available samples -- as defined by Nyquist's theorem. If this were not the case sample rate converters wouldn't work!

However, calculating the amplitude at every point in time is clearly very wasteful in terms of processor cycles and, more to the point, not actually necessary in the metering application defined in BS.1770. Instead, a simpler calculation to provide a very close approximation with a known error margin has been deemed acceptable, and an allowance is built in to the specification to accommodate it.

The key point that most people do not realise is that if you do not go over 0dBFS in the digital domain then you do not have a problem in the analog domain *IF* you lower the signal so your A/D can handle it without conversion problems.

You mean D-A, but yes, I agree -- as discussed earlier.

H
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