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The main requirements of Loudness can be summed up in 6 crucial point:


-K-Weighting

-LKFS,LUFS & LU

-Loudness Range, Program Loudness & Descriptors

-Gating

-Target Levels

-True-peak


K – Weightlifting

Rather than trying to measure audio level by counting samples (sample-peak level), circumstantial research has proven that even though two pieces of audio may be measured to be equally loud using the sample-peak method, they may very well be perceived as being very different in terms of level. Studies based on substantial listening tests performed by independent organizations such as Communications Research Centre (CRC) and McGill University in Canada have helped developing a method to measure audio level based on perceived loudness. Without getting into the technical details, a so-called K-weighted filter curve (based on the above-mentioned research results) is applied to each audio channel, which basically builds a bridge between subjective impression and objective measurement.

The K-weighting method is an essential part of a global, open standard defined by The International Telecommunication Union: ITU 85.1770 (now updated to BS.1770-3).


LKFS, LUFS & LU

When measuring loudness, three terms are essential to be aware of: LKFS, LUFS and LU.
What tends to create confusion is that these terms are very similar and basically aims at describing the exact same thing.


LKFS is an abbreviation of: Loudness K-weighted Full Scale, and one unit of LKFS is equal to one dB.
The LKFS term is used in the ITU BS.1770 standard and the ATSC A/85 standard also operates with this term.

Other organizations, such as The European Broadcast Union (EBU), uses the term LUFS, which is an abbreviation of Loudness Units Full Scale.
Despite the different names, LFKS and LUFS are identical.
Both terms describe the same phenomenon and just like LKFS, one unit of LUFS is equal to one dB.


LKFS/LUFS are absolute measures, and depending on which broadcast standard is in use, the loudness target level could be e.g. -24 LKFS or -23 LUFS. However, in order to aim for a more 'traditional number, a relative measure has been defined: Loudness Units (LU). Now, the broadcaster can set the target level (regardless of whether it is -23 or -24) to 0 LU, and again, one LU is equal to one dB.


Loudness Range, Program Loudness & Descriptors

Loudness Range - or LRA - describes the overall program material range: From the softest part to the loudest part The range is quantified in LU, and to avoid extreme events from affecting the overall result, the top 5% and the lowest 10% of the total loudness range is being excluded from the LRA measurement.
For example, a single gunshot or a long passage of silence in a movie would result in a very broad Loudness Range even though it would not be representative in the big picture.


The LRA parameter was originally developed by TC as the descriptor named 'Consistency: Later, it was adopted by e.g. EBU R128 standard and is currently under consideration for implementation at ITU as well.

Program Loudness aims at describing the average program material loudness. Sometimes, Program Loudness may also be referred to as Integrated Loudness.
Program Loudness is described using LUFS or LIKES.


Loudness meters featuring 'EBU Mode' will display the two above parameters, which will be projected as two numeric values - or descriptors - which will represent a valuable overview of the total 'loudness landscape' of the program material being measured.

Gating

When measuring Program Loudness, merely calculating the average level will not always be desirable as certain events - e.g. long passages of silence (or very soft background noise) in a movie - affect the Program Loudness parameter. Therefore, a gating scheme that pauses the measuring when the audio level drops below a threshold of -10 LU relative to an ungated measurement of the same program material has been developed.
The benefit of applying this gate is that the measurement becomes far more cross-genre friendly, allowing for example movies and classical music to be 'loudness aligned' with e.g. pop music and commercials. Note that to be efficient in a broadcast context, various types of program material with very different durations - such as a 2 hour movie and a 20 seconds commercial - must be able to be aligned in terms of loudness, and the gate is a very powerful tool in making this possible.


Target Levels

Target levels are specified in various broadcast standards, but only vary slightly. For instance, the ATSC AMS standard recommends a target of-24 and uses the LKFS term, whereas the EBU R128 standard sets the target level at -23 and uses the LUFS term. One of the reasons for this difference is that the R128 standard employs the above-mentioned gate, which in effect makes most measurements equivalent to -24 LKFS/LUFS without the gate-yet more useful for aligning loudness across genres.


True-peak

Since loudness measuring is based on an algorithm that builds on a study of subjective perception, in theory, program material that complies with the determined LRA and Program Loudness of a certain broadcast standard can in fact overload if normalized the traditional way (quasi-peak or sample-pea k). Therefore, normalization is also part of many broadcast standards, and to comply, broadcasters must use a true-peak meter.
Many loudness meters have a built-in true-peak meter, and what sets the true-peak meter apart from sample-peak meters is a special algorithm - donated by TC - that not only looks at the actual samples, but also inter sample peaks. In effect, the true-peak mater can unveil peaks in between actual samples that would otherwise cause distortion. Therefore, a true-peak meter actually 'goes beyond 0 dB'. A reading using a traditional sample-peak meter that displays a maxim of e.g. -0.2 dB could go as far as +3 dB on a true-peak meter reading.
Please note that this does not indicate acceptance of exceeding 0dB on a true-peak meter, but it provides a more precise reading that helps in normalizing program material without compromising the quality of the audio. As an example, the maxim value of normalized program material according to the EBU R128 standard is -1 dBTP (d B True-Peak).