Loudness Control

Recommendation ITU-R BS.1770 (Source: ITU)
EBU R128 - Loudness normalization and permitted maximum level of audio signals (Source: EBU) Level and Distortion in digital Broadcasting (Source: EBU)
EBU Tech3341 - Loudness Metering (Source: EBU) EBU Tech3342 - Loudness Range (Source: EBU) EBU Tech3343 - Practical Guidelines R128 (Source: EBU) Justice for Audio Forum


ITU-R BS.1770 and EBU R128 were introduced to establish a measurement procedure that takes account of audible loudness differences between different broadcast audio material. The intention is to match the audio levels of 'loud' advertizing clips and the audio levels of regular movies or talk shows.

The standards are:
Recommendation ITU-R BS.1770
EBU R128

An open algorithm for the measurement of loudness and the true peak levels of programs with simple implementation are the basis of both standards. A so-called 'K-weighting' filter curve is defined. This filter curve is applied to all audio channels, except LFE. The goal is to achieve an objective measurement for a subjective impression of loudness. The result of the 'K-weighting' filter curve is LKFS. LKFS is not a level, it is the loudness unit, the illustration of loudness referenced to the digital Full Scale. The single unit is 'LU', the Loudness Units. 1LU is equivalent to 1dB.
ITU-R BS.1770 recommends the use of LKFS with the unit LU, the EBU recommends LUFS (EBU Tech Doc 3341) with the unit LU. Generally LKFS and LUFS are equivalent with each other. One of the differences is the implementation of areas with 'no' audio. The EBU measurement recognizes zones with no audio and doesn't integrate these zones into the measurement. With the ITU measurement these zones must be kept control of manually.

loudness measurement
(here Tektronix)

Controlling loudness the original content should not be changed. If signals are more or less compressed they should stay this way unchanged. The goal of loudness control is to change the overall level of different program parts in a way that these program parts sound equal (in perceived level) but are only changed in relative level to each other. Critical are here the time constants chosen within which levels are compared.

K-Weighting and Measurement Algorithms

The LU measurement implements a multichannel loudness measurement algorithm. Usually the five main channels are used (left, centre, right, left surround and right surround). If less channels are used, these measurement channels would not be used. The LFE channel (low frequency effect) is not included in the measurement.
The algorithm implements two filters: a pre-filter with a high frequency increase (modelling the acoustic effects of the listener's head) and following high-pass filter (RBL).

channel processing and summation

    left: pre-filter and high-pass filter (RBL)
    right: combined filtering "K-weighting" compared to the
    dB(A) weighting curve

practical loudness control for similar received loudness