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ITU-R BS.1770-5

• Title: Algorithms to measure audio programme loudness and true-peak audio level
• URL: itu.int/rec/R-REC-BS.2051-3-202205-I/en

Scope

This Recommendation specifies audio measurement algorithms for the purpose of determining subjective programme loudness, and true-peak signal level.

Keywords

3/2 multichannel sound system, advanced sound system, loudness, true-peak signal level The ITU Radiocommunication Assembly, considering

• a) that modern digital sound transmission techniques offer an extremely wide dynamic range;
• b) that modern digital sound production and transmission techniques provide a mixture of mono, stereo and 3/2 multichannel formats specified in Recommendation ITU-R BS.775 and advanced sound formats including channel-, object- and scene-based input signals and their combination with metadata specified in Recommendation ITU-R BS.2051, and that sound programmes are produced in all of these formats;
• c) that listeners desire the subjective loudness of audio programmes to be uniform for different sources and programme types;
• d) that many methods are available for the measurement of audio levels but that existing measurement methods employed in programme production do not provide the indication of subjective loudness;
• e) that, for the purpose of loudness control in programme exchange, in order to reduce audience annoyance, it is essential to have a single recommended algorithm for the objective estimation of subjective loudness;
• f) that future complex algorithms based on psychoacoustic models may provide improved objective measures of loudness for a wide variety of audio programmes;
• g) that digital media overloads abruptly, and thus even momentary overload should be avoided, considering further
  • a) that peak signal levels may increase due to commonly applied processes such as filtering or bit-rate reduction;
  • b) that existing metering technologies do not reflect the true-peak level contained in a digital signal since the true-peak value may occur between samples;
  • c) that the state of digital signal processing makes it practical to implement an algorithm that closely estimates the true-peak level of a signal;
  • d) that the use of a true-peak level indicating algorithm will allow accurate indication of the headroom between the peak level of a digital audio signal and the clipping level,

recommends

• 1 that when an objective measure of the loudness of an audio channel or programme, produced with up to five main channels per Recommendation ITU-R BS.775 (mono source, stereo and 3/2 multichannel sound), is required to facilitate programme delivery and exchange, the algorithm specified in Annex 1 should be used;
• 2 that when an objective measure of the loudness of an audio programme produced with a larger number of channels (such as the channel configurations specified in Recommendation ITU-R BS.2051) is required, the algorithm specified in Annex 3 should be used;
• 3 that when an objective measure of the loudness of object-based audio signals or combination of channel- and object-based audio signals is required, the algorithm specified in Annex 4 should be used;
• 4 that methods employed in programme production and post-production to indicate programme loudness may be based on the algorithm specified in Annexes 1, 3 and 4;
• 5 that when an indication of true-peak level of a digital audio signal is required, the measurement method should be based on the guidelines shown in Annex 2, or on a method that gives similar or superior results,

further recommends

• 1 that consideration should be given to the possible need to update this Recommendation in the event that new loudness algorithms are shown to provide performance that is significantly improved over the algorithm specified in Annexes 1, 3 and 4;
• 2 that this Recommendation should be updated when new algorithms have been developed to enable the measurement of audio programme loudness for scene-based audio programmes.
• NOTE 1 – Users should be aware that measured loudness is an estimation of subjective loudness and involves some degree of uncertainty depending on listeners, audio material and listening conditions.
• NOTE 2 – For testing compliance of meters according to this Recommendation, test material from the set described in Report ITU-R BS.2217 may be used.

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