1、 Recommendation ITU-R BS.1770-4 (10/2015) Algorithms to measure audio programme loudness and true-peak audio level BS Series Broadcasting service (sound) ii Rec. ITU-R BS.1770-4 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of t
2、he radio-frequency spectrum by all radiocommunication services, including satellite services, and carry out studies without limit of frequency range on the basis of which Recommendations are adopted. The regulatory and policy functions of the Radiocommunication Sector are performed by World and Regi
3、onal Radiocommunication Conferences and Radiocommunication Assemblies supported by Study Groups. Policy on Intellectual Property Right (IPR) ITU-R policy on IPR is described in the Common Patent Policy for ITU-T/ITU-R/ISO/IEC referenced in Annex 1 of Resolution ITU-R 1. Forms to be used for the subm
4、ission of patent statements and licensing declarations by patent holders are available from http:/www.itu.int/ITU-R/go/patents/en where the Guidelines for Implementation of the Common Patent Policy for ITU-T/ITU-R/ISO/IEC and the ITU-R patent information database can also be found. Series of ITU-R R
5、ecommendations (Also available online at http:/www.itu.int/publ/R-REC/en) Series Title BO Satellite delivery BR Recording for production, archival and play-out; film for television BS Broadcasting service (sound) BT Broadcasting service (television) F Fixed service M Mobile, radiodetermination, amat
6、eur and related satellite services P Radiowave propagation RA Radio astronomy RS Remote sensing systems S Fixed-satellite service SA Space applications and meteorology SF Frequency sharing and coordination between fixed-satellite and fixed service systems SM Spectrum management SNG Satellite news ga
7、thering TF Time signals and frequency standards emissions V Vocabulary and related subjects Note: This ITU-R Recommendation was approved in English under the procedure detailed in Resolution ITU-R 1. Electronic Publication Geneva, 2015 ITU 2015 All rights reserved. No part of this publication may be
8、 reproduced, by any means whatsoever, without written permission of ITU. Rec. ITU-R BS.1770-4 1 RECOMMENDATION ITU-R BS.1770-4 Algorithms to measure audio programme loudness and true-peak audio level (Question ITU-R 2/6) (2006-2007-2011-2012-2015) Scope This Recommendation specifies audio measuremen
9、t algorithms for the purpose of determining subjective programme loudness, and 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
10、 techniques provide a mixture of mono, stereo and 3/2 multichannel formats specified in Recommendation ITU-R BS.775 and advanced sound formats 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 o
11、f audio programmes to be uniform for different sources and programme types; d) that many methods are available for measurement of audio levels but that existing measurement methods employed in programme production do not provide indication of subjective loudness; e) that, for the purpose of loudness
12、 control in programme exchange, in order to reduce audience annoyance, it is essential to have a single recommended algorithm for objective estimation of subjective loudness; f) that future complex algorithms based on psychoacoustic models may provide improved objective measures of loudness for a wi
13、de variety of audio programmes; g) that digital media overload abruptly, and thus even momentary overload should be avoided, considering further h) that peak signal levels may increase due to commonly applied processes such as filtering or bit-rate reduction; j) that existing metering technologies d
14、o not reflect the true-peak level contained in a digital signal since the true-peak value may occur in between samples; k) that the state of digital signal processing makes it practical to implement an algorithm that closely estimates the true-peak level of a signal; l) that use of a true-peak indic
15、ating algorithm will allow accurate indication of the headroom between the peak level of a digital audio signal and the clipping level, 2 Rec. ITU-R BS.1770-4 recommends 1 that when an objective measure of the loudness of an audio channel or programme, produced with up to 5 main channels per Recomme
16、ndation 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
17、 channel configurations specified in Recommendation ITU-R BS.2051) is required, the algorithm specified in Annex 3 should be used; 3 that methods employed in programme production and post-production to indicate programme loudness may be based on the algorithm specified in Annex 1 and Annex 3; 4 that
18、 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, NOTE 1 Users should be aware that measured loudness is an estimation of subjective loudness
19、and involves some degree of uncertainty depending on listeners, audio material and listening conditions. 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 signi
20、ficantly improved over the algorithm specified in Annex 1 and Annex 3; 2 that this Recommendation should be updated when new algorithms have been developed to enable the measurement of audio programme loudness for object and scene based audio programmes. NOTE 2 For testing compliance of meters accor
21、ding to this Recommendation, test material from the set described in Report ITU-R BS.2217 may be used. Annex 1 Specification of the objective multichannel loudness measurement algorithm This Annex specifies the multichannel loudness measurement modelling algorithm. The algorithm consists of four sta
22、ges “K” frequency weighting; mean square calculation for each channel; channel-weighted summation (surround channels have larger weights, and the LFE channel is excluded); gating of 400 ms blocks (overlapping by 75%), where two thresholds are used: the first at 70 LKFS; the second at 10 dB relative
23、to the level measured after application of the first threshold. Rec. ITU-R BS.1770-4 3 Figure 1 shows a block diagram of the various components of the algorithm. Labels are provided at different points along the signal flow path to aid in the description of the algorithm. The block diagram shows inp
24、uts for five main channels (left, centre, right, left surround and right surround); this allows monitoring of programmes containing from one to five channels. For a programme that has less than five channels some inputs would not be used. The low frequency effects (LFE) channel is not included in th
25、e measurement. FIGURE 1 Simplified block diagram of multichannel loudness algorithm BS .17 70 -01M e a s ur e dl ou dn e s sx LK - f i l t e r M e a n s qu a r ey LGLx RK - f i l t e r M e a n s qu a r ey RG Rx CK - f i l t e r M e a n s qu a r ey CGCx LsK - f i l t e r M e a n s qu a r ey LsG Lsx R
26、sK - f i l t e r M e a n s qu a r ey RsGRsz Lz Rz Cz sLz Rs10 L og 10 G a t eThe first step of the algorithm applies a 2-stage pre-filtering1 of the signal. The first stage of the pre-filtering accounts for the acoustic effects of the head, where the head is modelled as a rigid sphere. The response
27、is shown in Fig. 2. FIGURE 2 Response of stage 1 of the pre-filter used to account for the acoustic effects of the head BS .1770-02F r e que nc y ( H z )Relativelevel(dB)10864202468100 1021031041 The K-weighting filter is composed of two stages of filtering; a first stage shelving filter and a secon
28、d stage high-pass filter. 4 Rec. ITU-R BS.1770-4 Stage 1 of the pre-filter is defined by the filter shown in Fig. 3 with the coefficients specified in Table 1. FIGURE 3 Signal flow diagram as a 2nd order filter BS .177 0-03-+ b 1b 2a 1a 2Z 1b 0Z 1TABLE 1 Filter coefficients for stage 1 of the pre-fi
29、lter to model a spherical head b0 1.53512485958697 a1 1.69065929318241 b1 2.69169618940638 a2 0.73248077421585 b2 1.19839281085285 These filter coefficients are for a sampling rate of 48 kHz. Implementations at other sampling rates will require different coefficient values, which should be chosen to
30、 provide the same frequency response that the specified filter provides at 48 kHz. The values of these coefficients may need to be quantized due to the internal precision of the available hardware. Tests have shown that the performance of the algorithm is not sensitive to small variations in these c
31、oefficients. The second stage of the pre-filter applies a simple high-pass filter as shown in Fig. 4. The stage weighting curve is specified as a 2nd order filter as shown in Fig. 3, with the coefficients specified in Table 2. These filter coefficients are for a sampling rate of 48 kHz. Implementati
32、ons at other sampling rates will require different coefficient values, which should be chosen to provide the same frequency response that the specified filter provides at 48 kHz. Rec. ITU-R BS.1770-4 5 FIGURE 4 Second stage weighting curve BS .17 70 -04F r e que nc y ( H z )Relativelevel (dB)5051015
33、202530102103104101TABLE 2 Filter coefficients for the second stage weighting curve b0 1.0 a1 1.99004745483398 b1 2.0 a2 0.99007225036621 b2 1.0 The power, the mean square of the filtered input signal in a measurement interval T is measured as: tyTzTii d1 0 2(1) where yi is the input signal (filtered
34、 by the 2-stage pre-filter as described above), and i I where I = L,R,C,Ls,Rs, the set of input channels. The loudness over the measurement interval T is defined as: Loudness, LK = 0.691 + 10 log10 ii i zGLKFS (2) where Gi are the weighting coefficients for the individual channels. To calculate a ga
35、ted loudness measurement, the interval T is divided into a set of overlapping gating block intervals. A gating block is a set of contiguous audio samples of duration Tg = 400 ms, to the nearest sample. The overlap of each gating block shall be 75% of the gating block duration. 6 Rec. ITU-R BS.1770-4
36、 The measurement interval shall be constrained such that it ends at the end of a gating block. Incomplete gating blocks at the end of the measurement interval are not used. The power, the mean square of the jth gating block of the ith input channel in the interval T is: )1( 2 d1 s t epjTs t epjT igi
37、jggtyTz where step = 1-overlap and s te pT TTj g g,.2,1,0(3) The jth gating block loudness is defined as: iji ij zGl 10lo g10691.0(4) For a gating threshold there is a set of gating block indices Jg = j : lj where the gating block loudness is above the gating threshold. The number of elements in Jg
38、is |Jg|. The gated loudness of the measurement interval T is then defined as: L K F SzJGLl o u d n e s sG a t e d gJ ijgi iKG 1l o g10691.0,10(5) A two-stage process is used to make a gated measurement, first with an absolute threshold, then with a relative threshold. The relative threshold r is cal
39、culated by measuring the loudness using the absolute threshold, a = 70 LKFS, and subtracting 10 from the result, thus: L K F SzJG gJ ijgi ir 101l o g10691.0 10 where: L K FSljJ a ajg 70 : (6) The gated loudness can then be calculated using r: L K F SzJGLl o u d n e s sG a t e d gJ ijgi iKG 1l o g106
40、91.0,10where: Jg = j : lj r (7) Rec. ITU-R BS.1770-4 7 The frequency weighting in this measure, which is generated by the pre-filter (concatenation of the stage 1 filter to compensate for the acoustics effects of the head, and the stage 2 filter, the RLB weighting) is designated K-weighting. The num
41、erical result for the value of loudness that is calculated in equation (2) should be followed by the designation LKFS. This designation signifies: Loudness, K-weighted, relative to nominal full scale. The LKFS unit is equivalent to a decibel in that an increase in the level of a signal by 1 dB will
42、cause the loudness reading to increase by 1 LKFS. If a 0 dB FS 1 kHz sine wave is applied to the left, centre, or right channel input, the indicated loudness will equal 3.01 LKFS. The weighting coefficient for each channel is given in Table 3. TABLE 3 Weightings for the individual audio channels Cha
43、nnel Weighting, Gi Left (GL) 1.0 (0 dB) Right (GR) 1.0 (0 dB) Centre (GC) 1.0 (0 dB) Left surround (GLs) 1.41 ( +1.5 dB) Right surround (GRs) 1.41 ( +1.5 dB) It should be noted that while this algorithm has been shown to be effective for use on audio programmes that are typical of broadcast content,
44、 the algorithm is not, in general, suitable for use to estimate the subjective loudness of pure tones. Appendix 1 to Annex 1 (informative) Description and development of the multichannel measurement algorithm This Appendix describes a newly developed algorithm for objectively measuring the perceived
45、 loudness of audio signals. The algorithm can be used to accurately measure the loudness of mono, stereo and multichannel signals. A key benefit of the proposed algorithm is its simplicity, allowing it to be implemented at very low cost. This Appendix also describes the results of formal subjective
46、tests conducted to form a subjective database that was used to evaluate the performance of the algorithm. 1 Introduction There are many applications where it is necessary to measure and control the perceived loudness of audio signals. Examples of this include television and radio broadcast applicati
47、ons where the nature and content of the audio material changes frequently. In these applications the audio content can continually switch between music, speech and sound effects, or some combination of these. Such changes in the content of the programme material can result in significant changes in
48、8 Rec. ITU-R BS.1770-4 subjective loudness. Moreover, various forms of dynamics processing are frequently applied to the signals, which can have a significant effect on the perceived loudness of the signal. Of course, the matter of subjective loudness is also of great importance to the music industr
49、y where dynamics processing is commonly used to maximize the perceived loudness of a recording. There has been an ongoing effort within Radiocommunication Working Party 6P in recent years to identify an objective means of measuring the perceived loudness of typical programme material for broadcast applications. The first phase of ITU-Rs effort examined objective monophonic loudness algorithms exclusively, and a weighted m
