1、 ETSI TS 126 403 V14.0.0 (2017-04) Digital cellular telecommunications system (Phase 2+) (GSM); Universal Mobile Telecommunications System (UMTS); LTE; General audio codec audio processing functions; Enhanced aacPlus general audio codec; Encoder specification; Advanced Audio Coding (AAC) part (3GPP
2、TS 26.403 version 14.0.0 Release 14) TECHNICAL SPECIFICATION ETSI ETSI TS 126 403 V14.0.0 (2017-04)13GPP TS 26.403 version 14.0.0 Release 14Reference RTS/TSGS-0426403ve00 Keywords GSM,LTE,UMTS ETSI 650 Route des Lucioles F-06921 Sophia Antipolis Cedex - FRANCE Tel.: +33 4 92 94 42 00 Fax: +33 4 93 6
3、5 47 16 Siret N 348 623 562 00017 - NAF 742 C Association but non lucratif enregistre la Sous-Prfecture de Grasse (06) N 7803/88 Important notice The present document can be downloaded from: http:/www.etsi.org/standards-search The present document may be made available in electronic versions and/or
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7、lectronic or mechanical, including photocopying and microfilm except as authorized by written permission of ETSI. The content of the PDF version shall not be modified without the written authorization of ETSI. The copyright and the foregoing restriction extend to reproduction in all media. European
8、Telecommunications Standards Institute 2017. All rights reserved. DECTTM, PLUGTESTSTM, UMTSTMand the ETSI logo are Trade Marks of ETSI registered for the benefit of its Members. 3GPPTM and LTE are Trade Marks of ETSI registered for the benefit of its Members and of the 3GPP Organizational Partners.
9、GSM and the GSM logo are Trade Marks registered and owned by the GSM Association. ETSI ETSI TS 126 403 V14.0.0 (2017-04)23GPP TS 26.403 version 14.0.0 Release 14Intellectual Property Rights IPRs essential or potentially essential to the present document may have been declared to ETSI. The informatio
10、n pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found in ETSI SR 000 314: “Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in respect of ETSI standards“, which is available from the ETSI
11、Secretariat. Latest updates are available on the ETSI Web server (https:/ipr.etsi.org/). Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee can be given as to the existence of other IPRs not referenced in ETSI SR 000 314 (or the upda
12、tes on the ETSI Web server) which are, or may be, or may become, essential to the present document. Foreword This Technical Specification (TS) has been produced by ETSI 3rd Generation Partnership Project (3GPP). The present document may refer to technical specifications or reports using their 3GPP i
13、dentities, UMTS identities or GSM identities. These should be interpreted as being references to the corresponding ETSI deliverables. The cross reference between GSM, UMTS, 3GPP and ETSI identities can be found under http:/webapp.etsi.org/key/queryform.asp. Modal verbs terminology In the present doc
14、ument “shall“, “shall not“, “should“, “should not“, “may“, “need not“, “will“, “will not“, “can“ and “cannot“ are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of provisions). “must“ and “must not“ are NOT allowed in ETSI deliverables except
15、 when used in direct citation. ETSI ETSI TS 126 403 V14.0.0 (2017-04)33GPP TS 26.403 version 14.0.0 Release 14Contents Intellectual Property Rights 2g3Foreword . 2g3Modal verbs terminology 2g3Foreword . 5g31 Scope 6g32 Normative references . 6g33 Definitions, symbols and abbreviations . 7g33.1 Defin
16、itions 7g33.2 Symbols 7g33.3 Abbreviations . 7g34 Outline description . 7g35 AAC Encoder . 7g35.1 Overview 7g35.2 Stereo Preprocessing 8g35.3 Filterbank . 9g35.4 Psychoacoustic Model 9g35.4.1 Blockswitching . 9g35.4.2 Threshold Calculation . 11g35.4.2.1 Calculation of the energy spectrum . 11g35.4.2
17、.2 From energy to threshold 12g35.4.2.3 Spreading 12g35.4.2.4 Threshold in quiet . 12g35.4.2.5 Pre-echo control 12g35.4.3 Spreaded Energy Calculation 13g35.4.4 Grouping . 13g35.5 Tools . 13g35.5.1 Temporal Noise Shaping (TNS) . 13g35.5.1.1 TNS detection . 13g35.5.1.2 TNS Stereo Synchronization . 14g
18、35.5.1.3 TNS Order . 14g35.5.1.4 TNS Filtering 14g35.5.1.5 Threshold modification . 14g35.5.2 Mid/Side Stereo 14g35.6 Quantization and coding . 15g35.6.1 Reduction of psychoacoustic requirements . 15g35.6.1.1 Principle of the threshold reduction strategy . 15g35.6.1.1.1 Addition of noise with equal
19、loudness. 15g35.6.1.1.2 Avoidance of spectral holes . 15g35.6.1.1.3 Relation between bit demand and perceptual entropy . 16g35.6.1.2 Calculation of Bit Demand 16g35.6.1.3 Calculation of the reduction value 18g35.6.1.3.1 Preparatory steps of the perceptual entropy calculation 19g35.6.1.3.2 Calculatio
20、n of the desired perceptual entropy . 19g35.6.1.3.3 Selection of the bands for avoidance of holes . 19g35.6.1.3.4 First Estimation of the reduction value 19g35.6.1.3.5 Second Estimation of the reduction value . 20g35.6.1.3.6 Final threshold modification by linearization 20g35.6.1.3.7 Further percept
21、ual entropy reduction . 21g35.6.1.3.8 Possible failures . 21g35.6.2 Scalefactor determination . 21g35.6.2.1 Scalefactor Estimation 22g35.6.2.2 Scalefactor Improvement by Quantization 22g35.6.2.3 Scalefactor Difference Reduction . 22g3ETSI ETSI TS 126 403 V14.0.0 (2017-04)43GPP TS 26.403 version 14.0
22、.0 Release 145.6.2.4 Final scalefactor determination . 23g35.6.3 Noiseless coding . 23g35.6.4 Out of Bits Prevention 23g3Annex A (informative): Change history . 24g3History 25g3ETSI ETSI TS 126 403 V14.0.0 (2017-04)53GPP TS 26.403 version 14.0.0 Release 14Foreword The present document describes the
23、detailed mapping of the general audio service employing the aacPlus general audio codec within the 3GPP system. The contents of the present document are subject to continuing work within the TSG and may change following formal TSG approval. Should the TSG modify the contents of this TS, it will be r
24、e-released by the TSG with an identifying change of release date and an increase in version number as follows: Version x.y.z where: x the first digit: 1 presented to TSG for information; 2 presented to TSG for approval; 3 Indicates TSG approved document under change control. y the second digit is in
25、cremented for all changes of substance, i.e. technical enhancements, corrections, updates, etc. z the third digit is incremented when editorial only changes have been incorporated in the specification; ETSI ETSI TS 126 403 V14.0.0 (2017-04)63GPP TS 26.403 version 14.0.0 Release 141 Scope This Teleco
26、mmunication Standard (TS) describes the AAC encoder part of the Enhanced aacPlus general audio codec 1. 2 Normative references This TS incorporates by dated and undated reference, provisions from other publications. These normative references are cited in the appropriate places in the text and the p
27、ublications are listed hereafter. For dated references, subsequent amendments to or revisions of any of these publications apply to this TS only when incorporated in it by amendment or revision. For undated references, the latest edition of the publication referred to applies. 1 3GPP TS 26.401: “Enh
28、anced aacPlus general audio codec; General Description“. 2 ISO/IEC 14496-3:2001: “Information technology - Coding of audio-visual objects - Part 3: Audio“. 3 ISO/IEC 14496-3:2001/Amd.1:2003: “Bandwidth Extension“. 4 ISO/IEC 14496-3:2001/Amd.1:2003/DCOR1. 5 ISO/IEC 14496-3:2001/ Amd.2:2004: “Parametr
29、ic Coding for High Quality Audio“. ETSI ETSI TS 126 403 V14.0.0 (2017-04)73GPP TS 26.403 version 14.0.0 Release 143 Definitions, symbols and abbreviations 3.1 Definitions For the purposes of this TS, the following definitions apply: frame: time segment associated with one AAC single channel or chann
30、el pair element frequency coefficient: output value of the MDCT transform scalefactor band: a group of consecutive frequency coefficients, that will be coded with the same quantizer step size 3.2 Symbols For the purposes of this TS, the following symbols apply: is the current index for the spectral
31、coefficients is the index of the first spectral coefficient in scalefactorband is the current scalefactor band 3.3 Abbreviations For the purposes of this TS, the following abbreviations apply. AAC Advanced Audio Coding aacPlus Combination of MPEG-4 AAC and MPEG-4 Bandwidth extension (SBR) Enhanced a
32、acPlus Combination of MPEG-4 AAC, MPEG-4 Bandwidth extension (SBR) and MPEG-4 Parametric Stereo KBD Kaiser-Bessel derivedPE perceptual entropy SBR Spectral Band Replication TNS Temporal Noise Shaping 4 Outline description This TS is structured as follows: Section 5.1 gives an encoder overview descri
33、ption. Section 5.2 gives a detailed description of the stereo preprocessing. Section 5.3 gives a detailed description of the filterbank used in the encoder. Section 5.4 gives a detailed description of the psychoacoustic model. Section 5.5 gives a detailed description of the temporal noise shaping an
34、d mid/side stereo tools. Section 5.6 gives a detailed description of the quantization and coding procedure used in the encoder. 5 AAC Encoder 5.1 Overview The AAC encoder acts as the core encoding algorithm of the aacPlus system encoding at half the sampling rate of aacPlus. Since aacPlus implements
35、 the High Efficiency AAC Profile at Level 2 as defined in 3, the AAC LC object k()kOffset n nnETSI ETSI TS 126 403 V14.0.0 (2017-04)83GPP TS 26.403 version 14.0.0 Release 14type is used. The AAC LC object type does not implement the Long Term Predictor (LTP) tool. The Level 2 implies a restriction t
36、o a maximum of two channels. Furthermore in case of SBR being used, the maximum AAC sampling rate is restricted to 24 kHz whereas if SBR is not used the maximum AAC sampling rate is restricted to 48 kHz. The basic layout is depicted below. Figure 1: AAC Encoder Block Diagram 5.2 Stereo Preprocessing
37、 With stereo preprocessing, the stereo width of difficult to encode signals at low bitrates is reduced. Stereo preprocessing is active for bitrates less than 60kbit/s. The side channel is attenuated with influence of the following parameters: - The total perceptual entropy before the increase of the
38、 thresholds. This PE is smoothed over past frames and normalized. For a definition of the perceptual entropy see 5.6.1.1.3. . StereoPreprocessingFilterbankTNSM/SReduction ofpsychoacousticrequirementsscalefactors /quantizationNoiselessCodingOut of bitspreventionPsycho-acousticModelBitstreammultiplexB
39、itstreamInput signalQuantization k=lpcStartLine; k-) wfack = (wfack + wfack+1) / 2; and up: for (k=lpcStartLine+1; k= ( ) 20dB/ barkls n bark(n)= ( ) 15dB/ barkhs nbark(n)=bark(n) n()enn ()esn()X k () () ()wX kXkwfack=1()()wfac kenn=()ennETSI ETSI TS 126 403 V14.0.0 (2017-04)143GPP TS 26.403 version
40、 14.0.0 Release 14The lower and upper limits lpcStartLine and lpcStopLine depend on the bitrate and the blocktype. Next steps are an autocorrelation calculation and a LPC calculation using the Levinson-Durbin algorithm. As result so called parcor or reflection coefficients and the prediction gain ar
41、e available. TNS will be used only if the prediction gain is greater than a given threshold, which is bitrate dependent and varies between 1.2 and 1.41. 5.5.1.2 TNS Stereo Synchronization If prediction gains for the left and right channel differ only less than 3%, the same TNS filter coefficients ar
42、e chosen for both channels by copying the TNS data of the left channel to the right channel. 5.5.1.3 TNS Order The TNS parcor coefficients will be quantized with a resolution of 4 bits for long blocks and 3 bits for short blocks. The order of the coefficients is now determined by going down from the
43、 maximum order until the first coefficient that exceeds an absolute value of 0.1 has been reached. 5.5.1.4 TNS Filtering The spectral coefficients will now be replaced by filtering with the parcor coefficients. The first scalefactor band affected corresponds to a frequency of 1275Hz for long blocks
44、resp. 2750Hz for short blocks. The filtering is done with the help of a so called lattice filter, no conversion from parcor coefficients to linear prediction coefficients is required. 5.5.1.5 Threshold modification In the frequency range from 380Hz to the start frequency of the TNS filter the coding
45、 demands will be increased by multiplying a factor of 0.25 to the thresholds calculated by the psychoacoustic model. 5.5.2 Mid/Side Stereo Normal stereo operation, and thus Mid/Side Stereo, is only required when operating the encoder at bitrates at or above 44 kbit/s. Below 44 kbit/s the Parametric
46、Stereo coding tool 5 is used instead where the AAC core is operated in mono. Within Mid/Side Stereo, for each scalefactor band the left and right channel coefficients are either coded as L and R or as mid and side channel and . For stereo in the psychoacoustic model in addition to the left and right
47、 energies also the mid and side energies are calculated. The threshold for coding mid and side channel is simply the minimum of left and right thresholds . M/S coding is actually used if is fulfilled. In such a case left channel values for spectral coefficients, energies and thresholds will be repla
48、ced by the mid channel values, resp. right channel values will be replaced by the side channel values. The spreaded energy for mid and side channel will be the minium of the spreaded energy of left and right channel. rq()X krq()thr n2LRM+=2LRS=,()LRenn,()MSenn,()LRthr n2min( ( ), ( ) ( ) ( )( )() ()
49、()LR L RMS LRthr n thr n thr n thr nen n en n en n en n()esnETSI ETSI TS 126 403 V14.0.0 (2017-04)153GPP TS 26.403 version 14.0.0 Release 145.6 Quantization and coding 5.6.1 Reduction of psychoacoustic requirements Usually the requirements of the psychoacoustic model are too strong for the desired bitrate. Thus a threshold reduction strategy is necessary, i.e. the strategy reduces the requirements by increasing the thresholds given by the psychoacoustic model. An overcoding, i.e. decreasing the thresholds for a finer quantization,
