Document: Our Ref: Date: DEFAULT UK VOTE: UK ABSTAIN NO UK INTEREST COMMENTS TO BEFORE 2015/01/30

Transcription

1 Private Circulation Document: EPL/100_14_0226 BSI, 389 Chiswick High Road London W4 4AL Tel: +44 (0) Fax: +44 (0) Our Ref: Date: EPL/ /11/24 Direct details: Telephone: Dear Member, IEC NEW WORK ITEM PROPOSAL (NWIP) DEFAULT UK VOTE: UK ABSTAIN NO UK INTEREST COMMENTS TO BEFORE 2015/01/30 Please find attached the following New Work Item Proposal [NWIP]: 100/2432/NP, Digital audio - Interface for non-linear PCM encoded audio bitstreams applying IEC Part 14 - Non-linear PCM bitstreams according to the AC-4 format Please consider whether this project should be added to the work programme of IEC Committee and provide comments on the proposed scope and target date together with relevant details of standards or regulatory matters. If you think the UK should submit a positive or negative vote please give a justification. - if the UK is to vote positively or negatively, then BSI has to answer the questions attached in Annex A and provide justification. - If no comments are received, a default abstention vote and no UK participation in this work will be submitted on behalf of UK with the following justification: - Whilst stakeholders consulted have not indicated a definite market need for this standard in the UK, we have not received any objections to the work proceeding. The UK therefore abstains on this proposal. - If a positive vote is submitted without justification, we will provide the following statement on your behalf - The subject of this proposal is of importance to UK Industry - If the UK is to vote to participate in the project then BSI has to nominate an expert to the project team. If you want to be nominated please let us know by the date above. When submitting comments please ensure that they are entered into the comments template. If you have any queries on how to use the template then please do not hesitate to contact CSC. Please notify us if you are aware of any keywords that might assist in classifying or identifying the standard or if the content of this standard: i) has any issues related to 3 rd party IPR, patent or copyright ii) affects other national standard(s) iii) is likely to require additional national guidance or information If no comments are received by the above date, we will submit a vote to "abstain" on behalf of the UK. Yours sincerely Snezhana Halacheva Secretary to EPL/100-1-

2 Annex A - Ballot responses if the UK does not abstain No. Questions Possible Answers 1 Vote approve * disapprove * abstain Please provide a statement justifying your approval or disapproval vote on the market relevance of this proposal. If no such statement is provided, the vote will not be registered and considered. Justification statements for abstention votes are also encouraged * 2 If Approved, should the next stage be Committee Draft Committee Draft for Vote Either 3 If Approved, will the NC participate in the work? If the UK is to vote to participate in the project then BSI has to nominate an expert to the project team. If you want to be nominated please let us know by the deadline date Yes No 4 Comments (using the comment template) (*) A Comment is required for this answer value. -2-

3 Proposer USNC TC/SC TC 100 / TA 4 Date of circulation /2432/NP NEW WORK ITEM PROPOSAL Date of proposal Secretariat Korea Closing date for voting A proposal for a new work item within the scope of an existing technical committee or subcommittee shall be submitted to the Central Office. The proposal will be distributed to the P-members of the technical committee or subcommittee for voting on the introduction of it into the work programme, and to the O-members for information. The proposer may be a National Committee of the IEC, the secretariat itself, another technical committee or subcommittee, an organization in liaison, the Standardization Management Board or one of the advisory committees, or the General Secretary. Guidelines for proposing and justifying a new work item are given in ISO/IEC Directives, Part 1, Annex C (see extract overleaf). This form is not to be used for amendments or revisions to existing publications. The proposal (to be completed by the proposer) Title of proposal Digital audio Interface for non-linear PCM encoded audio bitstreams applying IEC Part 14: Non-linear PCM bitstreams according to the AC-4 format Standard Technical Specification Scope (as defined in ISO/IEC Directives, Part 2, 6.2.1) This part of IEC describes the method used to convey non-linear PCM bitstreams encoded acording to the AC-4 format. Purpose and justification, including the market relevance, whether it is a proposed horizontal standard (Guide 108) 1 ) and relationship to Safety (Guide 104), EMC (Guide 107), Environmental aspects (Guide 109) and Quality assurance (Guide 102). (attach a separate page as annex, if necessary) The AC-4 audio compression standard has recently been published by ETSI. This proposes to provide a mapping for AC-4 to IEC analogous to those already defined for the AC-3 and Enhanced AC-3 audio compression standards. Target date for first CD for IS/ TS Estimated number of meetings Frequency of meetings: per year Date and place of first meeting: Proposed working methods Collaboration tools Relevant documents to be considered ETSI TS V1.1.1 Relationship of project to activities of other international bodies ETSI/EBU JTC Broadcast Technical Committee SMPTE 32NF Liaison organizations Need for coordination within ISO or IEC Coordination within IEC to update for required data type and sub data type is needed. 1) Other TC/SCs are requested to indicate their interest, if any, in this NP to the TC/SC secretary. Copyright 2014 International Electrotechnical Commission, IEC. All rights reserved. It is permitted to download this electronic file, to make a copy and to print out the content for the sole purpose of preparing National Committee positions. You may not copy or "mirror" the file or printed version of the document, or any part of it, for any other purpose without permission in writing from

4 Preparatory work Ensure that all copyright issues are identified. Check one of the two following boxes A draft is attached for comment* An outline is attached * Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to provide supporting documentation. We nominate a project leader as follows in accordance with ISO/IEC Directives, Part 1, (name, address, fax and ): Mr. Michael Dolan PO Box 190, Del Mar, CA USA Fax: mdolan@newtbt.com Concerns known patented items (see ISO/IEC Directives, Part 2) Name and/or signature of the proposer Yes. If yes, provide full information as an annex no Mr. Michael Dolan (USNC) Comments and recommendations from the TC/SC officers 1) Work allocation Project team New working group Existing working group no: 2) Draft suitable for direct submission as CD CDV/ DTS 3) General quality of the draft (conformity to ISO/IEC Directives, Part 2) Little redrafting needed Substantial redrafting needed no draft (outline only) 4) Relationship with other activities In IEC MT under IEC TC 100 TA 4 In other organizations ETSI/EBU JTC Broadcast Technical Committee SMPTE 32NF 5) Proposed horizontal standard 1) Remarks from the TC/SC officers Please note that TA 4 reviewed this new work item and decided to submit an NP at the recent TA 4 meeting held in Tokyo, Japan on Nov. 7, (Please refer to the Action Items 9 in the document, 100/TA4(Tokyo/TS)16).) 1) Other TC/SCs are requested to indicate their interest, if any, in this NP to the TC/SC secretary. Approval criteria: Approval of the work item by a simple majority of the P-members voting; At least 4 P-members in the case of a committee with 16 or fewer P-members, or at least 5 P-members in the case of committees with more than 17 P-members, have nominated or confirmed the name of an expert and approved the new work item proposal. Elements to be clarified when proposing a new work item Title Indicate the subject matter of the proposed new standard or technical specification. Indicate whether it is intended to prepare a standardor a technical specification. Scope Give a clear indication of the coverage of the proposed new work item and, if necessary for clarity, exclusions. Indicate whether the subject proposed relates to one or more of the fields of safety, EMC, the environment or quality assurance. Purpose and justification Give details based on a critical study of the following elements wherever practicable.

5 a) The specific aims and reason for the standardization activity, with particular emphasis on the aspects of standardization to be covered, the problems it is expected to solve or the difficulties it is intended to overcome. b) The main interests that might benefit from or be affected by the activity, such as industry, consumers, trade, governments, distributors. c) Feasibility of the activity: Are there factors that could hinder the successful establishment or general application of the standard? d) Timeliness of the standard to be produced: Is the technology reasonably stabilized? If not, how much time is likely to be available before advances in technology may render the proposed standard outdated? Is the proposed standard required as a basis for the future development of the technology in question? e) Urgency of the activity, considering the needs of the market (industry, consumers, trade, governments etc.) as well as other fields or organizations. Indicate target date and, when a series of standards is proposed, suggest priorities. f) The benefits to be gained by the implementation of the proposed standard; alternatively, the loss or disadvantage(s) if no standard is established within a reasonable time. Data such as product volume of value of trade should be included and quantified. g) If the standardization activity is, or is likely to be, the subject of regulations or to require the harmonization of existing regulations, this should be indicated. If a series of new work items is proposed, the purpose and justification of which is common, a common proposal may be drafted including all elements to be clarified and enumerating the titles and scopes of each individual item. Relevant documents List any known relevant documents (such as standards and regulations), regardless of their source. When the proposer considers that an existing well-established document may be acceptable as a standard (with or without amendments), indicate this with appropriate justification and attach a copy to the proposal. Cooperation and liaison List relevant organizations or bodies with which cooperation and liaison should exist. Preparatory work Indicate the name of the project leader nominated by the proposer.

6 /Ed.1.0 CD IEC CONTENTS FOREWORD Scope Normative references Definition of terminology Definitions Abbreviations Mapping of the audio bit stream on to IEC General AC-4 burst-info Format of AC-4 data-bursts General Pause data-burst Audio data-burst The AC-4 data Latency of the AC-4 decoder The AC-4 HBR data Latency of the AC-4 HBR decoder Annex A A.1 AC-4 sync frame Figure 1 AC-4 data-burst... 5 Figure 2 Latency of AC-4 decoding... 8 Figure 3 AC-4 HBR data-burst... 9 Figure 4 Latency of AC-4 decoding Figure A.1 An AC-4 sync frame Table 1 Shorthand notation for fractional frame rates... 4 Table 2 Fields of burst-info... 5 Table 3 Repetition period of the Pause data-bursts... 5 Table 4 Data-type-dependent information for AC Table 5 AC-4 base sampling frequency, AC-4 audio frame rate and corresponding AC-4 data-burst repetition period... 6 Table 6 AC-4 data-burst sequence and repetition period variance to ensure AC-4 audio frame alignment at 29,97, 59,94 and 119,88 fps... 7 Table 7 Meaning of Pc bits 8 11 at an IEC frame rate of 48 khz... 7 Table 8 Meaning of Pc bits 8 11 at an IEC frame rate of 44.1 khz... 7 Table 9 Maximum burst-length values per AC-4 repetition period... 8 Table 10 Data-type-dependent information for AC-4 HBR Table 11 AC-4 audio frame rate and corresponding AC-4 HBR data-burst repetition period when bit 12 of Pc = Table 12 AC-4 audio frame rate and corresponding AC-4 HBR data-burst repetition period when bit 12 of Pc = Table 13 AC-4 HBR data-burst sequence and repetition period variance to ensure AC-4 audio frame alignment at 29,97, 59,94 and 119,88 fps... 11

7 Table 14 AC-4 HBR data-burst: Meaning of Pc bits 8 11 at an IEC frame rate of 192 khz (bit 12 of Pc = 0) Table 15 AC-4 HBR data-burst: Meaning of Pc bits 8 11 at an IEC frame rate of 176,4 khz (bit 12 of Pc = 0) Table 16 AC-4 HBR data-burst: Meaning of Pc bits 8 11 at an IEC frame rate of 768 khz (bit 12 of Pc = 1) Table 17 AC-4 HBR data-burst: Meaning of Pc bits 8 11 at an IEC frame rate of 705,6 khz (bit 12 of Pc = 1) Table 18 Maximum burst-length values per AC-4 HBR data-burst repetition period Table A.1 AC-4 sync frame syntax Table A.2 framesize() Table A.3 AC-4 sync word values... 17

8 /Ed.1.0 CD IEC INTERNATIONAL ELECTROTECHNICAL COMMISSION DIGITAL AUDIO INTERFACE FOR NON-LINEAR PCM ENCODED AUDIO BITSTREAMS APPLYING IEC Part 14: Non-linear PCM bit streams according to the AC-4 format FOREWORD 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as IEC Publication(s) ). Their preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with may participate in this preparatory work. International, governmental and nongovernmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for Standardization (ISO) in accordance with conditions determined by agreement between the two organizations. 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international consensus of opinion on the relevant subjects since each technical committee has representation from all interested IEC National Committees. 3) IEC Publications have the form of recommendations for international use and are accepted by IEC National Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any misinterpretation by any end user. 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications transparently to the maximum extent possible in their national and regional publications. Any divergence between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter. 5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any services carried out by independent certification bodies. 6) All users should ensure that they have the latest edition of this publication. 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and members of its technical committees and IEC National Committees for any personal injury, property damage or other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publications. 8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is indispensable for the correct application of this publication. 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent rights. IEC shall not be held responsible for identifying any or all such patent rights. International Standard IEC has been prepared by technical area 4: Digital system interfaces and protocols, of IEC technical committee 100: Audio, video and multimedia systems and equipment.: The text of this standard is based on the following documents: FDIS XX/XX/FDIS Report on voting XX/XX/RVD Full information on the voting for the approval of this standard can be found in the report on voting indicated in the above table.

9 This publication has been drafted in accordance with the ISO/IEC Directives, Part 2. A list of all parts of IEC 61937, under the general title Digital audio Interface for non-linear PCM encoded audio bitstreams applying IEC can be found on the IEC website. The committee has decided that the contents of this publication will remain unchanged until the stability date indicated on the IEC web site under " in the data related to the specific publication. At this date, the publication will be reconfirmed, withdrawn, replaced by a revised edition, or amended The National Committees are requested to note that for this publication the stability date is THIS TEXT IS INCLUDED FOR THE INFORMATION OF THE NATIONAL COMMITTEES AND WILL BE DELETED AT THE PUBLICATION STAGE.

10 DIGITAL AUDIO INTERFACE FOR NON-LINEAR PCM ENCODED AUDIO BITSTREAMS APPLYING IEC Part 14: Non-linear PCM bit streams according to the AC-4 format Scope This part of IEC describes the method to convey non-linear PCM bit streams encoded according to the AC-4 format Normative references The following Standards contain provisions which, through reference in the text, referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC (all parts), Digital audio interface IEC :2007, Digital audio interface for non-linear PCM encoded audio bitstreams applying IEC Part 1: General IEC :2007/A1:2011, Digital audio interface for non-linear PCM encoded audio bitstreams applying IEC Part 1: General IEC :201x, Digital audio interface for non-linear PCM encoded audio bitstreams applying IEC Part 2: Burst Information ETSI TS v.1.1.1, 2014, Digital Audio Compression (AC-4) Standard Definition of terminology For the purpose of this standard, the following definitions and abbreviations apply. 3.1 Definitions AC-4 frame An AC-4 sync frame, as specified in Annex A Audio Frame Rate The number of AC-4 audio frames per second, indicated by the value of the frame_rate_index AC-4 bit stream parameter as specified in ETSI TS Base sampling frequency The sampling frequency of the AC-4 bit stream, indicated by the value of the fs_index AC-4 bit stream parameter as specified in ETSI TS

11 Latency Delay time of an external audio decoder to decode an AC-4 data burst, defined as the sum of two values; the receiving delay time and the decoding delay time Fractional frame rates AC-4 supports a number of fractional audio frame rates. These frame rates are written in shorthand notation, as specified in Table 1. Table 1 Shorthand notation for fractional frame rates Fractional AC-4 audio frame rate (fps) Shorthand version 24 x / , x / ,97 48 x / , x / , x / , / , / , Abbreviations ETSI European Telecommunication Standards Institute fps frames per second HBR High Bit Rate IEC International Electrotechnical Committee UIMSBF Unsigned integer, most significant bit first Mapping of the audio bit stream on to IEC General The coding of the bit stream and data-burst is in accordance with IEC , IEC :2007/A1 and IEC AC-4 burst-info The 16-bit burst-info contains information about the data which will be found in the data-burst (see Table 2).

12 136 Table 2 Fields of burst-info Bits of Pc Data type value (bits 0 4) Sub Data type value (bits 5 6) Contents Reference point R Repetition period of data-burst in IEC frames According to IEC AC-4 bit 0 of Pa See Table AC-4 HBR bit 0 of Pa 2, 3 Reserved According to IEC See Table 11 and Table According to IEC Format of AC-4 data-bursts 5.1 General This clause specifies the AC-4 audio data-burst. Specific properties such as reference points, repetition period, the method of filling stream gaps, and decoding latency are specified. The decoding latency (or delay), indicated for the data-type, should be used by the transmitter to schedule data-bursts as necessary to establish synchronization between picture and decoded audio. 5.2 Pause data-burst The Pause data-burst for AC-4 is given in Table 3 Table 3 Repetition period of the Pause data-bursts Data-type of audio data-burst Repetition period of pause data-burst Mandatory Recommended AC-4-3 AC-4 HBR Audio data-burst The AC-4 data An AC-4 bit stream consists of a sequence of AC-4 frames. The AC-4 data-burst is headed with a burst-preamble, followed by the burst-payload. The structure of the AC-4 data-burst is shown in Figure 1. The data-type of an AC-4 data-burst is 24 [pending confirmation in ], and the sub data type is 0. When AC-4 data is being transmitted, the transmission device shall ensure that both the data-type and sub data-type values are set correctly. Additionally, the receiving device shall utilize both the data-type and sub data-type values to ensure that the content of the data-burst is correctly identified as AC-4. Pa Pb Pc Pd AC-4 burst-payload Pa Pb Pc Pd AC-4 burst-payload Bit 0 of Pa AC-4 frame Stuffing Bit 0 of Pa AC-4 frame Stuffing Regular rate of the AC-4 frames Repetition rate of the data-burst Figure 1 AC-4 data-burst

13 The data-type dependent information for AC-4 is specified in Table 4. Table 4 Data-type-dependent information for AC-4 Bits of Pc LSB..MSB Value Meaning 8 11 See Table 7 and Table 8 12 Reserved Repetition period of the data-burst in IEC frames The AC-4 burst-payload shall always contain a single AC-4 frame. The transmission device shall ensure that the AC-4 burst-payload is constructed only from a single complete AC-4 frame. It is prohibited to transmit a single AC-4 frame using multiple data-bursts. The length of the AC-4 data-burst will depend on the encoded bit rate (which determines the AC-4 frame length). The specification for the AC-4 bit stream may be found in ETSI document TS The reference point of an AC-4 data-burst is bit 0 of Pa. The repetition period of the AC-4 databurst shall be defined by the base sampling frequency and frame rate of the AC-4 audio bit stream. The IEC frame rate shall be equal to the base sampling frequency of the AC-4 bit stream. As AC-4 supports multiple audio frame rates that match commonly used video frame rates, the transmission device shall ensure that the selected AC-4 data-burst repetition rate is equal to the duration of the AC-4 frame, as specified in Table 5. Table 5 AC-4 base sampling frequency, AC-4 audio frame rate and corresponding AC-4 data-burst repetition period AC-4 base sampling frequency 48 khz AC-4 audio frame rate (fps) AC-4 data-burst repetition period in IEC frames 23, , / (see Table 6) , , / 801 (see Table 6) , / 401 (see Table 6) , ,1 khz 21, For AC-4 audio frame rates of 29,97, 59,94 and 119,88 fps, the duration of an AC-4 audio frame does not correspond to an integer number of IEC frames. To ensure that precise time alignment is maintained between the AC-4 data-burst and the AC-4 audio frames at these frame rates, the repetition period of data-bursts varies so that over a sequence of 5 data bursts, the AC-4 data-bursts are time-aligned with the corresponding 5 audio frames, as specified in Table 6.

14 Table 6 AC-4 data-burst sequence and repetition period variance to ensure AC-4 audio frame alignment at 29,97, 59,94 and 119,88 fps AC-4 audio frame rate Repetition periods for AC-4 data-burst sequence in IEC frames Data-burst 0 Data-burst 1 Data-burst 2 Data-burst 3 Data-burst 4 29,97 fps ,94 fps ,88 fps The value of bits 8 to 11 Pc shall indicate the repetition period of the AC-4 data-burst. The interpretation of these bits is dependent on the base sampling frequency of the AC-4 bit stream and the IEC frame rate, as specified in Table 7 and Table 8. Table 7 Meaning of Pc bits 8 11 at an IEC frame rate of 48 khz Value Meaning / (see Table 6) Repetition period of the AC-4 data-burst in IEC frames / 801 (see Table 6) / 401 (see Table 6) , 15 Reserved 185 Table 8 Meaning of Pc bits 8 11 at an IEC frame rate of 44.1 khz Value Meaning 0 12 Reserved 13 Repetition period of the AC-4 data-burst in IEC frames 14, 15 Reserved The units of burst-length shall be in bytes. The maximum size of an AC-4 burst-payload is dependent on the repetition rate of the data-burst, and is specified in Table 9. The maximum burst-length values shown in Table 9 assume a provision for two IEC frames for padding between bursts.

15 190 Table 9 Maximum burst-length values per AC-4 repetition period Encoded AC-4 base sampling rate / IEC frame rate Repetition period of the AC-4 data-burst in IEC frames Maximum burst-length of the AC-4 data-burst in bytes / / khz / / / / ,1 khz Latency of the AC-4 decoder The latency of an AC-4 decoder is defined as the sum of the receiving delay time and decoding delay time. The receiving delay time is the time taken to receive the complete AC-4 burst-payload, and is dependent on the encoded bitrate and audio frame rate of the AC-4 bit stream. For the purposes of maintaining synchronization with video, it is recommended that a constant value of receiving delay time for each audio frame rate and corresponding repetition period is assumed. This value is calculated based on the maximum possible size of an AC-4 burst-payload, and is equal to the time occupied by the duration of the AC-4 data-burst in IEC frames at the IEC frame rate. The decoding delay time is equal to the time occupied by PCM samples at the AC-4 base sampling frequency. Pa Pb Pc Pd AC-4 burstpayload Pa Pb Pc Pd AC-4 burstpayload Pa Pb Pc Pd AC-4 burstpayload Bit 0 of Pa AC-4 frame Stuffing Bit 0 of Pa AC-4 frame Stuffing Bit 0 of Pa AC-4 frame Stuffing Regular rate of the AC-4 frames Repetition rate of the data-burst Latency of the AC-4 decoder Figure 2 Latency of AC-4 decoding It is recommended that Pause data-bursts are used to fill stream gaps in the AC-4 bit stream as described in IEC , and that pause data-bursts be transmitted with a repetition period of 3 IEC frames, except when other repetition periods are necessary to fill the precise

16 stream gap length (which may not be a multiple of 3 IEC frames), or to meet the requirement on burst spacing (refer to IEC , 6.3.3). When a stream gap in an AC-4 stream is filled by a sequence of pause data-bursts, the Pa of the first pause data-burst shall be located one frame repetition period following the Pa of the previous AC-4 frame. It is recommended that the sequence(s) of Pause data-bursts which fill the stream gap should continue from this point up to (as close as possible considering the 3 IEC frame length of the Pause data-burst) the Pa of the first AC-4 data-burst which follows the stream gap. The gap length parameter contained in the pause data-burst is intended to be interpreted by the AC-4 decoder as an indication of the number of decoded PCM samples which are missing (due to the resulting audio gap). The gap-length parameter is intended to indicate the number of decoded PCM audio samples that are missing (due to the resulting audio gap). Some AC-4 decoders may be capable of concealing audio gaps. The indication of the audio gap length (gap-length) which may be included in the payload of the pause data-burst allows the decoder to know how long an audio gap will need to be concealed, and thus allow the decoder to optimize the concealment process for the actual audio gap length. AC-4 decoders will most easily conceal audio gaps that have a length equal to an audio frame. Thus, audio gap-length values that match the duration of the AC-4 audio frame rate (2 002, 2 000, 1 920, etc. IEC frames) are strongly preferred, and transmitters should provide stream gaps that represent audio gaps with this granularity The AC-4 HBR data When the required transmission rate for AC-4 data exceeds the maximum data rate supported by the AC-4 data burst, the AC-4 HBR data used instead. An AC-4 HBR bit stream consists of a sequence of AC-4 frames. The AC-4 HBR data-burst is headed with a burst-preamble, followed by the burst-payload. The structure of the AC-4 HBR data-burst is shown in Figure 1. The data-type of an AC-4 HBR data-burst is 24, and the sub data type is 1. When AC-4 HBR data is being transmitted, the transmission device shall ensure that both the data-type and sub data-type values are set correctly. Additionally, the receiving device shall utilize both the datatype and sub data-type values to ensure that the content of the data-burst is correctly identified as AC-4 HBR. Pa Pb Pc Pd AC-4 HBR burst-payload Pa Pb Pc Pd AC-4 HBR burst-payload Bit 0 of Pa AC-4 frame Stuffing Bit 0 of Pa AC-4 frame Stuffing Regular rate of the AC-4 frames Repetition rate of the data-burst Figure 3 AC-4 HBR data-burst The data-type dependent information for AC-4 HBR is specified in Table 10.

17 241 Table 10 Data-type-dependent information for AC-4 HBR Bits of Pc LSB..MSB Value See Table 14, Table 15, Table 16 and Table Meaning Repetition period of the data-burst in IEC frames IEC frame rate = 4 x AC-4 base sampling frequency IEC frame rate = 16 x AC-4 base sampling frequency The AC-4 HBR burst-payload shall always contain a single AC-4 frame. The transmission device shall ensure that the AC-4 HBR burst-payload is constructed only from a single complete AC-4 frame. It is prohibited to transmit a single AC-4 frame using multiple data-bursts. The length of the AC-4 HBR data-burst will depend on the encoded bit rate (which determines the AC-4 frame length). The specification for the AC-4 bit stream may be found in ETSI document TS The reference point of an AC-4 data-burst is bit 0 of Pa. The repetition period of the AC-4 databurst shall be defined by the base sampling frequency and frame rate of the AC-4 audio bit stream. When bit 12 of Pc is set to 0, the IEC frame rate shall be four times the base sampling frequency of the AC-4 bit stream. When bit 12 of Pc is set to 1, the IEC frame rate shall be sixteen times the base sampling frequency of the AC-4 bit stream. As AC-4 supports multiple audio frame rates that match commonly used video frame rates, the transmission device shall ensure that the selected AC-4 HBR data-burst repetition rate is equal to the duration of the AC-4 frame, as specified in Table 11 and Table 12. Table 11 AC-4 audio frame rate and corresponding AC-4 HBR data-burst repetition period when bit 12 of Pc = 0 AC-4 base sampling frequency IEC frame rate 48 khz 192 khz AC-4 audio frame rate (fps) AC-4 HBR data-burst repetition period in IEC frames 23, , / , , / , / , ,1 khz 176,4 khz 21,

18 Table 12 AC-4 audio frame rate and corresponding AC-4 HBR data-burst repetition period when bit 12 of Pc = 1 AC-4 base sampling frequency IEC frame rate 48 khz 768 khz AC-4 audio frame rate (fps) AC-4 HBR data-burst repetition period in IEC frames 23, , / , , / , / , ,1 khz 705,6 khz 21, For AC-4 audio frame rates of 29,97, 59,94 and 119,88 fps, the duration of an AC-4 audio frame does not correspond to an integer number of IEC frames. To ensure that precise time alignment is maintained between the AC-4 HBR data-burst and the AC-4 audio frames at these frame rates, the repetition period of data-bursts varies so that over a sequence of 5 data bursts, the AC-4 HBR data-bursts are time-aligned with the corresponding 5 audio frames, as specified in Table 13. Table 13 AC-4 HBR data-burst sequence and repetition period variance to ensure AC-4 audio frame alignment at 29,97, 59,94 and 119,88 fps IEC frame rate 192 khz 768 khz AC-4 audio frame rate (fps) Repetition periods for AC-4 HBR data-burst sequence in IEC frames Data-burst 0 Data-burst 1 Data-burst 2 Data-burst 3 Data-burst 4 29, , , , , , The value of bits 8 to 11 Pc shall indicate the repetition period of the AC-4 HBR data-burst. The interpretation of these bits is dependent on the base sampling rate of the AC-4 bit stream, bit 12 of Pc, and the IEC frame rate, as specified in Table 14, Table 15, Table 16 and Table 17.

19 Table 14 AC-4 HBR data-burst: Meaning of Pc bits 8 11 at an IEC frame rate of 192 khz (bit 12 of Pc = 0) Value Meaning / (see Table 13) Repetition period of the AC-4 data-burst in IEC frames / (see Table 13) / (see Table 13) , 15 Reserved Table 15 AC-4 HBR data-burst: Meaning of Pc bits 8 11 at an IEC frame rate of 176,4 khz (bit 12 of Pc = 0) Value Meaning 0 12 Reserved 13 Repetition period of the AC-4 data-burst in IEC frames 14, 15 Reserved Table 16 AC-4 HBR data-burst: Meaning of Pc bits 8 11 at an IEC frame rate of 768 khz (bit 12 of Pc = 1) Value Meaning / (see Table 13) Repetition period of the AC-4 data-burst in IEC frames / (see Table 13) / (see Table 13) , 15 Reserved

20 Table 17 AC-4 HBR data-burst: Meaning of Pc bits 8 11 at an IEC frame rate of 705,6 khz (bit 12 of Pc = 1) Value Meaning 0 12 Reserved 13 Repetition period of the AC-4 data-burst in IEC frames 14, 15 Reserved The units of burst-length shall be in 8-byte units. If the AC-4 frame is not equal to an integer number of 8-byte units, the remaining bytes of the AC-4 HBR burst-payload shall be set to zero. The maximum size of an AC-4 burst-payload is dependent on the repetition rate of the databurst, and is specified in Table 18. The maximum burst-length values shown in Table 18 assume a provision for two IEC frames for padding between bursts.

21 285 Table 18 Maximum burst-length values per AC-4 HBR data-burst repetition period AC-4 base sampling rate IEC frame rate Repetition period of the AC-4 HBR databurst in IEC frames Maximum burstlength of the AC-4 HBR data-burst in 8-byte units / / khz / / / / khz / / khz / / / / ,1 khz 176,4 khz ,6 khz Latency of the AC-4 HBR decoder The latency of an AC-4 HBR decoder is defined as the sum of the receiving delay time and decoding delay time. The receiving delay time is the time taken to receive the complete AC-4 HBR burst-payload, and is dependent on the encoded bitrate and audio frame rate of the AC-4 bit stream. For the purposes of maintaining synchronization with video, it is recommended that a constant value of receiving delay time for each repetition period be assumed. This value is calculated based on the maximum possible size of an AC-4 HBR burst-payload, and is equal to the time occupied by the duration of the AC-4 HBR data-burst in IEC frames at the IEC frame rate.

22 The decoding delay time is equal to the time occupied by PCM samples at the AC-4 base sampling frequency, which corresponds to IEC frames when bit 12 of Pc = 0, and IEC frames when bit 12 of Pc = 1. Pa Pb Pc Pd AC-4 HBR burst-payload Pa Pb Pc Pd AC-4 HBR burst-payload Pa Pb Pc Pd AC-4 HBR burst-payload Bit 0 of Pa AC-4 frame Stuffing Bit 0 of Pa AC-4 frame Stuffing Bit 0 of Pa AC-4 frame Stuffing Regular rate of the AC-4 frames Repetition rate of the data-burst Latency of the AC-4 HBR decoder Figure 4 Latency of AC-4 decoding It is recommended that Pause data-bursts are used to fill stream gaps in the AC-4 HBR bit stream as described in IEC , and that pause data-bursts be transmitted with a repetition period of 4 IEC frames, except when other repetition periods are necessary to fill the precise stream gap length (which may not be a multiple of 4 IEC frames), or to meet the requirement on burst spacing (refer to IEC , 6.3.3). When a stream gap in an AC-4 HBR bit stream is filled by a sequence of pause data-bursts, the Pa of the first pause data-burst shall be located one frame repetition period following the Pa of the previous AC-4 frame. It is recommended that the sequence(s) of Pause data-bursts which fill the stream gap should continue from this point up to (as close as possible considering the 4 IEC frame length of the Pause data-burst) the Pa of the first AC-4 HBR data-burst which follows the stream gap. The gap length parameter contained in the pause data-burst is intended to be interpreted by the AC-4 decoder as an indication of the number of decoded PCM samples which are missing (due to the resulting audio gap). The gap-length parameter is intended to indicate the number of decoded PCM audio samples that are missing (due to the resulting audio gap). Some AC-4 decoders may be capable of concealing audio gaps. The indication of the audio gap length (gap-length) which may be included in the payload of the pause data-burst allows the decoder to know how long an audio gap will need to be concealed, and thus allow the decoder to optimize the concealment process for the actual audio gap length. AC-4 decoders will most easily conceal audio gaps that have a length equal to an audio frame. Thus, audio gap-length values that match the duration of the AC-4 audio frame rate (8 008, 8 000, , etc. IEC frames) are strongly preferred, and transmitters should provide stream gaps that represent audio gaps with this granularity.

23 323 Annex A A.1 AC-4 sync frame Figure A.1 shows how synchronization, frame size and CRC fields are added to a raw AC-4 frame, as defined in ETSI TS , to create an AC-4 sync frame. Each AC-4 or AC-4 HBR burst-payload contains a single AC-4 sync frame. AC4SyncFrame syncword framesize RawAC4Frame CRC Figure A.1 An AC-4 sync frame The syntax of the AC-4 sync frame shall be as specified in Table A.1. Table A.1 AC-4 sync frame syntax Syntax No. of Bits Identifier ac4syncframe() { syncword 16 uimsbf framesize() RawAC4Frame() if (syncword == 0xAC41 ) { crc_word 16 uimsbf } } The value of the framesize field shall be equal to the size of the RawAC4Frame in bytes. The function for deriving the frame size of the RawAC4Frame in bytes is given in Table 26 below. Table A.2 framesize() Syntax No. of Bits Identifier framesize() { size 16 uimsbf if (size == 0xFFFF ) { size 24 uimsbf } return size } The 16-bit CRC word is optional for AC-4. Its presence shall be determined by the value of the syncword.

24 340 Table A.3 AC-4 sync word values Value 0xAC41 0xAC40 Description CRC word is present CRC word is not present The following CRC generator polynomial shall be used to generate the CRC word: x 16 + x 15 + x The CRC calculation on the decoder side may be implemented by one of several standard techniques, e.g. by a linear feedback shift register (LFSR). A CRC-protected AC-4 sync frame is considered valid if the register contains all zeros after the complete data of the AC-4 sync frame, excluding the syncword (but including the framesize), has been shifted in. 350