Digital audio - Interface for non-linear PCM encoded audio bitstreams applying IEC 60958 - Part 14: Non-linear PCM bitstreams according to the AC-4 format

IEC 61937-14:2017(E) describes the method to convey non-linear PCM bit streams encoded according to the AC-4 format.

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IEC 61937-14
Edition 1.0 2017-08
INTERNATIONAL
STANDARD
Digital audio – Interface for non-linear pcm encoded audio bitstreams applying
IEC 60958 –
Part 14: Non-linear PCM bit streams according to the AC-4 format
IEC 61937-14:2017-08(en)
---------------------- Page: 1 ----------------------
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IEC 61937-14
Edition 1.0 2017-08
INTERNATIONAL
STANDARD
Digital audio – Interface for non-linear pcm encoded audio bitstreams applying
IEC 60958 –
Part 14: Non-linear PCM bit streams according to the AC-4 format
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 33.160.30; 33.170 ISBN 978-2-8322-4705-1

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC 61937-14:2017 © IEC 2017
CONTENTS

FOREWORD ........................................................................................................................... 4

1 Scope .............................................................................................................................. 6

2 Normative references ...................................................................................................... 6

3 Terms, definitions and abbreviated terms ........................................................................ 6

3.6 Abbreviated terms ................................................................................................... 7

4 Mapping of the audio bit stream on to IEC 61937-1 ......................................................... 7

4.1 General ................................................................................................................... 7

4.2 AC-4 burst-info ....................................................................................................... 7

5 Format of AC-4 data-bursts ............................................................................................. 8

5.1 General ................................................................................................................... 8

5.2 Pause data-burst .................................................................................................... 8

5.3 Audio data-burst ..................................................................................................... 8

5.3.1 The AC-4 data ................................................................................................. 8

5.3.2 Latency of the AC-4 decoder ......................................................................... 12

5.3.3 The AC-4 HBR4 data ..................................................................................... 13

5.3.4 Latency of the AC-4 HBR4 decoder ............................................................... 16

5.3.5 The AC-4 HBR16 data ................................................................................... 17

5.3.6 Latency of the AC-4 HBR16 decoder ............................................................. 20

5.3.7 The AC-4 LD data .......................................................................................... 21

5.3.8 Latency of the AC-4 LD decoder .................................................................... 23

Annex A (normative) AC-4 sync frame ................................................................................. 25

Figure 1 – AC-4 data-burst ...................................................................................................... 9

Figure 2 – Latency of AC-4 decoding .................................................................................... 12

Figure 3 – AC-4 HBR4 data-burst ......................................................................................... 13

Figure 4 – Latency of AC-4 HBR4 decoding .......................................................................... 16

Figure 5 – AC-4 HBR16 data-burst........................................................................................ 17

Figure 6 – Latency of AC-4 HBR16 decoding ........................................................................ 21

Figure 7 – AC-4 LD data-burst .............................................................................................. 22

Figure 8 – Latency of AC-4 LD decoding ............................................................................... 24

Figure A.1 – An AC-4 sync frame .......................................................................................... 25

Table 1 – Shorthand notation for fractional frame rates .......................................................... 7

Table 2 – Fields of burst-info .................................................................................................. 8

Table 3 – Repetition period of the Pause data-bursts .............................................................. 8

Table 4 – Data-type-dependent information for AC-4............................................................... 9

Table 5 – AC-4 base sampling frequency, AC-4 audio framerate and corresponding

AC-4 data-burst repetition period .......................................................................................... 10

Table 6 – AC-4 data-burst sequence and repetition period variance at 29,97, 59,94

and 119,88 fps ...................................................................................................................... 10

Table 7 – AC-4 data-burst: meaning of Pc bits 8 to 11 at an IEC 60958 frame rate of

48 kHz .................................................................................................................................. 11

Table 8 – AC-4 data-burst: meaning of Pc bits 8 to 11 at an IEC 60958 frame rate of

44,1 kHz ............................................................................................................................... 11

Table 9 – Maximum burst-length values per AC-4 data-burst repetition period ..................... 12

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IEC 61937-14:2017 © IEC 2017 – 3 –

Table 10 – Data-type-dependent information for AC-4 HBR4................................................. 13

Table 11 – AC-4 base sampling frequency, AC-4 audio frame rate and corresponding

AC-4 HBR4 data-burst repetition period ................................................................................ 14

Table 12 – AC-4 HBR4 data-burst sequence and repetition period variance at 29,97,

59,94 and 119,88 fps ............................................................................................................ 14

Table 13 – AC-4 HBR4 data-burst: meaning of Pc bits 8 to 11 at an IEC 60958 frame

rate of 192 kHz ..................................................................................................................... 15

Table 14 – AC-4 HBR4 data-burst: meaning of Pc bits 8 to 11 at an IEC 60958 frame

rate of 176,4 kHz .................................................................................................................. 15

Table 15 – Maximum burst-length values per AC-4 HBR4 data-burst repetition period ......... 16

Table 16 – Data-type-dependent information for AC-4 HBR16 ............................................... 18

Table 17 – AC-4 base sampling frequency, AC-4 audio frame rate and corresponding

AC-4 HBR16 data-burst repetition period .............................................................................. 18

Table 18 – AC-4 HBR16 data-burst sequence and repetition period variance at 29,97,

59,94 and 119,88 fps ............................................................................................................ 19

Table 19 – AC-4 HBR16 data-burst: meaning of Pc bits 8 – 11 at an IEC 60958 frame

rate of 768 kHz ..................................................................................................................... 19

Table 20 – AC-4 HBR16 data-burst: meaning of Pc bits 8 to 11 at an IEC 60958 frame

rate of 705,6 kHz .................................................................................................................. 19

Table 21 – Maximum burst-length values per AC-4 HBR16 data-burst repetition period ....... 20

Table 22 – Data-type-dependent information for AC-4 LD ..................................................... 22

Table 23 – AC-4 base sampling frequency, AC-4 audio frame rate and corresponding

AC-4 LD data-burst repetition period .................................................................................... 22

Table 24 – AC-4 LD data-burst sequence and repetition period variance at 119,88 fps ........ 23

Table 25 – AC-4 LD data-burst: meaning of Pc bits 8 to 11 ................................................... 23

Table 26 – Maximum burst-length values per AC-4 data-burst repetition period ................... 23

Table A.1 – AC-4 sync frame syntax ..................................................................................... 25

Table A.2 – framesize() ......................................................................................................... 25

Table A.3 – AC-4 syncword values ........................................................................................ 26

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– 4 – IEC 61937-14:2017 © IEC 2017
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DIGITAL AUDIO –
INTERFACE FOR NON-LINEAR PCM ENCODED AUDIO BITSTREAMS
APPLYING IEC 60958 –
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

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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.

International Standard IEC 61937-14 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 International Standard is based on the following documents:
CDV Report on voting
100/2723/CDV 100/2932/RVC

Full information on the voting for the approval of this International Standard can be found in

the report on voting indicated in the above table.

This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

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IEC 61937-14:2017 © IEC 2017 – 5 –

A list of all parts of IEC 61937, under the general title Digital audio – Interface for non-linear

PCM encoded audio bitstreams applying IEC 60958 can be found on the IEC website.

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.
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– 6 – IEC 61937-14:2017 © IEC 2017
DIGITAL AUDIO –
INTERFACE FOR NON-LINEAR PCM ENCODED AUDIO BITSTREAMS
APPLYING IEC 60958 –
Part 14: Non-linear PCM bit streams according to the AC-4 format
1 Scope

This part of IEC 61937 describes the method to convey non-linear PCM bit streams encoded

according to the AC-4 format.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their

content constitutes requirements 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 60958 (all parts), Digital audio interface

IEC 61937-1:2007, Digital audio interface for non-linear PCM encoded audio bitstreams

applying IEC 60958 – Part 1: General

IEC 61937-1:2007/AMD1:2011, Digital audio interface for non-linear PCM encoded audio

bitstreams applying IEC 60958 – Part 1: General

IEC 61937-2, Digital audio interface for non-linear PCM encoded audio bitstreams applying

IEC 60958 – Part 2: Burst Information
ETSI TS 103 190 v.1.1.1, 2014, Digital Audio Compression (AC-4) Standard
3 Terms, definitions and abbreviated terms

For the purpose of this standard, the following definitions and abbreviations apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
AC-4 frame
AC-4 sync frame, as specified in Annex A
3.2
audio frame rate

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 103 190
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IEC 61937-14:2017 © IEC 2017 – 7 –
3.3
base sampling frequency

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 103 190

Note 1 to entry: AC-4 supports base sampling frequencies of 44.1 and 48 kHz only.

3.4
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
3.5
fractional frame rates
fractional audio frame rates supported by AC-4

Note 1 to entry: 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 Shorthand version
(fps)
1 000 23,976
24 × �
1 001
1 000 29,97
30 × �
1 001
1 000 47,952
48 × �
1 001
1 000 59,94
60 × �
1 001
1 000 119,88
120 × �
1 001
12 000
23,438
512
11 025
21,533
512
3.6 Abbreviated terms
ETSI European Telecommunication Standards Institute
fps frames per second
HBR high bit rate
IEC International Electrotechnical Commission
LD low delay
UIMSBF unsigned integer, most significant bit first
4 Mapping of the audio bit stream on to IEC 61937-1
4.1 General

The coding of the bit stream and data-burst is in accordance with IEC 61937-1, IEC 61937-

1:2007/AMD1 and IEC 61937-2, including field names such as "Pc", "Pa" and "R".
4.2 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).
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– 8 – IEC 61937-14:2017 © IEC 2017
Table 2 – Fields of burst-info
Data-type Data-type Repetition period of
Bits of Reference
value value Contents data-burst in
Pc point R
(bits 0 – 4) (bits 5 – 6) IEC 60958 frames
1 – 23 According to IEC 61937-2
0 AC-4 bit 0 of Pa See Table 4
1 AC-4 HBR4 bit 0 of Pa See Table 10
0 – 6 24
2 AC-4 HBR16 bit 0 of Pa See Table 16
3 AC-4 LD bit 0 of Pa See Table 22
25 – 31 According to IEC 61937-2
7 – 15 According to IEC 61937-1
5 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
Repetition period of pause data-burst
Data-type of audio data-burst
Mandatory Recommended
AC-4 - 3
AC-4 HBR4 - 4
AC-4 HBR16 - 4
AC-4 LD - 3
5.3 Audio data-burst
5.3.1 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 bits 0-4 of an AC-4 data-burst is 24, and the data-type bits

5-6 is 0. When AC-4 data is being transmitted, the transmission device shall ensure that both

the data-type bits 0-4 and data-type bits 5-6 values are set correctly. Additionally, the

receiving device shall utilize both the data-type bits 0-4 and data-type bits 5-6 values to

ensure that the content of the data-burst is correctly identified as AC-4.
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IEC 61937-14:2017 © IEC 2017 – 9 –
IEC
Figure 1 – AC-4 data-burst
The data-type bits 0-4 dependent information for AC-4 is specified in Table 4.
Table 4 – Data-type-dependent information for AC-4
Bits of Pc
Value Meaning
LSB..MSB
See Table 7 Repetition period of the data-burst in
8 – 11
and Table 8 IEC 60958 frames
12 Reserved

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 AC-4 bit stream is specified in ETSI TS 103 190.

The reference point of an AC-4 data-burst is bit 0 of Pa. The repetition period of the AC-4

data-burst shall be defined by the base sampling frequency (indicated by the value of the

fs_index AC-4 bit stream parameter as specified in ETSI TS 103 190) and frame rate

(indicated by the value of the frame_rate_index AC-4 bit stream parameter as specified in

ETSI TS 103 190) of the AC-4 audio bit stream. The IEC 60958 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.
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– 10 – IEC 61937-14:2017 © IEC 2017
Table 5 – AC-4 base sampling frequency, AC-4 audio framerate and
corresponding AC-4 data-burst repetition period
AC-4 base AC-4 audio frame AC-4 data-burst repetition period
sampling rate in IEC 60958 frames
frequency (fps)
23,976 2 002
24 2 000
25 1 920
29,97 1 601 / 1 602 (see Table 6)
30 1 600
47,952 1 001
48 1 000
48 kHz
50 960
59,94 800 / 801 (see Table 6)
60 800
100 480
119,88 400 / 401 (see Table 6)
120 400
23,438 2 048
44,1 kHz 21,533 2 048

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 60958 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, five AC-4 data-bursts are time-aligned with the corresponding 5 audio frames,

0…4
as specified in Table 6.
Table 6 – AC-4 data-burst sequence and repetition period variance
at 29,97, 59,94 and 119,88 fps
Repetition periods for AC-4 data-burst sequence in IEC 60958 frames
AC-4 audio
frame rate
Data-burst Data-burst Data-burst Data-burst Data-burst
0 1 2 3 4
29,97 fps 1 602 1 601 1 602 1 601 1 602
59,94 fps 801 801 800 801 801
119,88 fps 400 401 400 401 400

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 60958 frame rate, as specified in Table 7 and Table 8.
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IEC 61937-14:2017 © IEC 2017 – 11 –
Table 7 – AC-4 data-burst: meaning of Pc bits 8 to 11 at an
IEC 60958 frame rate of 48 kHz
Value Meaning
0 2 002
1 2 000
2 1 920
3 1 601 / 1 602 (see Table 6)
4 1 600
5 1 001
Repetition period of
6 1 000
the AC-4 data-burst in
7 960
IEC 60958 frames
8 800 / 801 (see Table 6)
9 800
10 480
11 400 / 401 (see Table 6)
12 400
13 2 048
14, 15 Reserved
Table 8 – AC-4 data-burst: meaning of Pc bits 8 to 11 at an
IEC 60958 frame rate of 44,1 kHz
Value Meaning
0 – 12 Reserved
Repetition period of
13 the AC-4 data-burst in 2 048
IEC 60958 frames
14, 15 Reserved

The units of burst-length shall be in bits. 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 60958 frames for

padding between bursts.
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– 12 – IEC 61937-14:2017 © IEC 2017
Table 9 – Maximum burst-length values per AC-4
data-burst repetition period
Encoded AC-4 base Repetition period of the Maximum burst-length
sampling frequency / AC-4 data-burst in of the AC-4 data-burst in
IEC 60958 frame rate IEC 60958 frames bits
2 002 63 936
2 000 63 872
1 920 61 312
1 601 / 1 602 51 104 / 51 136
1 600 51 072
1 001 31 904
1 000 31 872
48 kHz
960 30 592
800 / 801 25 472 / 25 504
800 25 472
480 15 232
400 / 401 12 672 / 12 704
400 12 672
2 048 65 408
44,1 kHz 2 048 65 408
5.3.2 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 60958 frames at the IEC 60958 frame rate.

The decoding delay time is equal to the time occupied by 3 195 PCM samples at the AC-4

base sampling frequency (see Figure 2).
IEC
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 61937-1, and that pause data-bursts be transmitted with a repetition

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IEC 61937-14:2017 © IEC 2017 – 13 –

period of 3 IEC 60958 frames, except when other repetition periods are necessary to fill the

precise stream gap length (which may not be a multiple of 3 IEC 60958 frames), or to meet

the requirement on burst spacing (refer to IEC 61937-1:2007, 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 data-burst repetition period following the Pa of

the previous AC-4 frame. It is recommended that the sequence(s) of pause data-bursts that fill

the stream gap should continue from this point up to (as close as possible considering the 3

IEC 60958 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 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 60958 frames) are strongly preferred, and
...

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