Digital audio - Interface for non-linear PCM encoded audio bitstreams applying IEC 60958 - Part 15: Non-linear PCM bit streams according to Auro-Cx format

IEC 61937-15:2021 describes the method to convey non-linear PCM bit streams in accordance with the Auro-Cx format.

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IEC 61937-15
Edition 1.0 2021-03
INTERNATIONAL
STANDARD
Digital audio – Interface for non-linear PCM encoded audio bitstreams applying
IEC 60958 –
Part 15: Non-linear PCM bit streams according to Auro-Cx format
IEC 61937-15:2021-03(en)
---------------------- Page: 1 ----------------------
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IEC 61937-15
Edition 1.0 2021-03
INTERNATIONAL
STANDARD
Digital audio – Interface for non-linear PCM encoded audio bitstreams applying
IEC 60958 –
Part 15: Non-linear PCM bit streams according to Auro-Cx format
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 35.040.40 ISBN 978-2-8322-9526-7

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

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC 61937-15:2021 © IEC 2021
CONTENTS

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

INTRODUCTION ..................................................................................................................... 6

1 Scope .............................................................................................................................. 7

2 Normative references ...................................................................................................... 7

3 Terms, definitions and abbreviated terms ........................................................................ 7

3.1 Terms and definitions .............................................................................................. 7

3.2 Abbreviated terms ................................................................................................... 8

4 Mapping of the audio bit stream on to IEC 61937-1 ......................................................... 8

4.1 General ................................................................................................................... 8

4.2 Auro-Cx burst-info ................................................................................................... 8

5 Format of Auro-Cx data-bursts ......................................................................................... 9

5.1 General ................................................................................................................... 9

5.2 Pause data-burst .................................................................................................... 9

5.3 Audio data-bursts .................................................................................................. 10

5.3.1 ACX data-burst .............................................................................................. 10

5.3.2 Latency of the Auro-Cx Decoder .................................................................... 13

5.3.3 ACX HBR2 data-burst .................................................................................... 14

5.3.4 Latency of the ACX HBR2 Decoder ................................................................ 17

5.3.5 ACX HBR4 data-burst .................................................................................... 18

5.3.6 Latency of the ACX HBR4 Decoder ................................................................ 21

5.3.7 ACX HBR8 data-burst .................................................................................... 22

5.3.8 Latency of the ACX HBR8 Decoder ................................................................ 25

Bibliography .......................................................................................................................... 27

Figure 1 – ACX data-burst .................................................................................................... 10

Figure 2 – ACX Delay Time ................................................................................................... 14

Figure 3 – Auro-Cx HBR2 data-burst ..................................................................................... 14

Figure 4 – ACX HBR2 Delay Time......................................................................................... 18

Figure 5 – Auro-Cx HBR4 data-burst ..................................................................................... 18

Figure 6 – ACX HBR4 Delay time .......................................................................................... 22

Figure 7 – Auro-Cx HBR8 data-burst ..................................................................................... 22

Figure 8 – ACX HBR8 Delay Time......................................................................................... 26

Table 1 – Fractional frame rates ............................................................................................. 8

Table 2 – Burst-info for Auro-Cx data-type and subdata-types ................................................ 9

Table 3 – Repetition period of the Pause data-bursts .............................................................. 9

Table 4 – Data-type-dependent information for ACX ............................................................. 10

Table 5 – ACX data-burst: meaning of Pc bits 8 – 12 ............................................................ 11

Table 6 – ACX IEC 60958 frame rate for high sample rates .................................................. 12

Table 7 – ACX repetition periods for fractional frame rates 29,97 fps, 59,94 fps and

119,88 fps ............................................................................................................................. 12

Table 8 – Maximum burst-length values per ACX data-burst repetition period ....................... 13

Table 9 – Data-type-dependent information for ACX HBR2 ................................................... 14

Table 10 – ACX HBR2 data-burst: meaning of Pc bits 8 – 12 ................................................ 15

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IEC 61937-15:2021 © IEC 2021 – 3 –

Table 11 – ACX HBR2 IEC 60958 frame rate for high sample rates ...................................... 16

Table 12 – ACX HBR2 repetition periods for fractional frame rates 29,97 fps, 59,94 fps

and 119,88 fps ...................................................................................................................... 16

Table 13 – Maximum burst-length values per ACX HBR2 data-burst repetition period ........... 17

Table 14 – Data-type-dependent information for ACX HBR4 ................................................. 18

Table 15 – ACX HBR4 data-burst: meaning of Pc bits 8–12 .................................................. 19

Table 16 – ACX HBR4 IEC 60958 frame rate for high sample rates ...................................... 20

Table 17 – ACX HBR4 repetition periods for fractional frame rates 29,97 fps, 59,94 fps

and 119,88 fps ...................................................................................................................... 20

Table 18 – Maximum burst-length values per ACX HBR4 data-burst repetition period ........... 21

Table 19 – Data-type-dependent information for ACX HBR8 ................................................. 22

Table 20 – ACX HBR8 data-burst: meaning of Pc bits 8 – 12 ................................................ 23

Table 21 – ACX HBR8 IEC 60958 frame rate for high sample rates ...................................... 24

Table 22 – ACX HBR8 repetition periods for fractional frame rates 29,97 fps, 59,94 fps

and 119,88 fps ...................................................................................................................... 24

Table 23 – Maximum burst-length values per ACX HBR8 data-burst repetition period ........... 25

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– 4 – IEC 61937-15:2021 © IEC 2021
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DIGITAL AUDIO – INTERFACE FOR NON-LINEAR PCM
ENCODED AUDIO BITSTREAMS APPLYING IEC 60958 –
Part 15: Non-linear PCM bit streams according to Auro-Cx 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

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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.

IEC 61937-15 has been prepared by technical area 20: Analogue and digital audio, of IEC

technical committee 100: Audio, video and multimedia systems and equipment. It is an

International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
100/3462/CDV 100/3536/RVC

Full information on the voting for its approval can be found in the report on voting indicated in

the above table.
The language used for the development of this International Standard is English.

This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in

accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available

---------------------- Page: 6 ----------------------
IEC 61937-15:2021 © IEC 2021 – 5 –

at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are

described in greater detail at www.iec.ch/standardsdev/publications.

A list of all parts in the IEC 61937 series, published 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 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.
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– 6 – IEC 61937-15:2021 © IEC 2021
INTRODUCTION

The Auro-3D® format has brought immersive 3D audio to the digital cinema and consumer

electronics markets since 2011, and started a revolution in the audio industry, bringing the next

generation audio experience to the user.

To accommodate the shift towards the use of streaming using set-top boxes, smart TVs and

other connected devices, a new audio codec was developed: Auro-Cx® .

To be able to pass the Auro-Cx bit stream from said connected devices to an AV-receiver for

decoding via the IEC 60958 interface, the IEC 61937 series must support the Auro-Cx format.

—————————

Auro-3D is a trademark owned by Auro Technologies NV. This information is given for the convenience of users

of this document and does not constitute an endorsement by IEC of the product named. Equivalent products may

be used if they can be shown to lead to the same results.

Auro-Cx is the trademark of a product supplied by Auro Technologies NV. This information is given for the

convenience of users of this document and does not constitute an endorsement by IEC of the product named.

Equivalent products may be used if they can be shown to lead to the same results.

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IEC 61937-15:2021 © IEC 2021 – 7 –
DIGITAL AUDIO – INTERFACE FOR NON-LINEAR PCM
ENCODED AUDIO BITSTREAMS APPLYING IEC 60958 –
Part 15: Non-linear PCM bit streams according to Auro-Cx format
1 Scope

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

accordance with the Auro-Cx 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 61937-1:2021, Digital audio interface for non-linear PCM encoded audio bitstreams

applying IEC 60958 – Part 1: General

IEC 61937-2:2021, Digital audio interface for non-linear PCM encoded audio bitstreams

applying IEC 60958 – Part 2: Burst Information
3 Terms, definitions and abbreviated terms
For the purposes of this document, the following terms and definitions 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 Terms and definitions
3.1.1
Auro-Cx block

block containing all Auro-Cx audio data and metadata, formatted in accordance with the Auro-

Cx Bit Stream format
3.1.2
block size

number of samples represented by an Auro-Cx block, indicated by the value of the BlockSize

Auro-Cx bit stream parameter
3.1.3
latency

delay time introduced by an external Auro-Cx decoder while decoding an Auro-Cx data block,

defined as the sum of the receiving delay time and the decoding delay time
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– 8 – IEC 61937-15:2021 © IEC 2021
3.1.4
sample rate

sampling frequency of the linear PCM samples represented in the Auro-Cx bit stream, as

indicated by the value of the acxSampleRateIdx Auro-Cx bit stream parameter
3.1.5
Auro-Cx audio frame rate
number of Auro-Cx blocks per second
3.1.6
fractional frame rate

audio frame rates with non-integer values, which are written in shorthand notation as shown in

Table 1
Table 1 – Fractional frame rates
Fractional Auro-Cx frame rate in fps Shorthand notation
1 000
24 × / 23,976
1 001
1 000
30 × /
29,97
1 001
1 000
48 × /
47,972
1 001
1 000
60 × /
59,94
1 001
1 000
120 × /
119,88
1 001
3.2 Abbreviated terms
ACX Auro-Cx
fps frames per second
fs sampling frequency
HBR high bit rate
spb samples per block
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 and IEC 61937‑2.

4.2 Auro-Cx burst-info

The 16-bit burst-info contains information about the data that will be found in the data-burst

(see Table 2).
—————————
Auro-Cx supports sample rates up to 96 kHz.
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IEC 61937-15:2021 © IEC 2021 – 9 –
Table 2 – Burst-info for Auro-Cx data-type and subdata-types
Data type Subdata-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
0–25 According to IEC 61937-2
0 ACX bit 0 of Pa See Table 7
1 ACX HBR2 bit 0 of Pa See Table 12
0–6 26
2 ACX HBR4 bit 0 of Pa See Table 17
3 ACX HBR8 bit 0 of Pa See Table 22
27–31 According to IEC 61937-2
7–15 According to IEC 61937-1

The burst-info (Pc) bits 0-4 signal the data-type used for the transmission. For Auro-Cx, the

data-type is set to 26.

Pc bits 5-6 indicate the Auro-Cx bit stream mode. The repetition periods of data-bursts in

IEC 60958 frames shall be derived from the data-type-dependent information specified in

Table 4, Table 9, Table 14 and Table 19.
5 Format of Auro-Cx data-bursts
5.1 General

This clause specifies the Auro-Cx data-burst. Specific properties such as reference points,

repetition period, the method of filling stream gaps and decoding latency are specified.

To establish synchronisation between the picture and decoded audio, the transmitter should

use the latency indicated for the data-type to schedule data-bursts as necessary.

5.2 Pause data-burst
The Pause data-burst for Auro-Cx 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
ACX - 3
ACX HBR2 - 5
ACX HBR4 - 5
ACX HBR8 - 5

If regular audio data-bursts are not being transmitted because of a pause condition, for example,

it is recommended that Pause data-bursts are used to fill the stream gaps in the ACX bit stream,

as described in IEC 61937-1. The Pause data-bursts should be transmitted with a repetition

rate period in accordance with Table 3, 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).

When a stream gap in an ACX 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

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– 10 – IEC 61937-15:2021 © IEC 2021

the previous Auro-Cx block. 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 total IEC 60958 frame length of the Pause data-burst) the Pa of the first ACX data-burst

that follows the stream gap.

The gap length parameter contained in the Pause data-burst is intended to be interpreted by

the Auro-Cx decoder as an indication of the number of decoded PCM samples that are missing

(due to the resulting audio gap).
5.3 Audio data-bursts
5.3.1 ACX data-burst

An ACX bit stream consists of a sequence of Auro-Cx blocks. The ACX data-burst is headed

with a burst-preamble, followed by the burst-payload and stuffing. The structure of the ACX

data-burst is shown in Figure 1. The data-type of an ACX data-burst is 26 and the subdata-type

is 0. When transmitting Auro-Cx data, the transmitter needs to set both the data-type and

subdata-type correctly. The receiving device shall then use both values to make sure that the

content of the data-burst is identified as Auro-Cx.
Figure 1 – ACX data-burst
The data-type dependent information for Auro-Cx is specified in Table 4.
Table 4 – Data-type-dependent information for ACX
Bits of Pc
Value Meaning
LSB..MSB
Repetition period of the
8–12 See Table 5. data-burst in IEC 60958
frames

The ACX burst-payload always contains a single Auro-Cx block. The transmitter shall therefore

make sure that the ACX burst-payload is constructed from a single Auro-Cx block only. It is not

allowed to transmit a single Auro-Cx block using multiple data-bursts. The length of the Auro-

Cx data-burst will depend on the encoded block size.

The reference point R of an Auro-Cx data-burst is bit 0 of Pa. The block size of the Auro-Cx bit

stream defines the repetition period of the Auro-Cx data-burst. The IEC 60958 frame rate shall

be equal to the sample rate of the ACX bit stream. Because Auro-Cx supports multiple block

sizes that match commonly used video frame rates, the transmitter shall ensure that the

selected ACX data-burst repetition rate is equal to the block size, as specified in Table 5.

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IEC 61937-15:2021 © IEC 2021 – 11 –
Table 5 – ACX data-burst: meaning of Pc bits 8 – 12
Repetition period
Audio IEC 60958
of Auro-Cx data- Auro-Cx audio
Value sample rate frame rate in Auro-Cx block size
burst in IEC 60958 frame rate in fps
in kHz kHz
frames
0 256 256 187,5
1 400 400 120
400 / 402 400 / 402
2 119,88
(see Table 7) (see Table 7)
3 480 480 100
4 500 500 96
5 512 512 93,75
6 768 768 62,5
7 800 800 60
800 / 802 800 / 802
8 59,94
(see Table 7) (see Table 7)
9 960 960 50
10 1 000 1 000 48
11 1 001 1 001 47,952
48 48
12 1 024 1 024 46,875
13 1 536 1 536 31,25
14 1 600 1 600 30
1 600 / 1 602 1 600 / 1 602
15 29,97
(see Table 7) (see Table 7)
16 1 920 1 920 25
17 2 000 2 000 24
18 2 002 2 002 23,976
19 2 048 2 048 23,438
20 3 072 3 072 15,625
21 3 840 3 840 12,5
22 4 096 4 096 11,72
23–31 Reserved Reserved -
0 256 256 172,27
1–4 Reserved Reserved -
5 512 512 86,13
6 768 768 57,42
7–11 Reserved Reserved -
12 1 024 1 024 43,066
13 1 536 1 536 28,71
44,1 44,1
14–15 Reserved Reserved -
16 1 920 1 920 22,97
17–18 Reserved Reserved -
19 2 048 2 048 21,53
20 3 072 3 072 14,36
21 3 840 3 840 11,48
22 4 096 4 096 10,76
23–31 Reserved Reserved -

For higher audio sample rates (88,2 and 96kHz), the IEC 60958 frame rate and Auro-Cx audio

frame rate are multiplied by 2, as shown in Table 6.
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– 12 – IEC 61937-15:2021 © IEC 2021
Table 6 – ACX IEC 60958 frame rate for high sample rates
Audio sample rate in kHz IEC 60958 frame rate in kHz
88,2 88,2
96 96

The various Auro-Cx audio frame rates are defined to provide full synchronization with many

possible video frame rates. For most video frame rates higher than 30 fps, a one-to-one match

exists between the video frames and Auro-Cx blocks. However, for frame rates up to 30 fps,

two Auro-Cx blocks are transmitted for each video frame. Note that several block sizes (256 spb,

512 spb, 768 spb, 1 024 spb, 1 536 spb, 2 048 spb, 3 072 spb, 3 840 spb and 4 096 spb) are

typically used in audio-only applications.

For Auro-Cx audio frame rates of 29,97 fps, 59,94 fps and 119,88 fps, the duration of an

Auro-Cx block does not correspond to an integer number of IEC 60958 frames. To ensure that

precise time alignment is maintained between the Auro-Cx data-burst and the Auro-Cx audio

frames at these frame rates, the repetition period of data-bursts varies so that over a sequence

of 5 data bursts, the Auro-Cx data-bursts are time-aligned with the corresponding 5 audio

frames, as specified in Table 7.
Table 7 – ACX repetition periods for fractional frame
rates 29,97 fps, 59,94 fps and 119,88 fps
Auro-Cx audio Repetition period of ACX data-burst sequence in IEC 60958 frames
frame rate in
Data-burst 0 Data-burst 1 Data-burst 2 Data-burst 3 Data-burst 4
fps
29,97 1 602 1 602 1 600 1 602 1 602
59,94 800 802 800 802 800
119,88 400 400 402 400 400

The burst-length (Pd) shall be expressed in bytes with a maximum value determined from

2 × 16 bits × R − (P + B ), where
P ABCD S
R repetition period of data-bursts in IEC 60958 frames
P = 4 × 16 bits size of preamble words Pa-Pd
ABCD
B = 4 × 16 bits burst spacing

The maximum size of an Auro-Cx burst-payload is dependent on the repetition rate of the data-

burst and is specified in Table 8. The values shown assume a provision for two IEC 60958

frames for padding between bursts.
---------------------- Page: 14 ----------------------
IEC 61937-15:2021 © IEC 2021 – 13 –
Table 8 – Maximum burst-length values per ACX data-burst repetition period
Maximum bit-rate in
Repetition period of the Maximum burst-length
Mbps
Auro-Cx block size Auro-Cx data-burst in of the Auro-Cx data-
IEC 60958 frames burst in bytes
(fs = 48 kHz)
256 256 1 008 1,512
400 400 1 584 1,521
400 / 402 400 / 402 1 584 / 1 592 1,521
480 480 1 904 1,523
500 500 1 984 1,524
512 512 2 032 1,524
768 768 3 056 1,528
800 800 3 184 1,528
800 / 802 800 / 802 3 184 / 3 192 1,528
960 960 3 824 1,530
1 000 1 000 3 984 1,530
1 001 1 001 3 988 1,530
1 024 1 024 4 080 1,530
1 536 1 536 6 128 1,532
1 600 1 600 6 384 1,532
1 600 / 1 602 1 600 / 1 602 6 384 / 6 392 1,532
1 920 1 920 7 664 1,533
2 000 2 000 7 984 1,533
2 002 2 002 7 992 1,533
2 048 2 048 8 176 1,533
3 072 3 072 12 272 1,534
3 840 3 840 15 344 1,534
4 096 4 096 16 368 1,535
5.3.2 Latency of the Auro-Cx Decoder

The latency of the Auro-Cx decoder is defined as the sum of the receiving delay time and

decoding delay time, as shown in Figure 2.

The receiving delay time is the time required to receive the complete Auro-Cx burst-payload

and is dependent on the encoded bitrate and audio frame rate of the Auro-Cx bit stream. To

maintain synchronisation between audio and video, a constant value of receiving delay time is

recommended for each block size and corresponding repetition period. The value is calculated

using the size of an Auro-Cx burst-payload and is equal to the time occupied by the duration of

the Auro-Cx data-burst in IEC 60958 frames at the IEC 60958 frame rate.

The decoding delay time is equal to the time occupied by the number of PCM samples

represented by an Auro-Cx block at the Auro-Cx sample rate, as defined by the Auro-Cx block

size, increased with the time occupied by 256 PCM samples at the Auro-Cx sample rate,

represented by 256 IEC 60958 frames at the IEC 60958 frame rate.
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– 14 – IEC 61937-15:2021 © IEC 2021
Figure 2 – ACX Delay T
...

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