ISO/IEC 23003-4:2020/Amd 2:2023

INTERNATIONAL ISO/IEC
STANDARD 23003-4
Second edition
2020-06
AMENDMENT 2
2023-10
Information technology — MPEG
audio technologies —
Part 4:
Dynamic range control
AMENDMENT 2: Loudness leveling
Technologies de l'information — Technologies audio MPEG —
Partie 4: Contrôle de gamme dynamique
AMENDEMENT 2: Égalisation de l’intensité sonore
Reference number
ISO/IEC 23003-4:2020/Amd. 2:2023(E)
© ISO/IEC 2023
ISO/IEC 23003-4:2020/Amd. 2:2023(E)
© ISO/IEC 2023
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ii
© ISO/IEC 2023 – All rights reserved

ISO/IEC 23003-4:2020/Amd. 2:2023(E)
Foreword
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This document was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 29, Coding of audio, picture, multimedia, and hypermedia.
A list of all parts in the ISO/IEC 23003 series can be found on the ISO and IEC websites.
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© ISO/IEC 2023 – All rights reserved

ISO/IEC 23003-4:2020/Amd. 2:2023(E)
Information technology — MPEG audio technologies —
Part 4:
Dynamic range control
AMENDMENT 2: Loudness leveling

Introduction
Replace the fourth sentence in the second paragraph with the following:
The DRC tool includes dedicated sections for clipping prevention, ducking/leveling, and for generating a
fade-in and fade-out to supplement the main dynamic range compression functionality.

6.1.1
Add, before the sentence starting with “For ISO/IEC 14496-12, configuration extension”, the following:
An extension payload of type UNIDRCCONFEXT_V1 shall precede an extension payload of type
UNIDRCCONFEXT_LEVELING in the bitstream if both payloads are present.

6.1.2.4
Replace last three sentences of the first paragraph with:
If the bit for the “Duck other” or the “Duck/Level self” drcSetEffect is set, the DRC set is applied before
any downmix specified by the downmix ID, i.e. the DRC set is always applied to the base layout and
the downmix is generated thereafter. The downmixId 0x7F is not permitted for a DRC set with “Duck
other” or “Duck/Level self” effect. In all other cases, the DRC set is applied to the channel configuration
indicated by the downmixId.
Replace the last sentence in the next-to-last paragraph with:
Similarly, if duckingScalingPresent == 1, the scaling factor shall be applied to the associated ducking/
leveling gain sequence for that channel group.

6.3.1
Replace the fourth and fifth paragraph with the following:
DRC sets with only a “Fade”, “Duck other” or “Duck/Level self” effect are automatically selected by the
decoder without using the three-stage selection process. DRC sets with other features can be requested
by using DRC decoder settings as described below.
The pool of DRC sets that is subject to the three-stage selection process comprises not only the DRC sets
defined in the bitstream (except for DRC sets with “Fade”, “Duck other” or a “Duck/Level self” effect)
but also virtual DRC sets generated in the DRC tool. The virtual DRC sets are placeholders for the cases
where no compression is applied to the audio signal, hence their drcSetEffect bits are zero and they
correspond to the DRC effect request “None”.
© ISO/IEC 2023 – All rights reserved

ISO/IEC 23003-4:2020/Amd. 2:2023(E)

6.3.2.1
Replace the first paragraph with the following:
The pre-selection selects all DRC sets that fulfil all requirements listed in Table 10. All available DRC
sets are analysed in the given order of steps. If no DRC set is selected, no DRC can be applied except for
DRC sets with “Fade”, “Duck other” or “Duck/Level self” effect.
Replace Entry #4 of Table 10 with the following:
4 The DRC set does not only have Always. DRC sets with “Fade”, “Duck
a “Fade”, “Duck other” or “Duck/ other” or “Duck/Level self”
Level self” effect enabled. effect are selected automati-
cally. They are not subject to
this selection process.
Replace NOTE of Table 10 with the following:
NOTE Pre-selection steps #8 and #9 are interpreted as pre-selection steps #7 and #8 in the first
edition of this document (ISO/IEC 23003-4:2015). Pre-selection step #7 related to EQ support is first
available with the second edition of this document (ISO/IEC 23003-4:2020).

6.3.2.2.1
Replace the last sentence of the second paragraph, starting with “If no matching downmix IDs can be
found…” with the following:
If no matching downmix IDs can be found, no DRC set can be applied except for DRC sets with “Fade”,
“Duck other” or a “Duck/Level self” effect.

6.3.4
Replace the first paragraph with the following:
This clause uses the term “multiple DRC sets” for DRC sets that are independent of each other and do not
include DRC sets with “Fade”, “Duck other” or a “Duck/Level self” effect.

6.3.5
Replace 6.3.5 with the following:

6.3.5  Applying multiple DRC sets
In the following cases, multiple DRC sets are applied simultaneously. First, if the DRC set selected in
6.3.4 carries a non-zero entry in the dependsOnDrcSet field, the depending DRC set is applied together
with the selected one. Second, if a DRC set with “Fade”, “Duck other” or “Duck/Level self” effect was
automatically selected, it is applied simultaneously with the DRC set selected in 6.3.4 Thus, if the DRC
set selected in 6.3.4 has a non-zero dependsOnDrcSet value, a total of three DRC sets are applied, which
is the maximum number permitted. If all three DRC sets are applied to the same layout (downmixId), the
DRC set referenced in the dependsOnDrcSet field shall be applied first, the DRC set selected in 6.3.4 shall
be applied thereafter, and the DRC set with “Fade”, “Duck other” or “Duck/Level self” effect shall be
applied last. If none of the applied DRC sets is a parametric DRC (see 6.6), the DRC set with “Fade”, “Duck
© ISO/IEC 2023 – All rights reserved

ISO/IEC 23003-4:2020/Amd. 2:2023(E)
other” or “Duck/Level self” effect can alternatively be processed first. If only two DRC sets are applied
to the same layout (downmixId), the same order applies. If a DRC set with “Fade” effect and another DRC
set with “Duck other” or “Duck/Level self” effect were both automatically selected, the DRC set with
“Fade” effect is ignored. A DRC set with “Fade” effect shall be applied after the downmix, if present, if
any of the applied DRC sets is a parametric DRC.

6.3.6
Replace second paragraph of 6.3.6 with the following:
In album mode, any DRC set with “Fade” effect is not applied. If not in album mode, if an applicable
DRC set with “Fade” effect exists, it shall be applied. The DRC set with “Fade” effect can be applied
simultaneously with any other DRC except for DRC sets with a drcSetEffect equal to “Duck other” or
“Duck/Level self”.
6.3.7
Replace 6.3.7 with the following:

6.3.7  Ducking and Loudness Leveling
The base layout and each specific downmix with a unique downmixId can have a maximum of one
applicable DRC set with a drcSetEffect equal to “Ducking other” or “Ducking/Leveling self”. During
configuration, the decoder scans all available DRC sets for the active downmix to identify the applicable
DRC set with a drcSetEffect equal to “Ducking other” or “Ducking/Leveling self” if present. If DRC sets
with a drcSetEffect equal to “Ducking other” or “Ducking/Leveling self” are defined for both the base
layout and the active downmix, the one that exactly matches the active downmix is selected.
If Loudness Leveling is switched off as defined in B.3.7.3, selected DRC sets with a drcSetEffect
equal to “Ducking/Leveling self” and levelingPresent equal to 1 are discarded from the selection. If
duckingOnlyDrcSetPresent is equal to 1, the “Ducking only” DRC set is selected instead.
If a DRC set with a drcSetEffect equal to “Ducking other” or “Ducking/Leveling self” is selected and the
associated overlaid audio signal is active, the ducking/leveling gain sequence is automatically applied
to all channels except those that are members of the channel group associated with the “Duck other”
DRC set (drcSetEffect equal to “Ducking other”) or alternatively to all channels that are members of the
channel group associated with the "Duck/Level self” DRC set (drcSetEffect equal to “Ducking/Leveling
self”). The overlaid audio is defined to be active if at least one non-zero downmix coefficient is applied
to it.
DRC sets with a drcSetEffect equal to “Ducking other” or "Ducking/Leveling self” are always applied
before any downmix, i.e to the base layout. Hence, the DRC channel groups for the ducking/leveling
process refer to the base layout. The downmixId of the corresponding drcInstructionsUniDrc()
indicates how to generate the downmix after the DRC set with a drcSetEffect equal to “Ducking other” or
“Ducking/Leveling self” was applied.
A DRC set with a drcSetEffect equal to “Ducking other” or “Ducking/Leveling self” with downmixId equal
to 0x0 (baseLayout) is automatically applied independent of the requested downmixId. It is therefore
recommended to define DRC sets with a drcSetEffect equal to “Ducking other” or "Ducking/Leveling
self” with downmixId equal to 0x0 only for specific use cases, where the DRC set with a drcSetEffect
equal to “Ducking other” or “Ducking/Leveling self” should be always applied when DRC processing is
enabled.
Table 15
© ISO/IEC 2023 – All rights reserved

ISO/IEC 23003-4:2020/Amd. 2:2023(E)
Replace the third line with the following:
if ((drcSetEffect & (3<<10)) != 0) {    /* Ducking other, Ducking/Leveling self */

6.4.6
Replace last three sentences of second paragraph, starting with” When ducking is active…” with the
following:
When ducking/leveling is active, the ducking/leveling gains in dB are scaled by the factor duckingScaling,
if present. The duckingScaling factors are conveyed in the drcInstructionsUniDrc() payload for the
channel they are applied to, which is in contrast to the bsGainSetIndex channel assignment for the “Duck
other” effect. User-supplied compression and boost factors shall be applied to all DRC sets except DRC
sets with a drcSetEffect equal to “Clipping”, “Fade”, “Ducking other” or “Ducking/Leveling self”.

Table 17
Replace the fifth line with the following:
EFFECT_BITS_DUCKING = 0x0400 | 0x0800;   /* drcSetEffect 11 or 12 (Ducking other, Ducking/
Leveling self) */
Table 35
Replace the 61st line with the following:
EFFECT_BITS_DUCKING = 0x0400 | 0x0800;   /* drcSetEffect 11 or 12 (Ducking other, Ducking/
Leveling self) */
6.8.4
Replace fourth sentence of first paragraph with the following:
DRCs that are automatically applied, such as for ducking/leveling or fading, are not affected by the EQ
selection.
6.9.2
Replace fifth parameter with the following:
L represents the complexity level of a DRC set with a drcSetEffect of “Fade”, “Ducking other”
C,DRC,3
or “Ducking/Leveling self”, if present. Otherwise, it is 0.

6.10.3
After 6.10.2, add the following subclause:

6.10.3  Loudness Leveling
© ISO/IEC 2023 – All rights reserved

ISO/IEC 23003-4:2020/Amd. 2:2023(E)
Loudness Leveling shall be applied if it is switched on as described in B.3.7.3 (default is on). If a DRC
set with a “Duck/Level self” effect exists and is selected according to 6.3.7, it is applied according to
6.3.5. If Loudness Leveling is turned off as described in B.3.7.3, the DRC set selection process for DRC
sets with a “Duck/Level self” effect is based on the metadata received in the applicable part of the
uniDrcConfigExtension()-structure (see case UNIDRCCONFEXT_LEVELING in Table 75).

7.3
Replace Table 75 with:
Table 75 — Syntax of uniDrcConfigExtension() payload
Syntax No. of bits Mnemonic
uniDrcConfigExtension()
{
while (uniDrcConfigExtType != UNIDRCCONFEXT_TERM) { 4 uimsbf
extSizeBits = bitSizeLen + 4; 4 uimsbf
extBitSize = bitSize + 1; extSizeBits uimsbf
switch (uniDrcConfigExtType) {
case UNIDRCCONFEXT_PARAM_DRC:
drcCoefficientsParametricDrc();
parametricDrcInstructionsCount; 4 uimsbf
for (i=0; i parametricDrcInstructions ();
}
break;
case UNIDRCCONFEXT_V1:
downmixInstructionsV1Present; 1 bslbf
if (downmixInstructionsV1Present==1) {
downmixInstructionsV1Count; 7 uimsbf
for (i=0; i downmixInstructionsV1();
}
}
drcCoeffsAndInstructionsUniDrcV1Present; 1 bslbf
if (drcCoeffsAndInstructionsUniDrcV1Present==1) {
drcCoefficientsUniDrcV1Count; 3
...