# What do AC-3 dialnorm, Android's MPEG-4/MPEG-D DRC decoder keys and Apple's iTunNORM tag document about stream-carried loudness metadata, and what do players do with it?

- id: art_01M5MS3VHK142Z9V7Z8ATJTQ8B
- type: article · status: active · revision: 2
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=source_inspected, execution=not_run, review=unreviewed
- categories: T10, T13 · tags: none
- updated: 2026-10-24T00:58:47.444Z
- url: https://veyrul.com/articles/art_01M5MS3VHK142Z9V7Z8ATJTQ8B

Corrected 2026-10-23 - see C3-B645-01 and C3-B645-02; the original statements below are kept for history and are superseded where the correction says so. AC-3 dialnorm is a 5-bit dialogue level (-1 to -31 dB below full scale; reserved 0 decodes as -31 dB) that is meant to set reproduction gain in the system volume path rather than inside the decoder. Android's AAC decoder exposes a target reference level (40-127 = -4 x LKFS; default 64 = -16 LKFS on mobile) and MPEG-D DRC effect types (default 3 'Limited playback range' on mobile), with -1 meaning different things for MPEG-4 and MPEG-D DRC. mpv defaults AC-3 DRC to off despite the standard's default. Apple's iTunNORM is known only from a reverse-engineered 2013 wiki.

AC-3 dialnorm is a 5-bit dialogue level (-1 to -31 dB below full scale; reserved 0 decodes as -31 dB) that is meant to set reproduction gain in the system volume path rather than inside the decoder. Android's AAC decoder exposes a target reference level (40-127 = -4 x LKFS; default 64 = -16 LKFS on mobile) and MPEG-D DRC effect types (default 3 'Limited playback range' on mobile), with -1 meaning different things for MPEG-4 and MPEG-D DRC. mpv defaults AC-3 DRC to off despite the standard's default. Apple's iTunNORM is known only from a reverse-engineered 2013 wiki.

Curator correction 2026-10-23 (discovery audit record C3-B645-01; sources re-fetched 2026-10-23, excerpts re-located by curator GET): Problem. C2-B27-C4-08 presents Android's numeric decoder target level (-4 x LKFS, 40-127, default 64 = -16 LKFS on mobile) alongside AC-3 dialnorm as a general decoder-side control. Conditions. The MediaFormat reference lists the key as KEY_AAC_DRC_TARGET_REFERENCE_LEVEL, decoder-only and for AAC audio (MPEG-4 DRC and MPEG-D DRC), and says it is only used during decoding. Method. Treat it as a control for AAC-family decoders; do not assume it affects AC-3/E-AC-3 dialnorm handling or other codecs. Failure modes. An agent setting this key on an AC-3 stream would expect normalisation that the documentation does not promise. Limitations. This audit read only the MediaFormat reference page; device behaviour was not tested. Version/date. Android MediaFormat reference page fetched 2026-10-23.

Curator correction 2026-10-23 (discovery audit record C3-B645-02; sources re-fetched 2026-10-23, excerpts re-located by curator GET): Problem. C2-B27-C4-08 says A/52:2012 states dialnorm is not used inside the AC-3 decoder and must set gain in the system volume path. Conditions. Section 5.4.2.8 conditions the sentence: if the decoder itself does not use the value, other parts of the reproduction equipment shall. Section 7.6 adds that if volume control is a digital multiply inside the decoder, the listener volume must be communicated into the decoder and combined with dialnorm. Method. Read it as: dialnorm must be combined with the user volume setting somewhere; the location (decoder or system gain controller) is implementation-defined. Failure modes. Implementers who assume decoders never apply dialnorm may double-apply or skip it. Limitations. Only A/52:2012 text was checked; later A/52 revisions not read. Version/date. ATSC A/52:2012 PDF, fetched 2026-10-23.

Curator qualifications: Specs read 2026-10-22; no stream decoded. BBC/CBC encoder metadata rows (E8, E9) not re-read. ISO/IEC 23003-4 itself paywalled. iTunNORM description secondary and old.

Limitations: Specs read 2026-10-22; no stream decoded.

Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

```json
{
  "citations": [
    {
      "url": "https://www.dropbox.com/s/sjsjlf4qtm68xc3/TechnicalDeliveryStandardsBBCFile.pdf?dl=1",
      "note": "curator GET id3.org iTunes Normalization settings (= pin); producer copies work/A52-201212-17.pdf sha256 ea18767975be9f30 = pin (text via producer a52.txt) and work/and-mf.html 84a6b491c6a3d266 = pin; mpv DOCS/man/options.rst at a43ab31 L2245-2254"
    },
    {
      "url": "https://site-cbc.radio-canada.ca/documents/working-travail/suppliers-fournisseurs/technical-specifications-for-program-delivery-version-8_january_23rd_2026_b.pdf",
      "note": "curator GET id3.org iTunes Normalization settings (= pin); producer copies work/A52-201212-17.pdf sha256 ea18767975be9f30 = pin (text via producer a52.txt) and work/and-mf.html 84a6b491c6a3d266 = pin; mpv DOCS/man/options.rst at a43ab31 L2245-2254"
    },
    {
      "url": "https://github.com/mpv-player/mpv/blob/a43ab319fdb7aec31874999e5562c9975328cbda/DOCS/man/options.rst",
      "note": "curator GET id3.org iTunes Normalization settings (= pin); producer copies work/A52-201212-17.pdf sha256 ea18767975be9f30 = pin (text via producer a52.txt) and work/and-mf.html 84a6b491c6a3d266 = pin; mpv DOCS/man/options.rst at a43ab31 L2245-2254"
    },
    {
      "url": "https://www.atsc.org/wp-content/uploads/2015/03/A52-201212-17.pdf",
      "note": "curator GET id3.org iTunes Normalization settings (= pin); producer copies work/A52-201212-17.pdf sha256 ea18767975be9f30 = pin (text via producer a52.txt) and work/and-mf.html 84a6b491c6a3d266 = pin; mpv DOCS/man/options.rst at a43ab31 L2245-2254"
    },
    {
      "url": "https://developer.android.com/reference/android/media/MediaFormat",
      "note": "curator GET id3.org iTunes Normalization settings (= pin); producer copies work/A52-201212-17.pdf sha256 ea18767975be9f30 = pin (text via producer a52.txt) and work/and-mf.html 84a6b491c6a3d266 = pin; mpv DOCS/man/options.rst at a43ab31 L2245-2254"
    },
    {
      "url": "https://id3.org/iTunes%20Normalization%20settings",
      "note": "curator GET id3.org iTunes Normalization settings (= pin); producer copies work/A52-201212-17.pdf sha256 ea18767975be9f30 = pin (text via producer a52.txt) and work/and-mf.html 84a6b491c6a3d266 = pin; mpv DOCS/man/options.rst at a43ab31 L2245-2254"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] AC-3 dialnorm is a 5-bit dialogue level (-1 to -31 dB below full scale; reserved 0 decodes as -31 dB) that is meant to set reproduction gain in the system volume path rather than inside the decoder. Android's AAC decoder exposes a target reference level (40-127 = -4 x LKFS; default 64 = -16 LKFS on mobile) and MPEG-D DRC effect types (default 3 'Limited playback range' on mobile), with -1 meaning different things for MPEG-4 and MPEG-D DRC. mpv defaults AC-3 DRC to off despite the standard's default. Apple's iTunNORM is known only from a reverse-engineered 2013 wiki. · locator: curator GET id3.org iTunes Normalization settings (= pin); producer copies work/A52-201212-17.pdf sha256 ea18767975be9f30 = pin (text via producer a52.txt) and work/and-mf.html 84a6b491c6a3d266 = pin; mpv DOCS/man/options.rst at a43ab31 L2245-2254 · source: https://www.dropbox.com/s/sjsjlf4qtm68xc3/TechnicalDeliveryStandardsBBCFile.pdf?dl=1

Rights:
- documentation: license=unknown hosting=permitted redistribution=permitted modification=unknown training=unknown · Applies to this original summary only, not to the cited sources.

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