Audio Normalization

Audio normalization adjusts the loudness of a program to a specific target level. Audio normalization requires a loudness measurement pass followed by a loudness correction pass.

Audio normalization is applied to an output using the following properties:

Property

Type

Description

MeteringMode

enum

Specifies the loudness measurement algorithm: bt1770_1 ungated loudness, bt1770_2 gated loudness, bt1770_3 improved true peak, bt1770_4 higher channel count.

DialogIntelligence

boolean

Indicates whether dialogue intelligence is enabled. Default is false.

SpeechThreshold

number

Specifies the speech content threshold as a percentage. If the percentage of speech detected in the program exceeds this threshold the speech gated loudness measurment is used for normalization.

NormalizedLoudness

number

Specifies the normalized target level in LUFS. Default is -23.0 LUFS.

RelativeGateOffset

number

Specifies the loudness gate to be used as silence. The default value is 0.

For example the following composition normalizes the output audio level to -18.0 LUFS using the BT 1770-2 metering mode. Note that each audio channel must have a speaker label:

---
input:
  container:
  - name: wav
    audio:
    - name: a1
      channels: 2
      label:
      - L
      - R
output:
  container:
  - name: aac
    type: 'mp4 '
    audio:
    - route:
        name: a1
      properties:
        Format: aac
        AudioNormalize:
          MeteringMode: bt1770_2
          DialogInitelligence: true
          SpeechThreshold: 25
          NormalizedLoudness: -18.0
          RelativeGateOffset: -10
      encode:
        properties:
          BitRate: 96000
          MpegVersion: 1
          Profile: lc
{
  "input": {
    "container": [
      {
        "name": "wav",
        "audio": [
          {
            "name": "a1",
            "channels": 2,
            "label": [
              "L",
              "R"
            ]
          }
        ]
      }
    ]
  },
  "output": {
    "container": [
      {
        "name": "aac",
        "type": "mp4 ",
        "audio": [
          {
            "route": {
              "name": "a1"
            },
            "properties": {
              "Format": "aac",
              "AudioNormalize": {
                "MeteringMode": "bt1770_2",
                "DialogInitelligence": true,
                "SpeechThreshold": 25,
                "NormalizedLoudness": -18.0,
                "RelativeGateOffset": -10
              }
            },
            "encode": {
              "properties": {
                "BitRate": 96000,
                "MpegVersion": 1,
                "Profile": "lc"
              }
            }
          }
        ]
      }
    ]
  }
}

Metering Modes

BT.1770-1 measures the average or integrated loudness over the entire duration of the program. The measurement is expressed in LU (loudness units) and loudness units referenced to full scale LUFS.

BT.1770-2 introduced a level based gating function to prevent long periods of silence from disproportionately influencing the integrated loudness measurement.

BT.1770-3 introduced a more accurate method to measure the true peak sample level of the program. The peak sample level prevents subsequent audio normalization from introducing distortion due to peak clipping.

BT.1770-4 includes additional surround channels.

Dialogue Intelligence

Dolby Dialogue Intelligence is an alternative speech based gating function used to measure the integrated loudness of a program.

By default the BT.1770-2 level gated measurement represents the integrated program loudness. When dialog intelligence is enabled and the percentage of speech content exceeds a specific threshold the speech gated measurement is selected as the integrated loudness.


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