AC-3 Metadata


The AC-3 metadata options are used to set parameters that describe the audio, but in most cases do not affect the audio encoding itself. Some of the options do directly affect or influence the decoding and playback of the resulting bitstream, while others are just for informational purposes. A few of the options will add bits to the output stream that could otherwise be used for audio data, and will thus affect the quality of the output. Those will be indicated accordingly with a note in the option list below.

These parameters are described in detail in several publicly-available documents.

'-per_frame_metadata boolean'

Allow Per-Frame Metadata. Specifies if the encoder should check for changing metadata for each frame.

'0'

The metadata values set at initialization will be used for every frame in the stream. (default)

'1'

Metadata values can be changed before encoding each frame.

'-center_mixlev level'

Center Mix Level. The amount of gain the decoder should apply to the center channel when downmixing to stereo. This field will only be written to the bitstream if a center channel is present. The value is specified as a scale factor. There are 3 valid values:

'0.707'

Apply -3dB gain

'0.595'

Apply -4.5dB gain (default)

'0.500'

Apply -6dB gain

'-surround_mixlev level'

Surround Mix Level. The amount of gain the decoder should apply to the surround channel(s) when downmixing to stereo. This field will only be written to the bitstream if one or more surround channels are present. The value is specified as a scale factor. There are 3 valid values:

'0.707'

Apply -3dB gain

'0.500'

Apply -6dB gain (default)

'0.000'

Silence Surround Channel(s)

Audio Production Information is optional information describing the mixing environment. Either none or both of the fields are written to the bitstream.

'-mixing_level number'

Mixing Level. Specifies peak sound pressure level (SPL) in the production environment when the mix was mastered. Valid values are 80 to 111, or -1 for unknown or not indicated. The default value is -1, but that value cannot be used if the Audio Production Information is written to the bitstream. Therefore, if the room_type option is not the default value, the mixing_level option must not be -1.

'-room_type type'

Room Type. Describes the equalization used during the final mixing session at the studio or on the dubbing stage. A large room is a dubbing stage with the industry standard X-curve equalization; a small room has flat equalization. This field will not be written to the bitstream if both the mixing_level option and the room_type option have the default values.

'0'
'notindicated'

Not Indicated (default)

'1'
'large'

Large Room

'2'
'small'

Small Room

'-copyright boolean'

Copyright Indicator. Specifies whether a copyright exists for this audio.

'0'
'off'

No Copyright Exists (default)

'1'
'on'

Copyright Exists

'-dialnorm value'

Dialogue Normalization. Indicates how far the average dialogue level of the program is below digital 100% full scale (0 dBFS). This parameter determines a level shift during audio reproduction that sets the average volume of the dialogue to a preset level. The goal is to match volume level between program sources. A value of -31dB will result in no volume level change, relative to the source volume, during audio reproduction. Valid values are whole numbers in the range -31 to -1, with -31 being the default.

'-dsur_mode mode'

Dolby Surround Mode. Specifies whether the stereo signal uses Dolby Surround (Pro Logic). This field will only be written to the bitstream if the audio stream is stereo. Using this option does NOT mean the encoder will actually apply Dolby Surround processing.

'0'
'notindicated'

Not Indicated (default)

'1'
'off'

Not Dolby Surround Encoded

'2'
'on'

Dolby Surround Encoded

'-original boolean'

Original Bit Stream Indicator. Specifies whether this audio is from the original source and not a copy.

'0'
'off'

Not Original Source

'1'
'on'

Original Source (default)