416 lines
12 KiB
Plaintext
416 lines
12 KiB
Plaintext
@chapter Encoders
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@c man begin ENCODERS
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Encoders are configured elements in Libav which allow the encoding of
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multimedia streams.
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When you configure your Libav build, all the supported native encoders
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are enabled by default. Encoders requiring an external library must be enabled
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manually via the corresponding @code{--enable-lib} option. You can list all
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available encoders using the configure option @code{--list-encoders}.
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You can disable all the encoders with the configure option
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@code{--disable-encoders} and selectively enable / disable single encoders
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with the options @code{--enable-encoder=@var{ENCODER}} /
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@code{--disable-encoder=@var{ENCODER}}.
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The option @code{-codecs} of the ff* tools will display the list of
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enabled encoders.
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@c man end ENCODERS
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@chapter Audio Encoders
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@c man begin AUDIO ENCODERS
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A description of some of the currently available audio encoders
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follows.
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@section ac3 and ac3_fixed
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AC-3 audio encoders.
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These encoders implement part of ATSC A/52:2010 and ETSI TS 102 366, as well as
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the undocumented RealAudio 3 (a.k.a. dnet).
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The @var{ac3} encoder uses floating-point math, while the @var{ac3_fixed}
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encoder only uses fixed-point integer math. This does not mean that one is
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always faster, just that one or the other may be better suited to a
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particular system. The floating-point encoder will generally produce better
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quality audio for a given bitrate. The @var{ac3_fixed} encoder is not the
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default codec for any of the output formats, so it must be specified explicitly
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using the option @code{-acodec ac3_fixed} in order to use it.
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@subsection AC-3 Metadata
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The AC-3 metadata options are used to set parameters that describe the audio,
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but in most cases do not affect the audio encoding itself. Some of the options
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do directly affect or influence the decoding and playback of the resulting
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bitstream, while others are just for informational purposes. A few of the
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options will add bits to the output stream that could otherwise be used for
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audio data, and will thus affect the quality of the output. Those will be
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indicated accordingly with a note in the option list below.
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These parameters are described in detail in several publicly-available
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documents.
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@itemize
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@item @uref{http://www.atsc.org/cms/standards/a_52-2010.pdf,A/52:2010 - Digital Audio Compression (AC-3) (E-AC-3) Standard}
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@item @uref{http://www.atsc.org/cms/standards/a_54a_with_corr_1.pdf,A/54 - Guide to the Use of the ATSC Digital Television Standard}
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@item @uref{http://www.dolby.com/uploadedFiles/zz-_Shared_Assets/English_PDFs/Professional/18_Metadata.Guide.pdf,Dolby Metadata Guide}
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@item @uref{http://www.dolby.com/uploadedFiles/zz-_Shared_Assets/English_PDFs/Professional/46_DDEncodingGuidelines.pdf,Dolby Digital Professional Encoding Guidelines}
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@end itemize
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@subsubsection Metadata Control Options
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@table @option
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@item -per_frame_metadata @var{boolean}
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Allow Per-Frame Metadata. Specifies if the encoder should check for changing
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metadata for each frame.
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@table @option
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@item 0
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The metadata values set at initialization will be used for every frame in the
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stream. (default)
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@item 1
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Metadata values can be changed before encoding each frame.
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@end table
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@end table
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@subsubsection Downmix Levels
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@table @option
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@item -center_mixlev @var{level}
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Center Mix Level. The amount of gain the decoder should apply to the center
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channel when downmixing to stereo. This field will only be written to the
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bitstream if a center channel is present. The value is specified as a scale
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factor. There are 3 valid values:
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@table @option
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@item 0.707
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Apply -3dB gain
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@item 0.595
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Apply -4.5dB gain (default)
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@item 0.500
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Apply -6dB gain
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@end table
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@item -surround_mixlev @var{level}
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Surround Mix Level. The amount of gain the decoder should apply to the surround
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channel(s) when downmixing to stereo. This field will only be written to the
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bitstream if one or more surround channels are present. The value is specified
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as a scale factor. There are 3 valid values:
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@table @option
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@item 0.707
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Apply -3dB gain
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@item 0.500
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Apply -6dB gain (default)
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@item 0.000
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Silence Surround Channel(s)
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@end table
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@end table
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@subsubsection Audio Production Information
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Audio Production Information is optional information describing the mixing
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environment. Either none or both of the fields are written to the bitstream.
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@table @option
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@item -mixing_level @var{number}
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Mixing Level. Specifies peak sound pressure level (SPL) in the production
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environment when the mix was mastered. Valid values are 80 to 111, or -1 for
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unknown or not indicated. The default value is -1, but that value cannot be
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used if the Audio Production Information is written to the bitstream. Therefore,
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if the @code{room_type} option is not the default value, the @code{mixing_level}
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option must not be -1.
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@item -room_type @var{type}
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Room Type. Describes the equalization used during the final mixing session at
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the studio or on the dubbing stage. A large room is a dubbing stage with the
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industry standard X-curve equalization; a small room has flat equalization.
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This field will not be written to the bitstream if both the @code{mixing_level}
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option and the @code{room_type} option have the default values.
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@table @option
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@item 0
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@itemx notindicated
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Not Indicated (default)
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@item 1
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@itemx large
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Large Room
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@item 2
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@itemx small
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Small Room
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@end table
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@end table
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@subsubsection Other Metadata Options
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@table @option
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@item -copyright @var{boolean}
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Copyright Indicator. Specifies whether a copyright exists for this audio.
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@table @option
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@item 0
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@itemx off
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No Copyright Exists (default)
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@item 1
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@itemx on
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Copyright Exists
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@end table
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@item -dialnorm @var{value}
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Dialogue Normalization. Indicates how far the average dialogue level of the
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program is below digital 100% full scale (0 dBFS). This parameter determines a
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level shift during audio reproduction that sets the average volume of the
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dialogue to a preset level. The goal is to match volume level between program
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sources. A value of -31dB will result in no volume level change, relative to
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the source volume, during audio reproduction. Valid values are whole numbers in
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the range -31 to -1, with -31 being the default.
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@item -dsur_mode @var{mode}
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Dolby Surround Mode. Specifies whether the stereo signal uses Dolby Surround
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(Pro Logic). This field will only be written to the bitstream if the audio
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stream is stereo. Using this option does @b{NOT} mean the encoder will actually
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apply Dolby Surround processing.
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@table @option
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@item 0
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@itemx notindicated
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Not Indicated (default)
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@item 1
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@itemx off
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Not Dolby Surround Encoded
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@item 2
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@itemx on
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Dolby Surround Encoded
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@end table
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@item -original @var{boolean}
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Original Bit Stream Indicator. Specifies whether this audio is from the
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original source and not a copy.
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@table @option
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@item 0
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@itemx off
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Not Original Source
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@item 1
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@itemx on
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Original Source (default)
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@end table
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@end table
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@subsection Extended Bitstream Information
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The extended bitstream options are part of the Alternate Bit Stream Syntax as
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specified in Annex D of the A/52:2010 standard. It is grouped into 2 parts.
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If any one parameter in a group is specified, all values in that group will be
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written to the bitstream. Default values are used for those that are written
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but have not been specified. If the mixing levels are written, the decoder
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will use these values instead of the ones specified in the @code{center_mixlev}
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and @code{surround_mixlev} options if it supports the Alternate Bit Stream
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Syntax.
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@subsubsection Extended Bitstream Information - Part 1
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@table @option
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@item -dmix_mode @var{mode}
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Preferred Stereo Downmix Mode. Allows the user to select either Lt/Rt
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(Dolby Surround) or Lo/Ro (normal stereo) as the preferred stereo downmix mode.
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@table @option
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@item 0
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@itemx notindicated
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Not Indicated (default)
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@item 1
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@itemx ltrt
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Lt/Rt Downmix Preferred
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@item 2
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@itemx loro
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Lo/Ro Downmix Preferred
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@end table
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@item -ltrt_cmixlev @var{level}
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Lt/Rt Center Mix Level. The amount of gain the decoder should apply to the
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center channel when downmixing to stereo in Lt/Rt mode.
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@table @option
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@item 1.414
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Apply +3dB gain
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@item 1.189
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Apply +1.5dB gain
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@item 1.000
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Apply 0dB gain
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@item 0.841
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Apply -1.5dB gain
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@item 0.707
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Apply -3.0dB gain
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@item 0.595
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Apply -4.5dB gain (default)
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@item 0.500
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Apply -6.0dB gain
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@item 0.000
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Silence Center Channel
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@end table
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@item -ltrt_surmixlev @var{level}
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Lt/Rt Surround Mix Level. The amount of gain the decoder should apply to the
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surround channel(s) when downmixing to stereo in Lt/Rt mode.
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@table @option
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@item 0.841
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Apply -1.5dB gain
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@item 0.707
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Apply -3.0dB gain
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@item 0.595
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Apply -4.5dB gain
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@item 0.500
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Apply -6.0dB gain (default)
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@item 0.000
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Silence Surround Channel(s)
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@end table
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@item -loro_cmixlev @var{level}
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Lo/Ro Center Mix Level. The amount of gain the decoder should apply to the
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center channel when downmixing to stereo in Lo/Ro mode.
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@table @option
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@item 1.414
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Apply +3dB gain
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@item 1.189
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Apply +1.5dB gain
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@item 1.000
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Apply 0dB gain
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@item 0.841
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Apply -1.5dB gain
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@item 0.707
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Apply -3.0dB gain
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@item 0.595
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Apply -4.5dB gain (default)
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@item 0.500
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Apply -6.0dB gain
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@item 0.000
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Silence Center Channel
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@end table
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@item -loro_surmixlev @var{level}
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Lo/Ro Surround Mix Level. The amount of gain the decoder should apply to the
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surround channel(s) when downmixing to stereo in Lo/Ro mode.
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@table @option
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@item 0.841
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Apply -1.5dB gain
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@item 0.707
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Apply -3.0dB gain
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@item 0.595
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Apply -4.5dB gain
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@item 0.500
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Apply -6.0dB gain (default)
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@item 0.000
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Silence Surround Channel(s)
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@end table
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@end table
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@subsubsection Extended Bitstream Information - Part 2
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@table @option
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@item -dsurex_mode @var{mode}
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Dolby Surround EX Mode. Indicates whether the stream uses Dolby Surround EX
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(7.1 matrixed to 5.1). Using this option does @b{NOT} mean the encoder will actually
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apply Dolby Surround EX processing.
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@table @option
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@item 0
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@itemx notindicated
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Not Indicated (default)
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@item 1
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@itemx on
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Dolby Surround EX Off
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@item 2
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@itemx off
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Dolby Surround EX On
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@end table
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@item -dheadphone_mode @var{mode}
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Dolby Headphone Mode. Indicates whether the stream uses Dolby Headphone
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encoding (multi-channel matrixed to 2.0 for use with headphones). Using this
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option does @b{NOT} mean the encoder will actually apply Dolby Headphone
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processing.
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@table @option
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@item 0
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@itemx notindicated
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Not Indicated (default)
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@item 1
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@itemx on
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Dolby Headphone Off
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@item 2
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@itemx off
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Dolby Headphone On
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@end table
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@item -ad_conv_type @var{type}
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A/D Converter Type. Indicates whether the audio has passed through HDCD A/D
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conversion.
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@table @option
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@item 0
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@itemx standard
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Standard A/D Converter (default)
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@item 1
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@itemx hdcd
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HDCD A/D Converter
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@end table
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@end table
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@subsection Other AC-3 Encoding Options
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@table @option
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@item -stereo_rematrixing @var{boolean}
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Stereo Rematrixing. Enables/Disables use of rematrixing for stereo input. This
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is an optional AC-3 feature that increases quality by selectively encoding
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the left/right channels as mid/side. This option is enabled by default, and it
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is highly recommended that it be left as enabled except for testing purposes.
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@end table
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@subheading Floating-Point-Only AC-3 Encoding Options
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These options are only valid for the floating-point encoder and do not exist
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for the fixed-point encoder due to the corresponding features not being
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implemented in fixed-point.
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@table @option
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@item -channel_coupling @var{boolean}
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Enables/Disables use of channel coupling, which is an optional AC-3 feature
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that increases quality by combining high frequency information from multiple
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channels into a single channel. The per-channel high frequency information is
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sent with less accuracy in both the frequency and time domains. This allows
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more bits to be used for lower frequencies while preserving enough information
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to reconstruct the high frequencies. This option is enabled by default for the
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floating-point encoder and should generally be left as enabled except for
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testing purposes or to increase encoding speed.
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@table @option
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@item -1
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@itemx auto
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Selected by Encoder (default)
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@item 0
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@itemx off
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Disable Channel Coupling
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@item 1
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@itemx on
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Enable Channel Coupling
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@end table
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@item -cpl_start_band @var{number}
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Coupling Start Band. Sets the channel coupling start band, from 1 to 15. If a
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value higher than the bandwidth is used, it will be reduced to 1 less than the
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coupling end band. If @var{auto} is used, the start band will be determined by
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the encoder based on the bit rate, sample rate, and channel layout. This option
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has no effect if channel coupling is disabled.
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@table @option
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@item -1
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@itemx auto
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Selected by Encoder (default)
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@end table
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@end table
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@c man end AUDIO ENCODERS
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