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@chapter Muxers
@c man begin MUXERS

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Muxers are configured elements in Libav which allow writing
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multimedia streams to a particular type of file.

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When you configure your Libav build, all the supported muxers
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are enabled by default. You can list all available muxers using the
configure option @code{--list-muxers}.

You can disable all the muxers with the configure option
@code{--disable-muxers} and selectively enable / disable single muxers
with the options @code{--enable-muxer=@var{MUXER}} /
@code{--disable-muxer=@var{MUXER}}.

The option @code{-formats} of the ff* tools will display the list of
enabled muxers.

A description of some of the currently available muxers follows.

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@anchor{crc}
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@section crc

CRC (Cyclic Redundancy Check) testing format.

This muxer computes and prints the Adler-32 CRC of all the input audio
and video frames. By default audio frames are converted to signed
16-bit raw audio and video frames to raw video before computing the
CRC.

The output of the muxer consists of a single line of the form:
CRC=0x@var{CRC}, where @var{CRC} is a hexadecimal number 0-padded to
8 digits containing the CRC for all the decoded input frames.

For example to compute the CRC of the input, and store it in the file
@file{out.crc}:
@example
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avconv -i INPUT -f crc out.crc
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@end example

You can print the CRC to stdout with the command:
@example
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avconv -i INPUT -f crc -
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@end example

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You can select the output format of each frame with @command{avconv} by
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specifying the audio and video codec and format. For example to
compute the CRC of the input audio converted to PCM unsigned 8-bit
and the input video converted to MPEG-2 video, use the command:
@example
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avconv -i INPUT -c:a pcm_u8 -c:v mpeg2video -f crc -
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@end example

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See also the @ref{framecrc} muxer.
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@anchor{framecrc}
@section framecrc

Per-frame CRC (Cyclic Redundancy Check) testing format.

This muxer computes and prints the Adler-32 CRC for each decoded audio
and video frame. By default audio frames are converted to signed
16-bit raw audio and video frames to raw video before computing the
CRC.

The output of the muxer consists of a line for each audio and video
frame of the form: @var{stream_index}, @var{frame_dts},
@var{frame_size}, 0x@var{CRC}, where @var{CRC} is a hexadecimal
number 0-padded to 8 digits containing the CRC of the decoded frame.

For example to compute the CRC of each decoded frame in the input, and
store it in the file @file{out.crc}:
@example
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avconv -i INPUT -f framecrc out.crc
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@end example

You can print the CRC of each decoded frame to stdout with the command:
@example
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avconv -i INPUT -f framecrc -
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@end example

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You can select the output format of each frame with @command{avconv} by
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specifying the audio and video codec and format. For example, to
compute the CRC of each decoded input audio frame converted to PCM
unsigned 8-bit and of each decoded input video frame converted to
MPEG-2 video, use the command:
@example
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avconv -i INPUT -c:a pcm_u8 -c:v mpeg2video -f framecrc -
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@end example

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See also the @ref{crc} muxer.
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@anchor{image2}
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@section image2

Image file muxer.

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The image file muxer writes video frames to image files.
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The output filenames are specified by a pattern, which can be used to
produce sequentially numbered series of files.
The pattern may contain the string "%d" or "%0@var{N}d", this string
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specifies the position of the characters representing a numbering in
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the filenames. If the form "%0@var{N}d" is used, the string
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representing the number in each filename is 0-padded to @var{N}
digits. The literal character '%' can be specified in the pattern with
the string "%%".

If the pattern contains "%d" or "%0@var{N}d", the first filename of
the file list specified will contain the number 1, all the following
numbers will be sequential.

The pattern may contain a suffix which is used to automatically
determine the format of the image files to write.

For example the pattern "img-%03d.bmp" will specify a sequence of
filenames of the form @file{img-001.bmp}, @file{img-002.bmp}, ...,
@file{img-010.bmp}, etc.
The pattern "img%%-%d.jpg" will specify a sequence of filenames of the
form @file{img%-1.jpg}, @file{img%-2.jpg}, ..., @file{img%-10.jpg},
etc.

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The following example shows how to use @command{avconv} for creating a
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sequence of files @file{img-001.jpeg}, @file{img-002.jpeg}, ...,
taking one image every second from the input video:
@example
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avconv -i in.avi -vsync 1 -r 1 -f image2 'img-%03d.jpeg'
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@end example

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Note that with @command{avconv}, if the format is not specified with the
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@code{-f} option and the output filename specifies an image file
format, the image2 muxer is automatically selected, so the previous
command can be written as:
@example
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avconv -i in.avi -vsync 1 -r 1 'img-%03d.jpeg'
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@end example

Note also that the pattern must not necessarily contain "%d" or
"%0@var{N}d", for example to create a single image file
@file{img.jpeg} from the input video you can employ the command:
@example
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avconv -i in.avi -f image2 -frames:v 1 img.jpeg
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@end example

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@section MOV/MP4/ISMV

The mov/mp4/ismv muxer supports fragmentation. Normally, a MOV/MP4
file has all the metadata about all packets stored in one location
(written at the end of the file, it can be moved to the start for
better playback using the @command{qt-faststart} tool). A fragmented
file consists of a number of fragments, where packets and metadata
about these packets are stored together. Writing a fragmented
file has the advantage that the file is decodable even if the
writing is interrupted (while a normal MOV/MP4 is undecodable if
it is not properly finished), and it requires less memory when writing
very long files (since writing normal MOV/MP4 files stores info about
every single packet in memory until the file is closed). The downside
is that it is less compatible with other applications.

Fragmentation is enabled by setting one of the AVOptions that define
how to cut the file into fragments:

@table @option
@item -movflags frag_keyframe
Start a new fragment at each video keyframe.
@item -frag_duration @var{duration}
Create fragments that are @var{duration} microseconds long.
@item -frag_size @var{size}
Create fragments that contain up to @var{size} bytes of payload data.
@item -movflags frag_custom
Allow the caller to manually choose when to cut fragments, by
calling @code{av_write_frame(ctx, NULL)} to write a fragment with
the packets written so far. (This is only useful with other
applications integrating libavformat, not from @command{avconv}.)
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@item -min_frag_duration @var{duration}
Don't create fragments that are shorter than @var{duration} microseconds long.
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@end table

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If more than one condition is specified, fragments are cut when
one of the specified conditions is fulfilled. The exception to this is
@code{-min_frag_duration}, which has to be fulfilled for any of the other
conditions to apply.

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Additionally, the way the output file is written can be adjusted
through a few other options:

@table @option
@item -movflags empty_moov
Write an initial moov atom directly at the start of the file, without
describing any samples in it. Generally, an mdat/moov pair is written
at the start of the file, as a normal MOV/MP4 file, containing only
a short portion of the file. With this option set, there is no initial
mdat atom, and the moov atom only describes the tracks but has
a zero duration.

Files written with this option set do not work in QuickTime.
This option is implicitly set when writing ismv (Smooth Streaming) files.
@item -movflags separate_moof
Write a separate moof (movie fragment) atom for each track. Normally,
packets for all tracks are written in a moof atom (which is slightly
more efficient), but with this option set, the muxer writes one moof/mdat
pair for each track, making it easier to separate tracks.

This option is implicitly set when writing ismv (Smooth Streaming) files.
@end table

Smooth Streaming content can be pushed in real time to a publishing
point on IIS with this muxer. Example:
@example
avconv -re @var{<normal input/transcoding options>} -movflags isml+frag_keyframe -f ismv http://server/publishingpoint.isml/Streams(Encoder1)
@end example

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@section mpegts

MPEG transport stream muxer.

This muxer implements ISO 13818-1 and part of ETSI EN 300 468.

The muxer options are:

@table @option
@item -mpegts_original_network_id @var{number}
Set the original_network_id (default 0x0001). This is unique identifier
of a network in DVB. Its main use is in the unique identification of a
service through the path Original_Network_ID, Transport_Stream_ID.
@item -mpegts_transport_stream_id @var{number}
Set the transport_stream_id (default 0x0001). This identifies a
transponder in DVB.
@item -mpegts_service_id @var{number}
Set the service_id (default 0x0001) also known as program in DVB.
@item -mpegts_pmt_start_pid @var{number}
Set the first PID for PMT (default 0x1000, max 0x1f00).
@item -mpegts_start_pid @var{number}
Set the first PID for data packets (default 0x0100, max 0x0f00).
@end table

The recognized metadata settings in mpegts muxer are @code{service_provider}
and @code{service_name}. If they are not set the default for
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@code{service_provider} is "Libav" and the default for
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@code{service_name} is "Service01".

@example
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avconv -i file.mpg -c copy \
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     -mpegts_original_network_id 0x1122 \
     -mpegts_transport_stream_id 0x3344 \
     -mpegts_service_id 0x5566 \
     -mpegts_pmt_start_pid 0x1500 \
     -mpegts_start_pid 0x150 \
     -metadata service_provider="Some provider" \
     -metadata service_name="Some Channel" \
     -y out.ts
@end example

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@section null

Null muxer.

This muxer does not generate any output file, it is mainly useful for
testing or benchmarking purposes.

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For example to benchmark decoding with @command{avconv} you can use the
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command:
@example
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avconv -benchmark -i INPUT -f null out.null
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@end example

Note that the above command does not read or write the @file{out.null}
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file, but specifying the output file is required by the @command{avconv}
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syntax.

Alternatively you can write the command as:
@example
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avconv -benchmark -i INPUT -f null -
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@end example

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@section matroska

Matroska container muxer.

This muxer implements the matroska and webm container specs.

The recognized metadata settings in this muxer are:

@table @option

@item title=@var{title name}
Name provided to a single track
@end table

@table @option

@item language=@var{language name}
Specifies the language of the track in the Matroska languages form
@end table

@table @option

@item STEREO_MODE=@var{mode}
Stereo 3D video layout of two views in a single video track
@table @option
@item mono
video is not stereo
@item left_right
Both views are arranged side by side, Left-eye view is on the left
@item bottom_top
Both views are arranged in top-bottom orientation, Left-eye view is at bottom
@item top_bottom
Both views are arranged in top-bottom orientation, Left-eye view is on top
@item checkerboard_rl
Each view is arranged in a checkerboard interleaved pattern, Left-eye view being first
@item checkerboard_lr
Each view is arranged in a checkerboard interleaved pattern, Right-eye view being first
@item row_interleaved_rl
Each view is constituted by a row based interleaving, Right-eye view is first row
@item row_interleaved_lr
Each view is constituted by a row based interleaving, Left-eye view is first row
@item col_interleaved_rl
Both views are arranged in a column based interleaving manner, Right-eye view is first column
@item col_interleaved_lr
Both views are arranged in a column based interleaving manner, Left-eye view is first column
@item anaglyph_cyan_red
All frames are in anaglyph format viewable through red-cyan filters
@item right_left
Both views are arranged side by side, Right-eye view is on the left
@item anaglyph_green_magenta
All frames are in anaglyph format viewable through green-magenta filters
@item block_lr
Both eyes laced in one Block, Left-eye view is first
@item block_rl
Both eyes laced in one Block, Right-eye view is first
@end table
@end table

For example a 3D WebM clip can be created using the following command line:
@example
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avconv -i sample_left_right_clip.mpg -an -c:v libvpx -metadata STEREO_MODE=left_right -y stereo_clip.webm
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@end example

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@section segment

Basic stream segmenter.

The segmenter muxer outputs streams to a number of separate files of nearly
fixed duration. Output filename pattern can be set in a fashion similar to
@ref{image2}.

Every segment starts with a video keyframe, if a video stream is present.
The segment muxer works best with a single constant frame rate video.

Optionally it can generate a flat list of the created segments, one segment
per line.

@table @option
@item segment_format @var{format}
Override the inner container format, by default it is guessed by the filename
extension.
@item segment_time @var{t}
Set segment duration to @var{t} seconds.
@item segment_list @var{name}
Generate also a listfile named @var{name}.
@item segment_list_size @var{size}
Overwrite the listfile once it reaches @var{size} entries.
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@item segment_wrap @var{limit}
Wrap around segment index once it reaches @var{limit}.
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@end table

@example
avconv -i in.mkv -c copy -map 0 -f segment -list out.list out%03d.nut
@end example

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@section mp3

The MP3 muxer writes a raw MP3 stream with an ID3v2 header at the beginning and
optionally an ID3v1 tag at the end. ID3v2.3 and ID3v2.4 are supported, the
@code{id3v2_version} option controls which one is used. The legacy ID3v1 tag is
not written by default, but may be enabled with the @code{write_id3v1} option.

For seekable output the muxer also writes a Xing frame at the beginning, which
contains the number of frames in the file. It is useful for computing duration
of VBR files.

The muxer supports writing ID3v2 attached pictures (APIC frames). The pictures
are supplied to the muxer in form of a video stream with a single packet. There
can be any number of those streams, each will correspond to a single APIC frame.
The stream metadata tags @var{title} and @var{comment} map to APIC
@var{description} and @var{picture type} respectively. See
@url{http://id3.org/id3v2.4.0-frames} for allowed picture types.

Note that the APIC frames must be written at the beginning, so the muxer will
buffer the audio frames until it gets all the pictures. It is therefore advised
to provide the pictures as soon as possible to avoid excessive buffering.

Examples:

Write an mp3 with an ID3v2.3 header and an ID3v1 footer:
@example
avconv -i INPUT -id3v2_version 3 -write_id3v1 1 out.mp3
@end example

Attach a picture to an mp3:
@example
avconv -i input.mp3 -i cover.png -c copy -metadata:s:v title="Album cover"
-metadata:s:v comment="Cover (Front)" out.mp3
@end example
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@c man end MUXERS