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\begin_body

\begin_layout Title
Transcoding Content With MediaTomb
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<copyright>
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  <year>2005</year>
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  <holder>Gena Batsyan</holder>
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  <holder>Sergey Bostandzhyan</holder>
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</copyright>
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<copyright>
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  <year>2006-2008</year>
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\begin_layout Standard

  <holder>Gena Batsyan</holder>
\end_layout

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  <holder>Sergey Bostandzhyan</holder>
\end_layout

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  <holder>Leonhard Wimmer</holder>
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</copyright>
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<releaseinfo>This documentation is valid for MediaTomb version 0.12.0.</releaseinfo
>
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<legalnotice>
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\begin_layout Standard
THIS SOFTWARE COMES WITH ABSOLUTELY NO WARRANTY! USE AT YOUR OWN RISK!
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</legalnotice>
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\begin_layout Section
Introduction
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\begin_layout Standard
MediaTomb version 0.11.0 introduces a new feature - transcoding.
 It allows you to perform format conversion of your content on the fly allowing
 you to view media that is otherwise not supported by your player.
 
\end_layout

\begin_layout Standard
For example, you might have your music collection stored in the OGG format,
 but your player only supports MP3 or you have your movies stored in DivX
 format, but your player only supports MPEG2 and MPEG4.
 Of course you could sit down and convert everything before viewing, but
 that is usually a time consuming procedure, besides, you often you want
 to keep your original data untouched and end up storing both, the converted
 and the original content - wasting space on your hard disk.
 That's where on the fly transcoding comes into play.
\end_layout

\begin_layout Standard
Another use case is online content - it is often presented in flv or asf
 formats, you may get mms or rtp streams which your player can not handle.
 The transcoding feature makes it possible to access such content.
\end_layout

\begin_layout Standard
Last but not least - subtitles.
 Only a few devices provide subtitle support, usually it's a proprietary
 solution not covered by UPnP.
 Using transcoding you can enable subtitles independent of the player device.
\end_layout

\begin_layout Section
Theory Of Operation
\end_layout

\begin_layout Standard
This chapter describes the idea behind the current transcoding implementation.
\end_layout

\begin_layout Subsection
What Happens On The User Level
\end_layout

\begin_layout Standard
So how does this work? First, let's look at the normal situation where you
 are playing content that is natively supported by your player, let's say
 a DivX movie.
 You add it to the server, browse the content on your device, hit play and
 start streaming the content.
 Content that the player can not handle is usually grayed out in the on
 screen display or marked as unsupported.
\end_layout

\begin_layout Standard
Now, what happens if transcoding is in place?
\end_layout

\begin_layout Standard
First, you define transcoding profiles, specifying which formats should
 be converted, let's assume that you have some music stored in the FLAC
 format, but your device only supports MP3 and WAV.
 So, you can define that all FLAC media should be transcoded to WAV.
 You then start MediaTomb and browse the content as usual on your device,
 if everything was set up correctly you should see that your FLAC files
 are marked as playable now.
 You hit play, just like usual, and you will see that your device starts
 playback.
 
\end_layout

\begin_layout Standard
Here is what happens in the background: when you browse MediaTomb, we will
 look at the transcoding profile that you specified and, assuming the example
 above, tell your player that each FLAC file is actually a WAV file.
 Remember, we assumed that the player is capable of playing WAV content,
 so it will display the items as playable.
 As soon as you press play, we will use the options defined in the transcoding
 profile to launch the transcoder, we will feed it the original FLAC file
 and serve the transcoded WAV output directly to your player.
 The transcoding is done on the fly, the files are not stored on disk and
 do not require additional disk space.
\end_layout

\begin_layout Subsection
Technical Background
\end_layout

\begin_layout Standard
The current implementation allows to plug in any application to do the transcodi
ng.
 The only important thing is, that the application is capable of writing
 the output to a FIFO.
 Additionally, if the application is not capable of accessing online content
 directly we can proxy the online data and provide a FIFO for reading.
\end_layout

\begin_layout Standard
The application can be any executable and is launched as a process with
 a set of given parameters that are defined in the profile configuration.
 The special command line tokes %in and %out that are used in the profile
 will be substituted by the input file name or input URL and the output
 FIFO name.
\end_layout

\begin_layout Standard
So, the parameters tell the transcoding application: read content from this
 file, transcode it, and write the output to this FIFO.
 MediaTomb will read the output from the FIFO and serve the transcoded stream
 to the player device.
\end_layout

\begin_layout Standard
Buffering is implemented to allow smooth playback and compensate for high
 bitrate scenes that may require more CPU power in the transcoding process.
\end_layout

\begin_layout Standard
Once you press stop or once you reach end of file we will make sure that
 the transcoding process is killed and we will clean up the FIFOs.
\end_layout

\begin_layout Standard
The chosen approach is extremely flexible and gives you maximum freedom
 of choice - you can also use this framework view mms and rtp streams even
 if this is originally not supported by your player, blend in subtitles
 or even listen to text documents using a text to speech processor.
\end_layout

\begin_layout Description
Note: it is possible and may be more convenient to call a wrapper script
 and not the transcoding application directly, however, in this case make
 sure that your shell script uses exec when calling the transcoder.
 Otherwise we will not be able to kill it.
\end_layout

\begin_layout Section
Sample Configuration
\end_layout

\begin_layout Standard
We will not go through all possible configuration tags here, they are described
 in detail in the main documentation.
 Instead, we will show an sample configuration and describe the creation
 process.
\end_layout

\begin_layout Standard
First of all you need to decide what content has to be transcoded.
 It makes no sens to transcode something that can be played natively by
 your device.
 Next, you have to figure out how smart your device is - UPnP defines a
 way in which it is possible to provide several resources (or several format
 representations) of the same content, however most devices only look at
 the first resource and ignore the rest.
 We implemented options to overcome this, however it may get tricky if you
 have several devices around and if each of them needs different settings.
\end_layout

\begin_layout Standard
All settings apply to your config.xml.
\end_layout

\begin_layout Subsection
Profile Selection
\end_layout

\begin_layout Standard
What do we want to transcode? Let's assume that you have some .flv files
 on your drive or that you want to watch YouTube videos on your device using
 MediaTomb.
 I have not yet heard of a UPnP player device that natively supports flash
 video, so let's tell MediaTomb what we want to transcode all .flv content
 to something that our device understands.
\end_layout

\begin_layout Standard
This can be done in the mimetype-profile section under transcoding, mappings:
\end_layout

\begin_layout Code
<transcode mimetype="video/x-flv" using="vlcprof"/>
\end_layout

\begin_layout Standard
So, we told MediaTomb to transcode all video/x-flv content using the profile
 named 
\begin_inset Quotes sld
\end_inset

vlcprof
\begin_inset Quotes srd
\end_inset

.
\end_layout

\begin_layout Subsection
Profile Definition
\end_layout

\begin_layout Standard
We define vlcprof in the profiles section:
\end_layout

\begin_layout Code
<profile name="vlcprof" enabled="yes" type="external">
\end_layout

\begin_layout Code
  <mimetype>video/mpeg</mimetype>
\end_layout

\begin_layout Code
  <agent command="vlc" arguments="-I dummy %in --sout #transcode{venc=ffmpeg,vco
dec=mp2v,vb=4096,fps=25,aenc=ffmpeg,acodec=mpga,ab=192,samplerate=44100,channels
=2}:standard{access=file,mux=ps,dst=%out} vlc:quit"/>
\end_layout

\begin_layout Code
  <buffer size="10485760" chunk-size="131072" fill-size="2621440"/>
\end_layout

\begin_layout Code
  <accept-url>yes</accept-url>
\end_layout

\begin_layout Code
  <first-resource>yes</first-resource>
\end_layout

\begin_layout Code
</profile>
\end_layout

\begin_layout Standard
Let's have a closer look:
\end_layout

\begin_layout Code
<profile name="vlcprof" enabled="yes" type="external">
\end_layout

\begin_layout Standard
The profile tag defines the name of the profile - in our example it's 
\begin_inset Quotes sld
\end_inset

vlcprof
\begin_inset Quotes srd
\end_inset

, it allows you to quickly switch the profile on and off by setting the
 enabled parameter to 
\begin_inset Quotes sld
\end_inset

yes
\begin_inset Quotes srd
\end_inset

 or 
\begin_inset Quotes sld
\end_inset

no
\begin_inset Quotes srd
\end_inset

 and also defines the profile type.
 Currently only one transcoding type is supported - 
\begin_inset Quotes sld
\end_inset

external
\begin_inset Quotes srd
\end_inset

.
\end_layout

\begin_layout Subsubsection
Specifying The Target Mime Type
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\begin_layout Standard
We need to define which mime type we are transcoding to - that's what the
 player device will see.
 It must be something it supports and there are also some other limitations:
 the output format must be streamable - meaning, it must be a format which
 can be played back without the need of seeking in the stream.
 AVI is a good example - it contains the index at the end of the file, so
 the player needs to seek (or use HTTP range requests) to read the index.
 Because of that you will not be able to transcode to AVI on the fly.
 A good target format is MPEG2 - it does not require the player to seek
 in the stream and it can be encoded on the fly with reasonable CPU power.
\end_layout

\begin_layout Standard
So, let's specify our target mime type:
\end_layout

\begin_layout Code
  <mimetype>video/mpeg</mimetype>
\end_layout

\begin_layout Standard
Bear in mind that this line only tells your player device about the content
 format, it does not tell anything to the transcoder application.
\end_layout

\begin_layout Subsubsection
Choosing The Transcoder
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\begin_layout Standard
Now it is time to look at the agent parameter - this tells us which application
 to execute and it also provides the necessary command line options for
 it:
\end_layout

\begin_layout Code
<agent command="vlc" arguments="-I dummy %in --sout #transcode{venc=ffmpeg,vcode
c=mp2v,vb=4096,fps=25,aenc=ffmpeg,acodec=mpga,ab=192,samplerate=44100,channels=2
}:standard{access=file,mux=ps,dst=%out} vlc:quit"/>
\end_layout

\begin_layout Standard
In the above example the command to be executed is 
\begin_inset Quotes sld
\end_inset

vlc, it will be called with parameter specified in the arguments attribute.
 Note the special 
\emph on
%in
\emph default
 and 
\emph on
%out
\emph default
 tokens - they are not part of the vlc command line but have a special meaning
 in MediaTomb.
 The 
\emph on
%in
\emph default
 token will be replaced by the input file name (i.e.
 the file that needs to be transcoded) and the 
\emph on
%out
\emph default
 token will be replaced by the output FIFO name, from where the transcoded
 content will be read by MediaTomb and sent to the player.
\end_layout

\begin_layout Standard
Just to make it clearer:
\end_layout

\begin_layout Code
<agent command="executable name" arguments="command line %in %out/>
\end_layout

\begin_layout Standard
So, an agent tag defines the command which is an executable (make sure that
 it is in $PATH and that you have permissions to run it), and arguments
 which are the command line options and where 
\emph on
%in
\emph default
 and 
\emph on
%out
\emph default
 tokens are used in the place of the input and output file names.
\end_layout

\begin_layout Description
Note: the output format produced by the transcoder must match the target
 mime type setting.
\end_layout

\begin_layout Subsubsection
Buffer Settings
\end_layout

\begin_layout Standard
There are no defaults for the buffer settings, they need to be tuned to
 the performance of your system and also to the type of transcoded media
 if you want to achieve the best result.
\end_layout

\begin_layout Standard
The idea behind buffering is the following: let's assume that you are transcodin
g a high quality video, the source format has a variable bitrate.
 Your CPU can handle most scenes in real time, but occasionally some scenes
 have a higher bitrate which require more processing power.
 Without buffering you would not have a fluent playback - you would see
 stuttering during those high bitrate scenes.
 That's where buffering comes into play.
 Before sending the data to your player for the very first time, we will
 delay the start of the playback until the buffer is filled to a certain
 amount.
 This should give you enough slack to overcome those higher bitrate scenes
 and watch the movie without any stuttering or dropouts.
 Also, your CPU will not transcode the stream as fast as it is being played
 (i.e.
 real time), but work as fast as it can, filling up the buffer during lower
 bitrate scenes and thus giving you the chance to overcome even long scenes
 with high bitrate.
\end_layout

\begin_layout Standard
The buffer accepts three parameters and is defined like this:
\end_layout

\begin_layout Code
<buffer size="5242880" chunk-size="102400" fill-size="1048576"/>
\end_layout

\begin_layout Standard
Size is the total size of the buffer, fill-size is the amount that has to
 be filled before sending out data from the buffer for the first time.
 Chunk-size is somewhat tricky, as you know we read the transcoded stream
 from a FIFO, we then put it into the buffer from where it gets served to
 the player.
 We read the data from the transcoder in chunks, once we fill up the chunk
 we put it into the buffer, so this setting is defining the size of those
 chunks.
 Lower values will make the buffer feel more responsive (i.e.
 it will be filled at a more fluent rate), however too low values will decrease
 performance.
 Also, do not set a too high value here since it may prevent smooth playback
 - data from the buffer is being played out, if you wait for a too big chunk
 at the same time you may empty the buffer.
\end_layout

\begin_layout Subsubsection
Accepting Or Proxying Online Content
\end_layout

\begin_layout Standard
With MediaTomb it is possible to add items that are not pointing to local
 content, but to online resources.
 It can be an mp3 stream, a YouTube video or some photos stored on the web.
 In case that the online media is stored in a format that is not supported
 by your player, you can use transcoding to convert it.
 Some transcoding applications, like VLC, handle online content pretty well,
 so you can give a URL directly to the transcoder and it will handle the
 data download itself.
 You can even use that to stream mms or rtsp streams, even if they are not
 directly supported by your player device.
 Some transcoders however, can not access online content directly but can
 only work with local data.
 For this situation we offer a special option:
\end_layout

\begin_layout Code
<accept-url>no</accept-url>
\end_layout

\begin_layout Standard
If this option is set to 
\begin_inset Quotes sld
\end_inset

no
\begin_inset Quotes srd
\end_inset

 MediaTomb will handle the download of the content and will feed the input
 to the transcoder via a FIFO.
 Of course the transcoding application must be capable of handling input
 from a FIFO.
 This only works for the HTTP protocol, we do not handle RTSP or MMS streams,
 use VLC is you want to handle those.
 When this option is set to 
\begin_inset Quotes sld
\end_inset

yes
\begin_inset Quotes srd
\end_inset

 we will give the URL to the transcoder.
\end_layout

\begin_layout Subsubsection
Resource Index
\end_layout

\begin_layout Standard
What is a resource? In this case it's the <res> tag in the XML that is being
 sent to the player when it browses the server.
 Each item can have one or more resources, each resource describes the type
 of the content by specifying it's mime type and also tells the player how
 and where to get the content.
 So, resources within the item point to same content, but allow to present
 it in different formats.
 In case of transcoding we will offer the original data as well as the transcode
d data by using the resource tags.
 A well implemented player will look at all resources that are available
 for the given item and choose the one that it supports.
 Unfortunately most players only look at the first resource and ignore the
 rest, this feature tells us to place the transcoded resource at the first
 position so that those renderers will see and take it.
\end_layout

\begin_layout Code
<first-resource>yes</first-resource>
\end_layout

\begin_layout Subsubsection
Hiding Original Resource
\end_layout

\begin_layout Standard
Sometimes it may be required that you only present the transcoded resource
 (read the previous section for explanation about resources) to the player.
 This option allows to do so:
\end_layout

\begin_layout Code
<hide-original-resource>yes</hide-original-resource>
\end_layout

\begin_layout Subsection
Advanced Settings
\end_layout

\begin_layout Standard
Sometimes you encounter a container format but want to transcode it only
 if it has a specific codec inside.
 Provided that MediaTomb was compiled with ffmpeg support we offer fourcc
 based transcoding settings for AVI files.
 A sample configuration for a profile with fourcc specific settings would
 look like that:
\end_layout

\begin_layout Code
<avi-fourcc-list mode="ignore">
\end_layout

\begin_layout Code
    <fourcc>XVID</fourcc>
\end_layout

\begin_layout Code
    <fourcc>DX50</fourcc>
\end_layout

\begin_layout Code
</avi-fourcc-list>
\end_layout

\begin_layout Standard
Please refer to the main documentation on more information regarding the
 options.
\end_layout

\begin_layout Standard
We also provide a way to specify that a profile should only process the
 Theora codec if an OGG container is encountered:
\end_layout

\begin_layout Code
<accept-ogg-theora>yes</accept-ogg-theora>
\end_layout

\begin_layout Standard
A new feature that was added in the 0.12 version possibility to specify that
 transcoded streams should be sent out using chunked HTTP encoding.
 This is now the default setting, since chunked encoding is preferred with
 content where the content length is not known.
 The setting can be controlled on a per profile basis using the following
 parameter:
\end_layout

\begin_layout Code
<use-chunked-encoding>yes</use-chunked-encoding>
\end_layout

\begin_layout Section
Testing And Troubleshooting
\end_layout

\begin_layout Standard
The external transcoding feature is very flexible, however there is a price
 for flexibility: a lot of things can go wrong.
 This section will try to cover the most common problems and present some
 methods on how things can be tested outside of MediaTomb.
\end_layout

\begin_layout Subsection
Testing The Transcoder
\end_layout

\begin_layout Standard
It's a good idea to test your transcoding application before putting together
 a profile.
 As described in the previous sections we get the transcoded stream via
 a FIFO, so it's important that the transcoder is capable of writing the
 output to a FIFO.
 This can be easily tested in the Linux command prompt.
\end_layout

\begin_layout Standard
Open a terminal and issue the following command:
\end_layout

\begin_layout Code
mkfifo /tmp/tr-test
\end_layout

\begin_layout Standard
This will create a FIFO called tr-test in the /tmp directory.
 Open a second terminal, we will use one terminal to run the transcoder,
 and another one to examine the output.
\end_layout

\begin_layout Standard
For this test we will assume that we want to transcode an OGG file to WAV,
 the easiest way to do so is to use the ogg123 program which is part of
 the vorbis-tools package.
 Running ogg123 with the -d wav -f outfile parameter is exactly what we
 want, just remember that our outfile is the FIFO.
 So, run the following command, replacing some audio file with an OGG file
 that is available on your system, in one of the terminals:
\end_layout

\begin_layout Code
ogg123 -d wav -f /tmp/tr-test /some/audio/file.ogg
\end_layout

\begin_layout Standard
The program will start and will appear to be hanging - it's blocked because
 noone is reading from the FIFO.
 While ogg123 is hanging, go to the second terminal and try playing directly
 from the FIFO (in this example we will use VLC to do that):
\end_layout

\begin_layout Code
vlc /tmp/tr-test
\end_layout

\begin_layout Standard
If all goes well you should see that ogg123 is coming to life and you should
 hear the output from VLC - it should play the transcoded WAV stream.
\end_layout

\begin_layout Subsection
Troubleshooting
\end_layout

\begin_layout Standard
This section will try to cover the most common problems related to the external
 transcoding feature.
\end_layout

\begin_layout Subsubsection
Media Is Unplayable
\end_layout

\begin_layout Standard
What if the resulting stream is unplayable? 
\end_layout

\begin_layout Standard
This can be the case with some media formats and contaeinrs.
 A good example is the AVI container - it contains the index at the very
 end of the file, meaning, that a player needs to seek to the end to get
 the index before rendering the video.
 Since seeking is not possible in transcoded streams you will not be able
 to transcode something to AVI and watch it from the FIFO.
\end_layout

\begin_layout Subsubsection
Transcoding Does Not Start
\end_layout

\begin_layout Standard
As explained in the previous sections, transcoding only starts when your
 player issues an HTTP GET request to the server.
 Further, the request must be made to the transcoding URL.
\end_layout

\begin_layout Standard
Most common cases are:
\end_layout

\begin_layout Standard
\begin_inset ERT
status open

\begin_layout Standard

<itemizedlist><listitem>
\end_layout

\end_inset


\end_layout

\begin_layout Standard
wrong mime type mapping: are you sure that you specified the source mime
 type correctly? Recheck the settings in the <mimetype-profile> section.
 If you are not sure about the source mime type of your media you can always
 check that via the web UI - just pick one of the files in question and
 click on the Edit icon.
\end_layout

\begin_layout Standard
\begin_inset ERT
status open

\begin_layout Standard

</listitem><listitem>
\end_layout

\end_inset


\end_layout

\begin_layout Standard
wrong output mime type: make sure that the mime type specified in the profile
 matches the media format that is produced by your transcoder.
\end_layout

\begin_layout Standard
\begin_inset ERT
status open

\begin_layout Standard

</listitem><listitem>
\end_layout

\end_inset


\end_layout

\begin_layout Standard
no permissions to execute the transcoding application: check that the user
 under which MediaTomb is running has sufficient permissions to run the
 transcoding script or application.
\end_layout

\begin_layout Standard
\begin_inset ERT
status open

\begin_layout Standard

</listitem><listitem>
\end_layout

\end_inset


\end_layout

\begin_layout Standard
transcoding script is not executable or is not in $PATH: if you use a wrapper
 script around your transcoder, make sure that it is executable and can
 be found in $PATH (unless you specified an absolute name)
\end_layout

\begin_layout Standard
\begin_inset ERT
status open

\begin_layout Standard

</listitem></itemizedlist>
\end_layout

\end_inset


\end_layout

\begin_layout Subsubsection
Problem Transcoding Online Streams
\end_layout

\begin_layout Standard
Some transcoding applications do not accept online content directly or have
 problems transcoding online media.
 If this is the case, set the <accept-url> option appropriately (currently
 MediaTomb only supports proxying of HTTP streams).
 This will put the transcoder between two FIFOs, the online content will
 be downloaded by MediaTomb and fed to the transcoder via a FIFO.
\end_layout

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