.. _transcode-a-file:

##################################
How to transcode a file?
##################################

This page describes how to transcode a file (re-encode the elementary audio, video, and subtitle streams) using VLC. 

We use the ``transcode`` module to re-encode the streams, and then the ``std`` module to mux them together into a container. Hence the basic structure of the code will look like the following:

.. code-block ::
   
   $ vlc sample.mp4 --sout="#transcode{ }:std{ }"

To know about how the components of transcode and standard output interact, refer to the :ref:`Combining Transcoding and Standard Output <combine-transcode-and-std-output>` section. 

This page provides the code for performing simple trancoding and saving the resultant file.

.. _transcode-and-save:

**********************************************
How to transcode a file and save it locally?
**********************************************

To save the file locally, we will set ``access=file`` in the ``std`` block.

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Case 1: Re-encode using different codecs but mux it in the same container
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Suppose you have a file named ``sample.mp4`` which has been encoded by ``h264`` video-codec and ``mp4a`` audio-codec. (You can find which codecs have been used to encode a file by playing it on VLC and navigating to :menuselection:``Tools -> Codec Information``).

For this example, let's say we wish to re-encode this media file by using the ``h265`` and ``mpga`` codecs, and want the final output to still be contained in an ``MP4 container``.

In this case, the following code will do the required transcoding:

.. code-block ::

   $ vlc sample.mp4 --sout="#transcode{acodec=mpga, vcodec=h265}:std{access=file, mux=mp4, dst=sample_new.mp4}"

Comparing the codec-information of the original and the new file will look something like the following: 

.. figure::  /images/advanced/transcode/transcode_how_to_h264_vs_h265_media_info.png
   :align:   center

   Even though both the files have the same extension, they have been encoded by different codecs.

|

As the H.265 codec offers better compression than H.264, the new file is significantly smaller than the old one, as can be seen in the following figure: 

.. figure::  /images/advanced/transcode/transcode_how_to_h264_vs_h265_properties.png
   :align:   center

   Notice how smaller the new file is in terms of size as compared to the original one.

|

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Case 2: Re-encode using different codecs and mux it in a different container
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Let's suppose that we want to encode the same ``sample.mp4`` file while using ``h265`` and ``mpga`` codecs, and we also want the final output to be contained in an ``MKV`` (matroska) container. Note that we can do this because the above mentioned codecs are compatible with the matroska container. 

For this conversion, the code will look like the following: 

.. code-block ::

   $ vlc sample.mp4 --sout="#transcode{acodec=mpga, vcodec=h265}:std{access=file, mux=mkv, dst=sample_new.mkv}"


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Another Example for Case 2
++++++++++++++++++++++++++++

For this example, we will transcode the ``sample.mp4`` file by encoding it using ``theo`` and ``vorb`` codecs, and mux the output in an ``OGG`` container. Notice that in this case, we *have* to change the container because ``theo`` and ``vorb`` are not compatible with ``MP4``.

We can do the required transcoding by running the following code:

.. code-block ::

   $ vlc sample.mp4 --sout="#transcode{acodec=vorb, vcodec=theo}:std{access=file, mux=ogg, dst=sample_new.ogg}"


**************************************
Additional parameters for Transcoding
**************************************

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General purpose transcode parameters
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In the ``transcode`` block, we can detail on other specifications if we want more customization, for example: 

 + Bitrate of the audio and video streams (``vb`` and ``ab``).
 + Frame rate (``fps``) of the video stream.
 + Sample rate (``samplerate``) of the audio stream.
 + Number of audio channels (``channels``), etc.

Extending the second example, if you wish to detail on the above mentioned specifications, then the code might look something like this:

.. code-block ::

   $ vlc sample.mp4 --sout="#transcode{vcodec=h265, vb=800, fps=24, acodec=mpga, ab=128, channels=2, samplerate=44100, scodec=none}:std{access=file, mux=mkv, dst=sample_new.mkv}"

Apart from mentioning the specifications while transcoding, we can also apply various filters and effects, including: 

 + Video filters (``vfilter``) like blur, ripple, rotate, adjust (contrast, brightness, hue, saturation, gamma), etc.
 + Subtitle filters (``sfilter``) like marq, logo, mosaic, etc. 
 + Audio filters (``afilter``) like equalizer, karaoke filter, etc.

Further, we can also :ref:`combine transcoding and streaming <transcode-and-stream>`, where instead of saving the final output (the transcoded and muxed stream), we directly stream it over the local network. 

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Changing encoder parameters in transcode block
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The ``transcode`` block allows you to define parameters specific to the encoder modules.

 + the ``venc`` parameter forces a specific video encoder module to use (ie, using the ``x264`` encoder to have H.264 video codec).
 + the ``aenc`` parameter forces a specific audio encoder module to use.
 + the ``senc`` parameter forces a spefific subtitle encore module to use.

Moreover, you can specify some custom encoder parameters using the following syntax: ``venc=encoder{parameter1=...,parameter2=...}``

For example, the following example will:

 + Force the H.264 video encoder to ``x264``.
 + Set the H.264 profile to ``main``.
 + Set the H.264 maximum GoP (Group of Pictures) to 30 (parameter is ``keyint``).

.. code-block ::

   $ vlc sample.mp4 --sout="#transcode{vcodec=h264, venc=x264{profile=main, keyint=30}, fps=30, acodec=none, scodec=none}:std{access=file, mux=mkv, dst=sample_new.mkv}"