Encoding Profile Configuration

A workflow defines which operations are applied to media ingested into Opencast and the order of these operations. An operation can be something general like “encode this video”. The encoding profiles then specify exactly how a media is encoded, which filters are applied, which codecs are used and in which container these will be stored, …

Opencast comes with a set of such profiles generating files for both online playback and download. These profiles are build to work for everyone, meaning that in most cases optimization can be done according to local needs. So modifying these profiles or building new ones often makes sense. This document will help you modify or augment Opencast's default encoding profiles for audio, video and still images.

Default Profiles and Possible Settings

This section contains some notes about the default profiles, explaining some thoughts behind those profiles and pointing at things you might want to change depending on your local set-up.

A/V-Muxing: From lossless to safe

The audio/video muxing (profile.mux-av.copy) is applied if audio and video is sent to Opencast separately. The basic idea behind this is, to combine these separate files into one file which can later be converted in one step.

Possible settings:

The safest option for muxing is to always re-encode the streams. It is far slower than re-using the existing bit streams. It also, always means a quality loss.

Create an Encoding Profile

This section will help you to understand how you can modify an existing profile or create a completely new one.

Creating a new encoding profile is a matter of creating a configuration file and placing it in the encoding profiles watch folder.

Step 1: Encoding Profile Folder

The <config_dir>/encoding folder allows you to quickly augment Opencast's existing behavior, simply by modifying or adding new configuration files. The file names should follow the pattern *.properties.

Step 2: The Encoding Profile

Encoding profiles consist of a set of key-value pairs that conform to the following pattern:

profile.<name>.<context>.<property> = <value>

For example:

profile.mp4.http.name = Enocde Mp4 files for download

All profiles should have the following properties:

.input  = [audio|visual|stream|image]
.output = [audio|visual|stream|image]

For example:

// My audio/video encoding profile
profile.my-av-profile.http.name           = my audio/video encoding profile
profile.my-av-profile.http.input          = visual
profile.my-av-profile.http.output         = visual
profile.my-av-profile.http.suffix         = -encoded.enc
profile.my-av-profile.http.ffmpeg.command = -i #{in.video.path} -c:v venc -c:a aenc #{out.dir}/#{out.name}#{out.suffix}

The most important part of this profile is the ffmpeg.command. This line specifies FFmpeg command line options using #{expression} for string replacement.

Step 3: FFmpeg

To create a new profile you have basically one task to do: Find an appropriate FFmpeg command line for whatever you want to do. For more information about FFmpeg, its options and how you can build FFmpeg with additional functionality have a look at the Official FFmpeg Wiki. For trying out new encoding settings, just call FFmpeg from the command line.

Using a Profile

Once defined, use your encoding profile in your workflow by setting the encoding-profile property to the profiles name:

    description="Encode presenter using my audio/video encoding profile">
    <configuration key="source-flavor">presenter/work</configuration>
    <configuration key="target-flavor">presenter/delivery</configuration>
    <configuration key="target-tags">rss, atom, captioning</configuration>
    <configuration key="encoding-profile">my-av-profile.http</configuration>

Have a look at the Workflow Configuration section for more details about workflows and workflow operations.