Transcoding, Formats and Codecs

This chapter provides a short primer on video files, formats and codecs – because it is often cause for confusion:

A video file is a container. It usually contains one video track, one or more audio tracks, and possibly subtitle tracks, chapters… The way these tracks are stored in the file is defined by the file format. Common formats are avi, mov, ogg, mkv, mpeg, mpeg-ts, mp4, flv, or vob.

Each of the tracks by itself is encoded using a Codec. Common video codecs are h264, mpeg2, mpeg4, theora, mjpeg, wmv3. Common audio codecs are mp2, mp3, dts, aac, wav/pcm.

Not all codecs can be packed into a given format. For example the mpeg format is limited to mpeg2, mpeg4 and mp3 codecs (not entirely true). DVDs do have stringent limitations as well. The opposite would be .avi: pretty much every audio/video codec combination can be contained in an avi file-format.

To make things worse, naming conventions for video codecs and formats are often identical (especially MPEG ones) which leads to confusion. All in all it is a very wide and deep field. Suffice there are different uses for different codecs and formats.

Ardour specific issues

Ardour supports a wide variety of video file formats codecs. More specifically, Ardour itself actually does not support any video at all but delegates handling of video files to ffmpeg, which supports over 350 different video codecs and more than 250 file formats.

When importing a video into Ardour, it will be transcoded (changed from one format and codec to another) to avi/mjpeg for internal use (this allows reliable seeking to frames at low CPU cost—the file size will increase, but hard disks are large and fast).

The export dialog includes presets for common format and codec combinations (such as DVD, web-video,..). If in doubt, one of the presets should be used.

As a last note: every time a video is transcoded, the quality can only get worse. Hence for the final mastering/muxing process, one should always go back and use the original source of the video.