What is Transcoding?

Digital storage of any media requires conversion from analog to a digital format. The initial digital format after media production is still a raw file, and for it to be stored and accessed across different devices it needs to be compressed in the particular digital format compatible across the device. This form of video compression for files to make them compatible with a target device is called encoding.

The terms encoding and transcoding are sometimes used interchangeably, inspite of the use-case difference between the two.

Generally speaking, encoding refers to the process of converting uncompressed data to a desired format. This is understood to be a lossy process. On the other hand, transcoding is the process of decoding a video file from one format to an uncompressed format, and then encoding the uncompressed data to the desired format. Video transcoding is commonly used when the video file is being moved from a source to a different destination, and when the two support different file formats.

One of the most important uses of video transcoding is in uploading video from one source – a desktop, to an online video hosting site, so that the format is supported by video hosting site.

Some other terminology in use regarding video transcoding and encoding are:

  • Transmuxing – Conversion to a different container format without changing the file itself
  • Transrating – Conversion to a different bitrate using the same file format

What is a Codec?

A video codec is any device/software that compresses a video file. A device/software that only compresses an analog file is called an encoder, whereas a device/software that only decompresses a compressed digital file to analog is a decoder. The term ‘codec’ comes out of the concatenation of the two terms encoding and decoding.

How do codecs work?

For any codec to work it needs to compress the frames. There are two types of frame compressions – inter-frame and intra-frame compression. In intra-frame compression, each frame is compressed independently of the adjacent frames. It is therefore essentially image-compression applied to video.

Inter-frame compression on the other hand identifies redundancies across frames to compress videos. This is includes any elements of the moving image that may be static – say a static background in a talking head video. Inter-frame compression is much more efficient than intra-frame compression, and so most codecs are optimized to identify redundancies across frames.

Transcoding on cloud server for secure video hosting

What are some of the most prominent codecs?

MPEG (Motion Picture Experts Group) is the most common family of video codecs. International Standards Organization (ISO), whose standards impact the computer and consumer electronics markets has had at the following list of codecs as its standards:

  • MPEG-1 in 1993
  • MPEG-2 in 1994
  • MPEG-4 in 1999
  • AVC/H.264 in 2002

H.264 is the most prevalent compression standard in use currently. For a period in the late 90s and early 2000s, the rivalry between RealNetworks and Microsoft was around creating their own proprietary formats as the standards for codecs. RealNetwork’s RealVideo, Microsoft’s Windows Media Video, ON2’s VP6 and Sorenson Video 3 were the dominant proprietary codecs. The H.264 codec was added to Apple’s QuickTime 7 in 2005, and in 2007 added H.264 support for Flash.

VP9 is the proprietary video compression format and codec developed by ON2 Technologies, which were acquired by Google in 2010. VP9 is available under new BSD License by Google with source code available as libvpx VP9 codec SDK. 

Theora is a free lossy video compression format, distributed without licensing fees. Theora is derived from VP3 codec, which has been released into the public domain. Ogg Video Container format uses Theora as its compression format.

How is a media file format distinct from a codec?

A file format is a container, inside which is the data that has been compressed by a video codec. A single file format may support multiple video codecs. For example the Audio Video Interleaved (AVI) file format contains data that compressed from any of a range of codecs.

What are some of the most prominent containers?

QuickTime File Format is a multimedia container format used by Apple’s multimedia framework.

MP4 is the most popular container format used for storing digital audio and video. Both QuickTime and MP4 file formats use the same MPEG-4 format.

FLV is the file container format used for video content using Adobe Flash Player. Flash video content can be embedded within SWF files.

WebM is the royalty-free container format, sponsored by Google. WebM uses the VP8 and VP9 codecs as compression formats.

Ogg container format is maintained by Xiph.Org foundation, and is not restricted by software patents used in H.264. Ogg is supported by the Wikipedia community.

Advanced Systems Format (ASF) is Microsoft’s proprietary video container format designed for streaming media.

 

Reference

“What is a Codec?” Jan Ozer, StreamingMedia.com