The MPEG user data feature provides a means to inject application-specific data into an MPEG elementary stream

Elementary stream

An elementary stream as defined by MPEG communication protocol is usually the output of an audio or video encoder. ES contains only one kind of data, e.g. audio, video or closed caption. An elementary stream is often referred to as "elementary", "data", "audio", or "video" bitstreams or streams...

. User data can be inserted on three different levels:

• The sequence level
• The group of pictures
  Group of pictures
  In Video coding, a group of pictures, or GOP structure, specifies the order in which intra- and inter-frames are arranged. The GOP is a group of successive pictures within a coded video stream. Each coded video stream consists of successive GOPs...
  
   (GOP) level
• The picture data level

Applications that process MPEG data do not need to be able to understand data encapsulated in this way, but should be able to preserve it.

Examples of information embedded in MPEG streams as user data are:

• Aspect ratio
  Aspect ratio (image)
  The aspect ratio of an image is the ratio of the width of the image to its height, expressed as two numbers separated by a colon. That is, for an x:y aspect ratio, no matter how big or small the image is, if the width is divided into x units of equal length and the height is measured using this...
  
   information
• "Hidden" information per the Active Format Descriptor specification
• Closed captioning per the EIA-708
  EIA-708
  CEA-708 is the standard for closed captioning for ATSC digital television streams in the United States and Canada. It was developed by the Electronic Industries Alliance.Unlike most DVB captions, CEA-708 captions are textual like traditional Line 21 captions...
  
  standard