MPEG user data

The MPEG user data feature provides a means to inject application-specific data into an MPEG elementary stream. User data can be inserted on three different levels: 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.
 * The sequence level
 * The group of pictures (GOP) level
 * The picture data level

Examples of information embedded in MPEG streams as user data are:
 * Aspect ratio information
 * "Hidden" information per the Active Format Description specification
 * Closed captioning per the EIA-708 standard