How do you tell an MPEG-1 bitstream from an MPEG-2 bitstream ?
A. All MPEG-2 bitstreams must contain specific extension headers that immediately follow MPEG-1 headers. At the highest layer, for example, the MPEG-1 style sequence_header() is followed by sequence_extension(). Some extension headers are specific to MPEG-2 profiles. For example, sequence_scalable_extension() is not allowed in Main Profile bitstreams. A simple program need only scan the coded bitstream for byte-aligned start codes to determine whether the stream is MPEG-1 or MPEG-2. What are start codes? These 32-bit byte-aligned codes provide a mechanism for cheaply searching coded bitstreams for commencement of various layers of video without having to actually parse variable-length codes or perform any decoder arithmetic. Start codes also provide a mechanism for re-synchronizing in the presence of bit errors. A start code may be preceded by an arbitrary number of zero bytes. The zero bytes can be use to guarantee that a start code occurs within a certain location, or by rate control
