ANSI SCTE-138-2009 pdf download STREAM CONDITIONING FOR SWITCHING OF ADDRESSABLE CONTENT IN DIGITAL TELEVISION RECEIVERS
6.2 The MPEG-2 Transport Stream The transmission of addressable advertising within a digital cable television system is in compliance with MPEG-2 and SCTE standards. This section reviews some of the basic concepts, the knowledge of which is fundamental to understanding how addressable advertising systems might operate.
Typically, a number of separate television channels are combined in a single transport stream, as described in ISO/IEC 13818-1 [4]. The MPEG-2 transport stream is a packetized multiplex. Packets are fixed in length, and are comprised of a header portion and a payload portion. Video and audio coded data from each elementary stream within the multiplex is carried in the payload portion. The header portion carries, among other items a unique PID value that indicates to which elementary stream the contents of the payload portion belongs.
In addition to the data comprising encoded video and audio information for each television channel, the transport stream carries System Information streams which describe the video and audio data and enable a Receiver to select the proper streams to be decoded based on channel selection. There may also be data associated with applications, sometimes specific to the television channels contained within the multiplex, and there may be data transmitted to support decryption of encrypted video and audio elementary streams.
In accordance with the MPEG-2 Systems standard [4], the transmission of video, audio, and data is timed to insure that data from each stream is available at the Receiver in time for it to be decoded without overflowing that stream’s buffer in the Receiver. Null packets may be used in the stream as necessary to satisfy QAM modulator requirements. The result, as seen at the input to the Receiver, is a continuous stream of packets.
6.3 Level 0 Systems 6.3.1 Level 0 Stream Conditioning Level 0 systems permit addressable advertisement streams designated for insertion into a particular Primary Channel to be transmitted in an MPEG-2 transport multiplex different from the one that is carrying the Primary Channel. The Receiver, then, acquires the alternate multiplex before it can select and decode the required addressable advertisement. Typically, this will involve tuning to the RF channel carrying the multiplex that carries the addressable advertisements.