ANSI/SCTE 35:2007 pdf download.Society of Cable Telecommunications Engineers.
5 Introduction
5.1 Splice points (Informative)
To enable the splicing of compressed bit streams, this standard defines Splice Points. Splice Points in an MPEG-2 transport stream provide opportunities to switch elementary streams from one source to another. They indicate a place to switch or a place in the bit stream where a switch can be made. Splicing at such splice points may or may not result in good visual and audio quality. Thai is determined by the performance of the splicing device,
Transport streams arc created by multiplexing PID streams. In this standard. two types of Splice Points for PID streams are defined: Out Points and In Points. In Points are places in the bit streams where it is acceptable to enter, from a splicing standpoint. Out Points are places where ii is acceptable to exit the bit stream. The grouping of In Points of individual PID streams into Program In Points in order to enable the switching of entire programs (video with audio) is defined. Program Out Points ti-w exiting a program are also defined.
Out Points and In Points are imaginary points in the bit stream located between two elementary stream presentation units. Out Points and In Points are not necessarily transport packet aligned and are not necessarily PES packet aligned. An Out Point and an In Point may be co-located: that is, a single presentation unit boundary may serve as both a safe place to leave a bit stream and a safe place to enter
The output of a simple switching operation will contain access unit data from one stream up until its Out Point followed by data from another stream starting with the first access unit following an In Point. More complex splicing operations may exist whereby data prior to an Out Point or data after an In Point are modified by a splicing device. Splicing devices may also insert data between one stream’s Out Point and the other stream’s In Point. The behavior of splicing devices will not be specified or constrained in any way by this standard.
5.2 Program Splice Points (Informative)
Program In Points and Program Out Points arc sets of PID stream In Points or Out Points that
correspond in presentation time.
Although Splice Points in a Program Splice Point coi-respond in presentation lime, they do not usually appear near each other in the transport stream. Ikcause compressed video takes much longer to decode than audio, the audio Splice Points may lag the video Splice Points by as much as hundreds of milliseconds and by an amount that can vary during a program.
This standard defines two ways of signaling which splice points within a program are to he spliced. A program_splice_flag, when true, denotes that the Program Splice Mode is active and that all PIDs of a program may be spliced (the splice information table PID is an exception: splicing or passage of these messages is beyond the scope of this standard). A program_splice_flag, when false, indicates that the Component Splice Mode is active and that the message will specii unambiguously which PIDs are to be spliced and may give a unique splice time for each. This is required to direct the splicing device to splice or not to splice various unspecified data types as well as video and audio.
While this standard allows for a unique splice time to be given for each component of a program. it is expected that most Component Splice Mode messages will utilize one splice time (a default splice time) for all components as described in section 7. The facility for optionally specitying a separate splice time for each component is intended to be used when one or more components differ significantly in their start or stop lime relative to other components within the same message. An example would be a downloaded applet that must arrive at a set-top box several seconds prior to an advertisernent
5.3 Splice events (Informative)
This standard provides a method ftw in-band signaling of splice events using splice commands to downstream splicing equipment. Signaling a splice event identities which Splice Point within a stream to use for a splice. A splicing device may choose to act or not act upon a signaled event (a signaled event should be interpreted as an opportunity to splice; not a command. A splice infirmation table carries the notice of splice event opportunities. Each signaled splice event is analogous to an analog cue tone. The splice infomiation table incorporates the functionality of cue tones and extends it to enable the scheduling of splice events in advance.
