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Section 2 Technical elements




2.1 Definitions

For the purposes of this Recommendation†|†International Standard, the following definitions apply. If specific to a part, this is parenthetically noted.


2.1.1 access unit [system]: A coded representation of a presentation unit. In the case of audio, an access unit is the coded representation of an audio frame.
In the case of video, an access unit includes all the coded data for a picture, and any stuffing that follows it, up to but not including the start of the next access unit. If a picture is not preceded by a group_start_code or a sequence_header_code, the access unit begins with the picture start code. If a picture is preceded by a group_start_code and/or a sequence_header_code, the access unit begins with the first byte of the first of these start codes. If it is the last picture preceding a sequence_end_code in the bitstream all bytes between the last byte of the coded picture and the sequence_end_code (including the sequence_end_code) belong to the access unit.
2.1.2 bitrate: The rate at which the compressed bit stream is delivered from the channel to the input of a decoder.
2.1.3 byte aligned: A bit in a coded bit stream is byte-aligned if its position is a multiple of 8-bits from the first bit in the stream.
2.1.4 channel: A digital medium that stores or transports an ITU T Rec. H.222.0†|†ISO/IEC 13818 1 stream.
2.1.5 coded representation: A data element as represented in its encoded form.
2.1.6 compression: Reduction in the number of bits used to represent an item of data.
2.1.7 constant bitrate: Operation where the bitrate is constant from start to finish of the compressed bit stream.
2.1.8 constrained system parameter stream; CSPS [system]: A Program Stream for which the constraints defined in 2.7.9 on page 83 apply.
2.1.9 CRC: The Cyclic Redundancy Check to verify the correctness of data.
2.1.10 data element: An item of data as represented before encoding and after decoding.
2.1.11 decoded stream: The decoded reconstruction of a compressed bit stream.
2.1.12 decoder: An embodiment of a decoding process.
2.1.13 decoding (process): The process defined in this Recommendation†|†International Standard that reads an input coded bit stream and outputs decoded pictures or audio samples.
2.1.14 decoding time-stamp; DTS [system]: A field that may be present in a PES packet header that indicates the time that an access unit is decoded in the system target decoder.
2.1.15 digital storage media; DSM: A digital storage or transmission device or system.
2.1.16 DSM-CC; digital storage media command and control.
2.1.17 entitlement control message; ECM: Entitlement Control Messages are private conditional access information which specify control words and possibly other, typically stream-specific, scrambling and/or control parameters.
2.1.18 entitlement management message; EMM: Entitlement Management Messages are private conditional access information which specify the authorization levels or the services of specific decoders. They may be addressed to single decoders or groups of decoders.
2.1.19 editing: The process by which one or more compressed bit streams are manipulated to produce a new compressed bit stream. Edited bit streams meet the same requirements as streams which are not edited.
2.1.20 elementary stream; ES [system]: A generic term for one of the coded video, coded audio or other coded bit streams in PES packets. One elementary stream is carried in a sequence of PES packets with one and only one stream_id.
2.1.21 Elementary Stream Clock Reference; ESCR [system]: A time stamp in the PES Stream from which decoders of PES streams may derive timing.
2.1.22 encoder: An embodiment of an encoding process.
2.1.23 encoding (process): A process, not specified in this Recommendation†|†International Standard, that reads a stream of input pictures or audio samples and produces a coded bit stream conforming to this Recommendation†|†International Standard.
2.1.24 entropy coding: Variable length lossless coding of the digital representation of a signal to reduce redundancy.
2.1.25 event: An event is defined as a collection of elementary streams with a common time base, an associated start time, and an associated end time.
2.1.26 fast forward playback [video]: The process of displaying a sequence, or parts of a sequence, of pictures in display-order faster than real-time.
2.1.27 forbidden: The term "forbidden", when used in the clauses defining the coded bit stream, indicates that the value specified shall never be used.
2.1.28 ITU T Rec. H.222.0†|†ISO/IEC 13818 (multiplexed) stream [system]: A bit stream composed of 0 or more elementary streams combined in a manner that conforms to this part of ITU T Rec. H.222.0†|†ISO/IEC 13818-1.
2.1.29 layer [video and systems]: One of the levels in the data hierarchy of the video and system specifications defined in parts 1 and 2 of this Recommendation†|†International Standard.
2.1.30 pack [system]: A pack consists of a pack header followed by zero or more packets. It is a layer in the system coding syntax described in 2.5.3.3 on page 58 of this Recommendation†|†International Standard.
2.1.31 packet data [system]: Contiguous bytes of data from an elementary stream present in a packet.
2.1.32 packet identifier; PID [system]: A unique integer value used to identify elementary streams of a program in a single or multi-program Transport Stream as described in 2.4.3 on page 20.
2.1.33 padding [audio]: A method to adjust the average length of an audio frame in time to the duration of the corresponding PCM samples, by conditionally adding a slot to the audio frame.
2.1.34 payload: Payload refers to the bytes which follow the header bytes in a packet. For example, the payload of some Transport Stream packets includes a PES_packet_header and its PES_packet_data_bytes, or pointer_field and PSI sections, or private data; but a PES_packet_payload consists of only PES_packet_data_bytes. The Transport Stream packet header and adaptation fields are not payload.
2.1.35 PES [system]: An abbreviation for Packetized Elementary Stream.
2.1.36 PES packet [system]: The data structure used to carry elementary stream data. A PES packet consists of a PES packet header followed by a number of contiguous bytes from an elementary data stream. It is a layer in the system coding syntax described in 2.4.3.6 on page 33 of this Specification.
2.1.37 PES packet header[system]: The leading fields in a PES packet up to and not including the PES_packet_data_byte fields, where the stream is not a padding stream. In the case of a padding stream the PES packet header is similarly defined as the leading fields in a PES packet up to and not including padding_byte fields.
2.1.38 PES Stream [system]: A PES Stream consists of PES packets, all of whose payloads consist of data from a single elementary stream, and all of which have the same stream_id. Specific semantic constraints apply. Refer to 0.4 on page xvi.
2.1.39 presentation time-stamp; PTS [system]: A field that may be present in a PES packet header that indicates the time that a presentation unit is presented in the system target decoder.
2.1.40 presentation unit; PU [system]: A decoded Audio Access Unit or a decoded picture.
2.1.41 program [system]: A program is a collection of program elements. Program elements may be elementary streams. Program elements need not have any defined time base; those that do, have a common time base and are intended for synchronized presentation.
2.1.42 Program Clock Reference; PCR [system]: A time stamp in the Transport Stream from which decoder timing is derived.
2.1.43 program element[system]:A generic term for one of the elementary streams or other data streams that may be included in a program.
2.1.44 Program Specific Information; PSI [system]: PSI consists of normative data which is necessary for the demultiplexing of Transport Streams and the successful regeneration of programs and is described in 2.4.4 on page 44. An example of privately defined PSI data is the non-mandatory network information table.
2.1.45 random access: The process of beginning to read and decode the coded bit stream at an arbitrary point.
2.1.46 reserved: The term "reserved", when used in the clauses defining the coded bit stream, indicates that the value may be used in the future for ISO defined extensions. Unless otherwise specified within this Recommendation†|†International Standard, all reserved bits shall be set to '1'.
2.1.47 scrambling[system]: The alteration of the characteristics of a video, audio or coded data stream in order to prevent unauthorized reception of the information in a clear form. This alteration is a specified process under the control of a conditional access system.
2.1.48 source stream: A single non-multiplexed stream of samples before compression coding.
2.1.49 splicing[system]: The concatenation, performed on the system level, of two different elementary streams. The resulting system stream conforms totally to this Recommendation†|†International Standard. The splice may result in discontinuities in timebase, continuity counter, PSI, and decoding.
2.1.50 start codes [system]: 32-bit codes embedded in the coded bit stream. They are used for several purposes including identifying some of the layers in the coding syntax. Start codes consist of a 24 bit prefix (0x000001) and an 8 bit stream_id as shown in table 2-18 on page 36.
2.1.51 STD input buffer [system]: A first-in first-out buffer at the input of a system target decoder for storage of compressed data from elementary streams before decoding.
2.1.52 still picture: A coded still picture consists of a video sequence containing exactly one coded picture which is intra-coded. This picture has an associated PTS and the presentation time of succeeding pictures, if any, is later than that of the still picture by at least two picture periods.
2.1.53 system header [system]: The system header is a data structure defined in 2.5.3.5 on page 60 of this Recommendation†|†International Standard that carries information summarizing the system characteristics of the ITU T Rec. H.222.0†|†ISO/IEC 13818 Program Stream.
2.1.54 System Clock Reference; SCR [system]: A time stamp in the Program Stream from which decoder timing is derived.
2.1.55 system target decoder; STD [system]: A hypothetical reference model of a decoding process used to define the semantics of an ITU T Rec. H.222.0†|†ISO/IEC 13818 multiplexed bit stream.
2.1.56 time-stamp [system]: A term that indicates the time of a specific action such as the arrival of a byte or the presentation of a Presentation Unit.
2.1.57 Transport Stream packet header [system]: The leading fields in a Transport Stream packet, up to and including the continuity_counter field.
2.1.58 variable bitrate: An attribute of Transport Streams or Program Streams wherein the rate of arrival of bytes at the input to a decoder varies with time.

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