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© NP 1999 MPEG-1 Multiplexing Nimrod Peleg Update: Nov. 1999
© NP 1999 Multiplex Organization System layer: –Audio and Video encoders deliver Elementary Streams (ES), from the “Compression Layer”. –Each ES carries Access Units (AU), which are the coded representations of Presentation Units (PU). –These streams, have to be combined in an organized manner (with additional information) to allow correct decoding. –“Private Data” streams also packed !
© NP 1999 Decoder Missions Separation of coded bit-stream into basic AU. Synchronization of picture and sound. Selection of the particular components of interest (done by the user). Part I of MPEG-1 defines the rules of system layer, grouping video, audio and private data ES into a single bit-stream.
© NP 1999 System Layer Functions Packetization of multiple streams into one. Addition of time stamps on ES for Synchronization at playback. Initialization and management of the required buffers.
© NP 1999 System Target Decoder The system layer should also indicate the necessary resources required for the decoding the bit-stream. One important example is the size of buffers needed in the decoder for each individual ES. This is done using a theoretical reference decoder known as System Target Decoder (STD).
© NP 1999 Packetized Elementary Stream Each ES is cut into packets to form a PES, starting with packet header and followed by data. Structure of MPEG1 packet: Start_code_prefixstart code ( Hex)24 b Stream_idPES type (4b) and number (4b)8 b Packet_lengthnumber of bytes to follow these 216 b Stuffing_bytesOptional stuffing0 to 16x8 B
© NP 1999 Structure of MPEG1 packet (cont’d) Start_STDbits 01: begin STD_buffer field2 b STD_buffer_scalebuffer size unit (0:128B,1:1024B1 b STD_buffer_sizein multiples of 128 or 1024 B13 b PTS (optional)Presentation time stamp40 b 4 code bits + 33 time bits + 3 marker bits DTS (optional)Decoding time stamp40 b 4 code bits + 33 time bits + 3 marker bits Packet_data_byteData (N=packet_length less the 6 following fields)Nx8 b
© NP 1999 Some explanations... Start code: 32b, including 4 for type (audio, video, private), and 4 for id number of the ES from which it comes. Packet length: 16b, hence maximum length of 64KB Buffer size: required by STD for decoding. DTS: Decoding time of 1st AU of the packet. PTS: The time at which the corresponding PU should be presented (displayed or made audible)
© NP 1999 Time Stamps Used for audio and video synchronization. Sent frequently :maximum interval of 0.7Sec between consecutive stamps of a PES. Time stamps are coded with 33 bits, which represent absolute time, expressed in periods of a 90KHz reference clock.
© NP 1999 Packet size A packet can carry a variable number of data bytes, within the limit of 64kB. The packet size depends on the characteristics of the transmission line or Digital Storage Medium (DSM). The standard allows adding of up to 16 stuffing bytes, e.g to align on the physical sectors of a storage medium.
© NP 1999 Packs Packets are grouped in packs. The pack header contains timing and bitrate information. SCR (system clock reference): used to sync. A 90KHz System Time Clock (STC), common to all ES, which is a time base and a measuring unit for the DTS and PTS time stamps ( that are sent in the packet).
© NP 1999 MPEG-1 Pack Header Pack_start_codestart code: BA Hex32 b SCR_start beginning of SCR filed: ‘ 0010 ’ 4 b SCR [32..30]Sys. Clock ref (3 MSb)3 b marker_bitalways ‘ 1 ’ 1 b SCR [29..15]15 intermediate bits15 b marker_bitalways ‘ 1 ’ 1 b SCR [14..0]15 LSb15 b marker_bitalways ‘ 1 ’ 1 b Mux_rateMux bitrate (in multiples of 50B/S)22 b marker_bitalways ‘ 1 ’ 1 b
© NP 1999 System Header It’s a special packet which delivers all system parameters used during the stream, and can optionally be repeated at any new pack (in order to ease access to a random point in the sequence). Example of some important parameters: Maximum bit-rate, id of audio, Video private data, minimum size of input buffers etc.
© NP 1999 System Header (cont’d) Number of video PES in MPEG-1 ES : 0-16, Audio PES: 0-32 and private PES: 0-2. Pack number 1Pack number 2 Pack Header System Header PacketFollowing Packet Last Packet End code SCR, Mux rate System info, number and type of ES Audio and Video PES, Private B9
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