MOD is reported to stand for

MPEG on disk.

Elementary stream (ES)

A coder – audio or video – produces a stream of bytes known as an elementary stream. The previous chapter outlines the information that is encoded into a video elementary stream. (Audio encoding is outside the scope of this book.)

An elementary stream can contain private streams.

Packetized elementary stream (PES)

An elementary stream is packetized into packets of 188 bytes, the first byte being MPEG’s sync byte valued 47h. Some systems construct 204 byte packets, expecting the channel coder to overwrite the final 16 bytes of each packet; in this case the sync byte will be B8h.

MPEG-2 program stream

An MPEG-2 program stream (PS) a relatively simple mechanism to multiplex video and audio of a single program for storage or transmission on relatively errorfree media such as computer disks or digital optical media. PS packets are variable-length; packets of

1 KByte or 2 KBytes are typical, though a packet can be as long as 64 KBytes. MPEG-2 program streams are used in applications such as these:

•DVD media uses a strict subset of MPEG-2 program stream encoding; the associated file extension is vob.

•The MOD consumer video format is essentially an MPEG-2 MP@ML SD program stream according to DVD conventions. On a computer, such files typically have extensions mpg or mpeg.

MPEG-2 transport stream

An MPEG-2 transport stream (TS) is a part of the MPEG-2 suite of standards that specifies a relatively complex mechanism of multiplexing video and audio for one or more programs into a data stream, typically having short packets, suitable for transmission through error-prone media where relatively powerful forward error-correction (FEC) is required. A transport stream is suitable for applications where a player connects to

a transmission in progress (like television), as opposed to reading a file from its beginning. For terrestrial over- the-air (OTA) or cable television, TS packets are expected to be suitably protected; however specifica-

ATM: Asynchronous transfer mode, a protocol for high performance networking.

ATSC Standard A/65, Program and

System Information Protocol.

TOD is reported to stand for transport stream on disk.

I write 29.97 Hz; expressed exactly, it’s 30/1.001.

tion of the FEC and channel coding lies outside the MPEG standards and ordinarily lies within the realm of digital television standards (for example, ATSC standards in North America, and DVB standards in Europe).

A transport stream packet (TSP) comprises 188 bytes – a 4-byte header (whose first byte has the value 47h), including a 13-bit packet identifier (PID), and 184 bytes of payload. Packet size was designed with ATM in mind: One TS packet fits into four ATM cells (48 bytes each). Owing to a lack of external interfaces for program streams, a single program transport stream

(SPTS) may be used to carry one program. For some applications, a multiple program transport stream

(MPTS) is used.

Transport stream packets with PID 0 contain the program association table (PAT), repeated a few times per second. The PAT lists one or more PIDs of subsequent packets containing program map tables (PMTs). A PMT lists PIDs of video and audio elementary streams associated with a single program.

An ATSC DTV transport stream contains a set of packets implementing the program and system information protocol (PSIP). PSIP identifies channels and programs, and conveys time-of-day and station callsign information. PSIP enables a receiver to provide an electronic program guide (EPG).

On a computer, 188-byte transport stream packets typically have a 4-byte timecode appended (resulting in 192-byte packets); a file comprising a sequence of such packets typically has the extension m2t, m2ts, or just ts. MPEG-2 transport streams are used in applications

such as these:

•The TOD consumer video format (essentially an MPEG-2 MP@HL HD transport stream)

•The BDAV container of Blu-ray

•H.264 compressed video

•AVCHD compressed video (in computing, the file extension mts is usual)

System clock

Synchronization in MPEG is achieved through a system clock reference (SCR). The lowest common multiple of 25 Hz and 29.97 Hz is 30 kHz; In MPEG-2, 90 kHz was

27 MHz divided by 90 kHz is 300.

Table 50.1 MPEG-2

PCR counts per frame

chosen as the basis for the program clock reference (PCR). A program clock value is represented in 33 bits, sufficient to provide unique PCR values over 24 hours.

MPEG system timing is based upon a 27 MHz reference clock, expressed by augmenting the PCR by

a nine-bit field taking a value from 0 through 299. Table 50.1 in the margin enumerates the number of PCR counts per frame at various frame rates.

Each program stream has a single reference clock. Different programs in an MPTS can have different program clocks, so provision is made for a transport stream to carry multiple independent PCRs.

Further reading

Chen, Xuemin (2002), Digital Video Transport System (Springer).

Whitaker, Jerry C. (2003), “DTV Service Multiplex and Transport Systems,” Chapter 13.2 in Standard Handbook of Video and Television Engineering, Fourth Edition (McGraw-Hill).

Whitaker, Jerry C. (2003), “DTV Program and System Information Protocol,” Chapter 13.4 in Standard Handbook of Video and Television Engineering, Fourth Edition (McGraw-Hill).