MPEG-2 Transport streams

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MPEG-2 Transport streams
• The MPEG-2 Systems Standard specifies two methods
for multiplexing the audio, video and other data into a
format suitable for transmission and storage, namely the
program stream and the transport stream.
• The Program Stream is designed for applications where
errors are unlikely.
• It contains audio, video and data bitstreams (also called
elementary bitstreams) all merged into a single
bitstream.
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• The program stream, as well as each of the elementary
bitstreams, may be a fixed or variable bit rate.
• DVDs use program streams, carrying the DVD-specific
data in private data streams interleaved with the various
video and audio streams.
• The Transport Stream, using fixed-size packets of 188
bytes, is designed for applications where data loss is
likely.
• Also containing audio, video and data bitstreams all
merged into a single bitstream, multiple programs can be
carried.
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• The DVB and ATSC digital television standards use
transport streams.
• Both the Transport Stream and Program Stream are
based on a common packet structure, facilitating
common decoder implementations and conversions.
• Both streams are designed to support a large number of
known and anticipated applications, while retaining
flexibility.
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• The MPEG-2 system layer includes a mechanism for
combining MPEG encoded video and audio with
associated data signals into transport streams.
• It can also allow the combination of any number of
different programs in to a single MPEG-2 transport
stream.
• A very important part of the system layer is the clock
reference information, which is used to enable the
recovery of individual video and audio clocks.
• With MPEG-2 encoding there can be variable delays
experienced for video and audio signals due to variable
length coding.
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• The system layer includes decoding time stamps (DTS)
and presentation time stamps (PTS) which help to
ensure the delay is consistent, and that the video, audio,
and data are kept in synchronization even though the
signals are packetized.
• The system layer also provides useful information about
the actual transmission network itself (Network
Information Table), about the programs being carried
(Program Allocation Tables), and finally Conditional
Access Tables which reference scrambled or encrypted
services.
• These facilities allow a decoder select a particular
television service from the services being delivered to
the consumer at any particular time.
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• In Figure 1 a conceptual single service MPEG-2 TS
multiplexer is shown.
• The video, audio, and data elements are encoded and
packetized separately.
• Then the resulting packets are combined together to
form a transport stream.
• Elementary streams (ES) are the most basic type of
MPEG-2 signals and are generated by the encoder.
They are separate data streams for video, audio, and
data signals.
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Video
source
MPEG-2
Elementary
Encoder
Packetizer
Audio
source
MPEG-2
Elementary
Encoder
Packetizer
PES
Systems layer
MUX
Data
source
Packetizer
PES
PSI
CLOCK
Figure 1. An MPEG-2 multiplexer.
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• Packetized elementary streams (PES) are variable
length packets of data which also contain stamps and
header information. They are separate data streams for
video, audio, and data signals.
• The structure of an MPEG-2 PES is shown in Figure 2.
• Program Clock Reference (PCR) is a reference for the
27 MHz clock regeneration and is transmitted at least
every 0,1 second.
• Program Specific Information (PSI) is data transmitted in
the TS for the receiver demultiplexer.
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Name
Size
Packet start code
prefix
Stream id
3 bytes
1 byte
PES Packet
length
Optional PES
header
Data
2 bytes
variable
length
Padding bytes
Figure 2. MPEG-2 PES structure.
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• A typical MPEG-2 TS packet structure is shown in Figure
3.
• A packet is the basic unit of data in a transport stream. It
consists of a sync byte, followed by three one-bit flags
and a 13-bit PID.
4 bytes
Header
184 bytes
Adaptation Field
(optional)
Payload
Figure 3. MPEG-2 TS packet structure.
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•
•
•
•
•
This is followed by a 4-bit continuity counter.
Additional optional transport fields may follow.
The rest of the packet consists of payload.
Packets are most often 188 bytes in length.
Each elementary stream in a transport stream is
identified by a 13-bit PID, Packet Identification.
• A demultiplexer extracts elementary streams from the
transport stream in part by looking for packets identified
by the same PID.
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• Transport stream has a concept of programs, which are
groups of one or more PIDs that are related to each
other.
• For instance, a transport stream used in digital television
might contain three programs, to represent three
television channels. Suppose each channel consists of
one video stream, one or two audio streams, and any
necessary metadata. A receiver wishing to tune to a
particular channel merely has to decode the payload of
the PIDs associated with its program.
• PAT stands for Program Association Table. The PAT lists
for all PMTs in the stream.
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• Program Map Tables (PMT) contain information about
programs.
• For each program, there is a PMT, with the PMT for each
program appearing on its own PID.
• The PMTs describe which PIDs contain data relevant to
the program.
• PMTs also provide metadata about the streams in their
constituent PIDs. For example, if a program contains an
MPEG-2 video stream, the PMT will list this PID,
describe it as a video stream, and provide the type of
video that it contains (in this case, MPEG-2).
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• To assist the decoder in presenting programs on time, at
the right speed, and with synchronization, programs
usually periodically provide a Program Clock Reference,
PCR, on one of the PIDs in the program.
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