Digital Signal Processing Reference
In-Depth Information
previous chapters, the order of decode is different from the order of
display in a typical GOP, with a mixture of I-, P- and B-frames.
Program streams can contain multiple video displays, sup-
porting picture in picture, or multiple video sources
such as an
anchor person on one part of the screen and a remote video feed in
another. Another example is sports coverage, wheremultipleangles
might be shown simultaneously. Programstreams are usually used
when little or no data loss is expected in the transmission,
and typically use long packets. Applications of program streams
include DVD players or within a production studio or trailer.
The transport stream is often used when the video is trans-
mitted over long distances, over different types of multi-user
networks. This could be satellite links, broadcast terrestrial links or
video over IP. Transport streams feature error correction schemes,
such as Reed Solomon. To facilitate error correction, fixed-length
packets of 188 bytes are used. Accounting for additional bytes in
error correction, this can increase to 204 or 208 bytes. Transport
streams can contain multiple elementary streams, and each is
identified with a packet identifier (PID). PIDs are used to differ-
entiate between the video, audio and data elementary streams
used to make up a complete program. To keep track of the PIDs,
two further structures are used. One is the program association
table (PAT), which is sent to provide the index of all the programs
in the transport stream (this is used if PID
e
0). The other structure
is multiple program map tables (PAT), one for each program,
giving the PID numbers for the video stream(s), audio stream(s)
and data stream(s) in that program.
The transport-stream packets are much smaller than the
allowable data-packet lengths used in IP. Even restricting the IP
packet length to 1500 bytes to prevent fragmentation on Ethernet,
there can be seven of the 208 byte long transport stream packets
encapsulated.
ΒΌ
18.4 Transport Protocols
Video streams are sent over IP networks. IP networks have
their own protocols, which are independent of what type of data
or application the IP network is being used for. These protocols
are not specific to video over IP, but are used for nearly all
internet applications.
The most common protocol used is transmission control
protocol (TCP), also commonly referred to as transmission control
protocol over internet protocol (TCP/IP). TCP establishes a reliable
connection between source and destination using defined hand-
shaking schemes. It keeps track of all IP packets by assigning
