Digital Signal Processing Reference
In-Depth Information
18.2 Encapsulation
The video data may be continuously streaming or form
a single large file. However it is formatted, it must be broken up
into chunks to fit into IP packets, a process called encapsulation.
A simple method could use the largest possible data size for each
IP packet. However this has several disadvantages. Using very
large IP packets increases latency, because the packet cannot be
sent until enough data is made available by the source. If a packet
gets corrupted over the network, a large and noticeable amount of
data is lost. Also, large packets may be fragmented, meaning they
may be broken into multiple packets during transmission so that
they can be used by some networks
for example, standard
Ethernet only allows transmission of up to 1500 bytes in packet.
Very short packets, on the other hand, are inefficient. Since the
packet header is a constant size, it uses a larger portion of the
total packet size. More packets also put more load on the routers
in the network to process and route, as each packet is treated
individually. Sensitivity to latency as well as the packet error rates
on a given network can be used to determine packet data lengths.
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18.3 Video Streams
Compressed video uses several types of streams for transport
over a network. The simplest is the elementary stream, which
contains just the compressed data output from the video encoder
and does not contain audio or synchronization data.
A program stream contains several elementary streams, for
video, audio or data. It contains everything needed for a given
program to be presented. The data might be for on-screen text
message overlay, or it might be used for production and recording
functions. Time stamps are added to each of the elementary
streams to synchronize them. These enhanced elementary
streams are called packetized elementary streams (PES) because
the elementary stream has been broken into packets, each asso-
ciated with a different timestamp. The PES should not be
confused with IP packets. PES is associated with how video data is
packaged to maintain synchronization in a specific protocol used
for compressed video, and has nothing to do with what method or
protocols are used to move data, of which the internet is just one
of several methods.
There are two time stamps: one is known as the presentation
timestamp, indicating when each video packet should be displayed
in the video; the other is the decode timestamp, indicating the
order that it should be processed by the decoder. As discussed in
