User's Manual Part 2

Tech-X Flex User Guide - Firmware v06.50 Tech-X Flex
®
(NG2)
6-42
Intro
Overview
Wi-Fi
Ethernet
System
IP/Video
MoCA
RF
Specs
Elementary stream - The elementary stream is the basic compressed audio or video bitstream. In
the case of a video stream, this is the original content segmented into macroblocks, slices, and
frames, then packetized with header information required to reconstruct the stream at the far end. An
elementary stream is a single stream of video or audio only, relying on the transport stream layer to
associate it with other streams and create the concept of a program.
Transport stream - Once constructed, one or more elementary streams are packetized into a
transport stream that provides all the instructions necessary to identify the data associated with a full
program, synchronize with the encoder, and reconstruct and present the audio/video program
properly. The transport stream includes the program clock reference or PCR, which provides the
critical data required for the decoder to synchronize its internal clock with that of the encoder. Without
synchronization, the decoder would be unable to recreate the video with the same timing as it was
encoded. Furthermore, the transport stream includes information such as:
Packet identifiers or PIDs - Used as unique identifiers for individual elementary streams, as well
as program-specific information as described below.
Program map table or PMT - Lists the elementary streams in the transport stream and identifies
the respective program(s) to which they belong. A program includes one or more elementary
streams, typically one video elementary stream and one or more audio elementary streams.
Program association table or PAT - Lists all the programs included in the transport stream, as a
high-level list of all programs available to the decoder (or in other words, channels available to the
end user). When a program is selected for decoding, the decoder uses the program identifier in
the PAT to look up the required streams in the PMT.
Conditional access table or CAT - Includes pointers to the PIDs that contain the entitlement
control/management messages needed to unscramble audio/video content, useful for
subscription-based services where access is limited.
Once completed, a transport stream is a sequence of 188-byte MPEG packets, ready for encapsulation
and transport over a communications network. The header data of transport streams, as well as that of
packetized elementary streams, is extremely useful for performing audio/video quality analysis, and
therefore provides the great majority of data used to calculate quality scores and other metrics.
With respect to degradation that may be caused during transport, the impact on audio/video quality
depends heavily upon the specific portion of the transport stream that is affected. For example, at the
lowest level, a loss of macroblock data may only cause a momentary anomaly in the display, perhaps not
even perceptible by the viewer. At the other extreme, a loss of MPEG transport header data, such as a
loss of synchronization, can cause the complete loss of the video altogether. For this reason, modern
analysis techniques must carefully consider the nature of loss and its respective impact on quality.
Overall, it should be noted that the descriptions here are highly-simplified, provided as a general
overview only. The full architecture of a complete MPEG transport stream is multi-layered and very
complex, beyond the scope of this document to describe.
Preliminary issue - Limited distribution only!