Advanced Transport Stream Settings; Transport Stream Overview; Overhead And Pts-Pcr Gap; Variable Mode - VBrick 9000 Admin Manual

Advanced Transport Stream Settings

Transport Stream Overview

In VBrick's implementation of transport stream (MPEG2TS) the encoder administrator has
unprecedented ability to configure the transport stream behavior optimally for the particular
requirements of his network and application. Transport Stream "mode" (See: Transport
Stream Settings) can be configured for constant or variable. In constant mode, the transport
stream is constrained to maintain a precise bit rate on the network at all times. In order to
assure this, it is required to add some level of empty (NULL) packets to handle the inevitable
bursts in bit rate generated by the encoding engine. In variable mode, empty packets are not
sent, but in VBrick's implementation, the peak bit rate is still as tightly constrained as it would
be in constant mode. Variable medium delay has precisely the same constraint on peak bit
rate as constant mode, however, since in constant mode that peak bit rate is always
maintained, the number of bits actually transmitted and averaged over time is increased for
constant mode.

Overhead and PTS-PCR Gap

Note: The default gap and latency settings for Transport Stream have not been
optimized. If you have a requirement for low overhead or low latency, you will need to
manually tune these settings for optimal performance.

Variable Mode

The three variable selections (Variable High Delay, Variable Medium Delay, and Variable
High Delay) differ in the peak bit rate permitted. All constrain the peak bit rate, however,
Variable High Delay limits the peak bit rate considerably more than Variable Low Delay. In
all cases, the bit rate averaged over time is the same. The peak bit rate is reflected in the
Overhead setting which selects how much the peak bit rate is allowed to exceed the average
bit rate on a percentage basis. Note that the average bit rate includes the video rate, the audio
rate, and any additional overhead required by the transport stream algorithm. Generally this
transport stream overhead tends to increase at lower bit rates. Selecting Variable High Delay
selects an overhead of 5%, Variable Medium Delay and Constant select an overhead of 10%,
and Variable Low Delay selects an overhead of 500%. Variable Low Delay therefore results in
very low latency, however, it is not appropriate for use in peak bandwidth constrained networks.
If the peak bit rate is more constrained, the inevitable variability in the size of the frames
output by video encoding algorithm will result in a longer time to transmit the larger frames,
and hence will increase latency. This trade off latency (Gap) and peak bit rate (Overhead)
desired by the user determines how the Mode, Overhead, and Gap should be set. It should be
noted that the settings for these parameters can also interact with the video quality. The Rate
Control Setting on the Video page allows you to decide if you wish to improve the latency at
a given overhead by sacrificing some video quality. A Rate Control Setting of 1 limits the
VBrick 9000 Encoder Admin Guide
Chapter 10
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