Media Server Technology: Karl Paulsen
Serving Up ITV, Part 1
Once the deployment of digital television (DTV) advances enough
to warrant meaningful data broadcasting and other services –
such as interactive television – content developers and broadcasters
will need a means to store, manage and process that data as it makes
its way through the DTV system.
Nonaudio- and video-centric information that is intended to be
carried in the ATSC-compliant MPEG-2 streams will require hardware
and software that can integrate and synchronize not only video and
audio elements, but also other supplemental data – in a harmonious
format that the broadcaster can efficiently and effectively use.
In turn, the receiving devices will need tools to use this supplemental
information in a way that viewers will find attractive, entertaining
and valuable.
Video servers and their associated sub-systems are a natural medium
for storing and playing-out the video/audio content, as well as
the metadata or associated subchannel information needed for broadcast
television video transmissions. This process is now recognized as
"data broadcasting."
The potential enrichment of program content – conveyed in
time coincidence with conventional broadcast programming –
is only one of the many features and aspects of DTV for which standards
are already in place. Applications for future enhancements in the
field of data broadcasting continue to be developed. This installment
of my column will look at aspects of auxiliary digital services
heretofore only marginally possible in the analog NTSC broadcast
domain.
DATA BROADCAST FRAMEWORK
Data broadcasting technology is structured by, and described within,
the ATSC A/90 data broadcast framework. In the United States, portions
of the software environment for the A/90’s work are contained
under the description of the Digital TV Application Software Environment
(DASE). The Media Home Platform (MHP) is the European equivalent
of digital video broadcast (DVB). Both activities are intended to
enable and unify technology aimed at standardizing interactive digital
television content and behavior.
DASE working groups are focused on providing three fundamental
levels of applications, described in Table 1. Each of these
DASE levels, which are intended to provide increased capabilities
and services, will be possible once the deployment of DTV is accelerated
and the technology is incorporated into set-top boxes and/or receivers
to a sufficient degree.
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Table 1: DASE levels
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Over time, television receivers will be outfitted with additional
integral or outboard component features that will enhance the entertainment
and information capabilities of digital television broadcasting.
These devices, – possibly referred to in the future as digital
appliances – will combine such subsystems as personal video
recorders (PVR), Internet or wireless communication interfaces for
two-way interactivity, complete computer-centric control systems,
as well as sophisticated viewer-sensitive selection systems that
target the individual user (in much the same way the Internet selects
and processes information according to a user’s profile).
DASE SYSTEM INTERCONNECT
The DASE System Interconnect incorporates the fundamentals of
the digital television receiver – from the demodulator to broadcast
(MPEG-2) transport and user control to display and audio; it also
incorporates the layer of applications (the DASE system) that supplements
the platform services. Fig.1 depicts how the DASE system
is interfaced to the DTV-user system.
The architecture of MPEG-2 was fundamentally created to carry
the data necessary to enable the feature sets envisioned for data
broadcasting. These applications – such as false video-on-demand,
interactivity and the appearance of directed information channeling
to the viewer – are possible because of the various systems’
levels embodied in the transport mechanism of MPEG-2.
Future developments in MPEG-4, -7 and —21 may eventually
offer even more capabilities as applications are developed and systems
are deployed. How this architecture is assembled, in terms of the
MPEG-2 systems and DASE, will be the focus of the remainder of this
installment.
The encapsulation and structure of information used for data broadcasting
relies primarily on the MPEG-2 systems and extensions to the Digital
Storage Media-Command and Control (DSM-CC) specifications (ISO/IEC 13818-6).
In the same fashion that the Internet has attracted millions of
users in part because of its customizable feature sets, objectives
for DTV audiences also include the ability to customize the viewing
experience on a user-by-user, and channel-by-channel, basis.
The systems necessary to implement user-selectable controls are
defined in the DSM-CC specification, whose original aim was to provide
VCR-like controls for two-way services, such as VOD. DSM-CC’s
download protocol is used to deliver files in formats that support
a delivery on either a one-time or a carousel basis; the files are
sent via a synchronized non-streaming download or an asynchronous
streaming basis.
The carousel concept is based on multiple sets of short snippets
of data being periodically sent, as well as repeatedly sent, so
that the receiver appliances collect sufficient data to respond
to the actions, without having to wait for a lengthy download period
before any of the information is usable.
The depth and complexities of MPEG-2 provide avenues for the deployment
of interactive television broadcasting. Under the MPEG-2 umbrella
are two packet-oriented coding formats that are described in MPEG-2
systems: the program stream and the transport stream. Both are format-optimized
for their particular application or environment.
Historically, MPEG-2 program streams were to be used for storage
systems (including video servers) and employed in VOD and interactive
multimedia applications.
Today, their main applications are in DVD. Program streams are
designed for high-reliability transmissions where their large- and
variable-sized payloads require low-loss/low-error performance.
On the other side, MPEG-2 transport streams carry multiple MPEG-2
programs and applications for environments that are hostile –
meaning the conditions are potentially lossy – the quality
of the transmission varies and the susceptibility to accumulate
errors is high (for such things as microwave, satellites or cable
systems). Transport stream coding formats are the standard that
the ATSC selected for program and emission transmissions.
An MPEG-2 program is a set of component streams or elements, generally
comprised of compressed audio and video, which share a common time
base. A program multiplexer assembles this collection of compressed
audio and video elements, which were created from baseband (digital)
media sent through video and audio encoders, into bitstreams that
are properly coded according to the MPEG-2 system’s standard.
It is this program multiplexer that adds clock references and
time stamps to the encapsulated elementary streams, generating an
MPEG-2 transport stream. For DTV emission, one or more sets of transport
streams, created from multiple program multiplexor outputs, are
multiplexed (muxed) to form a compliant stream (e.g., ATSC) that
suitably meets the needs of the transmission format it is intended
to serve.
The final stream comes together in this emission mux, which will
further accept streams from data servers, from PSIP generators and/or
form-conditional access generators – generally at the time
of transmission formatting. It is at this stage that any scheduling
or synchronization between MPEG-2 programs, in the form of transport
streams, and data (for data broadcast applications) needs to occur.
Often a third-party data management entity is employed, whose task
it is to automate, schedule and control the activities of the emission
mux such that the synchronization of outbound data and any return
channel data management is harmonized.
The data management tool may also be the mechanism that records
– for billing purposes – the specifics of all the transport
stream activities, since it is at this point in the process that
all the elements are combined into a single transport so as to be
ready for input to a transmission-specific device (e.g., fiber or
microwave link, or 8-VSB modulator).
In part two, we will continue with the integration of data servers
for DTV broadcasting and explain how future profiles and architectures
for interactive services are being configured.
Karl Paulsen is vice president of Engineering at Azcar (www.azcar.com).
He is the author of the book "Video and Media Servers: Technology
and Applications"–now in its Second Edition (published
by Focal Press). Contact him via e-mail at: karl.paulsen@azcar.com
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