Digital Broadcast Case Studies
Author: Michael Adams
Publisher: Cisco Press (53)
This chapter considers the implementation of digital broadcast cable
systems. The two predominant systems in North America are
DigiCableGeneral Instrument developed the DigiCable
system to meet TCI's requirements. DigiCable has been deployed by multiple
cable operators in North America, including TCI (now AT&T BIS), Comcast,
PegasusScientific Atlanta developed its Digital Broadband
Delivery System (DBDS) in response to Time Warner Cable's Pegasus system requirements.
Multiple cable operators in North America, including Time Warner Cable, Comcast,
Cox, Marcus, and Rogers Cable, have adopted it.
This chapter is organized into three main sections:
Architectural comparisonThis section compares and contrasts
the two system architectures and describes the different cable operator requirements
for digital broadcast.
DigiCableThis section describes the DigiCable system
and its components in detail.
PegasusThis section describes the Pegasus system and
its components in detail.
DigiCable and Pegasus systems are very different in their implementation
but share many common architectural features:
Satellite distribution to headendsDigital programming
is compressed at an uplink and delivered in compressed digital form to the
headend location via satellite.
Headend to subscriber distributionQAM modulation is
used for broadband transmission of the digital programming over an analog
A split security modelThe conditional
access system used to secure the satellite link operates independently from
the conditional access system used to secure the cable system.
An out-of-band data channelA separate QPSK carrier
is used to deliver common system information associated with all in-band channels.
In both systems, a satellite link distributes MPEG-2 compressed digital
channels to cable headends, as shown in Figure
7-1. The up-link equipment generates an MPEG-2 multi-program transport
stream (MPTS) for each satellite transponder. The processing for each
transponder is as follows:
A real-time encoder compresses the video and audio content,
generating an MPEG-2 single program transport stream (SPTS) for each program.
(The content may be received from an analog or digital satellite feed, or
played back from a video tape or digital file server.)
Each SPTS is secured using a conditional access system
(CA1). General Instrument's Digicipher II is by far
the most common system in use in North America, and Scientific Atlanta's PowerVu
is also used.
Multiplexing equipment combines a number of SPTSs to generate
an MPEG-2 multi-program transport stream (MPTS).
The MPTS bit rate is chosen to fill the entire payload of a satellite transponder,
which varies from between about 27 and 44 Mbps, depending on transponder bandwidth
and forward error correction coding.
The modulation equipment applies forward error correction
to the MPTS and is responsible for QPSK modulation. The output is at L-band.
The transmission equipment up-converts each L-band carrier
to the satellite transmission band (C-band or Ku-band). Typically, 24 carriers
are amplified, combined, and fed to the dish via a wave-guide.
At each headend, the processing of the digital payload is fundamentally the same in DigiCable and Pegasus
The satellite signal is received and down-converted to L-band by
a low-noise block (LNB) converter.
(See Chapter 6 of Modern Cable Television
Technology; Video, Voice, and Data Communications by Walter
Ciciora and others.) The satellite modulation is QPSK (see the section
Broadband Transmission in Chapter 4, “Digital
Each L-band carrier is tuned and demodulated to recover the
MPEG-2 MPTS by the demodulation equipment.
The de-multiplexing equipment separates the MPTS into its
The conditional access system (CA1)
decrypts each SPTS. Each SPTS is secured using a different key so that each
SPTS can be authorized separately.
The SPTS may be groomed by an Add/Drop
Multiplexer at the headend to build a new system multiplex (or MPTS). In many
systems, the MPTSs constructed at the up-link are designed to be passed through
the headend intact, so grooming is unnecessary, thereby reducing headend equipment
DigiCable and Pegasus systems use the same mechanisms to distribute
broadcast digital services to subscribers, using QAM modulation to carry MPEG-2
MPTS over the analog cable distribution network to a digital set-top (see
the section Digital Transmission in Chapter 5, “Adding
Digital Television Services to Cable Systems”).
The North American cable industry agreed to use the ITU J.83 Annex B
standard for QAM modulation developed by General Instrument. The headend-to-set-top
processing is shown in Figure 7-1:
Each SPTS is secured using a conditional access system (CA2).
The DigiCable system uses General Instrument's Digicipher II conditional access
system. The Pegasus system uses Scientific Atlanta's PowerKEY conditional
Multiplexing equipment combines a number of SPTSs to generate
an MPEG-2 multi-program transport stream (MPTS). The MPTS bit rate is chosen
to fill the entire payload of a QAM channel, which is 26.97035 Mbps for a
64-QAM channel or 38.81070 Mbps for a 256-QAM channel.
The modulation equipment converts the MPEG-2 MPTS into a 6
MHz 64-QAM or 256-QAM channel.
The up-converter places the QAM channel on the desired frequency.
The QAM channels are combined with existing analog NTSC channels and
sent over the cable distribution network. In the customer's home, a hybrid
digital/analog set-top is responsible for the final processing steps in the
signal path (see Chapter 6, “The Digital Set-Top
Converter,” for more details):
The chosen cable channel is selected by a tuner, which feeds
an intermediate frequency to a QAM demodulator.
The demodulator recovers the MPEG-2 MPTS.
The de-multiplexer selects a single SPTS from the MPTS.
The conditional access system (CA2)
decrypts the SPTS.
The audio and video elementary streams in the SPTS are decoded
and fed to the television receiver.
The signal path from the up-link to the customer has a great many processing
elements, but the original SPTS encoded at the up-link is unchanged when it
arrives at the set-top decoder. The great advantage of a digital system is
that so many processing steps are possible while maintaining the integrity
of every bit in the SPTS. Thus, the picture quality from a decoder placed
at the up-link is identical to the picture quality from a set-top in the customer's
home (assuming that the transmission system is properly maintained).
The split security model separates the conditional access system for
the satellite link from the conditional access for the cable system:
The digital channels are secured over the satellite delivery system
The digital channels are secured over the cable system by CA2.The
DigiCable system uses Digicipher II conditional access for the satellite link
(CA1) and for the cable system
(CA2). The Pegasus system uses Digicipher II or
PowerVu conditional access for the satellite link (CA1)
and PowerKEY conditional access for the cable system (CA2).
The split security model maintains complete independence between CA1
and CA2. This approach has significant operational
benefits for the content provider and the cable operator:
The content provider need only authorize equipment at the
cable headend. This enables the content provider to manage its
customer, the cable operator.
Cable operators can choose their cable conditional access
system independently of the satellite conditional access system.
The cable operator can insert locally encoded channels into
the channel lineup and secure them with the cable conditional access system.
(This is particularly important for video-on-demand services. See the section
Conditional Access in Chapter 10, “On-Demand Services”).
A security breach of either the CA1
or CA2 conditional access systems does not affect the
integrity of the other conditional access system.
Both the DigiCable
and Pegasus systems rely on an out-of-band data channel. The out-of-band channel
delivers common system information associated with all in-band channels to
the digital set-tops:
Entitlement management messages
(EMM)These messages are addressed to individual set-tops and carry
secure authorization instructions from the conditional access system in the
headend to the set-top.
Service information (SI)These messages provide the
set-top with information to support channel navigation.
Program guide informationThese messages provide the
program guide information to the EPG application in the set-top.
Emergency alert system (EAS) messagesIn response to
an EAS message, the set-top displays a text message, plays an audio message,
or force-tunes to an alert channel.
The DigiCable system
is designed to provide either central or local subscriber management. The
Pegasus system is designed to provide only local subscriber management.
In central subscriber management, the satellite distribution system
is used to control each cable headend by using part of a satellite transponder
as a data communications link to carry authorization and control messages.
In local subscriber management, a local headend controller provides all the
management function for the cable system.
In both cases, the satellite distribution also provides a way of obtaining
digital programming services from the content provider. However, in the centrally
managed system, the programming and the management functions may be provided
as a bundled package. In a locally managed system, multiple sources
of digital programming may be combined, or groomed,
as desired by the cable system operator.
One of the primary architects of OpenCable, Michael
Adams, explains the key concepts of this initiative in his book
Broadband, Second Edition
by George Abe
Introduces the topics surrounding high-speed networks
to the home. It is written for anyone seeking a broad-based familiarity
with the issues of residential broadband (RBB) including product
developers, engineers, network designers, business people, professionals
in legal and regulatory positions, and industry analysts.