Digital Broadcast Case Studies

Satellite Distribution to Headends

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 systems:

• 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 Technologies”).

• 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 component SPTS.

• 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 costs.

Headend-to-Subscriber Distribution

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 access system.

• 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).

Split Security Model

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 by CA₁.

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.

Out-of-Band Channel

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 information—These messages provide the program guide information to the EPG application in the set-top.

• Emergency alert system (EAS) messages—In response to an EAS message, the set-top displays a text message, plays an audio message, or force-tunes to an alert channel.

Central Versus Local Subscriber Management

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.