The total coaxial cable spectrum is close to 1 GHz; however, most cable systems use less than 1GHz. Cable systems were built with different capacities—54MHz–350MHz; 54MHz–450MHz; 54MHz–550MHz; and 54MHz–750MHz—based upon the need for channel capacity in different service areas. More recently some systems have been built with a capacity of up to 860 MHz. Traditionally the spectrum above 54 MHz has been chopped into a number of 6-MHz channels, each delivering one NTSC analog TV signal. A few years ago, the cable industry decided to add digital technology to their systems. To preserve existing analog services without interruption, a majority of the cable systems decided to use the spectrum above the current usage of 550 MHz–750 MHz for the delivery of digital services. Additionally, one 6-MHz channel (traditionally known as the out-of-band (OOB) channel) was reserved for forward signaling purposes, such as delivering conditional access (CA) and management messages, interactive program guides (IPG), and other private data to set-top boxes (STBs).

Compared to other broadcasting systems, the hybrid fiber/coaxial (HFC) cable network is much less susceptible to atmospheric noise and provides a better signal-to-noise ratio (SNR) at the receiving end. To take advantage of this, the cable industry selected a higher-order modulation system, quadrature amplitude modulation (QAM), as its digital modulation system; QAM provides larger digital bandwidth compared to 8-vestigial sideband (VSB) and quarternine phase shift keying (QPSK) used in terrestrial and satellite broadcasting systems, respectively. For example, 64-QAM modulation on a 6-MHz channel provides a digital bandwidth of 27 Mbps, whereas QPSK provides about 10 Mbps, and 8 VSB about 19.39 Mbps. However, for more robust transmission, QPSK modulation has been adopted for the OOB channel.

Because signaling channels are different from service channels, STBs need two tuners—one for tuning service channels and the other for the OOB channel. The cable industry made a decision to use a smaller digital bandwidth (1.5 Mbps-–3.0 Mbps) for the OOB channel for simpler and less expensive implementation of OOB channel data demodulation and decoding in the STB. In order to support interactive program guides and other interactive services, upstream channels are a necessity; it was decided that the 0 MHz–50 MHz portion of the cable spectrum should be used for upstream channels.

OpenCable network architecture and terminal philosophy are based on the following:

To promote compatibility with current implementations, most of the existing network architecture has been adopted, and enhancements have been added wherever necessary to support future extensibility. The spectrum allocation to support the OpenCable architecture is:

The 64/256 QAM demodulator in a digital channel will demodulate the channel to a digital baseband (27 Mbps for 64 QAM and 38.4 Mbps for 256 QAM). This digital baseband is a multiplex of programs, and each program in the multiplex can be encrypted or unencrypted. The multiplex will be passed on to the point-of-deployment (POD) security module. For a program in the clear, the POD does nothing to the multiplex and passes on to the demultiplex subsystem of the host. If the host-requested program is encrypted, the POD first checks with its access control subsystem, to verify that the host is authorized by the CA system for this content. If the host has the authorization, the POD picks the appropriate ECM (decryption key) from the inband multiplex and decrypts the selected multiplex program and passes the entire multiplex on to the demux (demultiplex) subsystem of the host.

Before transporting across the POD-host interface, the unencrypted program will be re-encrypted for copy protection without any link to the CA system. Before that, the POD sends the key to the host using a Diffie-Helman key exchange technique so that host can decrypt the program. If the program is Main Profile at Main Level (MP@ML or SDTV), the program will be demultiplexed into audio/video/data elementary streams (ESs). The ESs will be decoded by the respective decoders. The decoded video and audio will be encoded by the multimedia subsystem into an analog NTSC signal, which will be output to a coaxial connector. A SPTS may be created out of the selected program. The SPTS or the entire multiplex can then be transmitted over the 1394 port. If the program is Main Profile at High Level (MP@HL or HDTV), then it is classified as a passthrough signal and will be transmitted over the 1394 link. Just prior to passing across this 1394 link, additional encryption also may be applied to the streams based on copy control instructions (CCI) received from the POD module.

In the absence of a POD module, the output of the QAM demodulator will be input to the demux directly. If the desired program is in the clear (i.e., broadcast channels), the STB will transmit it over an RF channel or 1394 link. The multimedia subsystem also is responsible for creating a bitmap for the IPG. When an interactive program guide (IPG) is requested, the the system can overlay the IPG with analog video or it may transmit the IPG bitmap data over the 1394 link using the asynchronous transfer mode.

As mentioned earlier, the cable system has an OOB signaling channel in addition to a large number of service channels. All OpenCable-compatible STBs are required to have two tuners. One tuner tunes to the desired service channel, while the second tuner will tune to the OOB channel. The OOB channel will be demodulated by the QPSK demodulator, the output of which is raw digital data without forward error correction (FEC). The POD module receives the raw data, performs FEC and recovers the baseband signal. The OOB data carries EMM (a conditional access message) and other data, such as systems information (SI), interactive program guides, tables and texts, and other management messages. The POD will separate the CA information and send the CA-unrelated data (electronic program guide (EPG) data, SI, etc.) to the host via the data channel.

The CPU provides the user interface for STB and POD applications, in addition to other STB functionalities, such as coordinating host subsystem functions. In one-way cable networks, subscribers are required to contact their cable operator for services, such as PPV movies. The same is true for a one-way capable STB in a two-way cable network system. In a two-way system, the STB can communicate directly with headend equipment. This two-way capability enables subscribers to access digital interactive applications using a remote control unit. For example, while going through offerings using an IPG, subscribers can hi-light, select, and watch a channel by pressing one or more buttons.

A consumer buys a STB from a retail store and, if he is not a cable subscriber already, contacts the local cable operator for service. Once connected to the local cable network, the consumer can then connect the STB to the cable and connect his DTV to the STB’s digital output port. The STB alone (without a POD module) will enable viewing of clear channels (if available), such as terrestrial broadcast channels carried over cable. The STB has to be registered with the local cable operator before it can receive any encrypted channel. To view encrypted premium service channels, the consumer will need a POD module, and will need to contact their local cable operator to obtain one. Once the POD module is acquired, the consumer will need to insert it into the host (STB). The STB will detect the presence of a POD and using a piece of software, the STB also will verify the POD’s status. If the POD is good, the following steps will take place before the POD will decrypt any encrypted channel in the inband multiplex.

Existing STBs use coaxial cables and connectors as output interfaces for the delivery of analog NTSC signals. The cable industry has been looking for a low-cost, high-performance, robust interface to deliver digital signals from the STB to digital equipment. An interface based on IEEE 1394 is a good candidate for the following reasons:

Figure 4 depicts a simple home entertainment environment with a few consumer electronics devices connected to each other using 1394 ports. A STB is the terminating node for cable at home. The 1394 ports in the STB are used as the simple home digital network interface. A DTV, DVCR, and digital camcorder are connected to this interface; it is assumed that the 1394 interface in the STB will have the capabilities required to be a root node, isochronous resource manager (IRM), cycle master and, optionally, bus manager (BM). The DTV and DVCR may have similar capabilities; the camcorder will have a less expensive four-wire 1394 interface.

What will happen when two devices, such as a STB and a DTV with 1394 capabilities, are connected using a 1394 cable for television viewing? Any device attach/detach, or power turn on, will cause the bus to reset. Before the link is ready for data transfer, a sequence of handshake signals (packets) will be exchanged between the two devices to complete standard procedures, such as dynamic node address allocation, self-identification, arbitration for IRM, BM, and cycle master. The STB will then query the DTV to collect capability information. Some of the AV/C commands involved are shown in the following table.

Once the initialization sequence is completed, the STB/DTV pair is ready to interact. An example of how this interaction could occur is described here. A menu display, with buttons or icons on the DTV screen, will indicate that the link to the STB is ready; the following table details the basic sequence to view a television channel.

Figure 5. STB Implementation and Logical Interfaces