Digital Video Services over Cable

When John Malone, CEO of TCI, talked about the 500 channel service in 1994, we surmise that he was referring to MPEG-2-encoded digital TV over HFC. HFC systems would be built with 750 MHz of forward bandwidth, offering 110 or more passbands of 6 MHz each. Each 6 MHz passband would be MPEG-2-encoded to yield four to six digital channels. Here's how.

Take 6 MHz and remove 500 kHz from each side for guard band, yielding 5 MHz. Use QAM 64 modulation on the 5 MHz. QAM-64 is selected by American and European standards bodies as the modulation scheme of choice for cable systems. QAM-64 encodes 6 bits per Hertz, yielding just over 30 Mb of raw throughput. Use Reed-Solomon forward error correction, which consumes 10 percent of raw throughput for error detection and correction, yielding 27 Mb. Twenty-seven Mb can comfortably support four to six channels of MPEG-2 programming.

It is believed that 750 MHz, shared among 500 to 2000 households, would satisfy bandwidth requirements for legacy analog television, near video on demand (nVoD), video on demand (VoD), Internet access, and the dozens of new networks and local or public service programming networks. To achieve the full potential of this bandwidth, some standardization will be required to reduce costs to the operators.

Principles of Operation

At the MSO head end, programs are acquired from a variety of analog and digital means. Analog channels will be received from satellite or terrestrial over-the-air means, just as they are in a nonupgraded (fiberless) plant. Analog programs will be passed along transparently to the viewer, with scrambling applied as needed.

Digital channels could be handled in a variety of ways. Recall that the modulation schemes of over-the-air digital TV and cable TV differ. Over-the-air will be transmitted in 8-VSB modulation, and cable digital TV will be transmitted using QAM-64 or QAM-256.

The possible options for the retransmission over cable of digital over-the-air TV channels are listed here:

• Receive channels in 8-VSB at the cable head end and pass the digital programs transparently through to the consumer. At the consumer site will be a decoder, which will decode 8-VSB for the over-the-air channels and QAM-64 for the cable digital channels (ESPN, Discover, HBO, and so on). That is, the decoder must be aware of the format of the program and decode accordingly.

• Receive channels in 8-VSB at the cable head end and remodulate the programs to QAM-64 at the head end. All programs, whether originated from over-the-air or from cable, will be sent in QAM. The remodulation technology is available, and it relieves the set-top box from decoding different types of streams.

• Receive channels in pure MPEG baseband and modulate the channels onto QAM. This can happen, for example, if a cable head end is located near a network affiliate and arrangements can be made to drop fiber between the broadcaster site and the head end.

The analog channels and digital channels are combined, or frequency division mutiplexed, into the frequency range of 54 MHz up to perhaps 750 MHz, thereby forming a broad channel lineup of analog and digital channels.

To assist the consumer in navigating through the dozens or hundreds of channels, an electronic program guide ( EPG) will be produced by the operator. In the case where the broadcasters offer their own EPG to inform the user of their channels, some conflict may arise between the MSO and the broadcaster over how the EPGs are presented. For example, if Disney offers a channel multiplex, how will the cable operator display the various Disney options?

New functions must be added at the head end to enable digital program insertions. For example, advertisements must be produced in MPEG format and spliced into program streams at the head end to localize advertising. Emergency broadcasts must also be spliced into or preempt MPEG programming, and new conditional access must also be applied. Conditional access for analog differs from digital.

In the end, the issues confronting cable head end operators are similar to the issues confronting broadcast stations when going to digital. There will be new equipment, new costs, and new skills required, but lots of new opportunities for the operator as well.

Digital Set-Top Box

At the consumer site, an MPEG-2 set-top decoder transforms the digital transport stream so that an analog TV set can present the image, a process called decoding. Analog programs will pass through the decoder directly to the analog TV.

The basic functions of the digital set-top box are listed here:

• Diplex filters to separate upstream from downstream traffic

• Handle demodulation of downstream traffic from the HFC

• Handle modulation of upstream traffic onto the HFC

• Create an electronic program guide that displays programming options, including virtual channels and access to chat rooms linked to programs

• Perform MPEG-2 PID selection

• Perform MPEG-2 decoding

• Manage analog interfaces (RGB and audio) outlets to analog TV sets

• Descramble analog TV broadcasts

• Manage infrared controls (for channel selection) and VCR controls

• Transmit return path signals

• De-encrypt downstream traffic

• Encrypt upstream traffic

• Authenticate

• Manage separate, or out of band (OOB), control channel

• Perform IP stack and IP address filtering

Optional components include the following:

• Interactive game interfaces, such as joysticks

• Data communications interfaces, such as Universal Serial Bus (USB) or Ethernet

• Web browser interfaces to support interactive data services

• Java engines, which enhance Web browsing and provide an application interface

• Smart cards for authentication

• Copy protection circuitry, to prevent unauthorized copying of digital content

Digital TV set tops will require use of the return path for user-control purposes, such as channel selection, trick modes, pay-per-view ordering, and fault monitoring. Applications such as WebTV, Wink, or OpenTV also will have user interaction with a data application. At the time of this writing, a rough estimate of the cost of elements of digital set tops were as follows:

Microprocessor, with 4 MB of memory and flash memory	$90
Transmission elements (tuner, equalizer, modulator, and FEC)	$50
MPEG chips, graphics, and audio processor	$50
Chassis, power supply, final assembly, PCB, and test	$50
Analog and infrared interfaces	$10
Software licenses (OS, encryption)	$10
Total cost	$260

This total is the cost to the manufacturer; consumers would pay more, probably around $400. As time passes, the cost will decrease.

Set-top boxes for analog cable delivery have been required for conditional access for premium viewers. Viewers who did not choose premium services did not have need for a set-top box (STB); they would simple purchase a cable-ready TV. Those who don't have premium channels outnumber those who do.

The MSOs that needed to enforce conditional access were required to go to the Big 2 (General Instruments and Scientific Atlanta) for analog set-top boxes. General Instruments and Scientific Atlanta have incompatible conditional access systems, so if one MSO chose one vendor at the head end, it could not choose the other for the customer premises. Also, the set-top boxes were provided by the MSO, which meant that the MSO carried the customer set-top box on their balance sheets, which had negative financial implications.

The development of digital TV meant an opportunity for the MSOs to open the design of the set-top box so that there would be more competition among vendors. This presumably would lower their costs, improve functionality, and get the set-top box into retail distribution and off their balance sheets. All these were perceived as wins for the MSOs. Hence, they embarked on a standardization effort called OpenCable. Although it is unclear at the time of this writing whether OpenCable will succeed in establishing an industry standard, it illustrates the issues associated with digital TV on cable.

OpenCable Initiative

The OpenCable Initiative is an effort that began in September 1997 and is supported by CableLabs and the Society of Cable Television Engineers (SCTE) to standardize key elements of a digital set top for digital television service over HFC systems. OpenCable does not address standardization issues for current and future generations of analog set tops. It concentrates on digital standards to create interoperable set-top boxes and to create new functionality for digital TVs, VCRs, DVD players, and personal computer NICs that process video. It is intended that OpenCable-compliant devices be available at retail and also be provided by the MSO. However, due to differences in service offerings of cable operators, there is no guarantee that an OpenCable device can be used if the consumer moves to another system.

The key elements subject to standardization are listed here:

• Consistent services— Two OpenCable boxes from different manufacturers must provide the same services when deployed on a common cable system. In particular, the cable operator would like to provide a common look and feel, whether the box were purchased retail or leased from the operator. This means that the user interface should be downloaded to what could be boxes of different manufacture.

• Backward compatibility— An OpenCable device should be capable of passing through analog programming and should be interoperable with existing head end services.

• Portability— The 1996 Telecom Act mandated the retail availability of DTV navigation devices. Any device available at retail carries with it the presumption that it is usable when the consumer moves from system to system. This is not guaranteed because different cable systems may offer different services, but is a design goal of OpenCable.

• Conditional access— The security system should be replaced in the case of a security breach or should be otherwise modified at the discretion of the MSO. This implies some form of modularity, which is introduced by way of a smart card that plugs into the set top, called the Point of Deployment (PoD) module.

• Software development environment for navigation tools— This primarily applies to electronic program guides and video services, such as interactive TV, VoD, electronic commerce, and tiered services. For example, tiered services allow MSOs to package groups of channels independently from the manner in which the channels are acquired from the content providers. HBO may broadcast multiple channels in digital format, but the operators can be free to reaggregate the various HBO streams to suit their particular marketing purposes.

• Platform independence— An OpenCable device should operate on any hardware platform, such as a personal computer, PowerPC, or videogame processor.

Over time, it is also expected that OpenCable will evolve to include data and voice capability. As of this writing, however, the intention is to develop a specification for video.

Security and the Point of Deployment Module

The first step in the development of OpenCable was agreement between General Instruments and Scientific Atlanta to harmonize their respective conditional access systems. Accordingly, an agreement on a common specification was reached, called Harmony.

The implementation of Harmony was to be on a smart card that plugs into the set-top box. The card, called the Point of Deployment (PoD) module, contains keys necessary to view premium programs. It is also the repository for billing information for electronic commerce transactions. The PoD module is interrogated by the head end to release billing information from time to time. Another requirement of this security module is that the set-top box can operate without it for basic programs—that is, programs not requiring conditional access. Furthermore, it is required that if a premium channel is being viewed with the PoD module, that emergency alerts preempt programming; that is, the PoD module can interfere with the premium programming but not with emergency alerts. It turns out that the engineering of the PoD module to satisfy these two requirements is tricky because the PoD module is provided by the carrier and is renewed periodically. Figure 3-4 shows a schematic of an OpenCable set-top box from [Adams].

The boxes outlined with dashes are optional elements of OpenCable. The F-connector delivers cable programming over the RF in. There are two tuners: one to receive the video feed, either analog or video, and another to receive an out-of-band QPSK channel for control information, or data, marked OOB RX.

For legacy analog TV service, the tuner receives the analog signal and passes it to an NTSC demodulator and an optional VBI decoder. The VBI decoder is used to display text information at the bottom of the TV screen. Graphics may be introduced from a graphics processor and CPU, and the analog signal is sent to the TV monitor via a channel 3/4, an S-Video interface, or an optional component video interface

Digital TV programming is QAM demodulated and sent to an MPEG decoder for rendering and decryption. Graphics are also introduced, after which the video can pass through an analog or a digital interface to the TV monitor. If it sent out an analog interface, it uses the channel 3/4 or S-Video interface. This would seem to be a terrible waste because so much work has gone into a fully digital program stream, but at the last few feet, the whole apparatus provides a legacy, analog TV image. Therefore, there is the provision for a digital interface using the IEEE 1394A interface. This interface is also known as Firewire and will be discussed further in Chapter 7, "Home Networks." For now, we only indicate that Firewire is the digital interface of choice for OpenCable.

In the return path, there is a provision for an out-of-band transmitter, denoted OOB TX, which transmits QPSK modulated data to the head end. This transmitter can be used for impulse pay-per-view accounting, data services, VoD selection and control, and interactive television. A telephone return path can also be used. In addition, computing components are required to support what is actually a full functioning computer. Among these elements are a CPU, read-only memory, Flash memory, RAM, and non-volative RAM. These components are needed to support booting, remote management by the operator, software downloads, processing and display of electronic program guides, and above all, support for application development.

Conditional access information is received in the out-of-band channel (that is, independently of the programs) and is used to grant access to the MPEG stream through the CP or control program.

This box has all the costs of an analog set-top box, with MPEG decoding, a POD module, a QPSK tuner, a 1394 interface, and a lot of new software.

Challenges to Digital Video Service

Despite gains in two-way cable plant, fiber distribution, and standardization, challenges remain for TV service over cable.

Competition from Satellite

The DBS industry has acquired roughly 9 million subscribers in the United States as of June 1999. This is about 15 percent of the cable subscriber base. Overseas, DBS providers such as Sky are likewise enjoying strong subscriber growth in Europe and Asia. Furthermore, satellite operators have recently been permitted to offer local TV stations, a capability they were largely barred from providing on technical and regulatory grounds. Both the technology, solved by a technique called spot beaming, and the regulatory environment, which relaxes rules soon with the delivery of local channels, have provided the impetus to DirecTV (NYSE-GMH) and Echostar (Nasdaq-DISH) to provide local channels in major metropolitan areas of the United States. Many satellite homes in the United States continue to keep cable services, mainly for local channels. With local access on satellite, it remains to be seen how many of these homes with both services will unplug one or the other.

Copy Protection

Digital interfaces, such as IEEE 1394, facilitate the pristine copies of content with the attachment of a CD or DVD burner. Hollywood studios, recording companies, and their representatives, such as the Motion Picture Artists Association (MPAA) and the Recording Industry Artists Association (RIAA), need assurances that their copyrights are enforceable. They are discussing copy protection alternatives with hardware developers.

Mechanisms are being discussed to provide digital watermarks, which will enable the tracing of stolen content. A proposal from consumer electronics manufacturers in Japan and Intel addresses ways to limit the number of copies that can be made from a single source. This technology, called 5C, would permit no copies, one copy, or an unlimited number of copies to be made from a single consumer-purchased item. The recent flap in the music industry regarding MP3 copies of music has put the recording industry into fast forward on the development of copy protection. Without an agreement on copy protection, the content providers will be reluctant to provide content in digital form.

Must Carry Rules for Digital TV

MSOs are fearful that current Must Carry rules will apply to digital TV distribution. Under current Must Carry rules, cable operators are required to carry the analog channels in the serving area. Without this carriage, small broadcasters are fearful they will not be carried on cable and will lose their advertising. This would put many of the smaller broadcasters, who wield power within the NAB, out of business.

When broadcasting goes digital, things get complicated. Several new questions present themselves:

• Will the cable operators be required to carry the digital simulcast of an analog channel when they already carry the analog channel? Haven't they satisfied the Must Carry rule by simply carrying the analog?

• If they are required to carry the digital form, or when the broadcaster only has digital formatting, will the cable operator be obligated to carry the programming in the original digital format? That is, if the broadcaster carries HDTV, will the cable operator be required to carry HDTV format, or can the cable operator downgrade to standard-definition TV, thereby saving some bandwidth on the cable system? Cable operators complain loudly that it is bad enough that they may be required to carry digital; they would be really annoyed if they would be required to carry high-definition TV.

• Finally, if the broadcaster elects to transmit a channel multiplex, is the cable operator required to carry all the streams in the multiplex? Or can it carry just the main channel, whatever that is? For example, broadcasters could elect to transmit data services. Would the MSOs be required to carry such data services? This would pose a competitive problem inasmuch as the MSOs have their own data services already, such as Roadrunner and @Home.

At the time of this writing, these issues are before the FCC. No matter how the FCC decides, it is likely that the matter will be litigated and taken to the highest courts in the land. The original Must Carry rule was taken to the U.S. Supreme Court, where it was upheld on a narrow 5 to 4 margin. Legislative action may ensue as well.

Viruses and Bugs

Advanced digital set-top boxes are basically computers connected to a broadband network. They have processors, memory, and operating systems, so they could also be subject to the problems of personal computers on the Internet. In particular, set-top boxes may be subject to computer hacks, computer viruses, and plain old software bugs that could impair TV viewing.