A Brief History
The history of interactive TV contains a number of distinct threads that have intertwined from time to time, but on the whole represent separate sequences of development. Interaction within the context of broadcast TV has had little to do with interactive video as it has developed in the computer industry. Similarly, until the past few years, interactivity based on closed circuit TV has been a separate thread. The telephony industry independently developed interactive video in the form of telephones with pictures. The computer industry's form of interactive video consisted entirely of graphical user interfaces to data processing applications, until the recent addition of multimedia data types. Then, there is the videogame industry, which over the course of 25 years has added whole new dimensions to interactivity, including high fidelity sound, force feedback, body motion input devices and 3-D imaging headsets.

Ever since the very first flickering of television about 70 years ago, the engineers and promoters of TV have talked about 2-way video. Their vision of bi-directional video communications was realized in closed circuit and private wireless TV in the 1950s. In the 1960s, AT&T attempted to commercialize 2-way video in the form of "videophones", using an adaptation of their voice telephony system. Until recently, the high cost of video cameras, the limited broadcast bandwidth available, and the high cost of cable for 2-way TV limited its application to proprietary systems used mainly for security within or around a specific facility. In these systems, the interactivity is that of people communicating with each other, and additionally there can be a means to remotely control the cameras.

The utility of video for surveillance made what is now referred to as "telepresence" a major form of interactive closed-circuit TV. Telepresence systems enable people to interact safely from a distance with objects in a hostile or dangerous environment using remote-controlled video cameras in combination with robotic devices such as manipulator arms and grippers. From bomb handling and disposal to underwater salvage, telepresence has proven itself to be an economically viable form of interactive TV. Telepresence in the form of 2-way video chat plus user-directed performance is now a profitable segment of Internet e-commerce, mainly for the purpose of adult entertainment.

Interaction with the content of broadcast television began with the first use of telephone call-ins by the host of the "Today Show" in the late 1950s. Callers responded to questions broadcast over the air by dialing the originating TV station's switchboard, and if they were deemed suitable for going on-air, the phone on the host's desk would ring. A 7-second broadcast delay was used to prevent inappropriate material from being aired. The telephone call-in form of interactive participation continues to this day on a variety of TV programs, ranging from games to shopping.

In the 1960s, interactive video took on an entirely different meaning within the computer industry. Video in the computer industry meant "glass teletype", as cathode ray tubes (CRTs) started to take over from online printers as one of the primary output devices for mainframe computers. Almost immediately, in university computing centers everywhere, the text-based computer game, "Space War", previously confined to a single or multiple users at teletype machines, became a CRT and keyboard-based game. Within a few years, the text-only video displays of space warfare encounters gained primitive character-based graphics depicting ship and missile trajectories. Beginning in the 1970s with Atari's "Pong", computer games made the leap from mainframe computers to microcomputers that could use the home TV screen as the video display.

Videogame hardware morphed into home computers in the 1980s. Many of these early systems used the home TV as the display device. True to their heritage, games quickly became the second most popular use of home PCs. And, since computers can use modems to communicate with each other over telephone lines, interactivity extended to remote multi-users, so games could be played among a group of people at a distance from one another. Long before the rise of the Internet, bulletin board-based games offered chat between players and game downloading services. For a few years, a single architecture combined a broadcast receiver color TV with a microcomputer in a keyboard and a modem to deliver conventional TV, electronic messaging, and videogames to the living room.

By 1996, when the Internet and Web browsers became mainstream PC applications, home computers no longer used the TV as the video display, simply because TVs could not support SVGA, and then XVGA screen resolutions. The Web developed into a powerful interactive media in its own right, distinct from videogames, TV, and non-networked computers. Now, it is estimated that over 50% of American homes have both a TV and a PC - and it's the PC that is connected to the Web.

Over the past three years, interactivity has been added both to community-wide cable television and satellite TV. It is possible to connect to the Internet using a set-top box, a keyboard and the home TV for the display device. Typically, a telephone line is used by the set-top box to gain Internet access, but not in all cases. All Satellite TV systems require a phone line to support interactivity. Other interactive TV systems offer interactivity that enhances TV with "click to purchase" capability, without a keyboard, using only the remote control.

Levels of Interactivity
Aside from the hardware, it is useful to consider interactive TV on a purely functional basis. Acknowledging that some functions are more difficult and costly to implement than others, levels of interactivity can be defined, such as low, medium, and high. These distinctions are somewhat arbitrary, as the levels blend into one another and features that are easy to supply within one community system may be next to impossible to deliver in another.

Medium Interactivity TV
With advanced gaming, 2-way video, video on demand and support for all Internet services, the medium level of interactive TV features:

Commercialization of Interactive TV
Widely perceived as a potentially huge revenue generator for providers and consumer electronics companies, the implementation of interactive TV is being approached from a number of directions. Arguably, about 500,000 users already have interactive TV at the first level described above. Those users own a Microsoft WebTV set top box, a stereo TV, a stand-alone videogame system, and access a cable TV service with subscription or pay-per-view movies. In the near future, with some digital cable services, both the external videogame system and the WebTV box will not be required. The cable company will offer those features as options for digital cable, reducing living room clutter substantially. Indeed, some hotels offer such services, today.

Starting with the hardware, there is now a proposed digital TV hardware specification circulating among national committees of manufacturers. Last minute changes in the proposed standard have delayed shipments of digital TVs from some manufacturers. Other manufacturers are now shipping their best guess at compliant sets. On the PC hardware side, there is general agreement on XVGA as the screen display format. Note that both analog and digital TV have rectangular pixels and PCs have square pixels. PCs do not interlace video frames and TVs, including most of the proposed digital ones, do. On the other hand, many digital camcorders are capable of recording and playing non-interlaced video. DVD players generally support a range of image formats, including HDTV.

For transmission of NTSC digital TV, which can range from the same effective screen resolution as analog NTSC, right up to high-end HDTV, the proposed standard only covers the downstream side. This is because the user's back channel to the transmitter can not be within the allocated spectrum of the main signal. A variety of proprietary formats for user upstream data intended for use on cable or fiber are presently in trial deployments. The competing satellite TV systems each use their own downstream transmission format, with a fully standardized back channel of data via modem. Digital video on the Internet is transmitted via either HTTP or UDP, which are not compatible server data protocols. However, underlying both of those is a common data packet format, TCP/IP. For interactive TV on the Web, HTTP is more common because it handles both the downstream and upstream data with verification, even though UDP is more efficient for downstream-only video, which can be sent without any receiver verification.

On the software side, the three main standardization issues are video encoding/decoding ( the "codec"), stream format, and the user interface. Digital video must be encoded to make both storage and transmission practical. Digital broadcast, cable and satellite TV have all agreed on a type of codec, MPEG, that is used in different, sometimes compatible ways, by each segment. A version of MPEG 1 is used by satellite TV, digital cable and minimum standard broadcast digital TV. MPEG 2 is used by HDTV broadcasters and DVD-standard devices. On the Internet, MPEG 4 is used by about half of all streaming video providers. The other half is split between two other codecs, RealNetworks and Sorenson.

TV and PCs - Convergence or Divergence?
This author, among many others, has written that the convergence of TVs, PCs and the Internet seems likely within 10 years. A number of trends point in this direction, including the decreasing cost of PCs, the increasing image quality of TV, smart digital TVs, the number of homes with PCs wired to the Internet, increasing Internet usage per household, pay per view on the Internet, video chat, and on and on. Those trends are supported by plenty of surveys and statistics. On the other hand, there are trends in the opposite direction, that indicate TVs and their audience will remain separated and distinct. Perhaps the two most important trends that point to divergence are the popularity of home theater and proprietary videogame platforms.

Big screen home theaters with multi-channel sound are increasingly popular with consumers. As broadcast and cable signals improve in quality with digital TV and HDTV, the visual benefits of shear size will become even more obvious and attractive. It seems clear that without the additional cost and complexity burden of computer electronics, big TVs will always be less expensive than big-screen PCs. Not to mention the fact that TVs don't "crash" the way computers do. More reliability further enhances the visual entertainment bang for the buck advantage of TV over PCs.

The new generation of videogame platforms now on the market are supercomputers in their own right, complete with high-speed network connectivity, at the user's option. Compared head to head with PCs of similar game graphics capability, the proprietary platforms perform as well, if not better, at a fraction of the price, which is typically one-sixth that of a PC. And again, the game machines are more reliable and have virtually no maintenance, compared to a PC.

Conclusion
Interactive TV is already on the market in a variety of forms, ranging from PCs with Internet-based video, to digital set top boxes on cable TV, to videogame systems. At this point in time, in appears unlikely that any single flavor or configuration will dominate the consumer space in the foreseeable future. Instead, interactive TV will continue to gain popularity in a multitude of forms, serving its users within their budgetary constraints both at work and at home.