Interactive Television (iTV) is a hot topic these days. Almost all the heavy hitters - Microsoft, Intel, Sony and AOL - have made major investments in iTV. The major television networks and cable companies are participating in one way or another. Companies such as webtv, Liberate Technologies, Mixed Signals Technologies and Wink Communications are at the forefront, providing a first look at the interactive future.

This month's article focuses on interactive television for terrestrial broadcasters, with an emphasis on what is happening today. We will examine one interactive set-top box, webtv, employed in one environment, analog terrestrial broadcast.

webtv If you have seen a webtv demonstration, or have a webtv box, you may recognize the icon on the left. This icon appears on the television screen when the webtv set-top box (STB) recognizes that valid webtv interactive content is present. Clicking on the icon starts the interactive session.

A typical webtv STB installation is shown in Figure 1. Note that in this example, a program with interactive content may be recorded and played back later. (This is only true for systems using Transport A, as will be discussed.)

Transporting interactive data iTV involves moving data along with normal program content. Two data channels are typically employed in an iTV connection. One is the vertical blanking interval, or VBI (lines at the top of the television raster that are not normally displayed on a television set), and the other is a "backchannel," usually a telephone line.

The VBI is typically used to send triggers and links. The backchannel is used to retrieve interactive content to be displayed on the television screen.

In 1990, Congress passed the Television Decoder Circuitry Act. This legislation requires that all television transmission systems must pass closed caption data encoded in line 21 of the vertical interval from its source to the end viewer. While the original intent of the act was to provide closed captioning for the hearing impaired, the act opened up a data path from the broadcaster to the home. webtv uses this link to send interactive data to its STBs using the Text-2 channel of line 21.

There is a lot going on in line 21 of the vertical interval. There are four caption channels (CC-1 through CC-4), four data channels (Text-1 through Text-4), and another service called XDS or Extended Data Services. All of these channels are multiplexed on line 21. Field one of line 21 is used for CC-1, CC-2, Text-1 and Text-2. Field two of line 21 is used for CC-3, CC-4, Text-3, Text-4 and XDS.

Making matters more interesting, the bandwidth of this channel is limited. The line 21 full-field data rate is 840 bits per second, or 120 characters per second (7 bits per character). Because captions and text (CC-1 and Text-2) are only in field one, the data rate is half or 420 bits per second (approximately 60 characters per second). The available bandwidth is divided between all of the services in line 21 on a priority order. CC-1 is highest, and Text-2 is the lowest. The bandwidth used by captions varies greatly depending upon the show. Typically, captions use about 25 percent of the available bandwidth, but a very verbose show can take up to 75 percent or more.

The data formatting, protocol and channel priority of data encoded in line 21 of the vertical interval is described in the EIA-608 standard. Generally speaking, captions and data encoded in line 21 operate according to Transport A, as described in the ATVEF specification described below.

EIA-608 specifies that closed captioning data takes precedence over all other data. Given the restrictions on available bandwidth in the channel, synchronizing interactive data to the video while giving priority to closed caption data can be challenging.

The ATVEF specification has two transport mechanisms: Transport A and Transport B. Transport B is another method of transmitting data in the vertical interval. Transport B can use any available VBI lines.

A combination of standards, Internet requests for comment (RFC), and trade association specifications set the data rate for encoding data using Transport B, which operates at a higher data rate than Transport A. Transport B has one limitation for the consumer - it will not survive recording on a VCR. Transport A will.

Transport B has a data rate of approximately 10.5kb/s per line. Deriving this number is a little complicated. The Advanced Television Enhancement Forum (ATVEF) Transport B has no bandwidth limitations, since the specification is transport independent. However, using Internet RFC 2728, Transport B can be encoded into one or more lines in the VBI using the North American Basic Teletext Specification, EIA-516 (NABTS) encoding waveform.

There is a significant challenge to using Transport B. Transport B is not protected by the Television Decoder Circuitry Act. Therefore, it may not be carried all the way from your station to the home. In many cities, cable systems maintain ownership of the vertical interval with the exception of line 21. They can strip any data on lines other than line 21 and insert their own data. In any case, cable systems are under no obligation to deliver your vertical interval to the home unless you have a contract with the system that specifically guarantees this right. Benefits of Transport B are that it has much higher bandwidth, and that there is not a requirement to give priority to closed captioning data.

As a practical matter, many broadcasters will find that the vertical interval is a very busy place and that multiplexing iTV data on line 21 using Transport A may be the best solution available.

Unfortunately, any VBI transport mechanism is a one-way link. It is not possible for the viewer to transmit messages or responses from the STB back to the station using this path. The most common way for viewers to get a return connection (commonly called a backchannel) is to use a dial-up telephone connection to the Internet. This provides a two-way path between the STB and a website dedicated to providing interactive content.

Under the covers It may help to look at a specific example. Our example will be the game show Jeopardy running on webtv using Transport A, line 21.

When an interactive program begins, a special trigger is sent using Text-2. This trigger tells the STB that interactive content is available. The STB superimposes the interactive icon on the television screen. When the viewer selects the interactive icon, the STB initiates a connection to the Internet. Once connected, the STB downloads all of the interactive content for that particular Jeopardy show. Once the interactive content is downloaded into the STB, it shifts display modes. The television picture is reduced in size, and a portion of the screen is dedicated to interactive content. From this point on, the viewer uses the controls of the STB to interact with the game and to attempt to provide answers along with the contestants on the show.

As the show progresses, various triggers and links are transmitted in real time in the vertical interval. These cues change the display and control functions such as moving on to the next question and answer and revealing answers.

Once the STB enters interactive mode, the viewer is essentially looking at a webpage that has been provided over the Internet. HTML, Java scripts, Cascading Style Sheets and other typical Web authoring techniques are used to change the content of the page. Other iTV environments may employ different processes.

Authoring interactive content Authoring iTV content, which typically takes place in two steps, is quite different from authoring a typical webpage. A designer determines the look and feel of iTV, and appropriate page backgrounds are developed. Triggers and links are then added for each specific show.

Once the show is finished, it is delivered to an iTV workstation running software such as TV Link Creator by Mixed Signals. The workstation has inputs for video and timecode from the VTR. The workstation takes in a disk file containing closed captions that were prepared earlier. As the iTV workstation operator views the program, he inserts triggers and links at the appropriate times. (The software resolves conflicts between closed captions and iTV content.) At the end of the process, the operator can either write out a new disk file containing both closed captions and iTV content or encode the combined closed captions and iTV cues into a new tape using a Mixed Signals DV2000 encoder. In any case, the closed caption and iTV data are usually encoded into the vertical interval for later playback. In some cases, the disk file is played out to air through a caption server during playback of the show.

This article has focused on one example of what is possible today. There are other iTV systems available, such as the one produced by Wink Communications. (Not to be confused with WINK-TV of Fort Myers, FL. WINK started out in 1938 with WINK Radio at 1240 AM. WINK-TV is continually getting calls for Wink Communications and has asked that I make you aware of the difference.) Wink Communications has been working on interactive television solutions for many years. Its current focus is on interactivity and e-commerce solutions for television. Another vendor of interactive solutions is Liberate Technologies. New companies are entering the market all the time.

Bandwidth limitations place restrictions on the current capabilities of iTV. As DTV becomes more common, and high-speed cable, satellite and DSL technologies make their way into the home, the interactive TV experience is sure to change. New creative possibilities will arise. By studying the current technology you can get some idea of where iTV is headed.