Creating Interactive Television Links

June 09, 1999


Interactive television links provide an easy and powerful way to bring the richness and interactive nature of the World Wide Web to your television audience. They are transmitted as part of the television broadcast and work in conjunction with HTML tags in associated Web pages (see Displaying Television Broadcasts in Web Pages). You can use interactive TV links to show your television viewers Web content specifically designed to complement what they're watching on TV at any given time. To enhance the viewing experience even more, you can use interactive TV links to create truly interactive programming by integrating the content of your Web pages with the television broadcast itself.

Interactive TV links work on both televisions and computers. To view interactive TV links on your television, you need to use the WebTV-based Internet Receiver (a set-top box that uses the Microsoft® WebTV Network™ Plus service). To see them on a computer, you need to use Microsoft® WebTV® for Windows® (a feature of Microsoft® Windows® 98) with a compatible TV tuner card. Interactive TV links basically work the same on both receivers; however, there are a few differences in how they are handled. These differences are pointed out as appropriate in the following information.


What the Television Audience Sees—What interactive TV links look like to the television audience.

How Interactive TV Links Work—A general description of the components used to create interactive TV links.

Getting Help Creating Interactive TV Links—Companies that can help you create interactive TV links.

Creating Your Own Interactive TV Links—Technical instructions for creating your own interactive TV links and inserting them into the TV broadcast.

What the Television Audience Sees

An interactive TV link appears as an "i" icon on the screen when interactive TV programming is available for a TV show. Viewers can use their remote control or keyboard to follow the interactive TV link and access a new world of information. On a TV, this icon looks like the one shown below, however, on a computer, broadcasters may choose to use this icon or substitute their own icon.

              <td width=The interactive television link (sometimes called the "i" icon).

After the viewer clicks the link, one of two things will happen:

  • On a TV with a set-top box, a selection panel appears. The viewer can choose to see the interactive program or to return to the TV show.

  • On a computer, interactive content for the TV show appears, skipping the selection panel step.

  • Both receivers can display the interactive programming specified in the interactive TV link, although there may be differences in the way they look. One difference is that on your television, there is only one window and it is a fixed size. On WebTV for Windows, the interactive show can open in a separate window that you can resize.

Back to Contents

How Interactive TV Links Work

Interactive TV links contain text codes called Uniform Resource Locators (URLs). These point to specific content—Web pages—on the World Wide Web. Interactive TV links are transmitted as part of the television broadcast in the same manner as closed captioning text.

Web pages, in turn, are created using another type of text code called Hypertext Markup Language (HTML). HTML is nothing more than short strings of symbols and text, known as tags, that you place around the content you want viewers to see. These tags tell receivers how to display the content contained in the Web pages.

When a TV viewer chooses to display the interactive show, the receiver interprets the code in the interactive TV link and displays the appropriate Web page.

Back to Contents

Getting Help Creating Interactive TV Links

We have compiled a list of companies that can help you develop your interactive television programming. See Interactive Television Development Resources for companies that can create your interactive TV links or provide you with the software and/or hardware you need to get started. Note that this list is not intended to be comprehensive, but simply to help you get started.

Back to Contents

Creating Your Own Interactive TV Links

Interactive TV links are straight forward and easy to create. They consist of a URL and any of a number of attributes, depending on your design and the needs of the interactive show. There are abbreviations available for all of the attributes, except URL and checksum. Interactive TV links with abbreviated attributes are transmitted more quickly and more accurately.

To create your own interactive TV links, you'll need to know the following:

Interactive TV Link Syntax

The interactive TV link encoding format is specified by the Electronic Industries Association specification EIA-746-A, "Transport of Internet Uniform Resource Locator (URL) Information Using Text-2 (T-2) Service." This specification is available for purchase on the Global Engineering Documents Web site at

For United States and Canadian broadcasts, interactive TV links are transmitted on line 21 of the vertical blanking interval (VBI) of an analog television signal. Line 21 is the part of the VBI that is also used to send closed captioning text. The standard accepted by CEMA mandates that the interactive television link string be inserted in Text-2 and use the following general format:

link components

Size Limitations

The total number of characters you can transmit in an interactive TV link is limited both by the physical constraints of the VBI (2 characters/frame and 30 frames/second) and by the encoder. Check your encoder manual to determine how many characters per second it can encode, and therefore how long your interactive TV links can be.

Interactive TV Link Attributes

Interactive TV links can have any combination of a number of attributes. All of the attributes, except URL and Checksum, can and should be abbreviated to increase the speed and accuracy with which they are transmitted.

When received by WebTV for Windows, all of the attributes except the URL and Checksum are ignored, but the target Web page will still be displayed in a new browser window. In short, the experience that interactive TV links create in WebTV for Windows can be just as robust as on a WebTV-based Internet Receiver.

Of the following attributes the URL and Checksum are the only required fields:



The Web site address of the interactive TV content.






Used to categorize interactive TV links and show the TV audience any links that appeared during a program or time period.



Program—the current program.

Network—the broadcast network.

Station—the local station.

Sponsor—a commercial sponsor of the current program.

Operator—the service operator (for example, cable or satellite).




Note: CEMA allows you to omit the word "type" (for example, [sponsor]). With interactive TV links, however, if you want to include a type attribute, you must use either the word "type" or the abbreviation "t".



The message TV viewers see on the drop-down panel that displays after they select the interactive TV link icon. This is limited to 20 or 30 characters, depending on the characters used (a proportional font is used). The name is the only information your TV viewers will have about the interactivity before playing it. If you do not name your interactive program, viewers may have little reason to display it.



[n:WebTV Networks]

[name:WebTV Networks]



Triggers actions on the Web pages that the interactive TV link references. The value specifies a script fragment that will be sent to the target Web page and executed. The script is executed when the page is opened. If the page is currently being displayed, the script fragment is immediately executed. The context for the script is the root document corresponding to the URL specified in the link string.







Specifies the last date the interactive TV link is valid, after which the receiver should ignore it.

The format of the time conforms to ISO-8601 and is relative to Greenwich Mean Time (GMT). A valid compact format is yyyymmddThhmmss, where 'T' separates the date from the time. Note that you can shorten the date string by reducing the resolution (where yyyymmddThhmm becomes yyyymmdd).








Specifies how the receiver should display the linked resource (or how the TV audience should see it).

There are two ways to view Web pages, as television related content ([view:tv]) or as a conventional Web page ([view:web]). This information works in conjunction with the view attribute, described in Displaying Television Broadcasts in Web Pages.

Optional. If not specified, will default to [view:web].






Verifies the accuracy of the string and detects data corruption.

The checksum is established in the CEMA standard, is required for line 21 triggers only, it is not required for triggers sent in IP packets.

You can use our Interactive TV Link Checksum Tool to calculate the value. You can also view the C code for the tool.




Back to Contents

Trigger Matching

Trigger matching is a special syntax that enables the WebTV client to send a script contained in an interactive TV link (called a trigger) to a set of Web pages, instead of to a single Web page. In previous releases of the client, the trigger could only be sent to a single Web page.

In order for the client to execute the script on successive pages, it must determine that a match exists. The client determines that a match exists if the following conditions are met:

  • The final character in the trigger is an asterisk (‘*’), and
  • The URL of the top-level page of the current interactive TV program matches up to the position of the asterisk.

Note: a complete domain is required before the asterisk.

For instance, if the trigger


was received by a WebTV client currently on the page or

the client will execute the script on that page. However, for a client currently on the page

the client will not execute the script because the root path does not match the string up to the * in the URL passed with the trigger.

Inserting Interactive TV Links

Most software programs for closed captioning encoding provide a mechanism for inserting characters in Text-2, and some closed captioning tools have specific support for interactive TV links. With most encoding software, you simply type the text and encode it just as you would for a closed caption.

Important After calculating the checksum, do not do any of the following:

  • Change the case of any text.
  • Add or remove spaces.
  • Format the string in any way.

Note: If you do not observe these prohibitions, the checksum will fail and the interactive TV link won't work.

If you choose not to create your own interactive TV links, see Interactive Television Development Resources for a list of professional services that can create them for you.

Back to Contents

Technical Specifications


Characters per second: 60 characters can be transmitted per second.

String length: An average string is between 45 and 60 characters.

Bandwidth: EIA-746 recommends that interactive TV links be limited to 15% of the available bandwidth on field 1 of line 21. However, when the closed captioning is encoded before or during interactive TV link encoding, it is generally safe to use all of the remaining field 1 bandwidth for interactive TV links.

Back to Contents