Creating Interactive Television LinksJune 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 SeesWhat interactive TV links look like to the television audience.
How Interactive TV Links WorkA general description of the components used to create interactive TV links.
Getting Help Creating Interactive TV LinksCompanies that can help you create interactive TV links.
Creating Your Own Interactive TV LinksTechnical instructions for creating your own interactive TV links and inserting them into the TV broadcast.
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.
After the viewer clicks the link, one of two things will happen:
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.
Interactive TV links contain text codes called Uniform Resource Locators (URLs). These point to specific contentWeb pageson 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.
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.
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:
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 http://global.ihs.com/.
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:
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 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:
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:
Note: a complete domain is required before the asterisk.
For instance, if the trigger
was received by a WebTV client currently on the page
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
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.
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.
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.