Set-top boxes fall into several categories. From the point of view of a JavaScript developer, the most important category is the 'Browser in a box' model. This takes a basic Netscape Navigator or MSIE browser, places it inside a modest functionality PC and allows its page content to be overlaid on top of the video that is broadcast off-air. The video can also be placed into a cell within the page to allow the web page to be placed behind the video.

This is generally accomplished by allowing the <IMG> tag to take its source from a new URL type. Instead of an HTTP: protocol, the TV: protocol is used to trigger the video overlay hardware in place of a web request.

The WebTV boxes have been available for several years and are an analogue TV set-top box.

The JellyScript interpreter used in the WebTV box underwent an upgrade in late Spring 2000 and was released for public use during the Summer. It is generally referred to as the Summer 2000 release. You should be somewhat careful with releases of interpreters in set-top boxes, as a new release is likely to be presented for each manufacturing run. These will generally contain only minor changes. The set-top boxes are also designed to allow the interpreters to be upgraded remotely when they connect. This can lead to some interesting problems if you connect a UK PAL compatible box to an American NTSC service because the video hardware is reconfigured and is then rendered unusable.

The integration of the web content with the TV service is by means of crossover links that are URL values embedded into the video signal on a very low data rate channel that is part of the closed captioning signal.

The URL is encoded with a checksum and arrives at a rate of about 100 characters per second. Because the transfer is at such a low bit rate the URL values need to be short. When the box detects a cross-over link, it displays a small icon in the top right of the screen and the user can elect to request the associated page. The box then dials an ISP and the page is delivered in the normal way.

This is a quite good and workable enhancement to the TV service and although its deployment is limited mainly to the United States, boxes have been trialled in Europe and elsewhere.

There are a few limitations imposed on the JavaScript supported by the box and the embedded browser does not generally support extensions such as Java, plugins and ActiveX but nevertheless you canstill accomplish quite a lot.

Cross-references:

http://developer.webtv.net/authoring/javascript/javascript.htm