Interactive TV is one of those technologies that enable so many services but yet we know so little about what makes it tick, How does the simple push of a Red Button trigger events to happen on your TV? How do your votes get counted if you use your Sky Digital Remote to vote for a Reality TV contestant?

First of all its best to define what we mean by Interactive, true interaction is the two way exchange of data that allows for events or sequences of events to be changed dependant on the user input, in layman's terms if I push the button then and only then does the action occur and I am severed with data that has been uniquely requested by me. Now by this definition about 95% of all interactive TV is not truly interactive.

That's quite a bold statement but let me explain, one of the first interactive experiences available on TV was (and for that matter still is) analogue teletext, here the user selects from pages either by directly typing in a number or by 4 coloured button options to make navigation easier, before you start to ridicule teletext remember teletext holidays went on to sell some 15 - 20% of all UK breaks! The analogue television system uses a data carousel this is loaded with pages and much like a slide projector it spins round until a user selects which page (or slide) they wish to view it gives the impression of interactive but in truth all the data is there and constantly being sent to all television sets you select and in the case of analogue television wait for the page to be broadcast.

Now the heart of a modern "digital" interactive system is a streamer, this sends data out to the set top box and guess what? its a data carousel and the main manufacture is the same company who was the market leader in analogue teletext infrastructure, the main difference in the Sky Digital system is that the set top box runs small applications which use this data and control screen presentation to achieve some pretty impressive effects.

The Red button is the start key of the Sky Interactive platform, pressing this will load the application the broadcaster wants to run on that channel, the application will have already been downloaded to the unit when you last watched the channel, if the application is not present in your box it will take a while for it to pull this from the data carousel before it can present data on screen, this may result in a wait or loading message. Most Interactive apps shrink the video using X-Y commands so that the rest of the screen can be used to display text, the option son screen are all driven from your set top box and selection of any item will result in the unit pulling data from the carousel, again here you may get a delay as the unit waits for that data to be broadcast in sequence.

Unlike the teletext system where the refresh rate of the data carousel was limited by the number of data lines available in the vertical blanking interval (within the signal but outside the visible picture) the interactive streamers rely on allocated bandwidth, much as a channel selects the amount of bandwidth for its video and audio an interactive broadcaster must allow for data throughput for the application. This is a balance too little bandwidth allocated to the interactive stream and the application will be slow and users will switch off, allocate too much and you will be paying a high price for something you are not using (bandwidth isn't cheap), in practice while channels may be anywhere from 2.5mbits - 5mbits for video and audio 0.5 mbits is sufficient for a fairly complex interactive application.

Typical applications may provide further information of the programme you are watching, the BBC and Sky themselves use this heavily to cross promote series and competitions, this is a pure 1 way application, the user requests data which is served to them but in fact is being broadcast to all and they are merely selecting which data they wish to view, in there words teletext but less blocky! Then there are interactive adverts the "data" layer is used to provide additional information or perhaps let you order a sample of the product, these use the same technology but are far more difficult for the broadcaster as the application has to be pre loaded before the advert appears on screen and as adverts are typically 30 seconds there is no margin for error, you don't want to press the button on a Budweiser advert and get a Pepsi special offer.

Advertising is the one of the applications that starts to introduce an element of interactivity albeit limited, the concept of a back channel is used to transfer your request back to an aggregation point where actions can be taken based on your input, now DTH satellite is one way unlike Vsat or DVB RCS systems where users can send information back via satellite so Sky Digital use the domestic phone line as a back channel. This is the main reason your subsidised box has a clause making you keep the phone line connected, through this 56k dial up back channel data can be sent from the interactive app back to the broadcaster, in its simplest form this can request a brochure or order a film on pay per view, the call can be free but it is more likely charged which the on screen information must clearly show.

It is still rare that this data makes it back on screen, some applications will input this back to the studio, like voting on I'm a celebrity or other reality shows or perhaps as bet on interactive gambling, but true interactivity still eludes us, this is still glorified teletext.

"Interactive" is two way communication where what I press on my handset significantly changes what I see, perhaps changing what's playing in a video jukebox system, some systems exist that can deliver this they are typically cable based where the viewer has a one to one relationship with the broadcasters hub systems this allows video on demand and that is easily enhanced with an interactive layer so that you could for example change ending of a film.

Sky Digital does not have this one to one relationship and this leads to some clever workarounds to mimic interactive experiences.

Lets take the example of a big court case that is being re constructed though an "interactive broadcaster" we can enter the channel as normal where there would be a video stream playing telling us about what is contained within the service these are typically called "barkers" its job is to get us to push the red button, remember this could be a charged service.

Once we have pushed the button the interactive app springs to life and squeezes the barker back to quarter screen, we are then presented with a number of options, listen to the case for the prosecution / defence / hear witness statements / see forensic evidence. On selecting one of these the application chooses another channel which is not available on the EPG and switches to it while staying within the same look and feel, these are typically quarter screen video with text and images presented through the application. The video is on a loop which will repeat lets say every five minutes, now having one barker channel and 4 further video feeds is the equivalent of 5 TV channels (not cheap) so the broadcaster's second channel is actually 4 video streams quad spilt on the screen the interactive application selects which quarter it needs and uses the X-Y positioning to mask the others and select the correct audio.

There can be multiple levels of this kind of Interactive spoofing each requiring just one additional hidden channel, again this can be combined with a back channel to order some more information or enter your verdict which may be shown on screen as a graph later in the process.

This is undoubtedly a very intelligent use of technology but also a clear sign that the medium of Sky Digital is not really fully interactive otherwise why would you go to these lengths?

Authoring of these applications is currently a black art much like the early days of Web site development, but in practice there are only a few formats in use on each platform, Sky uses "Open TV" and most cable providers use Liberate, MHEG5 is in use on DTT but has the limitation of no back channel.

Recent industry reports have actually shown a slump in interactive viewers with a drop of about 600,000 in this last reporting period, could this be that we are actually realising this is not really very interactive? but then who wants to change the end of the film especially if I have to pay to do so. Interactive TV is a very clever piece of technology which has the might of the SKY marketing machine behind it but it has to do battle with the very nature of television as being a 1 way couch potato medium, viewers are far more likely to use the internet for true 2 way interaction.

The next few years will be the make or break of interactive services across all platforms nor just Sky Digital, its up to the broadcasters to make engaging content and to drive the use of technology to deliver the rich viewing experience that viewers want.

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