Active Format Descriptor

There are now more than 250,000 widescreen televisions in the UK and the overwhelming reaction is that it greatly enhances the viewing experience. Going back to 4 x 3 is like returning to black and white ... viewers wonder why they put up with 4 x 3 for so long. Their main complaint is the lack of widescreen transmissions. Herein lies a 'chicken and egg' problem; broadcasters maintain they cannot transmit more whilst there are so many 4x3 television sets still in use and viewers say they will not buy a widescreen television set until there is more widescreen programming!

Enter digital terrestrial (DTT). The new services require new equipment and broadcasters can broadcast much higher quantities of widescreen without disadvantaging existing viewers. The Broadcasters see widescreen as a sales feature of DTT and manufacturers have been quick to respond with advertising campaigns encouraging customers to purchase a widescreen television set as a sensible preparation for digital.One of the complications for broadcasters is that their archives are full of 4x3 programmes. Only in the last couple of years have most broadcasters been shooting some programming in widescreen 16x9 format, so there will continue to be some 4x3 'letterbox' programmes on digital, particularly on themed repeat channels. Therefore, manufacturers will continue to provide a variety of 'zoom' functions on digital television sets. The Active Formal Descriptor (AFD) is a signal that broadcasters will transmit with the picture to enable television sets to display the picture to best effect.What is best effect depends on the viewers display (4x3 or 16x9) and it depends on the viewer's preferences. Most viewers like to 'see a screenful' but many prefer to set their televisions so that not too much picture is cropped. Some like to see the whole picture, even if that means black bands at the sides of the screen. In general, widescreen televisions offer five options when transmission is not full 16x9 widescreen:

  • 4 by 3
  • 14 by 9
  • Zoom (16 by 9)
  • Smart/Panorama
  • Auto

The latter three modes need some explanation. Zoom will keep the correct shape but crops the top and bottom of a 4x3 picture quite considerably. Smart / panorama avoids the cropping by stretching the picture horizontally to fill the screen but makes everyone look as though they need to go on a diet. Neither of these modes is entirely satisfactory. What we need is a signal from the broadcaster to say, for example, 'This picture can be zoomed in and cropped as far as 14x9 (because we have shot it with this compromise in mind)' or 'this picture is actually letterbox so, even if you normally prefer to watch in 4x3, you can zoom it in to 14x9.' This is where the AFD come in. It is a way of giving the tv set more information so that it can adjust the zoom for best effect taking account of viewers preferences.

Lastly, when a viewer records a programme on an existing analogue VCR, the digital bitsream flag which indicates a 16x9 programme is lost. Therefore, the DTG is recommending that manufacturers generate a 'line 23' WSS signal on the output feed to the VCR. Without it, widescreen programmes will appear distorted. Use of the AFD in the WSS generation gives the viewer the same fine control of zoom when viewing recordings as when watching off-air transmissions.