TAKE a flawed technology, a rushed roll-out and overambitious commercial goals, and what do you get? According to the BBC, you force the world鈥檚 first digital terrestrial television station off the air. That鈥檚 its conclusion following extensive tests of the transmission technology used by ITV Digital, the defunct British broadcaster.
ITV Digital went bust in April, after only a million homes subscribed. One reason for its lack of popularity was the fact that reception was generally awful, often causing pictures to freeze.
The BBC has discovered that ITV Digital鈥檚 commercial aims far outstripped the technology鈥檚 capabilities. To make the service viable, it broadcast around 50 channels. This cut the data capacity available to each channel, damaging picture quality. The BBC says the service can live again, but only if broadcasters do not try to jam in too many channels.
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ITV Digital used a technology called DVB-T, which uses frequency bands 8 megahertz wide, enough for just one analogue channel, to carry several digital TV channels in a bundle called a multiplex.
The DVB-T multiplex is split into thousands of very narrow 鈥渃arrier鈥� signals. Each carries only a tiny portion of a digital TV picture. By switching these carriers through 64 different power levels, there鈥檚 room for 24 megabits of picture data per second in each multiplex, enough for six or seven channels. Or so ITV Digital thought.
Using the frequencies ITV Digital vacated, the BBC has shown that receivers cannot reliably tell the 64 levels of the signals apart, especially in areas where signal strength is low. But it found that DVB-T using just 16 levels is extremely robust, and would allow give viewers a much better chance of getting a picture 鈥� even in fringe reception areas. And all existing receivers should work with a 16-level signal.
But there鈥檚 a downside. The 16-level format only has room for four channels in a multiplex, and a total of 24 channels for the service as a whole, not the 50 ITV Digital was transmitting. The BBC doesn鈥檛 think this is enough to support both free and subscription channels. 鈥淭he best thing to do with digital terrestrial TV is to turn it into a free-to-air service,鈥� its director general, Greg Dyke, says.
The BBC鈥檚 tests have also highlighted another mistake. Although each DVB-T channel can be split into 8000 narrow carrier signals, ITV Digital鈥檚 rush to market led it to use a compromise system with just 2000 carriers, because the only microchips available at the time were for that system. But as only half the set-top receivers in use can receive 8k broadcasts, this mistake cannot readily be fixed.
The Independent Television Commission will this week decide who takes over the service.