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Originally Posted by towny
Why develop one system when you can have three? Heaven forbid that consumers might want to buy one set-top-box and then switch between satellite, cable and terrestrial as their needs, and the market, changes. This way, to switch, they must buy a new STB each time, or sign up with a broadcaster who will give them a 'free' STB in return for a minimum-term contract, and in the case of Sky also on condition that you connect the blasted thing to your phone line so it can run up your bill whenever it likes, without your permission. The winners here are the broadcasters, who have a way to lock you in to their services, and the manufacturers of the STBs, who get to sell more of them.
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Or could it be due to the transmission medium in each case ? Not really thought it through have you? A satellite signal has to travel approx 23500Miles to the satellite and then 23500 miles back to earth. Theres a lot of interference in the atmosphere and the signals involved aren't particularly high powered. With the noisy enviroment the transmission method is kept simple. A dish is required to focus the weak signals. QPSK modulation is simple and hence simple to recieve and decode.
DTT using rooftop or indoor aerials has to cope with lots of echos which cause ghosting in the analogue enviroment, hence they use a method which can cope with echoes (reflected signals). COFDM, is thought the best method.
Cable is a relatively noise free enviroment, when terminated properly it doesn't suffer from reflections. Therefore it can use a relatively complicated modulation method (QAM), without the 'echo cancellation' capabilities of COFDM.
So your comments about it being pure greed on the part of broadcasters/manufacturers are cr*p.