Unlike analog television, where a broadcast can often be viewed even through adverse conditions, such as a heavy "snow" effect, a digital receiver either manages to recognise the digital data stream, or it doesn't. If it does, you get a "perfect" picture. If it doesn't, you get nothing. In some marginal cases you may get "pixelation" (blocky squares similar to that used to deliberatly mask faces on the news), where the receiver gets some parts of the data and not others, although this can vary depending on the broadcast method used.
In this way it is similar to the difference between analog and mobile phones. With an analog, you can often talk through static noise... with a digital, it can just cut out completely. Having said that, you cannot take this analogy any further. Mobile phones are not designed as broadcast units and, as such, a digital timing method was used which places an upper limit on the usable range of a digital mobile, but at the same time works far better within that range. Digital Television is a broadcast system and has a different approach to suit different goals.
The other common misconception is that you must have a perfect picture now to be able to receive digital broadcasts in the future. If you have a modern VCR, it will have a "test signal" mode to assist you with tuning your television to the VCR. This signal is often two vertical white bars on a black background. Why was this chosen? The reason is that the human eye will beable to see it even through a very snowy picture. If the two bars formed the digital signal, it would be very easy to pick up indeed! Naturally, this is obviously over-simplified, but the principle is the same. It is this concept that allows digital broadcasts to use much lower power than convential broadcasts, while acheiving the same viewable broadcast coverage area.
In summary, the "Cliff Effect" is a concern, especially for people in poor reception areas who are still relying on Terrestrial (as opposed to Satellite) broadcasts. However, it is a concern which should not be blown out of proportion.
Digital Television isn't going to be introduced overnight. During the phase-in period, both public and private organisations as well as interested individuals will be able to monitor this situation and call for retention of some parts of the analog coverage while the digital coverage catches up. Sound familiar? The legislative framework being proposed already includes safeguards to this effect. Sometimes Government does learn from its mistakes. Commercial television stations would also not take the loss of viewers lightly.
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