Thin margin for digital TV perfection

It doesn't take much time for the perfect picture quality of digital TV to "fall off the cliff" and be lost altogether in some areas, a spokesman from the Australian Communications and Media Authority said.

His comments came in response to a Mercury story last week about digital TV reception issues on the South Coast.

The spokesman said the digital service was designed "to the greatest possible extent" to replicate the coverage and quality of the analog system.

"However, analog and digital television services are based on fundamentally different technologies and are affected differently by local environmental conditions, especially in areas where the signal coverage is marginal," the spokesman said.

The spokesman said perceptions that analog delivered a better picture than digital were due to a more sudden transition between a perfect digital picture and no signal at all.

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"This effect is known as the 'digital cliff edge effect'," the spokesman said.

"If the signal level is close to the threshold, a very small degradation can push a perfect digital picture to 'fall off the cliff'. On the other side, the same transition between perfect and non-existent analog picture is gradual and some viewers would still consider a very snowy or ghosted analog signal as watchable or even a good signal.

"Some viewers in the marginal reception areas who used to receive marginal, snowy or ghosted analog picture are likely to receive a digital signal that cannot be viewed [pixilation], a still picture or no picture at all.

"This creates a perception that the analog coverage was better than digital."

Those viewers in marginal areas would be close to the "digital cliff edge", the spokesman said, and a very small degradation, caused by local conditions, such as trees or even heavy rain, could push a perfect picture off the cliff.

The spokesman said homes in marginal - or variable - coverage areas could improve the picture quality by ensuring they had an appropriate antenna and it was directed to the transmitter that provided the best coverage.

The website myswitch.digitalready.gov.au allows people to see the strength of the signal in their area.

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