Please read the White Paper Copyright Notice

Download
WHP160 807 Kb

BBC R&D White Paper WHP160

Digital TV Switchover:
An aerial test that uses a teletext test page
John Salter

Keywords
reception, antenna, digital margin

Abstract
This white paper gives some of the technical background and detail of the work that we have been doing developing an on-screen indicator to assist digital switchover (DSO). We want to help viewers decide whether they need to improve their aerial installation to be reasonably certain of reliably receiving digital terrestrial transmissions (DTT) after DSO. Our work in this area forms a part of the overall work being conducted within the Digital Reception Prediction Group run by DigitalUK, the non-profit organisation leading the process of DSO in the UK.

The basic idea is very simple. Could the quality of existing analogue reception be used as a guide to the quality of future digital reception? A weak aerial signal results in snowy analogue pictures which may mean unreliable future digital reception. An assessment can be made of how snowy analogue pictures are in order to judge whether the aerial signal is too weak. However, viewers are not very comfortable making subjective assessments, so an indication using teletext as a test signal has now been developed. Less judgement is required for this test as this involves checking for missing blocks in a regular pattern. This so called ‘aerial test’ is currently being transmitted in teletext on page 284 on the four main analogue TV services in the UK.

A number of relevant relationships are considered in detail:

• Video signal-to-noise ratio versus RF vision carrier-to-noise ratio.
• Subjective analogue picture quality versus video signal-to-noise ratio.
• Video signal-to-noise ratio versus probability of error for teletext reception.
• RF vision carrier-to-noise ratio versus digital RF carrier-to-noise ratio.
• RF digital carrier-to-noise ratio versus error rate.
• Digital error rate versus subjective failure point.

There are some caveats that mean this test is not suitable for some switchover scenarios (the postcode database should be checked) and accuracy may be affected under certain conditions. Nevertheless, initial trials indicate that this ‘aerial test’ will be both a useful and simple test that will benefit the majority of viewers during DSO.