ÐÓ°ÉÔ­´´

Q: A few seconds before iTV or Channel 4 start to broadcast adverts some
televisions display a small black and white rectangle in the top right corner
of the screen. Why do they do this and could it be used to stop video
recorders from recording the adverts?

A: The small squares appearing on the top right of TV screens on ITV and
Channel 4 just before commercial breaks are called cue dots. They consist of
vertical black and white stripes, to make them visible on any background. They
are not locked to the picture, which is why they sometimes seem to spin
around.

Their main purpose is as a visual warning to operators who are controlling
video machines with manual rolls of tape. They appear 5 seconds before the
time when the next machine should be switched on. This is usually to allow the
operators to roll the adverts appearing in programme breaks. If cue dots
appear during the programme, they have been used for editing purposes.

The ones used by the BBC appear in the top left corner, coming on and off
twice to give a 5 and a 15 second roll, for older two-inch tape machines.
Film, tape and cassette machines now stabilise within 3 seconds of being
switched on, so 5 seconds is enough. Cue dots have been replaced at night by
audio tones sent over telephone lines, which start automated machines
remotely.

Cue dots don’t appear on all TV receivers because some have been adjusted
to make the raster (and hence the picture) larger than the screen.

As the cue dot is part of the jumble of picture information, it would be
hard to use it to avoid recording commercials on a VCR. The digital
information sent among the picture frames may be more useful, but it would
require complex circuitry. One problem would be that not all breaks are the
same length – especially when programmes are live – so restarting your machine
would be difficult. Another is that the tape and video heads would wear
quickly if paused.

Anyway, you are supposed to watch commercials avidly, spend lots of money
and make someone else rich.

A: If the signal is always the same it might be possible for video
recorders to detect the pattern (or more importantly, when it ends). However,
advertising breaks would have to be of a constant length, because there is no
such signal when programmes resume. There is also a chance that, on rare
occasions, the video recorder might be stimulated by the actual television
picture itself.