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Q: I realise this question may involve commercially secret information, but
could someone please explain the principles behind the VideoPlus codes used to
switch VCRs on and off when recording certain television programmes?

A: The Videoplus system works quite simply by using a reversible algorithm
– something well known to most computer programmers. These types of algorithms
are found in diverse applications, ranging from credit card machines to data
compression and encryption.

The VideoPlus algorithm itself is a closely guarded secret, but it’s
probably only a little more complicated than the example described below. I
just wish I’d had the marketing nous to think of it first.

A simple example of such an algorithm would work along the following lines.
Imagine you wish to record a programme that starts at 9.50 pm on 3 November.
VideoPlus probably uses a cyclic date system, rather than the standard way of
representing dates. If, for example, there are 50 days in a Videoplus “month”,
then it would be possible to use VideoPlus up to 50 days in advance of a
programme.

So, 3 November might be converted to, say, day 45. You would then convert
9.50 pm to the 24-hour clock and append the VideoPlus day to the converted
time. This gives 215045. The next step would be to work out by what single-
digit number 215045 could be divided by without leaving a fraction. Five is an
obvious answer, so let’s use that. Divide 215045 by five, and you get 43009.
Append the dividing factor to the end of that number, and you have a
“VideoPlus” number of 430095. Now, unless you know the algorithm, it won’t be
easy to convert that number into a time and date. (The dividing factor won’t
necessarily appear at the end of the VideoPlus number.) However, if you do
know the algorithm, applying it backwards becomes very easy.