[Milky Joe]

This is something ive always wondered! How do the vcrs know what the code means? as each one is different for the same programme each week i think. Is it sent through with the terrestrial broadcasts like teletext?

[TheCableGuy]

Quote:

Originally posted by Exulus
This is something ive always wondered! How do the vcrs know what the code means? as each one is different for the same programme each week i think. Is it sent through with the terrestrial broadcasts like teletext?

I dont know too much about it. But the data is carried in the spare VBI lines of a tv picture, the same lines Teletext uses.

I dont really knwo much else about it. But im sure if you Google™ it you will find all the information you need.

[edit]
Ah, here we go: http://www.newscientist.com/lastword...le.jsp?id=lw74

Here is the info from the above link:

Quote:

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 through 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.

[GaryB]

It's PDC (Programme Delivery Control) that's carried along with the teletext signal. VideoPlus doesn't need anything from the broadcaster, just the clever algorithm that's mentioned in the NS extract. PDC and VideoPlus are usually used together in modern VCRs.

[Milky Joe]

ah brilliant cheers guys

[chrisbartley]

Quote:

The VideoPlus algorithm itself is a closely guarded secret

Its not that closely guarded, these is code out on the web to encode / decode up to 5 digit codes, for some reson it get a bit more difficult beond that

(well there used to be, I can't find it now, was in C as I recall, an it was someones life work to reverse engineer the code - definatly exsisted, as I did fiddle with it for a while, as part of another project))

edit: Found it ! . seems it was longer ago than I thought
here
see here for explantion of the code/decode process

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Last edited by chrisbartley : 24-09-2003 at 11:54.

[kev]

Quote:

Originally posted by chrisbartley
Its not that closely guarded, these is code out on the web to encode / decode up to 5 digit codes, for some reson it get a bit more difficult beond that

(well there used to be, I can't find it now, was in C as I recall, an it was someones life work to reverse engineer the code - definatly exsisted, as I did fiddle with it for a while, as part of another project))

edit: Found it ! . seems it was longer ago than I thought
here
see the readme for explantion of the code/decode process

Thats VCR PLUS not VIDEO PLUS, there is a plug in for Digiguide to covert to the numbers...


Also, you would have to take into account duration, and channel number. Video Plus gives Channels 1 -> 4 (BBC1London, BBC2England, ITV London, Channel 4 in the UK - as most regions have shows at the same time you can use the same codes for multiple regions) and shows on them channels during primetime an advantage of shortcodes. (Thats why Corrie is nearly always 3 digits, while Familiy Affairs is about 8)

[kev]

Heres the Video Plus Jobbie

http://tallyho.bc.nu/~steve/videoplus.html

[chrisbartley]

All based on the original work by ken shirriff et al

not sure theres much difference between US and UK version apart from channel mappings
Edit : There is , I read your link now

All that crypto stuff is way over my head