hH? - rediffusion


Transmission Methods

The network consists of six-pair copper stranded cables of which four are now used for video transmission (refer to earlier regarding the four channels which used to be transmitted). Signals are sent out on VHF frequency, and are converted using a simple frequency shifter. This technique is referred to as 'Plain Jane' and is rarely used nowadays although it is assumed that the restrictive nature of the cabling prohibits more complex transmission protocols.

Channel selection is made using a twelve-pole dual-terminal rotary switch (channels marked A to L on the switch, many houses still have them usually near the window frames):

Outside 12-core goes in here
            |  |
          __|__|_________________
         |                       |
         |         F   G         |
         |    E   _______   H    |   
         |  D    /   |   \    I  |
         |      |    |    |      |      Twelve-channel rotary
         |  C   |         |   J  | <--- switch
         |       \_______/       |
         |   B              K    |
         |       A       L       |
         |_______________________|
            |  |
            |  |
Two-core selected cable comes out here

Hull used four channels, which were lettered ABCD.

What is surprising is that all the satellite channels are transmitted 'in the clear' (ie. unencoded) and any old Rediffusion box will pick up the signal if you wire it in. Alternatively a circuit is reproduced in this document for you to build your own ;-)

Note: Apparently these 'upconvertors' are freely available in Ireland as it uses both VHF and UHF distribution; the circuitry is actually extremely simple, often being composed of a transistor amplifier, a transistor oscillator, and a diode mixer. They are apparently available freely through adverts in TV/electronics magazines (such as 'Television' and 'ETi').

From the Rediffusion Engineering Journal, Vol.1 No.8 (September 1971):-

"In the days before transistor feeder amplifiers such as the A.623 and the AA.105, Rediffusion links were usually High Level Links carrying a voltage of up to 660V, supplied by large amplifiers, typically the A.126 and A.40. The object of the high voltage was of course to minimised the copper loss and so permit the use of wire of a moderate gauge. Service Level Links are the current system and, from the foregoing reasoning, is a logical development of the High Level Link system in that small, high-quality line amplifiers are used to drive the link system."

Hull appears to be on a High Level Link system, from the number of H.L.L. kiosks mentioned in the list at the end of this document and the lack of S.L.L. information.


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Disclaimer: All the info given here is incomplete, there's always something missing. This is basically to cover our own backs, but for someone with a bit of sense it shouldn't be too hard to fill in the blanks. The authors of this page and the carriers do not take any responsibility whatsoever for any action you may carry out as a result of you reading or using this information. Any documentation provided is given for research purposes only.


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