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rb:coventrycable [12/04/2018 20:01]
andrew created
rb:coventrycable [26/06/2018 00:37] (current)
andrew [Supertrunk]
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 ====== Coventry Cable TV ====== ====== Coventry Cable TV ======
  
 +Now defunct, the original build was for 450MHz bandwidth carrying about 30 channels. It was built and operated by BT and along with the likes of Milton Keynes was one of the first franchises in the UK.
 +
 +===== Headend =====
 +
 +Located at the end of Whitley Village, the building is now demolished.
 +
 +
 +Satellite TV receivers & cable tv modulators and Launch Amplifiers.
 +
 +{{:​rb:​covcable-headend_.png?​200|}} {{:​rb:​covcable-launchamp.png?​200|}}
 +
 +Master Control room - monitoring all the channels being transmitted and some of the playout equipment.
 +
 +{{:​rb:​covcablemcr1.png?​200|}} ​  ​{{:​rb:​covcablemcr2.png?​200|}}
 +===== Supertrunk =====
 +
 +The Supertrunks radiated out from the Head End to distribution sites prior to feeding the local distribution network. These consisted of several amplifiers with one input and one output boosting the signal. The amplifiers were nothing special, just the same as the DLA (Distribution Launch Amplifier) amplifiers but run at lower gain (ie. less noise added to the signal), the supertrunk coax was about 40mm in diameter for minimum signal loss. Due to the inflexibility of the low loss cable, the link to the cabinet was on reduced size cable with the joint in a chamber adjacent to the cabinet (left hand side next to road below)
 +The supertrunk amplifiers were located in smaller green BT fibreglass cabinets and were all line powered from either end of the trunk.
 +
 +{{rb:​20180519_115423.jpg?​400|}} ​ {{rb:​20180519_115350.jpg?​150|}}
 +
 +This cabinet seems abandoned, the door to this has been lying on the grass for several months.
 +
 +{{:​rb:​core1.jpg?​200|}} {{:​rb:​core2.jpg?​200|}} {{:​rb:​labelling-disc-spacing.jpg?​500|}}\\
 +This is the supertrunk cable, it is an aluminium core with a copper cladding (The aluminium will carry most of the AC line power, whilst the copper will carry the RF due to the "Skin Effect"​),​ the dielectric is polyethylene with air spacing to reduce loss but periodic "​Discs"​ of polyethylene to provide support for the core, the screen comprises a copper foil wrapped with an aluminised plastic enclosed in a black plastic sheath.
 +
 +The right side picture shows the cable manufacturer and codes, as well as showing the position of the dielectric discs supporting the core.
 +===== Distribution =====
 +
 +Example DLA (Distribution Launch Amplifier) cabinet, this appears completely abandoned, the door is missing, the mains electricity is severed and the input and output coax cables have been cut and removed. These were all housed in grey versions of the BT green fibreglass cabinet used sometimes for the phone distribution PCPs:- \\
 +{{:​rb:​20151006_175303.jpg?​200|Distribution Launch Amplifier}}
 +
 +From left to right:​- ​
 +  - Texscan wideband amplifier (White) (450MHz downsteam and approx 5MHz return path back to the headend), one input feed and either one loop through and two amplified outputs or one input and three outputs.
 +  - Fuba ferro resonant transformer,​ 240v input and  50V ac output. This would feed the amplifier in the cabinet and also line power any downstream line extender amplifiers, these were all located underground.
 +  - 240v power supply feed to Fuba transformer,​ via Wylex consumer unit.
 +
 +
 +===== Consumer Distribution =====
 +
 +==== External ====
 +
 +The Line extender amplifiers and underground splitters fed customer premises either overhead or underground. This was determined by how the BT phone line was cabled. ​
 +
 +Overhead distribution was brought to an 8 way directional coupler mounted at the top of telegraph poles or on the side of buildings (for low rise flats) This fed a twin "​figure of eight" cable with the phone and strength wires in one half and the tv coax in the other half. The unused outlets were capped with a ''​F''​ terminator to both provide a weather seal and prevent ingress / egress of RF.  \\
 +{{:​rb:​tap1.png?​200|}} ​ {{:​rb:​tap2.png?​200|}} ​ {{:​rb:​tap3.png?​200|}} {{:​rb:​20180503_134911.jpg?​200|}}
 +
 +Underground distribution was used where there were no telegraph poles allowed such as in new estates. In this case the directional couplers to feed consumers were mounted in BT's footway boxes. Some of these were always prone to run off rain flooding them, so sometimes waterproofing would fail and cause problems.
 +
 +
 +==== Indoor ====
 +
 +The coax feed from outside was passed to an amplified splitter in the house which also served to allow a changeover from outdoor grade, UV resistant cable to indoor grade cable which was a low smoke and fume cable in case of fire.
 +
 +The each output from the splitter would connect to a FM / CATV demux, the F socket going to the Set Top Box and the "​TV"​ style Belling Lee going to an FM radio. These are shown below with the covers on and off. The blue knob is to adjust the gain fed to the STB. On the right hand picture, the connector in the centre is the dropwire input, it feeds in to an AC/DC blocking isolator so any internal faults in the premises could not result in mains electricity being fed back in to the network.
 +
 +
 +{{:​rb:​20180526_183224.jpg?​200|}}
 +{{:​rb:​20180526_183725.jpg?​200|}}
 +{{:​rb:​20180526_183817.jpg?​200|}}
 +
 +The Set Top Boxes were Jerrold 450 made by General Instrument. These had a VHF output and were fitted with a VHF to UHF upconverter.
 +
 +There was also a "UHF bypass"​ adapter which could sit under the STB and upconverted the "Must Carries"​ (BBC and ITV/C4) from the VHF distribution band to the UHF band used in a TV.\\
 +
 +{{rb:​jerrold450.jpg?​200|}}
  

rb/coventrycable.1523559696.txt.gz ยท Last modified: 12/04/2018 20:01 by andrew