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Coventry Cable TV
Now defunct, the original build was for 450MHz bandwidth carrying about 30 TV channels as well as the FM radio spectrum. It was built and operated by BT and along with the likes of Milton Keynes was one of the first franchises in the UK.
Located at the end of Whitley Village off the London Road in Coventry (CV3 4HL), Blackburn House is now demolished and replaced with houses and flats.
Satellite TV receivers & cable tv modulators and Launch Amplifiers.
Master Control room - monitoring all the channels being transmitted and some of the playout equipment.
The Supertrunks radiated out from the Head End to distribution sites located in BT's district telephone exchanges prior to feeding the subscriber distribution network. These consisted of several amplifiers with one input and one output boosting the signal. The amplifiers were manufactured by Texscan in the USA, and were 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.
This cabinet seems abandoned, the door to this has been lying on the grass for several months.
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.
The DLA (Distribution Launch Amplifier) was made by Texscan in the USA and was locally powered by a Fuba ferro-resonant transformer which also provided phantom power to downstream amplifiers. Mains power was supplied from the utility provider with a consumer unit in the cabinet.
Example DLA 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:-
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.
Each DLA fed several trunks which were split as it radiated out with passive directional couplers. As required when the cable loss exceeded the allowed limits, a Line Extender amplifier was used, this typically had one input, one loop through (at -3.4db) which fed a tap either on a telegraph pole, building or an underground in a footway box, and two amplified outputs.
Reverse path and Status Monitoring
The Texscan module fitted inside the trunk and supertrunk amplifiers, and used the band up to 5MHz to communicate to the central monitoring system. I don't believe this was ever put in to active service however as the reports were that the status monitoring was less reliable than the amplifiers and reported false positives too often to be believed.
It looks like the address for the unit is set via the blue jumper strip, this all preceeds TCP/IP etc, so I have no idea what the address structure or protocols would be like. I don't even know if this was one way, or if the central controller could talk to each monitor individually to interrogate it.
Similar in principle to the Texscan unit, but I have no idea if they were compatible with a common central monitoring system. I don't know which designs of amplifier they would have fitted either.
The DLA fed local trunks which were boosted periodically by Line Extender Amplifiers, these were mounted underground in a waterproof sleeve in footway boxes. They were made by ProCATV in Denmark.
From a THG member:- “According to the Vocabulary of Stores Amp No. 320A is a 2-way line extender amplifier which includes case, power module and forward and reverse amplifier modules. This could be a later version.”
Some versions had a loop through on the input side and there was a two output version also. This one used a separate RF hybrid for each output leg.
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.
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 prone to run off rain flooding them, sometimes waterproofing would fail and cause problems due to water ingress. In many cases, prompt action to wash the affected unit in clear water and then dry it out with a hairdryer resulted in no lasting damage.
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. In addition, it included a galvanic isolator to prevent any egress on utility 240v power to enter the external network in the case of a fault with any internal equipment.
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.
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.