Fundemental Audio Input Link

Fundemental Audio Input Link

The TD.80 Wired Vision System
Contact: 
Gerald K Clode
REDIFFUSION TELEVISION LIMITED
email - Gerald Clode
The function of the Wired Vision System is to convert 'off - air'  Television and Radio broadcast signals to System Frequencies to be used for transmission over a wired network.  These signals are amplified to the maximum level which may be applied to the network.
The general scheme of signal distribution is that reception takes place at an Aerial Site, carefully selected to give the best possible reception conditions that can be provided economically.

Conversion of the received frequencies to the System Frequencies is carried out at the Aerial Site and, after some amplification, the signals are applied to the primary network or Vision Trunk Route (VTR) which carries them to the various points at which they are transferred to the Subscriber Network.  The locations of these distribution points are mainly decided by the feeder planning but Repeater Amplifiers  have to be included in the VTR in order to restore signal levels which ate attenuated by the cable itself.  It is often convenient to make the repeater point the one from which the subscriber network is also energised, the repeater having the dual function of Trunk Repeater and Feeder Repeater. There are however, repeaters for trunk use only. 
Click in plan for full-size image
The types of antennas used at the reception site depend largely on the strength of the UHF signals available at the site.Under favourable conditions a single wideband aerial (AE.108) can be used for up to four uhf channels from the same transmitter.  Where further signal amplification is necessary, the AR.110 wideband pre-amplifier may be used to raise the signal level by up to 26db.
Alternate antenna configurations are possible, ie. using a seperate antenna (AE109) for each individual channel particularly where co-channel interference proves to be a problem.
Aerial Site   Head-End Equipment
The Frequency Converter
Each received channel needs to be converted from the "Off Air"  received frequency to the System Frequencies (SF) of the network. This is carried out using the FC.100 frequency converter. This unit operates on the Superhetrodyne principle mixing the signal frequency with a locally generated oscillator to create an intermediate frequency of 38.9mhz, corresponding to the vision carrier with 32.9mhz being the sound carrier frequency. The vision carrier is then mixed with a 30mhz oscillator to provide an output at 8.9mhz being the system frequency.  A 44.8mhz local oscillator is uses where a 5.9mhz SF output is required.
The TD.80 system was introduced in 1968 for the distribution of UHF 625 line PAL Colour television signals on a
'tete - beche' principle using HF sub-carriers of 5.9mhz and 8.9mhz
Click on circuit for full-size image
The maximum vision S.F. output of the FC.100 is 3V rms on the 5.9mhz channel and 4.3v rms on 8.9mhz these levels being adequate for direct application to the Vision Trunk Route however, the 2v a.f. output would require amplification depending on what type of circuit is used for conveying the audio signal to the distribution points.
The SF band of frequency corresponding to each channel is not transmitted directly from the FC100 with flat frequency charachteristics because the attenuation of the cables used in the Vision Trunk Route is greater at the higher frequencies so it is desireable to add pre-emphasis to off-set this effect. The EQ108 (5.9mhz) and EQ109 (8.9mhz) provides the necessary shaping slope.
The Vision Trunk Route
The preferred arrangement is to have a seperate co-axial cable for each vision channel and, for the sound channels, a multipair cable operated as an input link and feeding audio amplifiers at the the individual distribution points.
Three types of co-axial cables are available for the VTR qand these differ by their size and attenuation.  CA.150 is the largest with a nominal diameter of 12.5mm and a planning attenuation of 1.17db per 100m. at 5.9mhz and 1.43db per 100m. at 8.9mhz. In normal circumstances a cable with attenuation as low as this is not required because it is possible to arrange the planned distribution points using cables with higher attenuation charachteristics. The CA.138 with a diameter  10mm and a respective attenuation of 1.54db and 1.9db is more widely used.
The preferred repeater combining the trunk repeater and feeder distribution repeater functions is the AR.107 with a maximum output od 3V at 5.9mhz and 4.3v at 8.9mhzand a maximum planning gain of 57db.
The practice of having high powered valve audio amplifiers, of between 600W and 2.5KW output, to cater for large areas of network by direct applicationof the audio to the feeder network and High Level transmissions over links to outlying Kiosk distribution points has been overtaken by the development of the transistor audio amplifier.  The earlier A.623 ammplifier with an r.m.s. music rationg of 150W output and the later AA.105 with an output of 200W are each capeable of feeding the numbers of subscribers ( 800 - 1000 )  associated with a diamond - planned area. Provision has been made for conveying the audio outputs of the FC.100s ath the Aerial Site to the various distribution points and this is the function of the Input Link. The link may be a direct route with several distribution points situated in tandem along its length or it may have spurs to conform to the VTR pattern.
Audio Input Links
Click on diagram for full-size image
Distribution Point Equipment
There are circumstances when the distribution point is not located at the position of a Trunk Repeateron the VTR; it is then necessary to feed the distribution point via a supr from the VTR, or if the point is immediately adjacent to the VTR, it can simply be fed by a suitable insert. In either case the distribution amplifier does not have to include the trunk repeater function.
The two current feeder repeaters are the AR.106 and the higher powered AR.113. Sound bypass circuits are built into the repeaters so they can operate directly in configurations where the vision and audio signals are combined. 
Having obtained the required level of vision signals with the correct pre-emphasis together with sufficient audio power at Service Level, this must now be combined for application to the Feeder Network.  The equipment for achieving this is the FI.137 and FI.138. Sound and Vision Combiners.  The two are similar but the FI.137 caters for two back-matched feeder outlets and the FI.138 for four. They each handle one vision channel and its associated sound plus one other sound only programme.
Click on diagram for full-size image
Click on diagram for full-size image
Feeder Network
It has been established that the most economical way of covering a built up area with a feeder network is to plan a number if interlocking diamond shaped areas. In practice it is not possible to achieve true diamond shapes because of natural and man made features in the area, such as parks, rivers, railways etc. but the aim is to make a planned area conform as near as possible to the ideal. The diamond may have two or four main stems (feeders) with the shape arising because of the spurs taken off the main stem through low attenuation spur inserts. Those spurs nearer to the distribution point are the longest with the spurs getting progressively shorter as the distance from the distribution point increases. All are fed through the same low attenuation inserts. The sum of the cable attenuation ( main stem and spur ) and the insert loss is substantially the same between the distribution point and the end of the spur.
The size of the diamond is set by the relative attenuation and the network is planned to meet those attenuation limits ensuring that the performance regarding slope, crossview and voltage drop on the sound programmes will be satisfactory. The objective of planning is to provide a minimum of 30mV on all channels at the subscriber to give adequate protection against sporadic interference fron in-band radio transmissions.
Click on diagram for full-size image
Click on diagram for full-size image
Carlton House,  Lower Regent Street
London SW1Y 4LS
Tel. 01-930-0221
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Schematic af a Distribution System

Schematic af a Distribution System

Part 2  To Follow
Block diagram of FC100

Block diagram of FC100

Two Feeder Stem Diamond

Two Feeder Stem Diamond

 
Four Feeder Stem Diamond

Four Feeder Stem Diamond

 
Two feeder Outlets  Combiner

Two feeder Outlets Combiner

Four feeder Outlets  Combiner

Four feeder Outlets Combiner

REDIFFUSION System Frequencies 5.9mhz / 8.9mhz
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