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Explaining the BBC Circuit Order Form
© Chris Youlden



On a warm and sunny March day in 1988 I took a morning flight from Penzance to St Mary's, the main island of the Isles of Scilly. The islands lie some 30 miles Southwest of Penzance and experience weather and plant life which are claimed to be subtropical.

The reason for the visit was to test lines for the John Dunn Show which was to be broadcast on Radio 2 on 4th April.

Live programmes of any description from the Islands in those days was a bit of an ask. Connecting with the mainland was via an undersea cable - not fibre then - and/or making special provision to use BT's microwave link to St. Just.

This was Easter weekend with the transmission scheduled for Easter Monday. The lines test would normally be a day or so before the programme, but there were no seats available by air, given that it was necessary to fly in and back on the same day

I arrived with some time to spare and had a look around. It was fascinating to be able to walk from the north coast of the island in Hugh Town, St Mary's to the south coast in 2 minutes! There was a superb view out from the battlements over the bays and islands, and down beneath me The Scillonian ferry was loading its cargo for the trip to Penzance.

Technical Requirements

The usual circuit requirement for a Radio Outside Broadcast such as this was one Music from OB to destination, and one Control line. The latter was usually a 2-wire equipped with voice-frequency ringers. Sending 17Hz down this line would activate the ringer which generated 520Hz at the exchange. Another ringer at the other end would convert the tone back to 17Hz to contact the destination.

St Mary's COF

No ringers were available on St Mary's however. But this was not a typical radio OB. It was the era when technical standards seemed to matter, and Radio 2 was a stereo network. With no chance of providing stereo circuits from the island, the solution was for John Dunn to be mono with discs played in at the London end. Therefore two way talk back was needed for cueing purposes and although not easy to read on this form, a DEL (direct exchange line) with an HMT (a telephone instrument) for communication.

Circuit Order Form

Now a look at the order form above. This is typical of the forms issued by the BBC to BT for all types of circuit, music, data, vision, and comms. For OBs the first element which is needed are local ends, pairs of wires from the location to the nearest BT exchange which has access to trunk circuits. In this case the exchange is St Mary's. Notice that 3 local ends are ordered but the music picks up the trunk circuit in Truro. This was because the music circuit was provided by BT on the SHF link from St Mary's to St Just, which was connected to Truro. What the SHF link was normally used for was not known, but it would have afforded some backup to the undersea cable. It wasn't unknown for ships' anchors to drag offshore and damage a cable.

But BT was probably using the link for telephony and data, and in so doing employing a frequency division multiplex hierarchical Group structure. Each channel on such a system had a nominal bandwidth of 3.4kHz with signalling at 3.825kHz.

See article: Frequency Division Multiplexing

It would certainly have been feasible for 300 voice/data channels to be on the link if a Mastergroup had been used, and if so 3 channels could have been combined to provide a music circuit.

So, in summary, the Form orders a Music circuit from the Star Castle Hotel to London using one local end to St Mary's plus an allocation on the SHF link to Truro, and then a trunk music circuit to London. BT trunk circuits such as these were termed Occasional Programme circuits (OPs) and were spares which could be booked per occasion. Two-way talkback was also required so a 4W CNR was ordered via local ends and circuits to/from St Mary's to London.


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