Cablefilm

In the late 1950's the first transatlantic telephone cable was laid and brought into service. Before this, there were transatlantic telegraph (teleprinter) cables but telephone calls were made on radio circuits which were unreliable and phenomenally expensive. The new telephone cable provided something like 36 telephone circuits and, by special arrangements with the Post Office (as it was then), two circuits could be combined to give a 'music circuit'. This provided the possibility of transmitting television type pictures very slowly. The BBC Engineering Research and Designs Departments co-operated in designing and manufacturing two sets of equipment for sending and recording short pieces of 16mm film over a transatlantic music circuit. One set was installed in Canada and the other at Alexandra Palace. The system, called 'Cablefilm' was first used when the Queen and President Eisenhower opened the St Lawrence seaway in 1959.

In order to save time during transmission and to fit the signal into the available bandwidth, the film was scanned with 200 lines per picture (compared with 405) and only alternate frames of film were transmitted. Each transmitted frame was then made into two similar frames at the receiving end. It took eight seconds to transmit each frame. The nett effect of these subterfuges was that film took a hundred times the normal running speed to transmit i.e. one minute of film would take one hour and forty minutes to send. The sending operation was very tedious; the pictures could be seen on a long persistence monitor tube (orange in colour) and the video signal could be heard on a small loudspeaker. Every eight seconds there would be a 'clack' as the film was advanced for the next frame. If there had been a shot change, the lift and gain controls would need adjusting to suit the new picture.

The sound, if any, was sent after the picture transmission.  This was not straightforward because film shot at 25 frames per second in the UK would be reproduced at 24 f.p.s. the other side of the Atlantic and vice versa.  Synchronisation was achieved by comparing tone from the record/replay  machine in America with similar tone from the replay/record machine at A.P. and adjusting our machine to make them the same.  Any slight 'drift' in the speeds did not matter over the short lengths of film.

The effect of only transmitting alternate frames was to make movement in the scene appear jerky. This often did not matter but, on Cablefilm's first programme use, one shot showed a ship (probably the Royal Yacht, Britannia) entering a lock to enter the St Lawrence seaway; instead of moving smoothly and majestically it jerked its way forward like a cartoon character.

Cablefilm remained  in Canada for a while but was then moved to NBC in New York. We then found ourselves more often transmitting material rather than receiving it because (a) NBC could afford the cost of renting the circuits better than the BBC and (b) the time difference worked more in favour of east to west transmission; daytime events in Europe could be shown on the evening news programmes in America.

It was then quite a novelty to be able to talk to the operators and engineers in New York. There was a perceptible delay in the transmission and you could hear an echo of your own voice coming back which sometimes made talking difficult. On one occasion NBC had a problem with their Cablefilm equipment and I had to get out our circuit diagrams and direct the fault finding from 3000 miles away. I was surprised that the NBC operator had to summon up an engineer to change the defective valve (tube).  BBC TV News employed engineer/operators.  Eventually, the problem was cleared and we made the transmission successfully.

One notable west-to-east Cablefilm transmission was of President Kennedy's assassination.  The pictures were very poor but the journalists were pleased that the BBC had beaent the opposition.

Cablefilm remained in service after the first communications satellites came into use because it was cheaper and because the satellites were not geo-synchronous and only offered limited 'windows' when they were within sight of both sides of the Atlantic as they passed over. Eventually, more
satellites became available, the costs came down and Cablefilm was scrapped.

The equipment performed as either a telecine or a film recorder.  Unusual, in that it operated as a fast pull down telecine (relative to the 8 second frame scan period). As a film recorder it was a 'twin lens' machine (in order to make two similar frames from one).  The scanning rate of 200 lines per frame and 8 seconds per frame implies a line frequency of 25Hz and a video bandwidth of about 6 kHz.  I think that some sort of modulation system was used over the telephone music circuit and that a special pulse and bar signal was used for equalisation.

Geoff Rowlands