The telerecording process works by aiming a film camera at a specially-adapted, large, flat television screen, with the camera shooting at a frame rate in synchronisation with the television frame rate. In the UK, television runs at 25 frames - or more correctly, 50 fields - per second, so the film camera would be run at 25 frames per second rather than the cinematic film standard of 24 frames.
Because television is a field rather than frame-based system, however, not all the information in the picture can be retained on film in the same way as it can on videotape. The time taken physically to move the film on by one frame and stop it so that the gate can be opened to expose a new frame of film to the two fields of television picture is much longer than the vertical blanking interval between these fields - so the film is still moving when the start of the next field is being displayed on the television screen. It is not possible to accelerate the film fast enough to get it there in time without destroying the perforations in the film stock - and the larger the film gauge used, the worse the problem becomes.
The problem of adapting the way the image is either displayed or captured on film, to get around the above, was solved in various different ways as time went on - improving the quality of the image.
This procedure had the advantage of capturing both fields of the frame on a film, but it was difficult to keep the mirrors running at the right speed and all the equipment adjusted correctly, which often resulted in poor quality output. An additional problem was that the whole procedure took place in an open room and it was known for insects to settle on the screen which were then permanently present on the film recording. The Mechau film magazine only held enough for nine minutes so two recorders were needed to run in sequence in order to record anything longer.
This method had several disadvantages. In missing out every other field of video, half the information of the picture was lost on such recordings. The resulting film consisted of fewer than 200 lines of picture information and as a result the line structure was very apparent; the missing field information also made movement look very 'jerky'.
This method came to be preferred. Another technique developed by the BBC known as 'spot wobble' involved the addition of an extremely high frequency but low voltage sine wave to the vertical deflection plate of the television screen, which changed the moving 'spot' through which the television picture was displayed into an elongated oval. While this made the image slightly blurred, it removed the visible line structure and resulted in a better image. It also prevented moiré patterns appearing when the resulting film was re-broadcast on television and the lines of the recording did not match the scan lines.
The telerecording system was actually of a very high quality, easily reproducing the full detail of the television picture. The only slight disadvantage of the system was that it removed the 'fluid' look of interlaced video and 'filmised' the picture, but this would generally not have made a great deal of difference to the viewing audiences.
The system was largely used for black and white reproduction. Although some colour telerecordings were made, they were generally in the minority as by the time colour programmes were widely needed for sale, video standards conversion was easier and higher quality and the price of videotape had become much reduced. Before videotape became the exclusive transmission format during the early to mid-1980s, any (colour) video recordings used in documentaries or filmed programme inserts were usually transferred onto film.
The first known surviving example of the telerecording process in Britain is from October 1947, showing the singer Adelaide Hall performing at the RadiOlympia event. The wedding of Princess Elizabeth to Prince Philip also survives, as do various early 1950s productions such as It is Midnight, Dr Schweitzer and the opening two episodes of The Quatermass Experiment, although in varying degrees of quality. A complete 7-hour set of telerecordings of Queen Elizabeth's 1953 coronation also exists.
There is some evidence to suggest that the BBC experimented with filming the output of the television monitor before the television service was placed on hiatus in 1939 - BBC executive Cecil Madden later recalled filming a production of The Scarlet Pimpernel in this way, only for film director Alexander Korda to order the burning of the negative as he owned the film rights to the book, which he felt had been infringed. However, the evidence for this is purely anecdotal, and indeed there is no written record of any BBC Television production of The Scarlet Pimpernel during the 1936–1939 period.
Up until the early 1960s, much of the BBC and British television in general's output was broadcast live, and telerecordings would be used to preserve a programme for repeat showings, which had previously required the entire production being performed live for a second time.
In the 1950s a home telerecording kit was introduced in Britain, allowing enthusiasts to make 16 mm film recordings of television programmes. The major drawback, apart from the short duration of a 16 mm film magazine, was that a large opaque frame had to be placed in front of the TV set in order to block out any stray reflections - making it impossible to watch the set normally while filming. It is not known if any recordings made using this equipment still exist.
British broadcasters used telerecordings for domestic purposes well into the 1960s, with 35 mm being the film gauge usually used as it produced a higher quality result. For overseas sales, 16 mm film would be used, as it was cheaper. Although domestic use of telerecording in the UK for repeat broadcasts dropped off sharply after the move to colour in the late 1960s, 16 mm black and white film telerecordings were still being offered for sale by British broadcasters well into the 1970s.
Telerecording was still being used internally at the BBC in the 1980s too, to preserve copies for posterity of programmes which were not necessarily of the highest importance, but which nonetheless their producers wanted to be preserved. If there were no videotape machines available on a given day, then a telerecording would be made - there is evidence to suggest that the children's magazine programme Blue Peter was occasionally being telerecorded as late as 1985. After this point, however, cheap domestic videotape formats such as VHS could more easily be used to keep a back-up reference copy of a programme.
Another occasional use of telerecording into the late 1980s was by documentary makers working in 16 mm film who wished to include a videotape-sourced excerpt in their work, although such use was again rare.
Telerecordings form an important part of British television heritage, preserving what would otherwise have been lost. Nearly every pre-1960s British television programme in the archives is in the form of a telerecording, along with the vast majority of existing 1960s output. Videotape was expensive and could be wiped and re-used; film was cheaper, smaller, and in practice more durable. Only a very small proportion of British television from the black and white era survives at all; perhaps 5% from the 1953-58 period and 8-10% from the 1960s.
Many recovered programmes, particularly those made by the BBC, have been returned as telerecordings by foreign broadcasters or private film collectors from the 1980s onwards, as the BBC has taken stock of the large gaps in its archive and sought to recover as much of the missing material as possible. Many of these surviving telerecorded programmes, such as episodes of Doctor Who, Steptoe and Son and Till Death Us Do Part continue to be transmitted on satellite television stations such as UKTV Gold, and many such programmes have been released on VHS and DVD.
Because videotape records at fifty interlaced fields per second and telerecordings at twenty-five progressive frames per second, videotaped programmes that exist now only as telerecordings look more "jerky" than the originals. One solution to this problem is VidFIRE, an electronic process to restore video-type motion.