How it works… Telecine
How film gets on to Rediffusion viewers’ television screens
The first use of the word telecine seems to have been lost in the mists of television antiquity. It indicates the conversion of any type of film picture to a television signal suitable for transmission.
It may surprise most people to learn that the one thing that television cannot do is to transmit a complete television picture.
This may sound utter nonsense until it is remembered that the television picture we see only really exists in the mind of the viewer because of nature’s foresight in providing the human eye with a degree of persistence of vision.
Once this faculty of the human eye was recognised it became possible to evolve a number of systems capable of producing animated pictures from a series or sequence of completely still pictures – each picture had a slight positional displacement of image corresponding to what would produce normal movement.
As practically everyone must know, the projection of a picture with movement, whether it is through a home movie or a cinema projector, is the result of a series of transparent still pictures being shown in sequence.
Each time a picture is flashed on to the screen it is a complete picture in every sense of the word. But with our present television system it is not possible to transmit at any one time a complete picture, so an alternative method had to be evolved. In this television system a picture is continuously being built up on the viewing tube, by a very fast-moving, single spot which sweeps from top to bottom in a series of horizontal lines. The brightness varies according to the picture information it carries.
The television system, therefore, examines the picture being televised in a systematic manner by scanning every point of it and at the same time sending out a signal corresponding to the tonal value of that part of the picture being examined at that particular instant.
So cinema film projectors and their television counterparts have, for the most part, little in common with each other.
The name telecine has now become synonymous with the transmission of all film material, and all film transmission equipment is now known generally as the telecine machine or film scanner.
At present there are two alternative methods of transmitting a film picture.
The first is known as a ‘Vidicon Storage System’ and the second as the ‘Flying Spot System’. Without going into too much technical description, the Vidicon Storage System basically consists of the normal type of intermittent motion film projector which throws its optical film image on to the face of a small, special television camera pick-up tube called a Vidicon.
The optical images formed on the Vidicon tube face instantly form an equivalent electrical image on a photo-conductive layer called the target, and from then on the electrical image is examined or scanned in practically the same way as an ordinary television studio camera tube transforms a live studio scene into a television picture.
The telecine projector shutter has to be modified from its usual method of operation so that the optical image of each film frame is flashed on to the vidicon tube face during a very short period – called the frame blanking period. This is the precise period of time allowed for the fly-back of the electronic scanning beam.
The vidicon target has the fortunate property of being able to retain its electronic image long enough (hence the term storage system) for one complete scanning sequence to take place and thereby produces all the necessary information to build up a television picture.
While the retained electronic image is being scanned within the vidicon tube, two other important operations are simultaneously taking place. They are: 1. The shutter is being made to cut off the light source completely, and 2. The projector mechanism is utilising this longish period of scanning time to move the film forward on to its next stationary frame position so that it can be subsequently flashed on to the vidicon tube face.
The second method utilises the flying spot principle and here we use a very much more straightforward scheme. Basically we illuminate only a very small part of the film picture at a time. This is done by passing through the film a small intense point of light produced by a special type of cathode ray tube called a flying spot scanning tube.
This intense spot of light scans the film picture area completely and emerges on the other side of the film with a variation in brightness corresponding to the density of the image it has had to pass through.
The modulated spot of light is then directed on to a device known as a photo-electric multiplier cell. This device reacts to light by generating electric currents directly proportional to the strength of light falling on it.
It is these changing amounts of electrical energy which, when transmitted in proper sequence, tell the home television receiver when and where to produce whites, greys or black tones.
The Associated-Rediffusion telecine section is very versatile in that it can handle practically any combination or type of film material available, be it either 35 mm or 16 mm. At present the telecine section is equipped at Wembley with one Cintel, two E.M.I. flying spot scanners and one R.C.A. vidicon film channel, known respectively as Cintel 6, E.M.I. 1, E.M.I. 2 and R.C.A. 4 machines. Television House is equipped with one E.M.I. and two R.I. flying spot scanners, besides one R.C.A. Vidicon film channel. These are known as E.M.I. 5, R.I. 7, R.I. 8 and R.C.A. 3 machines.
The standard or professional film is 35 mm, while 16 mm is generally known as substandard.
The 35 mm film can be either married, that is with picture and sound all on one film, or double-headed, sometimes called unmarried. This means that the picture and sound information is on two separate film reels but, of course, carefully synchronised. The separate sound track can be either magnetic or optical.
But now we are moving away from our subject. I hope the above has helped you to understand telecine a little better.
1 thought on “How it works… Telecine”
Very interesting. It’s a shame, though, that Mr James did not explain the relative virtues of vidicon storage over flying spot, and vice versa. And did vidicon give way to orthicon and plumbicon upgrades while flying spot stayed the same?