AP mast (2K)


by Arthur Dungate


The Suppressed Frame System
Some Fundamental Aspects of Telerecording
by C.B.B. Wood, Designs Dept., Engineering Division, BBC

Operational Technique

Given a satisfactory design for the telerccording apparatus, with scientific aids to ensure consistent exposure of the film and a careful control of the photographic process, it comes as a slight disappointment to find that art rather than science has the last word in determining picture quality.

Although the two main systems in use by the B.B.C. have shown that they can provide very satisfactory results under favourable conditions, it is clearly not possible to make a good telerecording if the incoming picture is lacking in quality, and it is also not sufficient to aim solely at the production of a good photographic record of the original programme.

Reproduction of the recording will involve a telecine machine, and although those in use by the B.B.C. have an excellent performance, it must be remembered that in producing a vision signal from 35 mm film these machines perform a task equivalent to reading a variable-density sound track little more than 0.001" wide. Under these difficult conditions the signal to noise ratio of the output is surprisingly good when the film to be scanned is a normal motion picture and therefore relatively free from noise, but where the film is a telerecording which has the signal to noise ratio of the original television picture, the result will be less satisfactory.

It is an unfortunate feature of television in its present state that very few decibels in signal to noise ratio separate the very best pictures and those that are unacceptable, and the addition of only three decibels of noise in reproduction can easily turn a moderate original into a very poor repeat programme.

With certain types of programme material, particularly in the case of dark scenes where the signal level is low or where the incoming signal is accompanied by more random noise than is usual, the production of an acceptable telerecording, which will reproduce well, calls for considerable skill on the part of the operators in "steering" the various controls to produce an artistic rather than a scientifically accurate result. The recording camera is in fact much more critical of picture quality than is the average viewer.


The author is indebted to the Chief Engineer of the British Broadcasting Corporation for permission to publish this paper, and to his colleagues in the Research Department of the Engineering Division for their assistance in preparing the material.


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