AP mast (2K)

DIRECT TELEVISION from ALEXANDRA PALACE

by Arthur Dungate

.

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




  Reference [5]
KEMP, W.D. British Kinematography, Vol. 19, No.

Tone Gradation

Perhaps the most important contribution to good quality telerecording is the preservation of linearity in grey scale reproduction: loss of detail in highlight or shadow due to crushing in the photographic process can quickly ruin the subjective quality of a recording.

The telerecording is again at a disadvantage to photography of the natural scene, since it is called upon to record a picture which is already limited in contrast range and has suffered some non-linear distortion. The inevitable non-linearity of the photographic process can be disguised by the skilled photographer in recording a natural scene, since his finished result includes only one set of distortions, but an uncorrected telerecording would include the product of television and photographic distortions.

In an admirable paper on the subject, W. D. Kemp [5] has analysed the distortions due to television camera, display tube and the photographic process, and has shown the degree of compensating non-linearity which must be introduced to provide a linear reproduction of tones from the television camera to the finished recording.

Since that paper was written, two new systems of telerecording have been placed in service by the B.B.C. and both have incorporated electrical circuits which permit the deliberate introduction of non-linearity in the amplitude characteristic of the signal fed to the display tube.

Recent experience has, however, led to the belief that where a variety of types of television cameras are in use it is not practicable to attempt more than a general correction aimed at compensating for the distortions of the photographic process, which results in the reproduced recording having properties of tone gradation similar to that of the original television programme.



Distortion in the Photographic Process: The solid curve shows a typical characteristic of the negative positive photographic process while the dotted curve shows the compensating characteristics given to the video amplifier which feeds the recording C.R. tube (13K)

Distortion in the Photographic Process: The solid curve shows a typical characteristic of the negative positive photographic process while the dotted curve shows the compensating characteristics given to the video amplifier which feeds the recording C.R. tube

The distortions to be corrected invariably take the form of reduced slope at each end of the scene-brightness print-transmission characteristic. The slope of the centre portion can be controlled when processing the film and it is not as a rule necessary to apply any electrical correction to the vision signal over this region. The main tone-gradation controls in a telerecording apparatus are therefore usually known as "black stretch" and "white stretch" and adjustment of these controls can only be carried out after a series of film tests have been made to determine the amount of correction required.

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