AP mast (2K)

DIRECT TELEVISION from ALEXANDRA PALACE

by Arthur Dungate

.

"Tilt and Bend"


  Information
BBC Engineering 1922-1972.
Edward Pawley.
(BBC Publications 1972) ISBN 0 563 12127 0

BBC Engineering 1922-1972 by Edward Pawley, Ch 3.6 p144 --

The camera developed by the EMI team was the Emitron, which used the charge-storage principle of the iconoscope, in which the scene was focused on to the target area of the mosaic. Since the geometry of the tube required the scanning beam to attack the target at an angle, the width of the line scan had to increase gradually as the beam travelled down the target.

The Emitron camera was later superseded by the Super Emitron (image iconoscope), the CPS Emitron (orthieon) and the image-orthicon, but in the early days the difficulties inherent in the Emitron had to be offset as far as possible by extreme care in the adjustment of the scanning circuits, so as to reduce geometrical distortion and shading.

The shading distortion in the form of ‘tilt and bend’ gave a great deal of trouble; the small control room at Alexandra Palace, which had to be kept darkened to allow the pictures to be seen on monitors, contained a series of apparatus bays at each of which an engineer sat to operate the controls.

Studio A Racks (8K)

To allow each of these engineers to see the monitor screen they sat on stools of graduated heights.


  Information
The Life and Works of Alan Dower Blumlein.
Robert C. Alexander.
(Focal Press 1999)
ISBN 0 240 51628 1

The Life and Works of Alan Dower Blumlein by Robert Charles Alexander,
Ch 5 p139 --


This shading typically took the form of unwanted superimposed signals (and thus brightness) increasing from top to bottom and from left to right (or vice versa). Looked at on a waveform monitor, the signals were sawtooth-like, and christened 'tilt'. There was usually a noticeable curvature of the sawtooth shape christened 'bend'. Both EMI and RCA provided sawtooth generators of adjustable amplitude to cancel the unwanted components. ... Each camera channel in the final equipment had to have its own 'tilt' and 'bend' controls, adjusted by hand (and often to taste) when there were significant changes of picture content.

BACK