GB794531A - Improvements in or relating to television transmission systems - Google Patents

Abstract

794,531. Television. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. July 5, 1956 [Oct. 24, 1955], No. 30320/55. Class 40 (3). In a television system the rate of transmission of brightness information is made substantially constant, irrespective of picture content, thus effecting a better utilization of available transmission bandwith. This is achieved by deriving from picture signals generated by conventional constant velocity scanning a further picture signal of the velocity modulated scan type in which the velocity of scanning is determined by changes in slope of the original picture waveform (i.e. by variations in the brightness content). As shown in Figs. 1 and 2, picture signals (a) derived by scanning a scene A with a conventional constant velocity scanning camera B are supplied to a picture tube J and there reproduced as a picture, also with constant velocity scanning, after a predetermined time interval determined by a delay device H in the picture signal circuit and a delay device H<SP>1</SP> in the scanning circuit. The reproduced picture is then rescanned by a camera tube K the scanning circuit L of which varies the velocity of scan in accordance with control signals supplied thereto. The control signals are produced in a quantizing circuit G which, in response to input signals (e) derived in a circuit F which differentiates a sawtooth wave (d) generated in E under the control of the twice differentiated (in C and D producing waveforms (b) and (c) respectively) signal (a) from camera B, supplies the waveform (f) consisting of a series of pulses which can occupy only N discrete amplitude levels (N being equal to 3 in the arrangement shown) each level being inversely proportional to the frequency content of the original signal and each pulse occurring after a fixed time after changes in slope in the original signal (a). The signal output of tube K when the velocity of scanning is controlled in accordance with the pulse train (f) is shown in curve (h) whence it can be seen by comparison with curve (g), which is identical with curve (a) delayed, that a better utilization of bandwidth results. To enable the signals from tube K to be reproduced at a co-operating receiver the scanning control signals (f) are also displayed on a picture tube M and scanned by a camera tube O operating with the same velocity modulated scan as tube K thus producing scanning control signals (j) which after modulation on to a sub-carrier in P are then employed together with the output of tube K to modulate a main carrier for transmissions from transmitter Q. At a co-operating receiver the picture signals (h) and scanning control signals (j) are separately derived in a conventional circuit S, the former signals being employed to modulate the intensity of the beam in reproducing tube U and the latter signals to control the velocity of the scanning waveform generated by unit V in accordance with their amplitude. Specifications 399,469, 413,401, 427,625 and 427,630, [all in Group XL], are referred to.