Patent GB-1125795-A (GB)
Title: Video signal circuit control arrangement
IPC: H04N5/16
Filed: 1965-11-08
Abstract: 1,125,795. Television; automatic gain control in transistor circuits. FERNSEH G.m.b.H. 8 Nov., 1965 [20 Nov., 1964], No. 47312/65. Headings H3T and H4F. In an arrangement for controlling the level of video signals derived by scanning positive kinematograph films in which an automatic control system responds to the maximum level of the signals and functions to stabilize the maxiumm level, the operation of the arrangement is disturbed when the sequence of television field signals 21/0, 21/1, 21/5, 21/6, Fig. 7, is interrupted by a series of black level signals 21/2, 21/3-21/4 produced by a series of black film frames separating the sequences of normal picture frames. The gain of the system is increased automatically during the sequence 21/2, 21/3-21/4 and due to the normal timeconstants involved remains high during the beginning of the following sequence 21/5, 21/6 causing the first few reproduced pictures to be excessively bright. According to the invention a control stage is provided which detects whether the maximum value of the signal is greater or less than a predetermined reference level S throughout a field interval and produces a corresponding control signal U. The control signal then functions to modify the operation of the automatic control system so that it furnishes a constant control potential throughout a field interval subsequent to a field interval when the maximum level is less than the predetermined level. As illustrated the constant potential is provided during the fileds 21/3 to 21/4 and 21/5. In one arrangement the time constant is increased from 1 second to 100 seconds, whereby the system gain is maintained at the level determined by the preceeding picture sequence 21/0, 21/1. In another arrangement the automatic control system is supplied with a fixed potential independent of picture signal amplitude. The reference level S is other than peak white and is preferably 10-40% of the black-white potential difference. In the control stage, Fig. 6 the maximum amplitude of the picture signal is determined by a peak detector 29 and the resulting potential on a capacitor 31 at the end of each television field scan is transferred to another capacitor 34 via gates 32, 35 which are actuated by pulses 46 and 49. The charge on capacitor 31 is then removed by pulse 48 so that the circuit is made ready to determine the maximum value for the next field. Depending whether the potential 64 transferred to capacitor 34 is greater or less than a predetermined potential applied to the emitter of transistor 75, the transistor does not or does conduct. As a consequence there is produced a potential variation U which is applied to a switching transistor 39. As a consequence, subsequent to black frames 21/2, 21/3-21/4, when the maximum signal is less than level S, transistor 39 is rendered conducting and a capacitor 40 is added in parallel with capacitor 86 which determines the time-constant of the automatic control system 16. To prevent switching transients capacitor 40 is maintained at the automatic control potential by a connection from point 27 via emitter followers 38. In the alternative arrangement point 70 is maintained at a predetermined fixed potential and capacitor 40 together with emitter follower 38 are dispensed with. The invention may be applied to an arrangement for producing signals from negative films. In this case however the peak detector is controlled so as to be responsive to signal during the field picture intervals only, i.e. it is arranged not to respond to signal levels during the field blanking intervals 20, Fig. 6. The detector measures the maximum signal amplitude with respect to peak white. The reference level S is chosen to be other than peak black. An arrangement may be provided with control arrangements suitable for positive and negative films, a switching arrangement, associated with the channel polarity reversing stage, being provided to select the appropriate control arrangement. The automatic control system may control the gain of an amplifier in the video signal channel or the intensity of the light source which illuminates the film.