Patent GB-1068132-A (GB)
Title: Video signal modifying apparatus
Inventor: ASKEW GERALD HAMILTON
IPC: H04N5/20
Filed: 1965-01-25
Abstract: 1,068,132. Television. RANK BUSH MURPHY Ltd. Jan. 25, 1966 [Jan. 25, 1965], No. 3247/65. Heading H4F. A signal derived by scanning a negative film is applied through a chain of circuits, Fig. 3, between input terminal 1 and output terminal 11, the signals at the various stages being illustrated in Fig. 4 in terms of a single line scan b separating blanking intervals a and c and representing a uniform transition from black (left-hand end of line) to white (righthand end of line). The chain primarily establishes a desired power-law (gamma) relationship by the combination of logarithmic amplifier 4, variable gain device 16 (which is also a signal inverter) and exponential amplifier 8. The initial variable gain device 2 merely serves to establish the input signal amplitude within the operating range of the gamma circuits 4, 16, 8. Clamp circuit 12 operates during the blanking intervals as indicated by arrows 96 on waveform L, whilst clamps 14 and 15 operate on pulses which are added during the blanking intervals (see below) as indicated by arrows 140 and 150 on waveforms R and S. The amplitude of the output signal may be varied (1) by adjusting the gain of device 16 (Gain I) in which case clamp 15 is omitted, (2) by varying the direct-voltage bias applied to amplifier 8 (Gain II) in which case clamp 14 is omitted, or (3) by a combination of (1) and (2). Method (1) changes the gamma value (the exponent of the power law is determined by the value of the gain due to 16), whilst method (2) maintains a constant gamma and varies the " black stretch." A pulse generator 13 add pulses 14 to the signal during blanking for the purposes of determining the amount of " lift " in the final signal, i.e. the difference 142 between the black (141) and blanking (143) levels as indicated in waveforms R and S. The pulses are arranged to have an amplitude 99 which exceeds by a desired amount (the " lift ") the overall amplitude 98 of the signal. By this means the " lift " is rendered independent of the " sit " 95 of the original signal K which is determined by the exposure of the film and may be subject to variation. The pulse generator includes a circuit, Fig. 5 (not shown), which measures the peak value of the signal during one field and stores it as a potential on a capacitor for use in determining the pulse amplitude for the next field. A correction of the " lift consequent upon a change of film exposure is thus effected with a delay of only one field. A similar variation of " lift " may be due to variations in the original signal introduced by the use of different optical paths for scanning odd and even field. In such case the generator may include a circuit, Fig. 7 (not shown), in which measurements during odd and even fields are stored separately and used with a delay of one field when the same optical path again comes into use. The measured amplitude from pulse generator 13 may also be used for automatic control of the gain of device 2. An arrangement for use with signals obtained from positive film is described with reference to Figs. 1 and 2 (not shown). In this case the added pulses (51), waveform (D), is made equal to the peak value of the signal, although it is stated that a variable pulse as in Figs. 3 and 4 may be used. The output signal amplitude is controlled by device (2) (Gain) whilst the " lift " is controlled by device (6) (Lift I) or by variable bias applied to amplifier (8) (Lift II). This arrangement with fixed amplitude added pulses may be applied to negative films by making device (6) an inverter.