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780069

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780069

  1. 1. * GB780069 (A) Description: GB780069 (A) ? 1957-07-31 Improvements in and relating to the inversion of television signals Description of GB780069 (A) PATENT SPECIFICATION Date of filing Complete Specification March 23, 1955. Application Date March 24, 1954. 780,069 No. 8650154. Complete Specification PublishedJuly 31, 1957. Index at acceptance.:-Classes 40(3), F2F2; and40(6), N(1B: 2M: 3M). International Classification:-HO3f. H04n. COMPLETE SPECIFICATION Improvements in and relating to the Inversion of Television Signals We, JOHN LLEWELYN BLISS, of Derwent House, Barnet Lane, London, N.20, and PETER RAINGER, of 1, Ormesby Way, Kenton, Harrow, Middlesex, both British Subjects, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particuarly described in and by the following statement: - The present invention relates to the inversion of television signals. In deriving television pictures from motion picture film it is usual to employ a standard positive film such as is used for optical projections. Considerable economy of time and money can be achieved if a negative film is used, and the resulting signal, hereinafter referred to as a negative signal, is inverted, that is to say transformed into a positive signal, by electrical means. The simplest means of effecting such a transformation is by a simple phase reversal, using a transformer or an additional amplifier stage. The resulting pictures are, however, most unsatisfactory, having extreme compression of the white end of the tone range. This distortion is most objectionable except in cases where the range of transmission of the negative is very small, as, for example, when the original subject is dull and lacking in contrast. This compression arises from the intrinsic nonlinearity of the
  2. 2. photographic process. In normal photography the non-linearity of the negative is compensated by a similar nonlinearity in the positive printing process. Thus over a range of subject brightnesses the density D obtained by exposure to a subject brightness B is of the form D=ylog[,, B/Ba, where y is a constant determined by the particular photographic emulsion employed and the processing conditions and B. is another constant. With normal cinematograph technique y is about 0.6 to 0.7. When a negative film is scanned the resulting signal is proportional to the transmission r of the film. Since the density D is equal to [Pric 3s. 6d.1 -log,0T, it follows that T is proportional to B 'y. In order to produce an acceptable picture it is necessary to derive a signal S such that S is 50 proportional to By,, where y7 has a value in the neighbourhood of 0.4. Assuming y=0,6 it is, therefore, necessary to introduce into the signal path a device yielding an output proportional to B04 when the input is proportional sS to B-6', or, in other words, such that the output voltage is proportional to the negative v power of the input voltage. Circuits designed to achieve this result have been proposed but those so far known have 60 been complicated, and it is the object of the present invention to provide a relatively simple circuit whereby a close approximation to the required result can be obtained. The invention is based upon the realisation 65 that a variable-mu pentode can be arranged to provide a relation between grid voltage and anode current suitable for effecting the required correction. The non-linear characteristics of a variablemu pentode are well known and it has been proposed to use circuits including a variablemu pentode to correct or modify a characteristic curve. Thus a circuit has been proposed for use in television apparatus, including 75 a variable-mu pentode operating in parallel with a triode to correct for the characteristics of a cathode ray tube. However the characteristics of such a circuit differ from those required for the solution of the particular 80 problem with which the present invention is concerned. Another circuit has been proposed for use in sound film apparatus to correct for the characteristics of a sound track on a film, The problm in this instance is different from 8S the present problem since a sound waveform is very different from a telvision waveform, excursions being made either side of the datum level corresponding to zero signal, and the conditions under which the variable-mu pentode was operated would not be suitable to deal with the present problem. 78,069 According to the present invention there is provided apparatus for deriving a positive signal from a negative motion picture film or vice versa comprising means for scanning a negative or positive picture film and deriving negative or positive picture signals,
  3. 3. respectively, therefrom and a circuit for inverting the picture signals comprising a variable-mu pentode having its control grid biased approximately to cathode potential, or slightly positive relatively to cathode potential, the picture signals being applied to drive the control grid negatively, and the circuit being so constituted that inversion of the signals takes place between the input and output of the circuit. The process when a positive picture film is scanned may be used in recording by applying the negative signals to produce a negative picture, and photographing this negative picture to produce, directly, a positive picture fihlm, thus avoiding the need for a printing process when a positive picture film is required. In film scanners of the "flying spot" type it is usual to suppress the scanning beam at the end of each line and frame in order to eliminate spurious signals which would otherwise occur during the retrace or flyback of the scanning spot. During these suppression intervals the output is zero, corresponding to true black when positive film is scanned, but corresponding to an intense white, exceeding any white intensity in tlie picture, when negative film is scanned. This white signal has to be removed by means of suitable blanking pulses. In "' flying spot" film scanners the signal is usually clamped prior to the insertion of blanking pulses, the clamp being driven during the flyback intervals, when, as has been stated already, the signal level corresponds to absolute black when positive film is scanned. When negative film is used the large white signal generated must be replaced by a signal at or near black level in order that the clamp circuits in the blanking pulse mixer can operate correctly. According to a feature of the invention the required signal is introduced by cutting off the anode current of the variable-mu pentode by means of a negative pulse applied to its suppressor grid. The pulse timing and duration must be such that it embraces the clamp pulse interval. It has been found possible to use standard synchronising pulses for this purpose. The resulting signal lies very near true black level as picture blacks reduce the anode current to a very low value. The invention will be described by way of example with reference to the accompanying drawings in which Fig. 1 is a circuit diagram of a preferred embodiment, and Fig. 2 contains idealised waveforms, not to scale, at various points in the circuit of Fig. 1 in response to an input of sawtooth waveform, A signal to be inverted, of the waveform (a) in Fig. 2, is applied, with black positive, through a terminal 10 to the control grid of a pentode V,. The amplified and inverted signal at the anode of the valve V1 (Fig. 2(b)) is applied to the control grid of a variable-mu pentode V. which is arranged by the flow of 7C
  4. 4. grid current and the action of a capacitor 13 and resistor 14 to restore the D.C. component in such a way that full picture white corresponds to earth potential. If preferred a conventional clamp circuit may be employed for 25 this purpose. The curvature of the grid voltage/anode current characteristic of the pentode V. approximates closely to the required negative power law, and inverted and corrected signals may be obtained at a terminal 15 from 80 the anode of a phase reversing output stage V3. However, the approximation to the desired law is improved, in the example illustrated, by applying to the screen grid of the pentode V. 8S a waveform as shown in Fig. 2(c) which is that of Fig. 2(b) inverted. This waveform is obtained by applying the signal from the anode of valve V, to the control grid of a pentode V,, the valve V. being coupled to the screen grid 90 of the pentode V through a cathode-follower triode Vi. The signal has its D.C. component restored by grid current in the pentode V4 and by the action of a capacitor 16 and resistor 17. The effect of D.C. restoration may, in this case 95 also, be obtained by means of a clamp circuit. Negative-going pulses as shown in Fig. 2(d), which may conveniently be mixed line and frame synchronising pulses, are applied at a terminal 18 to the suppressor grid of the pentode V_. Each pulse cuts off the anode current in pentode V. during an interval within the line suppression period and the waveform at terminal 15 is thus as shown in Fig. 2(e). The output voltage during each pulse is near pie- 10o ture black and may be used subsequently in the apparatus chain as a black reference voltage. In this way the signal may be dealt with in the same way as if it had been derived by scanning a positive picture. A diode V, pre- 11 vents the suppressor grid of the pentode V2 from being driven above earth potential. * Sitemap * Accessibility * Legal notice * Terms of use * Last updated: 08.04.2015 * Worldwide Database * 5.8.23.4; 93p

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