Patent GB-1325439-A (GB)
Title: Focusing
IPC: H04B15/00
Filed: 1970-10-29
Abstract: 1325439 Television camera automatic focusing IMAGE ANALYSING COMPUTERS Ltd 29 Oct 1970 [5 Nov 1969] 54114/69 Heading H4D General.-In television camera automatic focusing systems of the type maximizing the high-frequency content of the video signal, the level of said content can be effected by factors other than the degree of focusing. Such factors comprise:- (a) amount of detail in the scanned image, (b) size of change in the video signal at a black/white intersection, (c) the shape of such an intersection line with respect to the scanning lines, (d) the amount of noise in the video signal. The invention generally relates to normalizing the high frequency content signal with respect to such factors and describes embodiments dealing with each of the above particular factors. Amount of detail. Figs. 1, 2.-The highfrequency content in the video signal (a) is selected by filter 12 and rectifier 14 and the level thereof detected by integrator 16 to give level signal (e). A threshold detector 18 squares off the video signal and the amount of detail corresponding to the number of pulses in the squared off signal (c) determined by intercept counter 20 and integrator 22. The resultant signal (f) is used in divider 24 to normalize the level signal (e). As seen in Fig. 2, the normalized signal has a constant level (for a given degree of focus) irrespective of the amount of detail. Black/white video signal change, Figs. 3, 4.- The video signal A ((I), Fig. 4), exemplified as arriving from scanning across a dark object on a light background, is successively twice delayed by small amounts in delays 26 and 28 such that, whitest (highest) and blackest (lowest) level detectors 30 and 32 select the highest and lowest points respectively of the video signal out of three time spaced points A, B and C. The selected levels are fed to averaging means 54 and difference means 34. The difference signal is fed to inversely vary the amplification of an amplifier 36 receiving a unipolar signal F derived from the high-frequency content signal by high-pass differentiator filter 38, detector 40 (first pulse) and inverter/detector, 44/46 (second pulse). The video signal is quantized (D) by means of a comparator 52 which gives a step output when said signal crosses the average level output of means 54. Gating pulses E are derived from the step points in means 50 and gate the pulses F to the variable amplifier 36. Slope of black/white intersection, Figs. 5, 6.- Video signals A and B from two successive line scans L1, L2, of an intersection 62 are made concurrent at the inputs of an exclusive-or gate 60 by means of 1 line delay 58, to give a pulsed output C having a width, converted into a charge level D by integrator 64, proportional to the intersection slope. A store 66 holds the charge level E and applies it as a control signal to a variable gain amplifier 70 receiving the unipolar signal derived as described from the video signal which is delayed at 72 by an amount equivalent to the intersection slope envisaged. A similar delay 86 delays signal A to produce a gating signal J from generator 84. The store 66 is reset by a pulse M derived from high-pass filter 90, detector 92 and pulse generator 94. Integrator 64 is reset by loop 68 once the charge level has been stored. Noise, Figs. 7, 8.-The noisy video signal (a) passes through high-pass filter 106 and the resultant (b) is delayed by a small period ¢t to give the output signal C. The noise is gated out by pulses (f) produced by squaring off threshold detector 110, leading pulse edge detector and pulse generator 112 and pulse stretcher 114. The first three factors can be compensated for simultaneously in a system, Fig. 9 (not shown), comprising the circuits of Figs. 1, 3 and 5. In a complete focusing system, Fig. 10, system 132 may be any of those described. The normalized high-frequency content signal is fed to a comparator 140, directly, and also via a delay 138 having a delay of ¢ the period of a pertubation applied to the focusing lens of the camera 10. The high-frequency content signal is thus amplitude modulated in synchronism with the pertubation about an average value having a value and sign detectable by comparator 140 and indicative of the amount and sense respectively of the out of focus.