Patent GB-1372988-A (GB)
Title: Methods of and apparatus for presenting a moving patter'
IPC: A61B5/00
Filed: 1971-11-04
Abstract: 1372988 Cathode-ray tube displays P M GARTON 4 Nov 1971 [4 Aug 1970] 37498/70 Heading H4T Apparatus intended primarily for testing visual acuity and making use of the phenomenon of "opto-kinetic nystagmus"-i.e. the involuntarily following by a patient's eyes of one of a succession of objects moving across the field of view until it disappears at which point the eyes revert quickly to the original direction of gaze and pick up another one of the objects and follow that-comprises a cathode-ray display tube the beam of which is deflected to produce a vertical line raster and intensity modulated by a signal of cyclic character synchronized with the vertical (line) scan but not with the (horizontal) frame scan so that the pattern resulting from the intensity modulation moves across the screen in the horizontal direction. Means are also provided for varying a parameter of the display which affects the establishment or loss of establishment of the opto-kinetic phenomenon such as the speed of movement, the contrast between dark and light parts or a dimension of the pattern. The moving pattern may be supplemented by a stationary pattern in the path of the former to act as an interrupting means as the moving pattern passes across it and the parameter varied may be a characteristic of the stationary pattern.. In all cases the detection of establishment or of loss of establishment of the optokinetic phenomenon may be effected by observation of the patient's eyes or by the use of a modified electroencephalograph (EEG) and the value of the varied parameter is taken as a measure of visual acuity. Fig. 1 shows one embodiment in which the moving pattern, comprising four truncated wedge-shaped elements 10, 11 is produced by blanking the beam of the display C.R.T. under the control of trains of pulses from a generator 22 controlled by a primary pulse generator 17 which, after division in frequency divider 18, controls the vertical time base pulse generator 19 so that the generation of the pattern is synchronized with the vertical (line deflection). The output of pulse generator 22 consists of cyclically repeating trains of pulses having suitably varying durations for producing the elements 10, 11 and in order that the pattern should move across the screen the time of two cycles (one cycle producing the left-hand pair 10, 11 and the second cycle the right-hand pair 10, 11) is made just greater than the horizontal (frame scan) deflection time. Also, to ensure that the trains of pulses are only supplied to the tube during time periods corresponding to the zones occupied by the elements 10, 11 the output of generator 22 is supplied via a gate 25 controlled by pulses derived from the vertical pulse generator 19. The various waveforms operative in this embodiment are depicted in Fig. 2 (not shown). The stationary pattern 12 comprises a grid of horizontal dark lines and intervening bright spaces produced by blanking pulses from a pulse generator 23 (the width of the pulses being adjustable in order to vary the thickness of the lines) supplied with an input from the horizontal pulse generator 15 and supplied to OR gate 28 via a variable gain circuit (amplifier and/or attenuator) 23a for adjusting the intensity of the lines and AND gate 24 which is controlled by gate pulse generator 32 to pass signals during the central portion of each horizontal (frame) scan. The frequency of the horizontal pulse generator 15 is adjustable and stabilization of the frequency at which it is set is effected by means of an automatic frequency control generator (AFC) 29 receiving an input from a mixer 30 supplied with sinusoidal inputs, of frame frequency (conveniently from pulse generator 23) and of divided down line frequency, such as would result in a stationary pattern, via dividers 27 and 31, mixer 30 developing a difference frequency which is fed to network having an attenuation which is non- linear with respect to frequency changes so that the magnitude of the output signal is dependent on the frequency difference, i.e. the degree of "slip". Operation of divider 31 then allows the speed of movement of the pattern 10, 11 across the screen to be adjusted manually and the control means associated with divider 31 and with generator 23 and variable gain circuit 23a are provided with scale indicating means to enable the settings indicative of the onset or loss of opto-kinetic nystagmus to be read. Fig. 3 shows a second embodiment (elements corresponding to those shown in Fig. 1 having the same reference numerals prefixed, by "1") producing the stripes, 140, 141, by means of generator 144, the number of stripes being determined by the division factor of frequency divider 143, the widths of the stripes being adjustable by manual control means operating on the vertical time base pulse generator 119 and the speed of movement of the stripes across the tube screen being adjustable by manual control means operating on the frequency divider 145, the two manual control means being provided with scale indicating means. The various waveforms operative in this embodiment are depicted in Fig. 4 (not shown). In a further embodiment a conventional television receiver rotated physically through 90 degrees so that the line scan is vertical is employed and supplied with video signals from a tape which is pre-recorded with a signal for producing a vertical stripe pattern having different widths in different, identifiable, tape sections, together with line and frame synch. signals producing a fixed speed of pattern movement. Alternatively, a fixed display may be produced from the tape signals and movement of the pattern produced by varying the speed of the tape (Fig. 5, not shown).