Patent GB-779410-A (GB)
Title: Improvements in or relating to methods of testing deflection linearity in cathode ray tube apparatus
IPC: H04N17/00
Filed: 1954-06-10
Abstract: 779,410. Testing iconoscopes; phase measurements. FERNSEH GES. June 10, 1954 [June 10, 1953], No. 17084/54. Class 37. [Also in Groups XL (b) and XL (c)] In apparatus for testing the linearity of the time deflection of the cathode-ray beam scanning the target of a television camera tube, an optical image of a series of vertical bars (Fig. 1, A) is projected on the tube target for measurements in the horizontal direction while an image of horizontal bars is projected for vertical measurement. When the deflection is linear against time and the deflectional velocity is constant, the tube output comprises a train of equally spaced pulses, while when the deflection is non-linear (Fig. 1, B) the time intervals between pulses are inversely proportional to the respective instantaneous velocity values (Fig. 1, C). For horizontal linearity testing, the latter pulse train is applied positively to the grid of a relaxation oscillator valve whose anode circuit comprises a load resistor of resistance R and a storage capacitor of capacitance C which is discharged by blanking pulses-at the beginning and end of a train of measurement pulses, each of which triggers the oscillator to discharge the capacitor, which subsequently charges through the resistor until the arrival of the succeeding pulse. The resultant voltage waveform across the capacitor (which may be displayed against time by an oscillograph) is shown in Fig. 1, D, and if #T is the time interval between adjacent pulses and #T/RC <<I, the corresponding velocity v of the horizontal deflection is approximately V = K/RC. Vo/Vm, where K is a constant and Vo, Vm are respectively the anode supply voltage and the capacitor peak voltage at the end of the interval, so that the variations of deflectional velocity against time are given by the envelope 1 of the successive peaks. In a modification (Fig. 2), applicable to determine the vertical deflectional linearity, the tube output applied to terminal 5 contains, in addition to the pulse train arising from the horizontal bars of the test image, the line blanking pulses and pulse trains arising from the horizontal scanning of vertical bars of a grid test image, if used. A switchable lowpass filter 6 removes the horizontal scan pulse train, while the line blanking pulses are removed by mixing the resultant signal, after amplification by pentode 7, with anti-phased line blanking pulses from terminal 12 through amplifying pentode 8. The pulse trains derived from the vertical scan of the horizontal test bars are coupled through cathode follower 9 to the grid of pentode 10 whose anode is fed through a load resistor and variable shift potentiometer 15 and earthed through capacitor 13, which is discharged on arrival of each positive going pulse and charged between pulses to a voltage proportional to the pulse interval. The varying capacitor voltage is applied through a direct coupled amplifier pentode 11  to an oscillograph from whose display against a time axis (Fig. 3) the vertical deflection velocity may be derived for different scanning positions. Fig. 4 shows a circuit enabling the horizontal or vertical deflection velocity to be displayed directly on an oscillograph, wherein the input pulse train at terminal 19 from the camera tube output (Fig. 5, E) is fed through delay circuit 21 to the grid of triode 22 to energize (Fig. 5, F) a relaxation oscillator circuit comprising its load resistor and capacitor 23 to derive a variable amplitude saw-tooth voltage waveform as before (Fig. 5, G) which is cathode follower coupled to the centre point of the secondary of the exciting transformer of a known phase comparison circuit 25; the primary being excited directly by the amplified pulse train input at terminal 19. Diode rectifiers 26, 27 are rendered conductive for short periods corresponding to the input pulses, during which capacitors 28, 29 are charged to the instantaneous amplitude of the saw-tooth waveform. Capacitor 30 of long time constant integrates these instantaneous voltages to a stepped waveform (Fig. 5, H) for oscillograph display; the amplitude of each horizontal portion being inversely proportional to the deflectional velocity at a corresponding point of the camera tube scan. In a delay circuit for the input pulse train (Fig. 6, not shown), the anode load circuit of a pulse amplifier comprises an inductor in parallel with a capacitor and a diode whose anode is earthed so that on arrival of a positive-going pulse the parallel resonant circuit is damped by the conductive diode while between pulses it executes half-waves of sinusoidal oscillation which are synchronous with the input pulses but delayed in time with respect thereto. The apparatus is applicable to the determination of the linearity of a television picture tube, whose raster is viewed by a photo-cell through a transparent mask bearing vertical or horizontal test bars to produce a pulse train output which is measured as above.