Improvements in or relating to electrical testing apparatus

Abstract

Abstract 520,349. Television. SCOPHONY, Ltd., and THOMSON, A. F. H. Oct. 19, 1938, No. 30205. [Class 40 (iii)] For testing a television receiver, particularly for testing the response from a transmitter employing interlaced scanning, an artificial picture signal is introduced between the synchronising signals so as to produce on the receiving screen a regular check pattern if the receiver is responding correctly to the interlaced synchronising signals. The artificial signal picture may be produced by a mixing circuit comprising two hexode valves 1, 2, Fig. 1. One control grid of each valve is connected to a push-pull circuit shown as a transformer 4 to which is fed an oscillation of the required frequency for the artificial picture signal. The other control grid of each valve is connected to a second push-pull circuit shown as a transformer 5 to which is fed a squaretopped oscillation having a frequency equal to the picture frequency and of such amplitude that during each half-cycle one of the valves is rendered non-conducting, and during the other half-cycle the other valve is rendered non- conducting. Thus, during one picture traversal the artificial picture signal produced across terminals 6, 7 comprises an oscillation which is 180 degrees out of phase with the oscillation produced during the second picture traversal: This signal produces on the receiving screen a regular check pattern all over it. This pattern may be disposed in vertical or horizontal bands. as shown in Fig. 2, and can then be employed also for testing the uniformity and linearity of the line and frame scanning circuits. An arrangement for producing a pattern as shown in Fig. 2 is shown in Fig. 3. A sinusoidal frequency of 10,125 cycles per second is generated by a master generator G, preferably of the kind described in Specification 490,294 [Group XXXV]. This is fed to a pulse transformer PT which produces a square-topped pulse of a frequency of 20,250 cycles per second. From this pulse circuits F, L produce frame and line scanning impulses, the outputs from which are mixed in the circuit FLM. Generators A, B, C, D produce oscillations of 25 cycles, 1,012,500 cycles, 202.5 kilocycles, and 1.6 kilocycles per second respectively. Generators A, D are held synchronized to the framescanning frequency by a lead 8, the generator C is held synchronized to the line-scanning frequency by a lead 9, and the generator B is held synchronized to the generator C as indicated at 10. The generators are preferably multivibrators giving an oscillation of square shape. The outputs from the generators A, B are applied to the modulator MAB, the output of which is similar to that produced by the circuit of Fig. 1. The output from the generators C, D is applied to the modulator MCD, which may consist of a heptode valve, the oscillation from C being applied to one grid and that from D to the other grid with such a voltage that at every alternate half cycle of the 1.6 kilocycle of the oscillation from D the valve is rendered non-conducting, so that the oscillation from C is interrupted at the frequency of the oscillation from D. The oscillation from MCD is applied to the modulator ME, to which is also fed the oscillation from the modulator MAB, which is thereby interrupted once at every half-cycle of the oscillation from MCD, i.e., twenty times per line period. This produces a check pattern in the receiver in a series of vertical bars. The oscillation from D withholds this interruption 1,600 times a second so that for a number of lines the check pattern is produced right across the picture, producing the horizontal bars of the lattice pattern. The output from ME passes to a phase-inverting stage P1 and thence to a modulator MG. Frame and line blanking pulse generators FBP, LBP, produce square-topped pulses of the frame and line frequencies and are synchronized to the frame and line synchronizing signal generators. The outputs from FBH, LBP are mixed in the mixing circuit BPM and are then applied to the modulator MG. The blanking pulses suppress the artificial picture from the stage PI to enable the frame and line synchronizing pulses to be inserted by the synchronizing signal modulator SSM. The resulting signal is passed through a buffer stage BS and then modulates a carrier wave which is fed to the receiver under test. The circuit for the parts A, B, MAB is shown in Fig. 4. Two pentode valves 11, 12, corresponding to the push-pull circuit 4, Fig. 1, are arranged as a multivibrator oscillating at 1 megacycle per second and are synchronized to the line synchronizing signals applied at 14. The oscillation in the anode circuit of the valve 11 is fed to one control grid of a heptode 16, and the output of the valve 12 is applied to one control grid of a heptode 18. The grids of the valves 16, 18 are thus fed in opposite phase with an oscillation the frequency of which is an integral multiple of the line-scanning frequency and is for producing the check pattern on the receiver to be tested. A multivibrator valve 19, corresponding to the push-pull circuit 5, Fig. 1, is arranged to oscillate at 25 cycles per second and is synchronized to the frame scanning frequency which is applied at 20. The oscillation from one anode 21 is applied to the second control grid of the heptode 16, and the oscillation from the other anode 23 is applied to the second control grid of the valve 18. The strength of a negative pulse from the valve 19 is such as to render the valves 16, 18 non-conducting, so that an oscillation is produced at the output terminal 25 similar to that produced by the circuit of Fig. 1, i.e., its phase during 1/25th second is 180 degrees different from that during the succeeding 1/25th second. The frequency of the oscillator B, Fig. 3, may be varied by adjusting a resistance 13, Fig. 4, in order to test the highest definition of which the receiver is capable.

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