Patent GB-637722-A (GB)
Title: Improvements in and relating to electrical testing and indicating systems
Inventor: PYATT EDWARD CHARLES
IPC: H04L25/04
Filed: 1946-07-24
Abstract: 637,722. Testing-apparatus for telegraphy. TELEPHONE MANUFACTURING CO., Ltd. July 24, 1946, No. 22058. [Class 40 (iii)] A device for indicating or measuring the distortion of telegraph or like signals consisting of elements each of which are, or should be, an integral multiple of a telegraph unit, comprises comparison means responsive to said signal during a period immediately following a whole number of telegraph units after a reference point of said signal, further comparison means responsive during a period immediately preceding a whole number of telegraph units after a reference point of said signal, said comparison means each operating to. produce a current or voltage which is a measure of the departure of said signal from the correct form and indicating or measuring means responsive to said currents or voltages. Incoming start-stop telegraph signals are applied over terminals 10 to a differentiating circuit 11 which produces a pulse of one or other polarity at each signal transition. A selector circuit 12 inverts those pulses of one polarity so that the output consists of a series of uni-directional pulses, the first of which trips a trigger circuit 13 which opens a gate circuit 14 and allows oscillations at a hundred times the signal frequency to reach two parallel counting circuits A, B, each comprising two scale of five sections followed by two scale of two sections. Subsequent pulses from the selector 12 have no effect on the trigger 13 and the counting circuits continue to count, the final sections 19, 23 emitting a pulse after each 100 oscillations, i.e. at signal frequency. The pulses from the final section 19 of the " A " counter are reversed in polarity by a phase inverter 40 and applied to a mixer circuit 41 in opposition to the pulses received directly from the section 23 of the " B " counter. So long as the received signals are undistorted the inputs to the mixer 41 nullify each other and no output results. After each fifty oscillations the counter sections 18 and 22 each emit a pulse, which is mixed via suitable phase inversion devices 25, 27 with the pulse output from the final sections 19, 23. The resulting pulse trains are applied to respective trigger circuits 28, 30 and the square-wave outputs, which are at half-signal-frequency, unblock the gate circuits 29, 31, the former during the first half of the next 100 cycle signal interval and the latter during the last half. If distorted signals, as at Fig. 1b, are received, the operation is as follows. The first transition b1 starts the oscillator and consequently both counters A, B. At the next transition b2, the selector 12 produces an output pulse but this coincides with the pulses c2, d2 (Figs. 1c, 1d) produced by the final sections 19, 23 of the two counters. The mark element b2-b3 is longer than if undistorted and the next transition b3 therefore causes the selector 12 to produce a pulse c5 during the first half of the signal interval following the previous pulse c4 of the counter " A." Since the gate 29 is open during this period, the selector pulse is applied to all sections of counter " A " to re-start it counting. Gate 31 is closed at this time so the pulse due to transition b3 has no effect upon counter B. The next counter pulses c6, d5 therefore no longer coincide and the output of the mixer 41 consists of two pulses of opposite polarity e1, e2, separated by an amount representing the distortion of the received signal. This produces in the trigger 42 a square pulse f1 whose width is a measure of the signal distortion. A meter 43 is arranged to indicate the average value of the current applied to it by the pulses from the trigger circuit 42. The reading of this meter therefore indicates the distortion present,. and it may be calibrated to read directly as a percentage of the nominal signal interval. The signal b3-b4 (Fig. 1b), is shorter than if it were undistorted and the pulse from selector 12 caused by the transition b4 occurs during the last half of the signal periods following counterpulses c7 and d6. Counter A is therefore unaffected but counter B is restarted and the following counter pulses c9, d8 give rise to an output e6, e7 from mixer 41 spaced by the amount by which the transition b3 was late plus the amount by which the transition b4 was early, i.e. the total distortion of the space signal b3-b4. The counter A now continues to restart at signal intervals unless a signal transition arrives which is more displaced in the same sense than the previous transitions. Counter B operates similarly for transitions displaced in the opposite sense. - The meter therefore registers the maximum distortion which has been present in the received signal. A key is provided for re-setting the complete circuit. The circuit of the triggers is that described in Specification 637,716, and conventional thermionic circuits for the remainder of Fig. 3 are described, Figs. 4a, 4b, 4c (not shown).