GB960976A - Improvements in or relating to frequency discriminating telegraph signal detectors - Google Patents

Abstract

960,976. Telegraphy. MARCONI CO. Ltd. Feb. 21, 1963 [June 4, 1962], No. 21454/62. Heading H4P. A frequency discriminating frequency shift telegraph signal detector includes timing means for determining if a cycle of the signal exceeds a predetermined time duration, and means for producing one or other of two output signals depending on this result. As shown, an incoming signal (a) is amplified at 1 and shaped at 2 to give a rectangular wave output (b) which is differentiated at 3 to give a wave (c), the negative pulses of which are selected at 4 and shaped, inverted and amplified at 5 to give a pulse train (d). A short space in front of one of these pulses indicates that the pulse corresponds to a " space " signal, i.e. the higher frequency, while a longer space in front of a pulse indicates a " mark " frequency. The pulse train (d) is fed to a pair of gates G1 G2 and also to a timing device A1. The timing device A1 switches on when it receives a pulse and switches off again causing an end element 1A1 to act at the end of a period corresponding to the shortest possible time cycle of a signal. The end element 1A1 then operates to cause a second timing device A2 to produce an output to open the gate G1 during a range of time covering a cycle of any acceptable space signal, and then a third timer A3 operates for the range corresponding to the longer period " mark " signals. Thus, if after receiving one pulse which operates the timing devices A1,A2, A3 in sequence, the next pulse is a " space " pulse occurring a short distance behind the first it passes through the open gate G1 to trigger a bi-stable circuit BS to one state, while if it is a mark pulse it arrives later to find the gate G1 closed and the gate G2 open and passes through the latter to trigger the circuit BS to the other state. If the " space " frequency is more than twice the " mark " frequency, a second pulse could arrive and operate the timers A1 and A2 while the timer A3 is operating, thus opening both gates G1 and G2. In these circumstances an inhibit connection shown in broken lines ensures that the gate G2 is closed for this " space " pulse. In a modification the timers A1, A2 are combined as a single timer and the element 1 is dispensed with.