Patent GB-799360-A (GB)
Title: Improvements in or relating to pulse code recognition circuits
IPC: H04W88/02
Filed: 1956-02-07
Abstract: 799,360. Electric selective signalling. WESTERN ELECTRIC CO., Inc. Feb. 7, 1956 [Feb. 11, 1955], No. 3790/56. Class 40 (1). [Also in Group XL (c)] Relates to pulse code recognition circuits for use in selective calling systems and particularly to circuits of the type adapted to be carried on the person. Each subscriber has a radio receiver feeding intermediate frequency pulses to the input 31 of a decoder, Fig. 2, the groups of pulses having the form shown at 34 and 35, Fig. 3, pulses A and F always forming part of a pulse group and the information, being transmitted by the presence or absence of pulses B to E. The pulses are applied via, delay lines 41 to 45 and diodes 61 to 65 to normally cut off N-P-N transistors 71 to 75, and via diode 92 to normally cut off transistor 93. The output circuits of transistors 71 to 75 and 93 include transformers, 81 to 85 and 94 provided with corresponding first secondary windings 81B to 85B and 94B and second secondary windings 81C to 85C and 94C. The transformer secondaries " B " are connected as shown (to recognize code pattern 34) to load resistors 101 to 106 in the input circuit of P-N-P transistor 96, this transistor being biased 3 volts positive by battery 97. An appropriately poled pulse across one of the resistors 101 to 106 inserts a negative bias of 1.5 volts, therefore three appropriately poled pulses are required to energize transistor 96 and this only occurs at the instant marked " firing slot " on Fig. 3. When the transistor 96 is energized a pulse is applied to transformer 111 which through its stray capacitance 114 causes an oscillation to be established and maintained during the succeed. ing code group A<SP>1</SP> to F<SP>1</SP>. The transformer secondaries " C are connected to resistors 118 so as to recognize code pattern 35 which similarly overcomes the negative bias 119 associated with N-P-N transistor 121. The energization of transistor 121 together with the oscillations present in transformer 111 are sufficient to energize transistor 123 which in turn energizes transistor 124 to produce a pulse at the output speaker 126. The pulses may be produced at an audio rate or alternatively may be used to trigger an audio oscillator, Fig. 9 (not shown). The transformers may have a third secondary winding to produce a greater number of unique pulse combinations, An alternative arrangement, Fig. 6, employs a pulse position modulation arrangement of the form shown in Fig. 5, the spacing between pulses 133, 134 corresponding to the delay introduced by unit d11 and the remaining pairs of pulses 141, 142 and 146, 147 having a spacing corresponding to the delay introduced by units d12 and d13 respectively. The pulses are applied at terminal 131 and energize transistors 175 to 178. A transformer has its primary 183 connected in the zero delay output circuit and secondary windings 181, 191, 198 respectively connected in the output circuits of delay lines d11, d12, d13. When pulses corresponding to pulses 133, 134 appear simultaneously at the output of transistors 175, 176, the transistor 185 is energized and starts an oscillation in the circuit comprising the primary of transformer 188 and its stray capacitance 209. The commencement of the negative half cycle of the oscillation energizes transistor 187 and it acts as a non-linear resistance across the secondary of transformer 188 the current flow from the base being indicated by curve 216, Fig. 8. As the current from the transistor base decreases the base to emitter resistance increases and the decay is terminated rather abruptly as at 218. During the period of current flow in the base circuit of transistor 187, the pair of pulses 141, 142 produce coincident pulses at the output of transistors 175, 177 and produce a negative pulse bn the base of transistor 197 to energize its load circuit comprising the primary of transformer 197. Transformer 197 operates in a similar manner to transformer 188, pulses 146, 147 produce a negative pulse to energize transistor 201, and the speaker 207 is energized as in Fig. 2.