GB921680A - Tone-generator circuits - Google Patents

Abstract

921,680. Ringing tone generators. MULLARD Ltd. Dec. 18, 1961, No. 45250/61. Class 40 (4). A ringing tone generator circuit comprises a pulse generator arranged to frequency-divide an A.C. source, e.g. 50 c.p.s. mains supply, to produce pulses at a frequency less than that of the source, e.g. 25 c.p.s., which are used to trigger a cold-cathode tube whose anode and cathode are connected in series with a load impedance across the A.C. source so that pulses of current flow from the source through the load on selected, e.g. alternate, cycles of the A.C. source. Fig. 1 shows one embodiment in which a cold-cathode tube TC is connected as a selfextinguishing pulse generator the time constant of R4, C3, C4 being arranged so that the tube is synchronized by a 50 c.p.s. waveform applied over transformer S to generate pulses at 25 p.p.s. Tubes TA and TB form a coldcathode multivibrator oscillating with a time pattern such as to indicate e.g. " busy " or " ringing " and the output of the multivibrator, added to the 50 c.p.s. supply at transformer S, switches the pulse generator TC. The output of the pulse generator, limited by diode Q 1 is applied to the trigger electrode of a cold-cathode tube TJ, Fig. 2, in the subscriber's line equipment which may be of the type described in Specification 913,617. When the called subscriber's speech tube Z, Fig. 2, is ignited, current flows in R8 and R9 and provides a biasing voltage to allow tube TJ to fire on receipt of a pulse from the ringing generator on its trigger electrode. The resulting pulses appearing across R10 are applied to the transformer U and hence to the called subscriber over line J and back to the calling subscriber through the speech tube Z. In an alternative form of generator, Fig. 7, a multivibrator TD, TE oscillates at 25 c.p.s. synchronized by the 50 c/s. supply applied over transformer S and gated by multivibrator TA, TB, generating the "ring- ing " pattern. Cold-cathode tube TC is selfoscillating at about 250 c.p.s. and is switched on and off by the output from the 25 c.p.s. multivibrator TD, TE, so that the signal output from tube TC comprises bursts of 250 p.p.s. at a repetition rate of 25 c.p.s., modulated by the appropriate " ringing " time pattern. These pulses are limited by diode Q1 and applied to the subscriber's line apparatus as described above with respect to Fig. 2. It is stated that the bursts of pulses generated by the embodiment of Fig. 7 offers a greater margin of stability in the triggering of the cold-cathode tube in the subscriber's line apparatus. It is suggested that the " ringing " time pattern could be generated by cam-operated switches.