Method of and circuitry for detecting, at a telephone exchange, the lifting of a subscriber's handset in response to ringing current emitted by the exchange
First Claim
1. A method of detecting at a telephone exchange, provided with a generator ringing current of low audio frequency connectable across a line loop leading to a called subscriber station, a response of the called subscriber which involves closure of a normally open hook switch shunting a series capacitor in the line forming said loop whereby a continuous voltage applied to the line at the exchange gives rise to a direct-current component superimposed upon the alternating ringing current,comprising the steps of:
- (a) sensing the line current circulating in said loop during emission of ringing current in the presence of said continuous voltage, with generation of a monitoring current whose absolute amplitude varies proportionally with that of said line current;
(b) integrating a bipolar current, derived from said monitoring current and proportional to the sensed line current, during a recurrent measuring period spanning a whole number of ringing-current cycles;
(c) comparing the result of integration in step (b) at the end of each measuring period with a predetermined minimum value to determine the existence of a mean amplitude indicative of said direct-current component; and
(d) discontinuing the emission of ringing current in response to said mean amplitude.
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Accused Products
Abstract
In order to detect at an electronic telephone exchange the lifting of a subscriber'"'"'s handset in response to ringing current sent over the line loop from a generator at that exchange, a charging current proportional to the incoming line current is fed to a capacitor over a recurrent integration period encompassing one or more whole ringing-current cycles. When the handset has been taken off the hook, the capacitor charge present at the end of such a period differs from zero on account of a d-c component superimposed upon the alternating ringing signal. In order to enable the use of a relatively small storage capacitor, the charging current is periodically interrupted by a train of chopping pulses. When the line current is measured at the exchange by a sensor responsive only to absolute magnitude, the polarity of the charging current is reversed in alternate nonzero-amplitude periods of a cycle.
15 Citations
15 Claims
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1. A method of detecting at a telephone exchange, provided with a generator ringing current of low audio frequency connectable across a line loop leading to a called subscriber station, a response of the called subscriber which involves closure of a normally open hook switch shunting a series capacitor in the line forming said loop whereby a continuous voltage applied to the line at the exchange gives rise to a direct-current component superimposed upon the alternating ringing current,
comprising the steps of: -
(a) sensing the line current circulating in said loop during emission of ringing current in the presence of said continuous voltage, with generation of a monitoring current whose absolute amplitude varies proportionally with that of said line current; (b) integrating a bipolar current, derived from said monitoring current and proportional to the sensed line current, during a recurrent measuring period spanning a whole number of ringing-current cycles; (c) comparing the result of integration in step (b) at the end of each measuring period with a predetermined minimum value to determine the existence of a mean amplitude indicative of said direct-current component; and (d) discontinuing the emission of ringing current in response to said mean amplitude. - View Dependent Claims (2, 3, 4)
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5. In a telephone system wherein an exchange is linked with an associated subscriber station via a two-conductor line whose conductors are interconnected at said station by a ringer in series with a capacitor shunted by a normally open hook switch, said ringer being operable in an open state of said hook switch by alternating current of low audio frequency transmitted over said line along with a continuous voltage whereby a direct-current component is superimposed upon said alternating current on closure of said hook switch,
the combination therewith of circuitry for detecting said direct-current component as a signal that the hook switch has been closed, said circuitry comprising: -
sensing means coupled to said line for producing a monitoring current of a magnitude proportional at least in absolute magnitude to the line current; conversion means connected to said sensing means for deriving from said monitoring current a bipolar current proportional to said line current; timing means responsive to said alternating current for establishing a recurrent measuring period spanning a whole number of cycles of said low audio frequency; integrating means connected to said conversion means and controlled by said timing means for determining the mean amplitude of said bipolar current over each measuring period; and comparison means connected to said integrating means for signaling a surpassing of a predetermined minimum value by said mean amplitude. - View Dependent Claims (6, 7, 8, 9, 10, 11, 12, 13, 14, 15)
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Specification