Acoustic feedback suppression device particularly for auxiliary ringers in plug-in telephone systems

Abstract: An acoustic feedback suppression device, particularly for auxiliary ringers in plug-in telephone systems, includes diodes (17,18) in addition to a rectifier bridge (11) of the ringer (14). The circuit provides direct-current decoupling and limits the AC voltage present across the auxiliary ringer.

Description: The present invention relates to an acoustic feedback suppression device particularly for auxiliary ringers in plug-in telephone systems. As is known, an additional electromechanical ringer is usually present in plug-in telephone systems. This ringer is always connected to the telephone line even when no telephone sets are connected to the sockets, thereby allowing for detection of any incoming calls and connection of a telephone set. Unfortunately, the presence of the additional ringer introduces an unpleasant acoustic feedback when a telephone number is dialed with a pulse-dial selector while a telephone set is connected to the line. With the pulse-dial signaling, the line is in fact suddenly interrupted a number of times corresponding to each individual selected digit. The interruption frequency is normally equal to 10 Hz, with an open/close ratio of 1.5 to 2.0. Additionally, due to exchange inductance, voltage pulses of random amplitude (overvoltages) occur during the repeated opening of the telephone line. These voltage pulses are the pulses which can consequently activate the additional ringer in a spurious manner, causing the abovementioned unwanted acoustic feedback. In order to eliminate this type of problem, it is necessary to limit the voltage across the additional ringer. This limiting action must be performed in AC during pulse-dialling, whereas the limiting circuit must be decoupled from the telephone line when connection to the line occurs in the on-hook condition or during ringing. Some known solutions are discussed below. A first solution provides for the insertion, by means of an appropriate switch, of a voltage limiter, as more clearly described hereinafter, constituted by a pair of Zener diodes connected back to back, of a DC decoupling capacitor and of an insertion switch. In the case of an electronic insertion switch there is also the problem that the telephone line does not have a set polarity, and it is thus necessary to overcome this problem as well. A solution to this further problem is given by the insertion of an additional bridge which simplifies the execution of the electronic switch. The above described solutions are particularly complicated to implement on a single silicon substrate. From US-4,107,476 it is known a circuit for controlling the on-off ringing of a line powered electronic tone ringer in a telephone set. The circuit includes suppression means which prevents low repetition frequency high magnitude transient signals from falsely triggering the tone ringer. The aim of the present invention is to eliminate or substantially reduce the problems described above by providing an acoustic feedback suppression device, particularly for auxiliary ringers in plug-in telephone systems, which allows for the elimination or substantial suppression of the acoustic feedback of the additional ringer during pulse-dialing. Within the scope of the above aim, an object of the present invention is to provide a suppression device which controls the voltage across the additional ringer, limiting any spurious activation pulses. Another object of the present invention is to provide an acoustic feedback suppression device which is highly reliable, relatively easy to manufacture and at competitive costs with respect to mechanical switches. This aim, the objects mentioned and others which will become apparent hereinafter are achieved by an acoustic feedback suppression device, particularly for auxiliary ringers in plug-in telephone systems as defined in claim 1. Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment of an acoustic feedback suppression device, particularly for auxiliary ringers in plug-in telephone systems, according to the invention, illustrated only by way of a non-limiting example in the accompanying drawings, wherein: figure 1 is an electrical schematic diagram of a first prior art circuit; figure 2 is an electrical schematic diagram of a second prior art circuit; figure 3 is an electrical schematic diagram of a third prior art circuit; figure 4 is an electrical schematic diagram of a suppression device circuit according to the present invention; and figure 5 is a sectional elevation view of one of the diodes of the circuit of figure 4. With reference to figures 1 to 3, these figures illustrate prior art solutions, as previously mentioned. In the prior art embodiment of figure 1, it can be seen that an attempt is made to eliminate acoustic feedback on the auxiliary ringer by using a pair of Zener diodes 1 and 2 which are mutually connected by means of their respective anode terminals, and the direct-current decoupling of the ringer is provided by a capacitor 3 and mechanical switches 4. Another known solution is the one illustrated in the prior art embodiment of figure 2, which includes a pair of Zener diodes 1 and 2 (numbering in the prior art embodiments is maintained consistent for like elements for the sake of simplicity in description) and a capacitor 3, whereas the mechanical switches of the embodiment of figure 1 are replaced with electronic switches 5. A rectifier bridge 6 is furthermore provided in order to supply power to the ringer. A severe problem of this solution is related to the polarity imposed by the telephone line. The polarity of the line is in fact not definite, and thus the problem relates to the biasing of the electronic switch, which generally comprises a bipolar transistor or a MOSFET transistor. Another known solution, shown in the prior art embodiment of figure 3, includes a single Zener diode 1 and uses a double rectifier bridge arrangement 6 and 7. A capacitor 3 is present for direct-current decoupling, and the switches 5 are preferably electronic. The execution of the switches 5 is simplified, since the polarity is ensured by the bridge 7. With reference to figure 4, an acoustic feedback suppression device particularly for auxiliary ringers in plug-in telephone systems according to the present invention is shown. A first rectifier bridge and a second rectifier bridge, respectively designated by the reference numerals 10 and 11, are located between input terminals 12 and 13 of a telephone line. An auxiliary ringer 33 is powered through a first terminal 29 and a second terminal 13. The cathode terminals of the diodes 17 and 18 are connected to the ends of the first diagonal of the second rectifier bridge 11. Means are provided for connecting the first rectifier bridge 10 and the second rectifier bridge 11 and for connecting the second rectifier bridge 11 and the ringer of the telephone 14. The diodes 17 and 18 are preferably of the low-leakage type. The second rectifier bridge 11 has a first terminal 15 connected to a second terminal 19 through a first switch 20 and a Zener diode 21. The first switch 20 is controlled by a talk circuit 25 which closes the first switch 20 if the handset is unhooked, inserting an additional voltage limiter between the terminals 29 and 13 of the ringer. A first terminal 23 of the first rectifier 10, preferably constituted by a diode bridge, can be connected, by means of a second switch 24, to the node 16 and a second terminal 32 can be connected to a talk circuit 25 by means of a third switch 31. The first switch 20, the second switch 24 and the third switch 31 are controlled by the unhooking of the handset and by the talk circuit 25, and constitute the connection means. A fourth switch 26, which is controlled by a call detector 22, is interposed between the first terminal 15 of the second bridge 11 and the ringer 14 of the telephone. A resistor 27 and a capacitor 28 are arranged along a branch which connects the input terminal 12 to the first diagonal of the rectifier bridge 11. The capacitor 28 acts as direct-current decoupler for the ringer 14 with respect to the telephone line. The diodes 17 and 18 are of the type shown in figure 5, those being LLDs (low leakage diodes). In the device according to the invention, which can be either produced with discrete components or integrated on a single substrate, each one of the diodes 17 and 18 has an associated parasitic diode 30 which has a PN junction. In the above described embodiment, the P side of the junction is the substrate itself, whereas the N side is the node 19 (shown in figure 4). With these characteristics, if a DC voltage is applied to the terminals 12 and 13, the voltage of the node 19 coincides with the voltage of the substrate in the on-hook condition, i.e. in the condition of a telephone which has its handset hooked, with no call in progress. During reception, i.e. when a call is arriving, the presence of the diodes 17 and 18 does not interfere with the operation of the circuit, since the first switch 20, the second switch 24 and the third switch 31 are open while the fourth switch 26 is closed. During a call, i.e. with the handset unhooked, the first switch 20, the second switch 24 and the third switch 31 are closed, the fourth switch 26 is open, and a voltage set by the talk circuit 25, except for particular cases provided for by national statutory provisions, is established on the terminals 12 and 13. During a call, terminals 29 and 13 of the power supply of the additional ringer are AC-limited by the presence of the Zener diode 21. Advantageously, the voltage of the node 19 follows the voltage of the substrate (node 16) on which the device according to the invention is provided, when the voltage of the terminal 12 is positive with respect to the terminal 13, whereas the voltage of node 19 is positive with respect to the substrate (node 16) when the voltage of the terminal 13 is positive with respect to the voltage of the terminal 12. In the latter case, the described condition does not constitute a problem for the integrated diodes 17 and 18, since the parasitic diodes 30 are reverse-biased. It is thus possible to use both integrated and discrete diodes 17 and 18 in the circuit of the present invention including the insertion of the Zener diode 21 through a switch 20, for the purpose of limiting the AC voltage across the additional ringer. Conveniently, the present invention is useful in the case of the production of talk circuits and ringer circuits on a single substrate, i.e., so-called "monochip solutions". Practical tests have shown that the present invention achieves the intended aim and objects, constituting a suppression device which can limit, by means of the Zener diode 21, the AC voltage across the auxiliary ringer, while simultaneously eliminating the possibility of acoustic feedbacks. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: An acoustic feedback suppression device, particularly for auxiliary ringers in plug-in telephone systems, comprising a first rectifier bridge (10) and a second rectifier bridge (11) connected between input terminals (12, 13) of a telephone line, said first and second bridges (10, 11) having respectively a first (23) and a second (15) terminals connected to a telephone ringer (14), a first terminal (16) of said second bridge (11) being connected to the first terminal (23) of said first bridge (10), an auxiliary ringer being connected between an input terminal (13) of the telephone line and said second bridge (11), characterized in that said device comprises two series-connected diodes (17, 18) connected with their cathode terminals to the ends of the first diagonal of said second rectifier bridge (11), a first switch (20) and a Zener diode (21) connected between said second terminal (15) of the second rectifier bridge (11) and an intermediate point between said series-connection of diodes (17, 18), said first terminal (23) of the first rectifier bridge (10) being connected, by means of a second switch (24) to the first terminal (16) of said second rectifier bridge (11), and a third switch (31) connected between a second terminal (32) of said first rectifier bridge (10) and a talk circuit (25), said first switch (20), said second switch (24) and said third switch (31) being controlled by the unhooking of the handset of a telephone set and by said talk circuit (25). A device according to claim 1, characterized in that a fourth switch (26) is interposed between the second terminal (15) of said second rectifier bridge (11) and said telephone ringer (14) and is controlled by a call detector. A device according to claim 1, characterized in that said series-connected diodes (17, 18) are low leakage diodes. A device according to claim 1, characterized in that said series-connected diodes (17, 18) are integrated monolithically.

Inventors: ANTONINI CARLO, CONSIGLIO PIETRO, ANTONINI, CARLO, CONSIGLIO, PIETRO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H04M1
IPC codes: H04M1, H04M19