DE1155490B - Circuit arrangement for preventing the interception of conversations via a telephone set with a polarized alarm clock drive system which is in the idle state ready for call - Google Patents

Description

Circuit arrangement to prevent eavesdropping on conversations a telephone set with a polarized Alarm clock drive system The invention relates to a circuit arrangement for preventing eavesdropping on conversations via an idle state that is ready to call Telephone set whose AC alarm clock has a polarized drive system.

Telephone sets of this type are known to show one more or less strong "microphone effect", which consists in the fact that those in the vicinity of the telephone set Calls made even with the handset on-hook, d. H. from the subscriber line Detached earpiece and speech capsule, at any point on the subscriber line can be monitored with the aid of an amplifier.

This "microphone effect", which is extremely undesirable for various reasons is due to the fact that the anchor carrying the movable alarm clapper the alarm clock drive system is excited by the sound vibrations, so that the magnetic resistance of the magnetic circuit pre-magnetized by the permanent magnet changes. This causes the speech vibrations in the coil winding of the drive system caused proportional alternating current, which in itself is relatively small is, with sufficient amplification, however, easily recorded and reproduced can be.

To avoid this microphonic effect, it is already known to connect a voltage-dependent resistor upstream of the alarm clock, which is very low tone-frequency voltages opposed a high resistance, but below that Effect of the high alternating ring voltage is low impedance and the ringing. alternating current weakens only insignificantly. However, this known circuit arrangement has the disadvantage that there is still a very low residual voltage on the wires, so that with sufficient amplification there is still the risk of eavesdropping.

In order to largely suppress these residual stresses as well, is it is also already known in the above-mentioned known circuit arrangement parallel to the series connection of alarm clock and voltage-dependent resistor Provide a capacitor. Since this capacitor, however, has the hook switch contact in series with the usually used to separate the alarm clock from DC the subscriber line serving capacitor between the a-b wires of the subscriber line is switched, this arrangement has the disadvantage that the ringing currents for the alarm clock are relatively weakened.

The use of a voltage dependent resistor and one in parallel switched capacitor is a relatively expensive solution with the rest take up a considerable amount of space.

In order to avoid the microphonic effect, one could continue to use it think instead of a polarized drive system an ordinary neutral, d. H. to use a non-premagnetized drive system. However, to deal with such a To achieve a sufficiently loud alarm clock sound, the alarm clapper would have to be used load-bearing anchor to be attached to a relatively strong spring, so what require a considerable ringing current to generate the required alarm volume would; the responsiveness of such an alarm clock drive system becomes namely extremely low.

According to an older proposal by the inventor, the microphone effect thereby avoid that the drive system consists of two electromagnets, the attract a common armature in opposite directions and their with two diodes interconnected windings for successive half-waves of the alternating ring voltage alternately conduct electricity.

Such a design is characterized above all by a manufacturing and assembly-technically simple design as well as one - by eliminating any Due to return springs - great operational reliability.

Further developments by the inventor have now shown that the initially mentioned unwanted »microphony -Effect «according to the invention too thereby avoiding the need for a simple, non-polarized (neutral) relay Commercially available design is provided that the alarm clock drive system in itself known way only when a call arrives via a relay contact to the Subscriber line turns on.

The use of a telephone set when the telephone is ready to be called connected to the subscriber line; non-polarized relay that drives the alarm clock only when a call arrives via a relay contact on the subscriber line turns on is in itself no longer new. However, this circuit has been used so far used only for the purpose of a temporally shifted response to achieve an optical and an acoustic signaling device. Accordingly is in the known version a specially designed, very expensive mercury relay provided, the response and release times of which are adjustable.

In contrast, the essential idea of the invention is that a simple, non-polarized (neutral) relay of commercially available design, which is used in is switched in the manner mentioned, is sufficient to avoid the undesirable microphonic effect turn off.

As tests have shown, a Protective tube contact relays of the simplest type are used in mass production can be produced cheaply and practically only from the protective tube contact and the him surrounding winding. Instead of the protective tube contact relay, however, any other quick-response, neutral relay of conventional design can be used.

When dimensioning the relay it must be taken into account that the Winding must have a high inductance (at 800 Hz an impedance of at least 20 kQ) so that the speech streams are only imperceptibly attenuated.

The main advantages of the solution according to the invention are thus can be seen in the following: a) The microphonic effect is definitely switched off, because the alarm clock drive system when hung up. Handset from the subscriber line is separated; b) the basic advantages of a polarized drive system, in particular, the high response sensitivity, are almost complete obtain; c) more commercially available through the use of a simple, non-polarized relay Construction, the manufacturing costs are relatively low; the omission of one else for the suppression of residual voltages serving capacitor also leads to a significant reduction in space requirements compared to the known at the outset Solution; d) because the alarm clock in the circuit arrangement according to the invention is galvanic and is completely disconnected from the subscriber line in terms of alternating current there are no residual microphonic stresses, no matter how small.

These and other details of the invention are evident from the description two exemplary embodiments illustrated in the drawings. It shows Fig. 1 is a circuit diagram of the telephone set according to the invention in a first Embodiment, Fig. 2 is a circuit diagram of another embodiment, Fig. 3 is a for Explanatory sketch of the current flow.

The circuit diagram of the telephone set according to FIG. 1 contains in the usual representation - apart from the components not of interest here such as microphone M, earpiece F, hook switch U, nsa, nsi and nsr contact - above all the alternating current alarm clock provided with a polarized drive system W. , a non-polarized (neutral) relay with winding H and capacitors C 1 and C2.

The relay winding H is continuously connected between the wires a and b of the subscriber line via the capacitor C1; the alarm clock drive system W, on the other hand, is only connected to the subscriber line when a call arrives, ie when the relay H responds via the contact h 1 and the capacitor C2.

3 illustrates the mode of operation of the circuit, which is in itself readily apparent from the above: If the ringing current (of, for example, 25 Hz) reaches the value ! 1, the relay H responds and switches the alarm clock drive system W on via the contact h 1 . When the current drops to the value i 2 , the contact h 1 opens again, so that the alarm clock drive system is de-energized. As a result of the already existing drop-out delay of the relay H, the current value i 2 is somewhat smaller than the value 11. The game then repeats itself in the next half-wave.

In the exemplary embodiment according to FIG. 2, the non-polarized (neutral) relay H has two contacts h 1 and h 2 . The contact h 1 assumes the position illustrated in FIG. 2 when the telephone set is ready for a call, ie when the handset is on-hook, so that the winding H of the non-polarized relay is connected via the capacitor C 1 between the wires a, b of the subscriber line.

When a call arrives, the relay H responds, so that the contact h 1 designed as a changeover contact now connects the drive system W of the alarm clock to the subscriber line. Relay H is switched off at the same time.

In order to immediately switch back the contact h 1 to the position shown in Fig. 2 to prevent the position illustrated, d. H. about the already existing fall delay of relay H, contact h2 is provided parallel to the relay winding, which closes when the relay responds, which occurs in the circuit formed by it creates a current.

This last-mentioned embodiment has, compared to the circuit according to FIG. 1 has the advantage that the alarm clock drive system W is not switched on is attenuated in parallel by the neutral relay H and that only a single capacitor (C 1) is required.

In contrast, the circuit according to FIG. 1 is characterized by a longer one Duty cycle ta (see. Fig. 3) of the alarm clock drive system W during the individual Half-waves of the ringing alternating current.

Claims (5)

PATENT CLAIMS: 1. Circuit arrangement for preventing the eavesdropping of conversations via a telephone set in the idle state ready to call, the alternating current alarm clock of which has a polarized drive system, characterized in that a simple, non-polarized (neutral) relay (H) of commercially available design to avoid the microphone effect it is provided that the alarm clock drive system (W) is only switched on in a manner known per se when a call arrives via a relay contact (h 1) on the subscriber line (a, b) . 2. Circuit arrangement according to claim 1, characterized in that the winding (H) of the relay via a capacitor (C1) is constantly connected between the wires (a, b) of the subscriber line and that the one in series with another capacitor (C2) Alarm clock drive system (W) when the relay responds via the relay contact (h1) in parallel to the branch containing the relay winding (H) and the upstream capacitor (C1) connected to the subscriber line. 3. Circuit arrangement according to claim 1, characterized in that that the non-polarized (neutral) relay (H) has two contacts (h 1, h 2), one of which (hl) is designed as a changeover contact and the alarm clock drive system (W) when the relay picks up, switching off the relay winding (H) to the subscriber line (a, b) turns on, while the one used to increase the drop-out delay of the relay second contact (h 2) is connected in parallel to the relay winding (H) and when it is tightened of the relay is closed. 4. Circuit arrangement according to claim 3, characterized in that that the drop-out delay of the relay (h) by means known per se, such as short-circuit windings and the like, is increased. 5. Circuit arrangement according to claim 1, characterized in that that the relay (H) is designed as a protective tube contact relay. Considered Publications: German Auslegeschrift No. 1066 624; U.S. Patent No. 2,428 007.