1. Field of the Invention
The invention relates to a circuit arrangement for the transmission of telecommunication signals between a switching center of a telecommunication network and at least one subscriber.
2. Description of the Prior Art
In telecommunications networks, subscribers are connected to a switching center by means of a connecting line. At the subscriber's end of the connecting line, there is connected a device in which, in addition to other passive and active internal circuit elements, a direct-current source and a transformer with a core, an input winding, and an output winding are arranged. The input winding is connected with the connecting line via the internal circuit elements of the device. The output winding is connected to the subscriber by means of another connecting line. The output winding of the transformer is subdivided into two partial windings of identical size, between which there are connected a first capacitor and a switch connected in series with the windings. The switch is to be actuated by a component for detecting the loop current. The transformer is equipped with at least one additional winding, whose number of turns is equal to the total of the number of turns of the two partial windings. In the additional winding, the current flows in a direction opposite to that in which it flows in the two partial windings. The direct-current source is connected with one of the partial windings at the connection point of the first capacitor, with the interposition of the additional winding, as shown in German OS 37 15 552.
A circuit arrangement of this type is used, for example, in the so-called "PCM 2" process, in which two subscribers in adjacent locations of a telecommunication network are jointly connected to a switching center by means of a connecting line. The letters "PCM" stand for the known pulse code modulation process, by means of which telecommunication signals are transmitted as digital signals. The additional explanations apply to this process, representing all possible other cases of application of the invention.
In the PCM 2 process, analog signals are digitalized in the switching center and fed into the connecting line in coded form. At the end of the connecting line there is installed a device in which the incoming digital signals are decoded and transformed back into analog signals. This, of course, applies also to the opposite direction. The two subscribers are connected to the device by means of connecting lines. For each subscriber, the device contains a transformer whose input winding is connected to the connecting line with the interposition of an analog-digital converter and other circuit elements. Each particular subscriber is connected to the output winding of the transformer by means of a connecting line. Each subscriber is assigned his own direct-current source for supplying his telephone, which is connected at the output side of the transformer. Particularly in the cases where the subscribers are connected to the device by means of long connecting lines, interferences can be induced by external electric and magnetic fields, which can lead to cross-talk in other channels and to humming sounds.
To prevent these interferences, in the known circuit arrangement according to the previously mentioned German OS 37 15 552, the output winding of the transformer is subdivided into two partial windings of equal size, between which the first capacitor and the switch are connected in series. By dividing the output winding into two partial windings, the whole arrangement is made symmetrical in such a way that identical conditions always result for the forward and return lines of the connecting line with respect to a reference potential--for example, the ground. Interferences induced by electrical or magnetic fields can therefore no longer have an effect, regardless of the length of the connecting lines. The first capacitor makes certain that the direct current supplied flows only in the desired direction.
The brief interruptions in the feed current occurring during a dialing process cannot be rapidly determined by the component detecting the loop current, because current initially continues to flow through the first capacitor. It may therefore happen that the switch, depending on the capacity of the first capacitor, is opened with such a delay that a distortion of the dialing characters takes place. This can lead to wrong connections.