1. Field of the Invention
The present invention relates to a circuit arrangement for testing the operability of a data transmission facility connected, on the one hand, to a data signal output line and to a data signal receiving line and, on the other hand, to a transmission line usable for a full-duplex data signal transmission, the data transmission facility comprising an echo compensation circuit between the data signal output line and the data signal receiving line with whose assistance the signal components proceeding onto the data signal receiving line in the data signal transmission from the data signal output line onto the transmission line may be compensated, whereby the data signal output line and the data signal receiving line, as well as the transmission line, are connected to one another via a hybrid circuit.
2. Description of the Prior Art
Data transmission facilities of the aforementioned type are already generally known for example from "IEEE Transactions on Acoustics, Speech, and Signal Processing", Vol. ASSP-27, No. 6, December 1979, pp. 768-781, particularly FIG. 2, from U.S. Pat. No. 4,355,214, and from U.S. Pat. No. 4,355,406, all of which are fully incorporated herein by this reference. Such data transmission facilities, however, cannot be checked for operability without further techniques when only the data signal output line and the data signal receiving line are available for test purposes. When, in particular, a data signal to be referred to as a test word is emitted via the data output line to the respective transmission facility, then the signal components of this test word which proceed onto the data signal receiving line are compensated by the echo compensation circuit. The echo compensation circuit could, in fact, be disabled in order to in turn accept the test word via the data signal receiving line, namely the test word that had been previously output via the data signal output line. Connected therewith, however, would be the disadvantage that the echo compensation circuit would not be involved in the testing procedure.