Telephonic communications – Subscriber line or transmission line interface – Network interface device
Patent
1988-02-18
1989-08-01
Levy, Stewart J.
Telephonic communications
Subscriber line or transmission line interface
Network interface device
379399, H04M 1900
Patent
active
048539597
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention comprises a method of feeding power to a telephone line such that for restricted battery voltage there is enabled the transmission of an undistorted speech signal, a line voltage between the connections of the telephone line being sensed, a first current corresponding to the line voltage generated, a first reference current generated and a first difference current generated, which answers to the difference between the first reference current and the first current, and an apparaus for carrying out the method.
BACKGROUND ART
In telephone networks with resistively powered telephone lines, the line current is determined by the pole voltage of a feed battery, the line resistance and the resistance of feed resistors through which the line is connected to the feed battery. The strength of the line current and also that of the transmitted speech signal will be lesser the longer the telephone line is. The telephone sets connected to the network sense the line current and amplify the speech signal so that the sound apprehended by the subscriber obtains the desired strength irrespective of the line length. In feeding power to telephone lines from electronic line circuits, the voltage characteristic of the resistively fed telephone network is imitated. An example of this kind of electronic line circuit is described in "Ericsson Review", No 4 1983, on pages 192-200, in the article "Line Circuit Component SLIC for AXE10", by A. Rydin and J. Sundvall. It is desirable that the power losses in the line circuits will be small, and low battery voltages are therefore used. The result of this is, however, that only short telephone lines can be powered as described in the reference. To avoid this problem, the electronic line circuit may be equipped with a means for restricting the line current so that a weakened speech signal can always be transmitted on a long telephone line. It is known here to allow this restriction to be dependent on the battery voltage, to enable transmission of a speech signal of full strength for as long lines as possible. This brings with it the disadvantage that a variation in battery voltage affects the power supply and constitutes an interference of the transmitted speech signal.
DISCLOSURE OF INVENTION
The aforementioned problems are solved in accordance with the invention by the restriction in the line current being introduced at a fixed point in the voltage characteristic of the telephone line independently of the battery voltage. The invention is characterised by the disclosures of the accompanying claims.
BRIEF DESCRIPTION OF DRAWINGS
An embodiment of the invention will be described in more detail with reference to a drawing, on which FIG. 1 schematically illustrates a known circuit for resistively powering a telephone line, FIG. 2 is a diagram of a voltage characterisitic for this line, FIG. 3 is a block diagram of a line circuit in accordance with the invention and, FIG. 4 is a circuit diagram showing this line circuit in more detail.
BEST MODE FOR CARRYING OUT THE INVENTION
A known apparatus for power supply in a telephone system, so called resistive power feed, is schematically illustrated in FIG. 1. A telephone set 1 is connected to a line circuit 3 via a line 2. The circuit has two feed resistors 4, connected to the two wires of the line 2 via respective points a and b. A battery voltage U.sub.B is connected across the resistors 4 and the connected line 2, and between the connection points a and b there are means 5 for transmitting a speech signal. The telephone set 1 and the line 2 have a total resistance, which is the line resistance R.sub.L. The battery voltage U.sub.B is voltage-divided between the resistor 4 and the line resistance R.sub.L so that a line voltage U.sub.L occurs between the connection points a and b and the line is powered by a line current I.sub.L. For an increased line length and accordingly increased line resistance R.sub.L, the line voltage U.sub.L increases, as illustrated by the diagram of FIG. 2. The line characteristic
REFERENCES:
patent: 4431874 (1984-02-01), Zobel et al.
patent: 4476350 (1984-10-01), Aull et al.
patent: 4631359 (1986-12-01), Johansson et al.
patent: 4727574 (1988-02-01), Jakab
Gray, Paul and Messerschmitt, David G., "Integrated Circuits for Telephony", Aug. 1980, pp. 1006-1007.
A Rydin et al., "Line Circuit Component SLIC for Axe 10", Ericsson Review, No. 4, 1983, pp. 192-200.
Levy Stewart J.
Simons Michele
Telefonaktiebolaget L M Ericsson
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