Telephonic communications – Substation or terminal circuitry – For loudspeaking terminal
Patent
1988-06-21
1989-08-08
Dwyer, James L.
Telephonic communications
Substation or terminal circuitry
For loudspeaking terminal
379413, H04M 160
Patent
active
048560570
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to an apparatus in a loudspeaking telephone set for supplying power to an amplifier connected to the loudspeaker, said telephone set being DC fed by a line current via its subscriber line and provided with a speech circuit having electronic semiconductor circuits, the receiver and microphone of the set being connected to the speech circuit, which also has a voltage sensing circuit connected to the subscriber line for sensing a line voltage across it, as well as an amplifier means connected via a first impedance to the subscriber line for maintaining a current through the first impedance at a constant value responsive to the line voltage.
BACKGROUND ART
In the transmission of speech signals in a telephone network between an exchange and a telephone set, it is a known problem that a varying line current causes distorsion of the transmitted speech signal. The line current, which is a direct current, is therefore kept at a constant level by electrical circuits included in the speech circuit of the telephone set, e.g. as illustrated in an article "Alla telefonens functioner i ett chip, PL3780" by Bengt Berg in "Ericsson Review", No 3, 1985, pp 138-143. For the transmission of speech signals over long telephone lines with low line current there have been developed feed circuits, e.g. for the receiver in a telephone set, which enable the speech signal to be transmitted without distortion. Such a circuit has been described, e.g. in IEEE, Vol. CE-28, No 3, 22. August 1982, M. Yoshitoshi et al, "All-electric Telephone Set with Low-Voltage Operating Circuit and Piezoelectric Transducers", and in the U.S. Pat. No. 4,319,094 cited in this article. According to this patent, a constant portion of the line current is taken off to charge a constant voltage circuit. The energy from this circuit is utilized when the speech signal has a large amplitude and can be distorted due to the line voltage being low. The apparatus is relatively complicated and has the drawback that it continuously takes off the contant portion of the line current, ever when the constant voltage circuit is fully charged.
DISCLOSURE OF THE INVENTION
The above-mentioned drawbacks are avoided with an apparatus according to the invention, that utilizes existing circuits in a telephone set and takes the line current to a reservoir capacitor only during short time intervals. The apparatus is distinguished by the disclosures of the accompanying claims.
BRIEF DESCRIPTION OF DRAWINGS
An example of an embodiment of the invention will now be described in more detail below, and with reference to a drawing, on which FIG. 1 schematically illustrates a part of a telephone system, FIG. 2 shows a graph of line current and voltage for this telephone system and FIG. 3 is a part of a circuit diagram for a telephone set.
BEST MODE FOR CARRYING OUT THE INVENTION
In FIG. 1 there is schematically illustrated a telephone exchange 1 and a subscriber telephone 2, which has line connections a and b for a subscriber line 3, connecting the set 2 to the exchange 1. During a call the set 2 is powered by DC line current IL and a line voltage U is maintained between the line connections a and b. The relationship between line current IL and voltage U is illustrated in the graph of FIG. 2. Different telephone administrations have requirements that the line voltage U and associated current IL shall lie within a given area on the graph, e.g. that bounded by two lines A and B. These requirements are set up to ensure that different functions in the exchange 1 can be performed with great reliability. In a given telephone system there is selected a well-defined relationship between line voltage U and current IL, e.g. according to the curve C in FIG. 2. For a given subscriber, where the subscriber line 3 has a definite line resistance RL, the line current has a value ILO and the line voltage a corresponding value UO, as indicated in the Figure. It is essential for the sound quality in speech transmission that the constant value of the line curre
REFERENCES:
patent: 4319094 (1982-03-01), Naganawa
patent: 4376876 (1983-03-01), Versteeg et al.
"All Telephone Functions in One Chip, PBL3780" by Bengt Berg in Ericsson Review, No. 3, 1985.
"All-Electronic Telephone Set With Low-Voltage Operating Circuit and Piezoelectric Transducers" by M. Yoshitoshi, Y. Ichinose and T. Naganawa.
Berg Bengt O.
Snith Lars A.
Dwyer James L.
Telefonaktiebolaget L M Ericsson
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