Telephonic communications – Subscriber line or transmission line interface – Power supply
Reexamination Certificate
1999-05-26
2004-06-01
Harvey, Minsun Oh (Department: 2644)
Telephonic communications
Subscriber line or transmission line interface
Power supply
C379S146000
Reexamination Certificate
active
06744889
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a subscriber circuit which accommodates a public (coin) telephone set, and more particularly to a technique for moderating, when a public telephone set accepts a coin, the influence of the acceptance of the coil coin upon a voice signal.
2. Description of the Related Art
A subscriber circuit which accommodates a public telephone set reverses, as a signal when it accepts a coin thrown in by a call originating person in accordance with a telephone rate, the polarity of loop current slowly upon acceptance of the coin and then reverses, after the coin is confirmed, the polarity of loop current slowly again. Since a variation in voltage between two subscriber lines then is superposed with a voice signal which is transmitted similarly as a variation in voltage between the two subscriber lines, the system must be designed so that the voltage variation as a signal of acceptance of a coin may not disturb transmission of the voice signal.
FIG. 1
shows a basic construction of a conventional subscriber circuit. Referring to
FIG. 1
, the frequency band usually handled as a voice signal on a telephone circuit ranges from 300 Hz to 3,400 Hz, and a voltage variation of a frequency lower than 300 Hz which appears between subscriber lines
1
and
2
is suppressed principally by a channel filter
6
.
However, usually a high-pass filter
5
for removing a high dc voltage applied to the subscriber lines to supply loop current is placed between a line voltage detection circuit
4
for detecting a voltage variation between the subscriber lines
1
and
2
and the channel filter
6
. Here, designing for preventing such slow reversal of the polarity of loop current which is preformed by a loop current supply circuit
3
is performed, for example, in such a manner as described below.
The slowness of reversal of the polarity of loop current is first set linearly so that components of the voice band may be reduced to such a magnitude that they cannot be sensed as noise by a speaker. Although a slow voltage variation by such setting is attenuated by the high-pass filter
5
, since the high-pass filter
5
is designed such that it does not have an overlapping function with the channel filter
6
and is not complicated, the attenuation amount is limited, and a slow voltage variation remains with the output of the high-pass filter
5
. The sensitivity of the line voltage detection circuit
4
is set so that a sum voltage variation of the slow voltage variation and the voice signal may not exceed the range of linear operation of the channel filter
6
.
Therefore, the voice signal introduced to the channel filter
6
decreases by an amount corresponding to the slow voltage variation remaining with the output of the high-pass filter
5
. The voice signal is required to be amplified to a predetermined level by the channel filter
6
, and noise generated by the line voltage detection circuit
4
and the high-pass filter
5
and input equivalent noise generated by the channel filter
6
must be low as much. Accordingly, with such a designing example as described above, various severe requirements for a noise characteristic must be satisfied.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a subscriber circuit for a public telephone set which eliminates the necessity to make a voice signal to be introduced to a channel filter a small signal.
In order to attain the object described above, according to the present invention, a slow voltage variation which remains with an output of a high-pass filter is estimated to produce an estimation signal, and the estimation signal is subtracted from an input signal to a channel filter.
In particular, according to the present invention, there is provided a subscriber circuit for a public telephone set, comprising a loop current supply circuit for supplying loop current between two subscriber lines, a line voltage detection circuit for detecting a voltage variation between the two subscriber lines, a high-pass filter for principally removing a high dc voltage from an output signal of the line voltage detection circuit, a channel filter for setting a voice signal band, variation signal generation means for generating a variation signal corresponding to a voltage variation applied between the two subscriber lines upon and caused by acceptance of a coin, and subtraction means for subtracting the variation signal from a signal of the voltage variation between the two subscriber lines detected by the line voltage detection circuit.
The variation signal generation means may include a voltage holding circuit for holding a voltage detected by the line voltage detection circuit prior to a reversal of the polarity of loop current, a reversal circuit for reversing the polarity of the voltage held by the voltage holding circuit, and a low-pass filter for buffering the voltage of the reversed polarity from the reversal circuit with a characteristic same as that of a low-pass filter which buffers the polarity reversal in the loop current supply circuit.
As an alternative, the variation signal generation means may include a voltage holding circuit for holding a voltage detected by the line voltage detection circuit prior to a reversal of the polarity of loop current, a reversal circuit for reversing the polarity of the voltage held by the voltage holding circuit, a high-pass filter for attenuating the voltage of the reversed polarity from the reversal circuit with a characteristic same as that of a high-pass filter for removing a high dc voltage, and a low-pass filter for buffering the output voltage of the high-pass filter with a characteristic same as that of a low-pass filter which buffers the polarity reversal in the loop current supply circuit.
As another alternative, the variation signal generation means may include a voltage holding circuit for holding a voltage detected by the line voltage detection circuit prior to a reversal of the polarity of loop current, a buffer amplifier to which the voltage held by the voltage holding circuit is inputted, a reversal amplifier for reversing the polarity of an output of the buffer amplifier, a pair of switches for extracting the outputs of the buffer amplifier and the reversal amplifier in response to a signal which indicates reversal of the polarity of loop current and another signal representing acceptance of a coin, respectively, and a low-pass filter for buffering the voltage extracted by the switches with a characteristic same as that of a low-pass filter which buffers the polarity reversal in the loop current supply circuit.
As a further alternative, the variation signal generation means may include a voltage holding circuit for holding a voltage detected by the line voltage detection circuit prior to a reversal of the polarity of loop current, a buffer amplifier to which the voltage held by the voltage holding circuit is inputted, a reversal amplifier for reversing the polarity of an output of the buffer amplifier, a pair of switches for extracting the outputs of the buffer amplifier and the reversal amplifier in response to a signal which indicates reversal of the polarity of loop current and another signal representing acceptance of a coin, respectively, a high-pass filter for attenuating the voltage extracted by one of the switches with a characteristic same as that of a high-pass filter for removing a high dc voltage, and a low-pass filter for buffering the output voltage of the high-pass filter with a characteristic same as that of a low-pass filter which buffers the polarity reversal in the loop current supply circuit.
In the subscriber circuit for a public telephone set, a signal which is equivalent to a voltage variation between the two subscriber lines upon acceptance of a coin from which a voice signal is removed is produced, and the signal is subtracted from a voltage detected from the two subscriber lines to extract only the voice signal from the voltage variation between the two subscriber lines. Consequently,
Foley & Lardner LLP
Harvey Minsun Oh
NEC Corporation
Swerdlow Daniel
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