The transfer condition from the on-hook state to the off-hook state of a terminal connected to the telephone line while the terminal is called is detected by the ring trip detecting circuit by connecting in series a ring trip circuit detecting resistor to a ringer generator. The voltage change generated by a change of current flowing into the ring trip detecting resistor during transfer between the on-hook state and the off-hook state depending on the response from a terminal is then detected.
However, a current value of the ring trip resistor and a rate of change of such current between the on-hook state and off-hook state are different depending on the condition in a side of load, such as the line length of telephone line and the number of terminals connected. Therefore, the accurate detection of the on-hook and off-hook conditions has been very difficult.
FIG. 1 is an explanatory diagram of a ring trip deciding method.
In FIG. 1, element 10 is a subscriber circuit; element 11 is an equivalent circuit of a telephone set forming a series resonant circuit; element 12 is a ring trip detecting resistance (RS) for detecting a change of current generated depending on the transfer condition between the on-hook state and off-hook state of the telephone set; element 13 is a ring trip detector; element 14 is a ringer generator for generating a low frequency AC voltage for ring trip decision. The letter B indicates a switch that opens for the on-hook condition and closes for the off-hook condition.
Operations of the ring trip deciding method shown in FIG. 1 will be explained hereunder.
The ringer generator 14 outputs an AC voltage superposed on a low frequency (about 20 Hz) DC voltage of -48 V. A current flowing into the ring trip detecting resistance (RS) 12 is detected by a detector 13 as an interterminal voltage, and the detected voltage is outputted to the output point A.
In the on-hook state, a low level AC voltage is outputted from the output point A since a DC element is zero.
In the off-hook state, the switch B in the subsrcriber circuit 10 closes, applying a DC element to the circuit and increasing a current flowing into the ring trip detecting resistance (RS) 12 and an outputting an AC voltage in which an AC element is superposed on the DC element from the output point A.
The on-hook state and off-hook state can be decided by detecting the voltage at the point A, which is different depending on these conditions.
FIGS. 2A and 2B show an example of a conventional ring trip deciding circuit. FIG. 2A shows a ring trip deciding circuit and FIG. 2B shows an output voltage waveform at the point A of the detector 14. In FIG. 2A, element 11 is a ringer generator; element 12 is a ring trip detecting resistance (RS); element 13 is a ring trip circuit for deciding the on-hook and off-hook states; element 14 is a detector for detecting and outputting a voltage change across the ring trip detecting resistance (RS) 12 generated depending on the on-hook state and off-hook state; element 15 is a detecting circuit for detecting a voltage across the ring trip detecting resistance (RE) 12; element 16 is a filter/integrating circuit consisting of R, C circuit for eliminating an AC element of the voltage detected by the detecting circuit 15 and integrating the signal; element 17 is a decision circuit for deciding the on-hook state and off-hook state depending on an output voltage of the detector 14.
Operations of the structure of FIG. 2A are then explained hereunder.
The detecting circuit 15 detects a low frequency AC voltage generated across the ring trip detecting resistance (RS) 12. The filter/integrating circuit 16 removes an AC element and integrates the signal to output the integrated signal.
In the structure of the conventional converter 14, the filter/integrating circuit 16 cannot sufficiently remove the AC element and provides an output at the point A having the voltage waveform shown in FIG. 2B. In the filter/integrating circuit 16, since an AC element from the ringer generator 11 cannot be removed perfectly, the low frequency AC voltage not including the DC element is outputted under the on-hook state, while the low frequency AC voltage in which an AC element is superposed on the DC element is outputted under the off-hook state. The deciding circuit 17 discriminates the on-hook state and off-hook state by detecting the output voltages at the point A including different DC elements.
FIGS. 3A-3D are explanatory diagrams for explaining a problem to be solved by the present invention. FIG. 3A schematically illustrates that a plurality of telephone sets are connected to a short line length. FIG. 3B shows an output voltage at the point A in FIG. 3A. FIG. 3C schematically illustrates that only one telephone set is connected to a long line length. FIG. 3D shows an output voltage at the point A in FIG. 3C.
In FIG. 3A, the point A is an output point of the detector for detecting the on-hook and off-hook states of a subscriber terminal (a telephone set, in this case) when the subscriber circuit is called. Elements 30 to 32 are telephone sets; element 33 is a subscriber circuit. FIG. 3B shows an output voltage at the point A of FIG. 3A.
In FIG. 3B, 1 indicates the on-hook period and, 2 indicates the off-hook period. In the case of FIG. 3A, a difference of DC voltage elements under the on-hook and off-hook states and an AC element voltage AO under the on-hook state are both in high level.
In FIG. 3C, element 34 is a telephone set; element 35 is a subscriber circuit; elements 36 and 37 are resistance elements of lines. The point A is an output point of the detector for detecting the on-hook and off-hook states of the subscriber terminal, while the subscribed circuit 35 is called.
In FIG. 3D, 3 indicates the on-hook period and 4 indicates the off-hook state.
In the FIG. 3C, a difference of DC elements under the on-hook and off-hook states and an AC element voltage AO under the off-hook period are both in low level. A voltage amplitude AO of an AC element under the on-hook state is large in the case of FIG. 3A, and the voltage amplitudes AO of the DC element and the AC element under the off-hook state are small in the case of FIG. 3C.
In the prior art, it has been difficult to discriminate the on-hook state and off-hook state in FIG. 3D (discrimination between 1 and 4) by directly comparing the voltage values with the reference value.