Telephone signal detecting circuit

#1. Field of the Invention

#This invention relates to a telephone signal detecting circuit, which can distinguish a line reversal signal and a ring signal.

#2. Description of Prior Art

#The following detailed description, given by way of example and not intended to limit the invention solely to the embodiments described herein, will best be understood in conjunction with the accompanying drawings in which:

#FIG. 1 is a timing diagram of call ID signal in a communication standard;

#FIG. 2 is a diagram illustrating the detection circuit for detecting the line reversal signal and the ring signal in a conventional telephone set;

#FIG. 5 illustrates the circuit diagram for the telephone signal detecting circuit according to this invention, in which the circuit for inputting RNGDI signal is omitted to simplify the description. In the drawings, the flip-flops A, B, and C constitute a ripple detecting circuit 40, which can effectively absorb the bouncing pulse produced by activating the line reversal signal or ring signal, and can detect the ripple signal of the capacitor while ringing. This invention uses the Schmitt trigger 31 and 32 of two different trigger levels V1 and V2, wherein the trigger level V1<V2, and the trigger having the trigger level V2 is used to detect the envelope of the ring signal. The trigger having the trigger level V1 is used to detect the ripple signal produced by the charge/discharge of the capacitor while ringing. A signal, which can be used to identify the line reversal signal or the ring signal, can be obtained after the flip-flops are triggered.

#FIG. 6 illustrates the timing diagram of the above circuit, wherein RNGDI represents input signal, Q1 is a transistor, V2 SCHMITT O/P indicates the output of the Schmitt trigger having the trigger level V2, V1 SCHMITT O/P indicates the output of the Schmitt trigger having the trigger level V1, RING represents the ringing signal. In FIG. 6, the bouncing at point I is absorbed by flip-flop A, the bouncing results in a small charge/discharge for the capacitor. If the voltage change of the charge/discharge is V1, the flip-flop is triggered at point J, then the flip-flop B absorbs the bouncing of the voltage. The ripple signal, which is produced when the ring signal causes the charge/discharge of the capacitor, is received by the flip-flop C at point K. This causes the ring signal RING to be high level. The input voltage RNGDI is changed along with the input voltage RNGDI after passing through the Schmitt trigger 30. The transistor Q1 opens to discharge the capacitor C3, or is closed so that the power supply VCC can charge the capacitor C3 through the resistor R5. The Schmitt's trigger 32 having the trigger level V2 is used to detect the envelope of the ring signal, i.e., the output of the Schmitt trigger 32 becomes high level when the input voltage is lower than V2. The Schmitt trigger 31 having the trigger level V1 is used to detect the ripple signal produced by the charge/discharge of capacitor while detecting the ringing.

#In the circuit of FIG. 5, the flip-flops A, B, C of the ripple detecting circuit 40 can also be negative flip-flops to obtain an identical result. The ripple detecting circuit 40, which can be a counter having the functions of Enable and Clear, can determine a ring signal only if the counter can count the pulse signal output by the Schmitt trigger 31 having the trigger level V1 corresponding to the ripple signal produced by the charge/discharge of the capacitor by the ring signal.