Ringing signal detection circuit

A ringing signal detection circuit for a subscriber's apparatus which is connected to a telephone office line together with an attached telephone set. An incoming signal on the telephone line is converted to a pulse signal with a repetition frequency twice the incoming signal by a full-wave rectifier and a differentiating circuit. The pulse signal is applied to a retriggerable mono-stable multivibrator through a photo-coupler and an inverter. The time period of a metastable state of the multivibrator by one trigger pulse is selected to be longer than the break period in the dial pulse signal but longer than a half cycle of the ringing signal. Therefore, the multivibrator is maintained at the high level for a longer time period at a time when the incoming signal is the ringing signal than the time when the incoming signal is the dial pulse signal. Therefore, the ringing signal can be detected from the time period when the high level signal is continuously maintained and distinctly distinguished from the dial pulse signal. The time period of the high level signal can be detected by the use of a clock pulse generator, an AND gate to which the high level signal and the clock pulse are applied, and a pulse counter connected to the output of the AND gate. The time period is, thus, represented by the number counted in the counter.

BACKGROUND OF THE INVENTION 
This invention relates to ringing signal detection circuits for 
subscriber's apparatus each being used together with a telephone set for 
an office line. 
In the present specification, the terms of the subscriber's apparatus 
includes a facsimile device, a telephone automatic answering device, a 
key-telephone-system control unit, a direct connection telephone set and 
all other apparatus or devices which has ringing signal detection circuits 
and which are used together with telephone sets to common office lines. 
In those apparatus, when a dialling device is operated at the attached 
telephone set for calling another party, the ringing signal detection 
circuit of the subscriber's apparatus and the attached telephone set are 
connected to a common office line. Therefore, the dial pulse signal sent 
out from the attached telephone set is also applied to the ringing signal 
detection circuit of the apparatus. 
For example, the key-telephone-system control unit has a ringing signal 
detection circuit which is connected to an office line. When a ringing 
signal is detected on the office line, the control unit operates to drive 
a bell, indicator lamps and other devices for noticing the call. When one 
of key telephone sets in the system is connected to the office line for 
calling another subscriber, the telephone set is also connected to the 
ringing signal detection circuit. Therefore, a dial pulse signal sent out 
from the telephone set to the office line is also applied to the ringing 
signal detection circuit. 
A facsimile device is also connected to a telephone office line together 
with a telephone set. In reception of a call by another party, the 
apparatus detects the ringing signal incoming on the telephone line and 
automatically responds to the call. When calling another subscriber's 
apparatus, the subscriber operates a dialling device of an attached 
telephone set to send out the dial pulse signal to the telephone office 
line. Then, the dial pulse signal is also applied to the apparatus 
connected to the telephone office line together with the telephone. 
In order to prevent such a subscriber's apparatus to erroneously operate in 
response to the dial pulse signal sent out from the attached telephone set 
connected to the office line together with the subscriber's apparatus, the 
ringing signal detection circuit of the subscriber's apparatus must be 
provided with a function for distinctly distinguishing the ringing signal 
from the dial pulse signal. 
In many public telephone systems, a sinusoidal AC signal is used for the 
ringing signal. The sinusoidal ringing signal has usually a frequency of 
16 Hz and a voltage of, typically, 65 volts (V). The ringing signal is 
sent out for a time period of one second and thereafter pauses for a time 
period of two or three seconds, and the sending for one second and the 
pause for two seconds are repeated. 
While, the dial pulse signal is generated by switching on and off the 
office telephone line, to which a DC voltage of 48 volts (V) is usually 
applied, by operation of the dialling device of the telephone set. The 
repetition frequency of the dial pulse signal is usually 10 Hz. 
As described above, since the ringing signal and the dial pulse signal are 
different in voltage level, a known ringing signal detection circuit is 
made to detect the voltage of the ringing signal. 
However, when the dial pulse signal arrives at a telephone office through 
the telephone office line, an electromotive force is induced due to an 
inductance of the exchanging unit in the telephone office. The induced 
voltage signal is superposed on the dial pulse signal and is applied to 
the calling party's subscriber's apparatus. Since the voltage of the dial 
pulse signal on which the induced voltage signal is superposed is nearly 
equal to the voltage of the ringing signal, the detection circuit of the 
calling party's subscriber's apparatus may erroneously detect a ringing 
signal. 
In another known method, a capacitor is used in the detection circuit so as 
to be charged by the incoming signal. Since the ringing signal and the 
dial pulse signal are different from one another in the voltage and the 
frequency, capacitor voltages charged by respective signals. Therefore, 
the ringing signal is detected and distinguished from the dial pulse 
signal by the capacitor voltage as charged. However, since the capacitor 
is not always charged to a constant level by ringing signals and since the 
capacitor voltage charged by the ringing signal is often nearly equal to 
the capacitor voltage charged by the dial pulse signal due to the 
above-described induced voltage signal, it is quite difficult to 
predetermine a capacitor voltage level for identifying the ringing signal. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a ringing signal detection 
circuit for a subscriber's apparatus connected to an office line together 
with a telephone set wherein the ringing signal can be detected and 
distinctly distinguished from a dial pulse signal sent out from the 
telephone set. 
It is another object of this invention to provide such a ringing signal 
detection circuit which is simple in circuit formation and realizes the 
above-mentioned object. 
According to this invention, a ringing signal detection circuit for a 
subscriber's apparatus connected to a telephone office line together with 
a telephone set having a rotary dialling device for sending out a dial 
pulse signal is obtained wherein the incoming ringing signal is detected 
and is reliably distinctly distinguished from the dial pulse signal from 
the attached telephone set. 
The ringing signal detection circuit comprises first means for receiving an 
incoming signal on the telephone office line and producing a pulse signal 
with a repetition frequency twice the incoming signal. Second means is 
coupled to an output of the first means and produces a high level signal 
at an output thereof in response to each pulse form the first means. The 
second means maintains the high level signal continuously during a time 
period when pulses are applied thereto with a pulse interval smaller than 
a predetermined break period in the dial pulse signal. Third means is 
connected to the output of the second means and detects a time period 
during which the output of the second means is maintained at the high 
level. The third means provides a ringing signal detection output at a 
time when the detected time period is equal to a time period predetermined 
in relation to the ringing signal. 
The third means can comprise a clock pulse generator means for generating 
clock pulses at a predetermined repetition frequency higher than the dial 
pulse signal, and fourth means coupled with the output of the second means 
and the clock pulse generator means. The fourth means permits the clock 
pulses to pass therethrough a time period when the output signal from the 
second means is maintained at the high level. Counter means is connected 
to the output of the fourth means and counts clock pulses passing through 
the fourth means during a fixed time period, or a time period when the 
output of the second means is maintained at the high level. The ringing 
signal is detected and is distinctly distinguished from the dial pulse 
signal in dependence of the difference of numbers counted at the counter 
means in response to inputs of respective signals. 
Further objects, features and other aspects of this invention will be 
understood from the following detailed description of preferred 
embodiments referring to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a subscriber's apparatus 1 such as a facsimile device 
is connected to an office line l together with a telephone set 2. The 
apparatus 1 includes a ringing signal detection circuit, a load impedance 
3 such as a resistor connected to office line and other functional 
portion. The functional portion is not shown because the present invention 
relates not to the functional portion but to the ringing signal detection 
circuit. 
The ringing signal detection circuit comprises a DC-cut capacitor 11 and a 
rectifier circuit 12 having four diodes, which are connected in the 
connecting circuit between the office line l and load impedance 3. A light 
emission diode portion 13 of a photocoupler is also connected in the 
circuit connecting rectifier circuit 12 and data input and output circuit 
3. An output of a photo-transistor portion 14 of the photo-coupler is 
connected to an inverter 15, an output of which is coupled with a 
retriggerable mono-stable multivibrator 16 having a predetermined 
metastable time period. As well known in the art, the retriggerable 
mono-stable multivibrator is a type wherein when an input pulse is applied 
to the multivibrator which is in its mestastable condition, the 
multivibrator is retriggered to start a new metastable state. The 
metastable state is selected to be longer than a half cycle of the ringing 
signal but shorter than a break period in the dial pulse signal. An output 
of multivibrator 16 is applied to one of the input terminals of an AND 
gate 18, while clock pulses from a clock pulse oscillator 17 are applied 
to the other input terminal of AND gate 18. 
Operation of the ringing signal detection circuit will be described 
hereinafter, referring to FIGS. 2 and 3 in addition to FIG. 1. 
When the ringing signal as shown at A in FIG. 2 comes into the apparatus 1 
through office line l, the signal is full-wave rectified at rectifier 
circuit 12 after passing capacitor 11, so that the full-wave rectified 
current flows through data input and output circuit 3 and light emission 
diode 13. The full-wave rectified current as shown at B in FIG. 2. 
The ringing signal is periodically sent out with intervals. The sending 
time T.sub.1 is usually one second while the interval T.sub.2 three 
seconds, as described above. Although the frequency of the ringing signal 
is usually 16 Hz, three-cycle wave is drawn for simplification of the 
drawings. 
Light emission diode 13 is luminescent by the current flowing therethrough, 
and photo-transistor 14 turns on when the light from light emission diode 
13 exceeds a threshold level of the photo-transistor. Accordingly, pulse 
signal as shown at C in FIG. 2 presents at the output of photo-transistor 
14 
It will be readily understood that the repetition frequency of the pulse 
signal is twice the frequency of the ringing signal because light emission 
diode 13 is driven by the full-wave rectified current. 
The pulse signal is phase-inverted at inverter 15, and the phase-inverted 
signal, as shown at D in FIG. 2 is applied to retriggerable mono-stable 
multivibrator 16. 
As described above, multivibrator 16 is a retriggerable one and its 
metastable time period is selected to be longer than a half cycle of the 
ringing signal but shorter than a break period of the dial pulse signal 
(that is a time interval between adjacent two pulses in the dial pulse 
signal). Therefore, multivibrator 16 is triggered by a first pulse from 
inverter 15 and is sequentially retriggered by subsequent pulses as shown 
at D in FIG. 2. When no pulse is inputted to multivibrator 16 over a time 
period longer than the metastable state as designed, the multivibrator 
returns to its stable state. Accordingly, the output of multivibrator 16 
is maintained at a high level during a time period of T.sub.3, which is a 
fixed period corresponding to the ringing signal sending time period 
T.sub.1. 
Since multivibrator 16 is maintained at the metastable state for a time 
period after triggered by an input pulse as described above, the time 
period T.sub.3 is slightly longer than T.sub.1. 
During a time interval T.sub.2 of each pause of the ringing signal, no 
pulse is not applied to multivibrator 16, so that the output of the 
multivibrator 16 is maintained at a low level during a time period T.sub.4 
corresponding to the time interval T.sub.2. 
Referring to FIG. 3, when a dial pulse signal as shown at A is applied to 
the ringing signal circuit in FIG. 1, each pulse is differentiated by 
capacitor 11 and load impedance 3 and the differentiated signal is 
rectified by full-wave rectifier circuit 12. Therefore, current pulses as 
shown at B in FIG. 3 flow through light emission diode 13. 
In connection with the current pulses, it will be understood that two 
pulses are produced in response to a dial pulse. The two pulses are spaced 
from one another by a time period t.sub.2 corresponding to the pulse width 
t.sub.1 of a dial pulse. When the dial pulse signal includes a plurality 
of pulses, for example, 10 pulses in response to dialling operation of a 
number "0", a time interval t.sub.3 between adjacent two dial pulses, or 
the break period, is constant. The dial pulse signal is determined to have 
usually a repetition frequency of 10 Hz as described above. Therefore, a 
time interval t.sub.4 between a pair pulse generated from a dial pulse and 
another pair pulse generated from a subsequent dial pulse is corresponding 
to, and equal to, the time interval T.sub.3. 
The pulse signal at B in FIG. 3 flows through light emission diode 13, and 
therefore, a corresponding pulse signal is obtained at a collector of 
photo-transistor 14, as shown at C in FIG. 3. The signal is phase inverted 
at inverter 15 to a signal as shown at D in FIG. 3, and is applied to 
multivibrator 16. 
Multivibrator 16 is repeatedly triggered by input pulses of D in FIG. 3. 
However, since the metastable time period of multivibrator 16 is longer 
than a half cycle of the ringing signal but shorter than a break period 
t.sub.3, no trigger pulse is applied to multivibrator 16 which is still in 
the metastable condition after it is triggered by two pulses generated by 
a dial pulse. Therefore, the output of multivibrator 16 is maintained at 
high level during a time period t.sub.5 corresponding to the pulse width 
t.sub.1 of one dial pulse and is at low level during a time period t.sub.6 
corresponding to the break period t.sub.3 of the dial pulse signal. The 
high level period t.sub.5 is longer than t.sub.1 and the low level period 
is shorter than t.sub.3 because of the metastable time period of the 
multivibrator. 
Comparing the time period T.sub.3 in FIG. 2 and the time period t.sub.5, 
T.sub.3 is quite longer than t.sub.5 because T.sub.3 is nearly equal to 
the sending time period (usually one second) of the ringing signal while 
t.sub.5 being nearly equal to the pulse width (t.sub.1) of one dial pulse. 
Therefore, the ringing signal can be detected and distinctly distinguished 
from the dial pulse signal by utilizing the time difference between 
T.sub.3 and t.sub.5. 
In order to detect the time period of the high level output of 
multivibrator 16, clock pulse generator 17 and AND gate 18 are used. 
The output signal of multivibrator 16 is applied to one input of AND gate 
18 to open the AND gate by the high level signal. While, clock pulse 
signal, as shown at F in FIG. 2 and in FIG. 3, is also applied to the 
other input of AND gate 18, so that clock pulses are outputted from AND 
gate 18 during a time period when AND gate 18 is open. This means that the 
high level period of the output signal from the multivibrator 16 is 
represented by number of clock pulses passing through AND gate 18. 
The pulse signal obtained from AND gate 18 during each period T.sub.3 and 
another pulse signal obtained during each period t.sub.5 are shown at G in 
FIG. 2 and FIG. 3, respectively. 
The number of pulses obtained during the time period T.sub.3 is larger than 
that during the timer period t.sub.5. Therefore, the ringing signal can be 
detected and distinctly distinguished from the dial pulse signal by 
employing a pulse counter at the output of the AND gate 18. 
Referring to FIG. 4, a pulse counter 19 is connected to the output of AND 
gate 18. Two pulse forming circuits 20 and 21 are connected to the output 
of multivibrator 16. Circuit 20 is for producing a pulse corresponding to 
the rising end of the high level signal from multivibrator 16, and the 
pulse is applied to counter 19 as a start pulse. Circuit 21 generates a 
reset pulse for counter 19 in correspondence to the lowering end of the 
high level signal from multivibrator. Thus, counter 19 counts up pulses 
applied thereto from AND gate 18 during a time period from the start pulse 
to the reset pulse. 
The output signal of multivibrator 16, the start and reset pulses and 
pulses counted up at counter 19 are shown at A, B and C, respectively, in 
FIG. 5. 
When the incoming signal to the ringing signal detection circuit of FIG. 1 
is the ringing signal, the time period of the high level signal continued 
from multivibrator 16 is T.sub.3 which is a constant time period, as 
described above in connection with FIG. 2. Therefore, the number counted 
at counter 19 from the start pulse to the reset pulse is the number M of 
clock pulses passing through AND gate 18 during the time period T.sub.3 
and is constant. 
While the dial pulse signal is applied to the circuit of FIG. 1, 
multivibrator 16 maintains a high level signal for a time period t.sub.5 
in response to one dial pulse, as described above in connection with FIG. 
3. The time period t.sub.5 is quite shorter than the time period T.sub.3. 
Accordingly, the pulse number N counted at counter 19 during the time 
period t.sub.5 is smaller than the number M. 
Therefore, if the counter 19 is made to an output signal at a time when the 
count number is M, the output signal represents that the ringing signal is 
inputted to the subscriber's apparatus 1 of FIG. 1. 
Referring to FIG. 6, according to another arrangement, a similar counter 
19' is connected to the output of AND gate 18. The counter 19' is 
controlled by a timer 22 to operate during a fixed time period. The timer 
22 is connected to the output of multivibrator 16 to be started by the 
rising end of the high level signal. If the operating time is set equal to 
the above-described time period T.sub.3, M pulses are counted up at 
counter 19' during the operating time T.sub.3 of timer 22 at a time when 
the ringing signal is applied to the subscriber's apparatus. While, when 
the dial pulse signal is applied to the subscriber's apparatus, the number 
of pulses counted up at counter 19 during the timer operating time T.sub.3 
is smaller than M, because clock pulses do not pass through AND gate 18 
during the time interval t.sub.6 (in FIG. 3) corresponding to each break 
period t.sub.3 in the dial pulse signal. 
Therefore, if counter 19' is made to produce an output signal at a time 
when the count number is M, the ringing signal detection signal is 
obtained from the counter 19'. 
It will be understood that the timer operating timer can be set shorter 
than T.sub.3 but longer than one period (t.sub.1 +t.sub.3) of the dial 
pulse signal. According to this, the count number at a time when the 
ringing signal detection signal is outputted is changed. 
As to the repetition frequency of the clock pulses, since at least one 
clock pulse must be present during a low level duration t.sub.6 (FIG. 3) 
of the output signal of multivibrator 16 according to a break period 
t.sub.3 of the dial pulse signal, it is selected to be sufficiently higher 
than the dial pulse repetition frequency.