Alarm system

In an alarm system utilizing a bidirectional wired television system, a large number of subscribers are combined in a plurality of group units, whereby the subscribers in each group unit are assigned with specified frequencies which are common to the respective group units but are different for the respective subscribers in the group unit, and the respective group units are assigned with another specified frequencies which are different for the respective group units. The group units are sequentially polled by interrogating signals comprising said another specified frequencies and applied from a central station, whereby when any subscriber in the polled group has an abnormal condition at its location, the subscriber answers to the polling by a signal of the specified frequency assigned to it. The central station discriminates and displays the answering subscriber in accordance with the combination of the frequency of the received answer signal and the specified frequency being generated for polling at the time that the answer signal was received.

BACKGROUND OF THE INVENTION 
The present invention relates to an alarm system utilizing a bidirectional 
wired television system, such as, a bidirectional community antenna 
television system or CATV system. 
Systems for providing an alarm service to a large number of subscribers 
from a remotely located central station are known in the art, as for 
example, polling systems of the type disclosed in U.S. Pat. No. 3,765,016 
in which a plurality of subscribers are sequentially interrogated to 
answer as to the occurrence of an abnormal condition and contention 
systems of the type disclosed in U.S. Pat. No. 3,996,578 in which the 
subscribers each detecting the occurrence of an abnormal condition in its 
monitoring area sends an alarm signal to the central station, and many 
systems of these types have been put in practical use. However, a system 
which is capable of handling a relatively large number of subscribers 
inexpensively is very scarce. With the coaxial cable network of the CATV 
system, the coaxial cables are generally installed to branch off the 
community antenna to the respective subscribers. In the system disclosed 
in U.S. Pat. No. 3,765,016, the central station is connected to a 
transmission loop having a plurality of series connected subscribers and a 
normally closed line relay is connected in series with each subscriber in 
the loop, thus making it impossible to utilize the previously mentioned 
CATV network of the branched-off coaxial cables as such. Another 
disadvantage is that since the interrogating signals for polling are pulse 
code signals and since each subscriber is connected in series with the 
transmission line, the cycle time required for the polling is long so that 
if the number of subscribers is increased, the time interval from the time 
that each subscriber receives the interrogating signal until the 
subscriber receives the next interrogating signal is increased, thus 
making it impossible to make an early alarming. 
On the other hand, in the contention system of the type disclosed in U.S. 
Pat. No. 3,996,578, different frequencies are assigned to the respective 
subscribers and another different frequencies are assigned to the 
respective groups each including a plurality of the subscribers so as to 
discriminate the subscriber which has sent an alarm in accordance with the 
values of the frequencies associated therewith, thus requiring a large 
number of frequency discriminating devices, such as, demodulators, tuners 
or band-pass filters. Another disadvantage is that particularly where an 
alarm signal includes a signal indicative of the type of abnormal 
condition, such as, fire, burglary or gas leakage and the type of abnormal 
condition must be discriminated at the central station, the assignment of 
frequencies tends to become more difficult as the number of subscribers is 
increased, thus setting a limit to the maximum number of subscribers which 
would permit effective discrimination of the terminals and types of 
abnormal condition with a limited frequency band. While this disadvantage 
involved in the discrimination of the types of abnormal condition can be 
overcome by polling the subscribers for every type of abnormal condition, 
the interrogating signals for polling must also contain signal components 
indicative of the types of abnormal condition, and the types of abnormal 
condition are generally discriminated in accordance with different 
frequency component, thus further increasing the frequency band required 
for the polling interrogating signals. As a result, the discrimination of 
a large number of subscribers and a plurality of types of abnormal 
condition in accordance with different frequency signals within the 
limited transmission band of the CATV cable network, sets by itself a 
limit. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an alarm system which 
requires a comparatively small number of types of frequencies used and 
hence capable of serving a large number of terminals, can be constructed 
inexpensively and ensures a reduction in the required time for alarm. 
It is another object of the invention to provide an alarm system in which a 
large number of subscribers are combined in a plurality of group units so 
as to simultaneously interrogate all the subscribers in every group unit, 
whereby when a plurality of the subscribers in the same group unit have 
answered, their answer signals are simultaneously sent to the central 
station and the subscribers are discriminated from one another in 
accordance with the specified frequencies assigned to the subscribers, 
thus reducing the polling cycle time. 
It is still another object of the invention to provide an inexpensive alarm 
system which is well suited for use with a CATV system for the subscribers 
crowded in a small area such as a building, wherein each subscriber is 
adapted to answer by the one and only alarm signal to the polling and the 
type of the abnormal condition which has occurred at the location of the 
answering subscriber is confirmed by the person in charge who visits the 
location or by interrogating the answering subscriber through an 
interphone, thus giving the highest priority to early alarm of the 
occurrence of an abnormal condition. 
In accordance with the invention, there is thus provided an alarm system 
wherein a central station is connected to the output side of a head end of 
a coaxial cable network of a bidirectional wired television system, 
whereby the interrogating signals for polling are delivered as 
down-signals from the central station to a large number of subscribers 
through the coaxial cable network and the answer signals from the 
subscribers are received as up-signals by the central station through the 
coaxial cable network. The subscribers are combined in a plurality of 
group units, and assigned to the subscribers in each group unit are 
specified frequencies which are common to the respective group units but 
are different for the subscribers in each group unit. Another different 
specified frequencies are assigned to the respective group units, and 
consequently any subscriber in any group unit can be specified in 
accordance with the combination of the corresponding assigned frequencies. 
The central station transmits interrogating signals comprising the signal 
components of the specified frequencies assigned to the group units, and 
all the subscribers in each group unit are simultaneously polled by the 
applied interrogating signal. In other words, after all the subscribers in 
one group unit have been simultaneously polled, all the subscribers in the 
next group unit are similarly polled simultaneously. In this way, all of 
the subscribers can be simultaneously polled for every group unit to which 
they belong, and the group units may be the respective floors of a 
building or the various sections of a sectionalized building floor. 
When any subscriber detects the occurrence of an abnormal condition, such 
as, a fire, gas leakage or burglary, the subscriber transmits to the 
central station an alarm signal of the specified frequency assigned 
thereto in response to the receipt of the interrogating signal of the 
specified frequency corresponding to the group unit to which the 
subscriber belongs. When the answer by this alarm signal is received, the 
central station discriminates the specified frequency of the alarm signal 
and then the location of the subscriber which has transmitted the alarm 
signal is discriminated and displayed in accordance with the conbination 
of the discriminated specified frequency and the frequency component of 
the interrogating signal causing the answer signal. In accordance with the 
present invention, the subscribers are polled for every group unit instead 
of polling the subscribers one by one. As a result, the total number of 
times of polling is equal to the number of the group units which is 
usually much smaller than the total number of the subscribers, thus 
reducing the required polling cycle time and providing to be effective in 
ensuring early alarming. When a plurality of the subscribers in the same 
group unit simultaneously answer to the polling, the subscribers can be 
discriminated in accordance with the specified frequencies assigned to 
them.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, a signal separator 3 is disposed on the front side 
of a coaxial cable 10 which is connected through a head end 2 to a 
community antenna 1 of a CATV system, and a central station 4 is connected 
to the signal separator 3. The coaxial cable 10 is connected by way of a 
bidirectional amplifier 6 to a branching unit 5 where the coaxial cable 10 
is divided into a plurality (four in this embodiment) of coaxial cables 
10-1, 10-2, 10-3 and 10-4. As shown in the Figure only for the cable 10-1, 
each of the branch coaxial cable 10-1 to 10-4 is connected to a plurality 
of bidirectional splitters 7, and subscriber stations 9 each including a 
television receiving set 29 are connected to the splitters 7. Although 
only the single subscriber station 9 is shown in the Figure, a plurality 
of such subscriber stations are connected to each of the splitters 7, and 
consequently there is provided a CATV coaxial cable network including a 
large number of the subscriber stations which are combined in a plurality 
of the group units with each branch coaxial cable as a unit. 
To generate and transmit the necessary interrogating signals for accessing 
all of the plurality of subscribers of the branch coaxial cables 10-1 to 
10-4 group unit by group unit, namely, for sequentially accessing the 
branch coaxial cables 10-1 to 10-4 one by one, the central station 4 
includes a combination of a switching circuit 20 and an oscillator circuit 
30 adapted to sequentially generate four signal components respectively 
having specified high frequencies F.sub.1, F.sub.2, F.sub.3 and F.sub.4 
which are respectively corresponding to the branch coaxial cables 10-1 to 
10-4 and which are different from one another, and a combination of a 
shift register 19 and a clock pulse generator 21 adapted to sequentially 
apply the required control signals to the switching circuit 20. Also the 
central station 4 includes a discriminator 22 for receiving answer signals 
or alarm signals from the subscriber stations 9 to discriminate the 
location of each subscriber station which has generated the alarm signal 
and generate the corresponding output, and a display panel 23 for 
displaying the location of the subscriber station which has generated the 
alarm signal in response to the output of the discriminator 22. The 
oscillator circuit 30 comprises four high frequency oscillators 11, 12, 13 
and 14 which respectively generate signal components of the different high 
frequencies F.sub.1, F.sub.2, F.sub.3 and F.sub.4, and the switching 
circuit 20 comprises four switching devices 15, 16, 17 and 18 which are 
respectively responsive to control signals t.sub.1, t.sub.2, t.sub.3 and 
t.sub.4 to selectively supply the generated outputs of the oscillators to 
a terminal T.sub.3 of the signal separator 3. In the presennt embodiment, 
the high frequencies F.sub.1 to F.sub.4 are assigned so that F.sub.1 is 
assigned to all the subscribers connected to the branch coaxial cable 
10-1, F.sub.2 is assigned to all the subscribers connected to the cable 
10-2, F.sub.3 is assigned to all the subscribers connected to the cable 
10-3 and F.sub.4 is assigned to all the subscribers connected to the cable 
10-4. As will be described later, the four high frequency components must 
be transmitted to the terminals of the CATV system and consequently a 
frequency band which is close to that of the television signal should 
preferably be selected as the required frequency band so as to effectively 
utilize the transmission characteristic of the CATV coaxial cable network. 
For example, if the previously mentioned high frequencies F.sub.1 to 
F.sub.4 consist of the frequencies which fall within the frequency band 
(108 to 174 MH.sub.z) between the upper limit of the FM broadcasting band 
and the lower limit of the high channel VHF television broadcasting band, 
the second higher harmonic components of the F.sub.1 to F.sub.4 will also 
fall within the frequency band (216 to 470 MHz) between the upper limit of 
the high channel VHF television broadcasting band and the lower limit of 
the UHF television broadcasting band, thus preventing the occurrence of 
beat interference on the television screen due to the second higher 
harmonic components of the F.sub.1 to F.sub.4. 
Each of the switching devices 15 to 18 may for example be a transistor high 
frequency class-A amplifier circuit in which the biasing of the amplifying 
transistor is controlled in such a manner that the class-A amplifier 
circuit is biased deeper than the class-C amplifier circuit when the 
control signal (t.sub.1 to t.sub.4) is not applied, and the amplifier 
circuit performs the class-A operation when the control signal (t.sub.1 to 
t.sub.4) is applied, thus transmitting the input signals F.sub.1 to 
F.sub.4 to the signal separator 3. The shift register 19 which generates 
the previously mentioned control signals t.sub.1 to t.sub.4 is actuated by 
the clock pulse generator 21 which generates clock pulses of a 
predetermined period, whereby the shift register 19 generates sequentially 
"1" signals or control signals t.sub.1, t.sub.2, t.sub.3 and t.sub.4 
respectively at its four output terminals, that is, one at a time in 
response to each clock pulse applied, thus sequentially bringing the 
switching devices 15 to 18 into operation. The discriminator 22 and the 
display panel 23 whose constructions are shown in detail in FIG. 2, are 
designed so that at least one alarm signal or up-signal having the 
specified frequency and received from any subscriber station 9 through the 
signal separator 3, is discriminated and the AND operation is performed on 
the result of the discrimination and any of the control signals t.sub.1 to 
t.sub.4 to indicate which subscriber of which group unit has generated the 
alarm signal. 
In FIG. 2, the discriminator 22 comprises a plurality of band-pass filters 
225-1 to 225-40 for receiving the alarm signals through the signal 
separator 3 to separate the signals into frequency components f.sub.1 
through f.sub.40, A/D converters 226-1 to 226-40 for respectively 
subjecting the outputs of the band-pass filters to analog-to-digital 
conversion, and a large number of AND gates 221-1 to 221-40, 222-1 to 
222-40, 223-1 to 223-40 and 224-1 to 224-40 which are connected to the A/D 
converters 226-1 to 226-40 so that the output terminal of each A/D 
converter is connected to four of the AND gates to perform the AND 
operation on the output of the A/D converter and each of the control 
signals t.sub.1, t.sub.2, t.sub.3 and t.sub.4. In other words, it is 
assumed that each of the four group units or the branch coaxial cables 
10-1 to 10-4 includes the fourty subscriber stations 9 connected thereto 
and that the specified frequencies f.sub.1 to f.sub.40 which are common to 
all the group units are assigned to the forty subscriber stations of every 
branch coaxial cable. The AND gates 221-1 to 221-40 are each adapted to 
receive at its one input terminal the control signal t.sub.1 and at its 
other input terminal the output of the corresponding one of the A/D 
converters 226-1 to 226-40. The AND gates 222-1 to 222-40, 223-1 to 223-40 
and 224-1 to 224-40 respectively receive at their one input terminals the 
control signals t.sub.2, t.sub.3 and t.sub.4 and at their other input 
terminals the outputs of the corresponding ones of the A/D converters 
226-1 to 226-40 in the similar manner as the AND gates 221-1 to 221-40. As 
a result, when the signal F.sub.1 is transmitted to the coaxial cable 10, 
the AND gates 221-1 to 221-40 are accessed by the control signal t.sub.1 
so that when an alarm signal is applied to any band-pass filter, one of 
the AND gates 221-1 to 221-40 receives an input through the band-pass 
filter and the A/D converter corresponding to the specific frequency of 
the applied alarm signal, and this AND gate generates an output. In other 
words, the AND gates 221-1 to 221-40 serve to discriminate all the 
subscriber stations in the group unit of the branch coaxial cable 10-1, 
and similarly the AND gates 221-1 to 222-40 discriminate the subscriber 
stations connected to the branch coaxial cable 10-2, the AND gates 223-1 
to 223-40 the subscriber stations connected to the branch coaxial cable 
10-3 and the AND gates 224-1 to 224-40 the subscriber stations connected 
to the cable 10-4. 
The display panel 23 comprises holding devices 235-1 to 235-40, 236-1 to 
236-40, 237-1 to 237-40 and 238-1 to 238-40 consisting for example of 
thyristors which are triggered by the outputs of the associated AND gates 
and indicators 231-1 to 231-40, 232-1 to 232-40, 233-1 to 233-40 and 234-1 
to 234-40 which are actuated by the outputs of the associated holding 
devices, and each of the indicators is adapted to separately indicate 
which subscriber station of which group unit has generated the alarm 
signal in response to the indication of such frequency combination as 
F.sub.1 .multidot.f.sub.1 which is affixed at right side of indicators in 
FIG. 2. 
Each of the subscriber stations 9 comprises a television receiving set 29, 
an alarm signal generator 24 for generating a low frequency signal having 
the specified one of the low frequencies f.sub.1 to f.sub.40 which is 
assigned to the subscriber station, an alarm switch 25 connected to the 
output of the generator 24 and consisting for example of a manual 
transmission switch or automatic switch controlled by the action of an 
automatic sensor for sensing a fire, burglary or the like, an analogue 
gate device 26 which when actuated selectively passes the alarm signal 
applied to it through the alarm switch 25, a band-pass filter 27 which 
passes only one of the high frequency signals F.sub.1 to F.sub.4 
corresponding to the group unit to which the subscriber station belongs, 
and a detector 28 adapted to detect the output of the band-pass filter 27 
and actuate the analogue gate device 26 by the resulting detected output. 
The subscriber station 9 also comprises a coupling unit 8, whereby the 
television signal fed through the antenna is applied to the TV set 29, the 
signals F.sub.1 to F.sub.4 from the central station 4 are applied to the 
band-pass filter 27 and the alarm signal is transmitted to the coaxial 
cable through the splitter 7. 
In this connection, as for example, each of the forty subscriber stations 
connected to the branch coaxial cable 10-1 includes the band-pass filter 
27 which passes only the signal F.sub.1, and the frequencies f.sub.1 to 
f.sub.40 are respectively assigned to the output frequencies of the alarm 
signal generators 24 of the subscriber stations. Similarly, the band-pass 
filters of the subscriber stations connected to the branch coaxial cable 
10-2 are adapted to pass only the signal F.sub.2, the band-pass filters of 
the subscriber stations connected to the cable 10-3 pass only the signal 
F.sub.3, and the band-pass filters of the subscriber stations connected to 
the cable 10-4 pass only the signal F.sub.4. 
By for example using the low frequencies within the band of 1 to 4 KHz, 
separating them at a spacing of 30 to 100 Hz and assigning them as the 
previously mentioned low frequencies f.sub.1 to f.sub.40, the band-pass 
filters 225-1 to 225-40 may each be comprised of an inexpensive mechanical 
filter, such as, a tone filter employing a tuning fork. 
FIG. 3 shows an exemplary circuit construction of the signal separator 3 
connected to the coaxial cable 10 to connect it to the central station 4. 
More specifically, the signal separator 3 is so designed that a television 
signal TVS applied to a first terminal T.sub.1 from the head end 2 is 
transmitted to the coaxial cable 10 through a coupling line 10a and a 
high-pass filter 31 and through a second terminal T.sub.2, the signals 
F.sub.1 to F.sub.4 applied to a third terminal T.sub.3 from the central 
station 4 are transmitted to the coaxial cable 10 through a directional 
coupler 33 comprising an inductive coupling conductor 34 wired parallel to 
the coupling line 10a, a tapped coil 35 and a resistor 36 and the 
high-pass filter 31 and through the second terminal T.sub.2, and the alarm 
signal or signals (having one or more of the frequencies f.sub.1 to 
f.sub.40) applied to the second terminal T.sub.2 are transmitted as the up 
signal or signals to the central station 4 through a low-pass filter 32 
and a fourth terminal T.sub.4. 
The bidirectional amplifier 6 shown in FIG. 1 may be comprised of an AC 
power-passing type up-signal amplifying line amplifier which is used with 
the ordinary CATV system so as to transmit the up-signals through an AC 
power passage or alternatively a combination of an down-signal high 
frequency amplifier and a up-signal low frequency amplifier may be used. 
Also the branching unit 5 may be comprised of the AC power passing type 
which is used with the ordinary CATV coaxial cable network and employing a 
hybrid coil so as to divide and pass the television signal and the 
interrogating signals F.sub.1 to F.sub.4 to the branch coaxial cables 10-1 
to 10-4 with reduced power loss. Also the bidirectional splitters 7 may 
each be comprised for example of an AC power-passing type hybrid spitter. 
On the other hand, the coupling unit 8 in each subscriber station 9 is 
constructed as shown in FIG. 4. In the Figure, the coupling unit 8 is 
designed so that the television signal TVS and the interrogating signals 
F.sub.1 to F.sub.4 which are applied to a first terminal T.sub.1 connected 
to the splitter 7 are divided, that is, the television signal TVS is 
transmitted to the TV set 29 through a high-pass filter 81 and a second 
terminal T.sub.2 and the signals F.sub.1 to F.sub.4 are transmitted to the 
band-pass filter 27 through an attenuator 82 and an amplifier 83 and 
through a third terminal T.sub.3, and the alarm signal (the signal of the 
frequency f.sub.1 is shown in the Figure) applied to a fourth terminal 
T.sub.4 from the alarm signal generator 24 is transmitted as an up-signal 
to the splitter 7 through the first terminal T.sub.1. 
Next, the operation of this embodiment will be described hereunder with 
reference to FIGS. 1 to 4. 
Firstly, the television signal TVS is received by the community antenna 1 
from which the signal is applied to the first terminal T.sub.1 of the 
signal separator 3 through the head end 2. In the signal separator 3, the 
television signal TVS applied to the first terminal T.sub.1 is delivered 
as such to the second terminal T.sub.2 through the coupling line 10a and 
the high-pass filter 31. The television signal TVS is then delivered 
through the second terminal T.sub.2 to the coaxial cable 10, and after 
having been amplified by the bidirectional amplifier 6 to a desired gain 
as occasions demand, the television signal TVS entering the branching unit 
5 divides and passes to the branch coaxial cables 10-1, 10-2, 10-3 and 
10-4. The television signal TVS transmitted to each of the branch coaxial 
cables is further divided and passed to the subscriber stations 9 through 
the bidirectional splitters 7. 
When applied to each subscriber station 9, the television signal TVS is 
received by the television receiving set 29 connected to the second 
terminal T.sub.2 through the first terminal T.sub.1 of the coupling unit 8 
and the high-pass filter 81. With the illustrated embodiment, assuming 
that the VHF band of 108 to 107 MHz is assigned to the high frequency 
signal components F.sub.1 to F.sub.4 of interrogating signals as mentioned 
previously and that the voice frequency band, e.g., the low frequency band 
of 1 to 4 KHz is assigned to the output signal frequencies f.sub.1 to 
f.sub.40 of the alarm signal generators 24 in the subscriber stations 
connected to each branch coaxial cable, it is necessary that the high-pass 
filters 31 and 81 of the signal separator 3 and the coupling units 8 each 
has a pass band extending from 54 MHz up to 870 MHz and the low-pass 
filter 32 of the signal separator 3 passes only the low frequency signals 
lower than 4 KHz. 
With the above-mentioned selection of the filters, the television signal 
TVS is received by the television receiving set 24 of each subscriber 
station through the above-mentioned route. 
With the system described, the transmission and reception of alarm signals 
are effected in the following manner. In the central station 4, the 
oscillators 11 to 14 of the oscillator circuit 30 respectively generate 
output signals of the frequencies F.sub.1 to F.sub.4, respectively. On the 
other hand, the clock pulse generator 21 generates clock pulses of a 
predetermined period, and the shift register 19 sequentially generates "1" 
signals at its output terminals, one at a time in response to every clock 
pulse applied, so that starting with the control signal t.sub.1, the 
control signals t.sub.1 to t.sub.4 are sequentially changed to a "1" 
signal, that is, the control signal t.sub.1 changes to a "0" signal when 
the next control signal t.sub.2 changes to a "1" signal and this process 
is repeated to change the signals periodically to shift the "1" signal. 
As the control signals t.sub.1 to t.sub.4 are sequentially changed to "1" 
signal, the switching devices 15 to 18 are sequentially actuated, and 
consequently the output of the oscillator in the oscillator circuit 30 
which is connected to the actuated switching device is applied to the 
third terminal T.sub.3 of the signal separator 3. At the same time, the 
control signals t.sub.1 to t.sub.4 are applied as gate input signals to 
the AND gates 221-1 to 224-40 of the discriminator 22. By virtue of this 
sequential actuation of the switching devices 15 to 18, the high frequency 
signals F.sub.1 to F.sub.4 are sequentially applied from the central 
station 4 to the third terminal T.sub.3 of the signal separator 3, and 
then the signals are transmitted as interrogating signals or polling 
down-signals to the coaxial cable 10 through the directional coupler 33 
and the high-pass filter 31. The high frequency signals F.sub.1 to F.sub.4 
are amplified by the bidirectional amplifier 6, and then the branching 
unit 5 sequentially divides and transmits the signals to the branch 
coaxial cables 10-1 to 10-4. As a result, the signals F.sub.1 to F.sub.4 
are sequentially transmitted to all the subscriber stations simultaneously 
as in the case of the television signal TVS. In each subscriber station 9, 
the coupling unit 8 applies the television signal TVS to the television 
set 29 through the high-pass filter 81 and it also applies the signals 
F.sub.1 to F.sub.4 to the band-pass filter 27 through the attenuator 82 
and the amplifier 83. Of these signal components F.sub.1, F.sub.2, F.sub.3 
and F.sub.4, only that signal component which corresponds to the branch 
coaxial cable or the group unit to which the subscriber station belongs, 
e.g., the signal component F.sub.1 is passed through the bandpass filter 
27 and it is then detected by the detector 28. In other words, since the 
signals F.sub.1, F.sub.2 , F.sub.3 and F.sub.4 are sequentially shifted 
periodically to "1" signal for a predetermined duration time in response 
to the operation of the shift register 19, the detector 28 generates a 
detection output only during the duration time of the corresponding one of 
the four signals, and this process is repeated periodically. This 
detection output is applied to the analogue gate device 26 and 
consequently the analogue gate device 26 is acuated periodically. Thus 
when the analogue gate device 26 is in operation, the output signal of the 
alarm signal generator 24 or the alarm signal can be transmitted to the 
fourth terminal T.sub.4 of the coupling unit 8 through the alarm switch 
25. As a result, when the alarm switch 25 of the subscriber station 9 is 
actuated in response to the occurrence of an abnormal condition, the alarm 
signal is delivered to the fourth terminal T.sub.4 of the coupling unit 8 
in synchronism with the detection output, and then the signal is 
transmitted to the associated branch coaxial cable through the first 
terminal T.sub.1 of the coupling unit 8 and the splitter 7. The alarm 
signal is further transmitted through the branching unit 5, the coaxial 
cable 10 and the bidirectional amplifier 6 to the second terminal T.sub.2 
of the signal separator 3, and then the signal is applied through the 
low-pass filter 32 and the fourth terminal T.sub.4 of the signal separator 
3 to the discriminator 22 of the central station 4. In the discriminator 
22, the alarm signal is applied to the band-pass filters 225-1 to 225-40, 
so that the alarm signal passes through one of these filters or that 
filter which passes a frequency corresponding to the specified frequency 
of the alarm signal, and then the signal is applied only to one of the A/D 
convertors 226-1 to 226-40 which is connected to that particular filter. 
Consequently, this particular A/D converter generates a "1" signal which 
in turn is passed, along with one of the control signals t.sub.1, t.sub.2, 
t.sub.3 and t.sub.4, through one of the AND gates to actuate the 
corresponding indicator through one of the holding devices. In this way, 
the group unit to which the subscriber station generating the alarm signal 
belongs, is discriminated in accordance with one of the signals F.sub.1, 
F.sub.2, F.sub.3 and F.sub.4 or the control signals t.sub.1, t.sub.2, 
t.sub.3 and t.sub.4, and the location of the subscriber station in the 
group unit is discriminated in accordance with the specified low frequency 
of the alarm signal (one of the frequencies F.sub.1 to F.sub.40), thus 
causing the actuated indicator to give an indication corresponding to the 
subscriber station in accordance with the combination of the 
discrimination results. For instance, with the interrogating signal or 
high frequency signal F.sub.1 being transmitted from the central station 4 
to the subscriber stations, when the alarm switch is closed in the 
subscriber station which is connected to the branch coaxial cable 10-1 and 
to which the low frequency f.sub.1 is assigned, the signal f.sub.1 is 
passed through the band-pass filter 225-1 during the time that the control 
signal t.sub.1 is applied from the shift register 19 to one input 
terminals of the AND gates 221-1 to 221-40 in the discriminator 22, so 
that the A/D convertor 226-1 applies a "1" signal to the other imput 
terminals of the AND gates 221-1, 222-1, 223-1 and 224-1 and only the AND 
gate 221-1 generates an output. Consequently, the holding device 235-1 is 
triggered and only the indicator 231-3 is brought into operation, thus 
indicating the occurrence of the abnormal condition at the location of the 
subscriber station specified by the low frequency f.sub.1 and belonging to 
the group unit specified by the high frequency F.sub.1. 
On the other hand, when an alarm signal is generated from the subscriber 
station which is connected to the branch coaxial cable 10-1 and to which 
the low frequency f.sub.40 is assigned, similarly the indicator 231-40 is 
brought into operation. When an alarm signal is generated from the 
subscriber station which is connected to the branch coaxial cable 10-4 and 
to which the low frequency f.sub.2 is assigned, the indicator 234-2 is 
brought into operation. Even if these signals are generated 
simultaneously, the corresponding indicators are sequentially started to 
operate in accordance with the shift timing of the shift register 19. 
It will thus be seen from the foregoing that in accordance with the alarm 
system of this invention, a large number of terminals (4.times.40 
terminals in the illustrated embodiment) can be discriminated with a 
relatively small number (4+40) of frequencies alone, and the 
discrimination of the terminals is simplified without using a large number 
of modulators, demodulators and filter as in the case of the known system 
in which the group discrimination frequencies F.sub.1 to F.sub.4 are 
modulated by the terminal discrimination frequencies f.sub.1 to f.sub.40. 
While this system is not adapted to give the types of alarm, this 
deficiency can be satisfactorily overcome by adapting the system for use 
over a short distance so that the person in charge can immediately visit 
the location where an abnormal condition has occurred, and there is 
another advantage that each subscriber is required to simply depress the 
switch in case of need, thus simplifying the operation and eliminating the 
occurrence of erroneous operations. 
Of course, the number of terminals as well as the number of terminals in 
each group unit can be changed as desired. Further, while the group 
designating polling signals are delivered as down-signals and the alarm 
signals from the terminals are delivered as up-signals, by assigning for 
example high frequencies which are close to the television frequency band 
to the former and low frequencies in the voice signal band to the latter, 
the required signal separation, amplification and the like may be 
separately effected simply by means of filters.