Abstract:
An intrusion alarm system has a number of satellites each connected to a master by an unshielded cable having only four conductors, namely two power conductors, a drive signal conductor which supplies drive to the satellite transmitter and the satellite control circuit, and an alarm conductor. When an intrusion is detected by a satellite, its control circuit transmits a high level alarm signal on the alarm conductor to the master and also enables (but does not operate) a speaker in the satellite. The master has a rocker switch which an investigator turns off when he arrives at the supervised premises. This turns off the drive signal to the satellite. The control circuit of the satellite that caused the alarm responds by sounding its speaker, enabling determination of where the alarm occurred. The rocker switch may also be placed in a walk test position, enabling the system to be walk tested, and then may be returned to its initial supervisory position applying a reset signal to restore the satellite control circuits all to their normal supervisory condition.

Description:
This invention relates to a four wire satellite control system for a multi-satellite intrusion alarm. 
     Intrusion alarms normally operate by transmitting a wave field (typically ultrasonic or electramagnetic radiation) into an area under supervision, receiving a portion of the reflected field, and comparing the two. If a moving intruder is present, a doppler shift is detected in the received field. The doppler frequency is processed and is used to produce an alarm signal. 
     Since each transmitter-receiver unit can supervise only a relatively small area, such as a room or hall, therefore in large buildings a number of separate units must be used. To reduce duplication of equipment, it is usual to provide a master unit and a number of separate satellites connected to the master unit. A satellite unit is placed in each area to be supervised. The satellites carry out some signal processing, but it is the practice to place some of the signal processing circuits in the master where they provide common processing for several satellites. 
     A disadvantage of conventional multi-satellite intrusion alarms is that the cables connecting the satellites to the master usually require numerous separate conductors. These cables are therefore expensive, bulky, and difficult to instal. In addition, conventional systems usually require that the connecting cables between the satellites and the master be shielded (since low level signals are being transmitted to the master for further analysis), increasing further the expense and bulk of the cable. The bulky shielded cables are also difficult to instal unobtrusively. 
     A still further disadvantage of conventional intrusion alarm systems relates to the practice, in large buildings where many satellites are required, of connecting the satellites to the master in groups. Each group of satellites constitutes a &#34;zone&#34;. When one satellite in a zone generates an alarm signal, it is normally difficult to determine which of the satellites in the zone actually generated the alarm. In conventional systems, when a security guard investigates these zones in question, his movement usually causes other satellites in the zone to be triggered. This adds to the difficulty of tracing the movements of the intruder or of analyzing the false alarm which caused the alarm signal. 
     Accordingly, it is an object of the invention to provide an intrusion alarm control system in which, in a preferred embodiment, each satellite is connected to the master by only four wires, which normally need not be shielded. One of the wires supplies power to the satellite; a second is a common return line; the third wire is a drive signal line which carries a drive signal from the master to the satellite; and the fourth is an alarm line which carries a high level alarm signal from the satellite to the master. In a preferred embodiment of the invention, means are also provided in the master and in each satellite so that when an alarm signal is produced by a satellite, the system can be switched to a condition in which the satellite in question enunciates the alarm, but none of the other satellites in the system will produce an alarm signal while an authorized person checks the supervised premises. This facilitates analysis of the cause of the alarm signal. 
    
    
     Further objects and advantages of the invention will appear from the following disclosure, taken together with the accompanied drawings, in which: 
     FIG. 1 is a block diagram showing a conventional connection arrangement of satellites to a master control unit; 
     FIG. 2 is a block diagram of a satellite according to the invention; 
     FIG. 3 is a block diagram showing a portion of a master control unit of the invention; 
     FIG. 4 is a schematic of a control circuit of a satellite; 
     FIG. 5 shows a drive wave form produced at a satellite; 
     FIG. 6 shows a portion of the control circuit of FIG. 4 in supervisory condition with the condition of certain logic elements indicated thereon; 
     FIG. 7 shows the FIG. 6 circuit in alarm condition; 
     FIG. 8 shows the FIG. 6 circuit with the drive signal off; 
     FIG. 9 shows the FIG. 6 circuit in walk test condition; 
     FIG. 10 is a schematic of a logic circuit of a master control unit; and 
     FIG. 11 shows wave forms produced by the logic circuit of FIG. 10. 
    
    
     GENERAL DESCRIPTION 
     Reference is first made to FIG. 1, which shows a typical connection system for a master control unit and satellites. The connection system of FIG. 1 has been used in most conventional alarm systems and is preferably also used in the alarm system of the invention. As shown, a master control unit 22 is connected to four satellite zones 24, 26, 28, 30. Each satellite zone typically consists of five satellites, which are indicated as satellites 1 to 5, 6 to 10, 11 to 15 and 16 to 20. The satellites of each zone are connected together and to the master control 22 by cables 32. 
     In operation, if an intruder is detected by any satellite in a zone, for example in satellite 1 of zone 24, a signal (which in conventional systems usually requires further analysis) is sent to the master control unit 22. The master control unit 22 generates an appropriate alarm signal, which may be sent by a telephone line 34 either to the alarm company whose duty it is to supervise the premises in question, or to police headquarters, or as desired. 
     As indicated previously, the cables 32 connecting the satellites to the master are usually shielded, and usually contain numerous conductors. Because of this, installation of the satellites is usually a difficult and expensive task. 
     According to the invention, means are provided in the satellites and in the master control unit 22 so that the cable 32 need contain only four conductors. These means are shown in block diagram form in FIGS. 2 and 3. FIG. 2 shows a typical satellite, for example satellite 1 of zone 24. Satellite 1 includes four terminals 40a, 40b, 40c, 40d which are connected by conductors 42a, 42b, 42c, 42d to four corresponding terminals 44a, 44b, 44c, 44d in the master 22 (FIG. 3). As will be explained, conductor 42a is a drive conductor, conductor 42b is an alarm conductor, and conductors 42c, 42d are power supply conductors. 
     In the example here illustrated, it is assumed that the transmitted field is ultrasonic sound at a frequency of 40 KHz. Accordingly, the master 22 includes a 40 KHz transmitter 46, which forms part of a logic circuit 48. The transmitter 46 applies a 40 KHz drive signal to terminal 44a, and thence through drive conductor 42a to terminal 40a of the satellite 1. In satellite 1 the 40 KHz signal is squared by a Schmidt trigger 50, which improves the waveform of the drive signal and ensures that its peak amplitude is constant. The 40 KHz signal is then sent to transmitter 52 (FIG. 2), which radiates a 40 KHz ultrasonic sound field. 
     A portion of the reflected field is received by a transducer-receiver 54, amplified by amplifier 56, and then directed to a synchronous detector consisting of transistor Q1. The base of transistor Q1 is driven by the 40 KHz drive signal. The signal from the transistor Q1 collector is passed to a band pass filter 58, which removes the 40 KHz component and also removes very low frequencies. The signal from the band pass filter 58 is then directed to a signal processor 60. The signal processor 60 processes the signal from filter 58 and produces an alarm signal if the signal from filter 58 contains the doppler frequencies which are likely to have been generated by a moving intruder (40 Hz to 300 Hz for a 40 KHz transmitted sound field). Various known forms of signal processing circuits may be used for processor 60. A preferred signal processing circuit is shown in my co-pending application Ser. No. 742,048 filed concurrently herewith, and the description and drawings of that application are hereby incorporated by reference into this specification. 
     The alarm signal (if any) from signal processor 60 may be of extremely short duration, so it is directed into a pulse stretcher 62 (typically a Schmidt trigger) which produces a pulse of fixed length when it is triggered. The pulses (if any) from the pulse stretcher 62 are fed to a control circuit 64, which then sends an appropriate high level signal back to the master 22 via conductor 66, terminal 40b, alarm conductor 42b, and terminal 44b. This signal is received in the master by a detector 68, which then provides a signal to operate an alarm signal generator 70. The signal from generator 70 may be of any desired form, e.g. it may operate a telephone to alert the alarm company. 
     As shown in FIG. 3, the logic circuit 48 includes a three position rocker switch 72 having a rocker element 74. The three positions of rocker switch 72 are (i) the position shown in FIG. 3, in which rocker element 74 contacts lower terminal 76, (ii) a position in which rocker element 74 contacts upper terminal 78, (iii) a central position in which rocker element 74 contacts neither of terminals 76, 78. The position shown in FIG. 3 is the normal supervisory position. In this position, rocker switch 72 controls logic circuit 48 so that the 40 KHz signal from oscillator 46 is applied to terminal 44a. 
     If an alarm signal is produced by generator 70, and after an authorized person arrives at the premises to investigate, he will place rocker switch 72 in its central position, in which rocker element 74 does not contact either of terminals 76, 78. As will be explained, the logic circuit 48 then removes the 40 KHz drive signal from terminal 44a, so that the satellite 1 (and the other satellites in zone 1) will no longer generate an alarm. This enables the person to investigate the premises protected by zone 1 without creating additional alarms. (He may also switch the corresponding rocker switches for the other zones to their central positions, thus also preventing any of the other satellites from generating an alarm signal. One switch actuator may be used for the rocker switches of all the zones.) 
     When the authorized person switches off the 40 KHz drive signal at the master 22, the control circuit 64 of FIG. 2 responds to the combination of the terminated 40 KHz drive signal, and the alarm signal which was previously received from pulse stretcher 62, and operates a speaker 80 in the satellite. Thus, when the investigating person walks through the area supervised by the satellite which generated the alarm signal, he will hear the speaker 80 and will know which satellite generated the alarm. 
     When the investigator moves rocker element 74, power is removed from terminal 82 of alarm signal generator 70. This, by conventional means, places generator 70 in a constant alarm condition, so that the alarm company will know that the system is not in its normal supervisory condition. 
     After the investigating person has completed his investigation, he can then place the system in a &#34;walk test&#34; condition by moving the rocker switch 72 so that the rocker element 74 contacts terminal 78. This operates the logic circuit 48 of FIG. 3 to resume supply of the 40 KHz drive signal to the satellites, including the satellite 1 of FIG. 2. The control circuit 64 of satellite 1 reacts to the resumption of the 40 KHz drive signal at terminal 40a by altering the control of the speaker 80, so that the speaker 80 will now be operated whenever an alarm pulse from pulse stretcher 62 is produced. Therefore, as the investigator walks through the area supervised by satellite 1, he can test and determine the extent of coverage of satellite 1 and whether it is operating properly or generating false alarms. The same applies to the other satellites in zone 1 (and in any other zones where rocker switches have been moved to the &#34;walk test&#34; position). 
     After the walk test has been completed, the investigator moves the rocker switch 72 back to its original position, in which rocker element 74 contacts terminal 76. This causes the logic circuit 48 to send a timed reset signal along drive conductor 42a, as will be described, to actuate the control circuit 64 of each satellite to resume its original supervisory mode of operation. Power is also reapplied via terminal 82 to the alarm signal generator 70. 
     As will also be described, during the time when the satellite is not generating an alarm signal, it sends a supervisory signal over alarm conductor 42b to the master 22. If the supervisory signal ceases for example because the cable 32 is cut or short circuited, this operates the detector 68 which causes the alarm signal generator 70 to operate. Similarly, if for some reason the 40 KHz drive signal from the master 22 to the satellite 1 ceases, the control circuit 64 of the satellite reacts by ceasing to supply the supervisory signal to terminal 40b, again causing detector 68 to operate. 
     The remaining two conductors 42c, 42d of FIGS. 2, 3 supply +6 volts and a common return respectively to the various components shown in FIG. 2. These two conductors are shown as connected directly to a six volt power supply 90 in the master 22, and are indicated as being connected to the components of FIG. 3 by the diagramatic showing of these components as being connected to +6 volts and ground. 
     DETAILED DESCRIPTION 
     A -- Circuit Description 
     Reference is next made to FIG. 4, which shows in detail the satellite control circuit 64. As shown in FIG. 4, the 40 KHz drive signal from terminal 40a is fed through resistor R1 to the Schmidt trigger 50, which produces a constant peak amplitude square wave train 98 (FIG. 5) from the drive signal. The wave train 98 is fed to the transmitter 52 and the base of transistor Q1, as described, and is also fed through diode D1 to the input of a second Schmidt trigger 100. The positive side of diode D1 is connected through resistor R2 to the +6 volt supply and is also connected through capacitor C1 to ground. 
     The output of Schmidt trigger 100 is fed to an inverter 102 and also through capacitor C2 to the reset terminal 104 of a memory latch 106. The output of the inventer 102 is fed to one input 107 of a NAND gate 108, and also to the set terminal 109 of a second memory latch 110. The output of latch 110 is directed to one terminal 112 of a NAND gate 114. The output of the NAND gate 114 is fed to one input 116 of NOR gate 118. The output of NOR gate 18 is directed to the input of an oscillator 120 (typically 5 KHz) consisting of NAND gate 112, inverter 124, resistors R3 and R4, and capacitor C3. The output of the oscillator 120 is fed through amplifier 128 to the speaker 80. 
     The square wave train from the input trigger 50 is also fed through a second diode D2 to the input 130 of another Schmidt trigger 132. The input 130 of the Schmidt trigger 132 is connected to ground, through the parallel combination of resistor R5 and capacitor C4. The output of Schmidt trigger 132 is connected to the reset terminal 134 of the memory latch 110. 
     The output of Schmidt trigger 62 (the pulse stretcher) is connected to the set terminal 138 of memory latch 106 and also to an input terminal 142 of NAND gate 114. The output of latch 106 is connected to an input terminal 140 of NAND gate 108. The output of NAND gate 108 is connected to input 148 of NOR gate 118. 
     The output of Schmidt trigger 62 is also connected through resistor R6 to the base of transistor Q2, the collector-emitter circuit of which is connected between ground and terminal 42b. 
     Finally, the FIG. 4 circuit includes a 10 Hz oscillator 150, consisting of NAND gates 152, 154, timing resistors R7 R8, and timing capacitor C5. Oscillator 150 applies a 10 Hz signal to terminal 40b so long as the 40 KHz signal is present at terminal 40a, as will be explained. 
     B -- Operation -- Supervisory Condition 
     The detailed operation of the FIG. 4 circuit is as follows. So long as the 40 KHz driving signal is present at terminal 40a, Schmidt trigger 50 produces the square wave signal 98 shown in FIG. 5, varying between +6 volts (when the driving signal is low), and ground (when the driving signal is high). Signal 98 maintains capacitor C1 discharged so long as the 40 KHz driving signal is present at terminal 40a. This is because diode D1 is reversed by the &#34;on&#34; half cycles of signal 98, permitting capacitor C1 to charge slowly through resistor R2 during &#34;on&#34; half cycles, but during &#34;off&#34; half cycles diode D1 is forward biased, discharging capacitor C1 through diode D1 and through a low resistance connection to ground (not shown) which is made in the Schmidt trigger 50. 
     The opposite situation prevails with regard to capacitor C4. This capacitor is normally charged, since during &#34;on&#34; half cycles of signal 98, diode D2 is forward biased, permitting rapid charging of capacitor C4, while during &#34;off&#34; half cycles, diode D2 is reverse biased, causing capacitor C4 to discharge slowly through resistor R5. 
     So long as capacitor C1 remains discharged, the output from Schmidt trigger 100 is high (i.e., +6 volts), since it is an inverting trigger, and the output from inverter 102 is low (i.e., ground), so the memory latch 110 is not set. Latch 110 therefore applies a low to input 112 of NAND gate 114. Inverter 102 also applies a low to input 107 of NAND gate 108. The output of NAND gate 108 is now high, applying a high to the second input 148 of NOR gate 118. So long as both inputs of NOR gate 118 are high, the output of gate 118 is low, inhibiting oscillator 120 (Gate 118 is a negative logic NOR gate, as indicated by the two open circles at its inputs, and therefore functions as if it were a positive logic NAND gate.) The speaker 80 therefore remains silent. This situation is shown in FIG. 6, in which highs are indicated by + signs and lows are indicated by -- signs. 
     In addition, so long as the 40 KHz driving signal is present, a high is applied from Schmidt trigger 100 to NAND gate 152 of 10 Hz oscillator 150, and a second high is applied from the input of trigger 132 to NAND gate 154 of oscillator 150. Oscillator 150 operates in conventional manner to apply a 10 Hz square wave train of about 6 volts amplitude to terminal 40b. The 10 Hz wave train is transmitted to the master 22 (FIG. 3) and received by the detector 68. So long as the detector 68 receives the 10 Hz signal, it will not operate the alarm signal generator 70. 
     C -- Supervisory Condition -- Alarm 
     If an intrusion occurs, causing a high pulse (+ 6 volts) from Schmidt trigger 62, this pulse turns on transistor Q2 for the duration of the pulse. Transistor Q2 grounds terminal 40b, stopping transmission of the 10 Hz signal from oscillator 150 to the detector 68. The absence of the 10 Hz signal triggers the detector 68, causing it to operate the alarm signal generator 70. 
     In addition, the high from Schmidt trigger 62 is applied to one input 142 of NAND gate 114 (See FIG. 7). However, since the other input 112 to gate 114 remains low (since latch 110 has not been set), the output from gate 114 remains high. There is, therefore, no change in the output of NAND gate 114 that would cause NOR gate 118 to remove the inhibit signal (a low) from oscillator 120. 
     The high from Schmidt trigger 62 also acts to set latch 106 (see FIG. 7) placing a high on input 140 of NAND gate 108. However, input 107 of NAND gate 108 remains low (due to inverter 102) and the output of gate 108 remains high, and again there is no change in the condition of NOR gate 118. The speaker 80 thus remains silent, so as not to alert the intruder, although an alarm has been transmitted to the master and hence to the alarm company. 
     D -- Drive Signal OFF 
     When an authorized person responds to the alarm and arrives at the supervised premises to investigate, he will move the rocker switch 72 (FIG. 3) to its intermediate position to shut off the 40 KHz drive signal. This shuts off the transmitters of all of the satellites and prevents them from responding to further movement. In addition, when the 40 KHz drive signal is shut off, the output from trigger 50 stays high; diode D1 remains reverse biased, and capacitor C1 charges through resistor R2, producing a low at the output of trigger 100. 
     The low at the output of trigger 100 produces a high at the output of inverter 102 (see FIG. 8) setting latch 110 and also applying a high to input 107 of NAND gate 108. Since latch 106 was set by the previous alarm pulse from trigger 60, and applies a second high to input 140 of NAND gate 108 (see FIG. 8) gate 108 now has two high inputs. Its output therefore goes low, applying a low to input 148 of NOR gate 118. The output of NOR gate 118 now goes high, enabling oscillator 120. The output of the oscillator 120 is amplified by amplifier 128 and is fed to speaker 80. Thus, when the investigator reaches the area which the satellite 1 supervises, he will hear its speaker and will know that the intruder was in that area or that it generated a false alarm. If the speakers of any other satellites are sounding, he will also know that these satellites generated alarm signals. No other alarm signals will be generated, because the 40 KHz driving signal has been turned off. It will be seen that speaker 80 sounded when two conditions occurred, namely (1) an intrusion was previously detected, and (2) the 40 KHz drive signal was turned off. 
     E -- Walk Test 
     After the investigation has been completed, it will normally be desired to walk test the system, to ensure that it is operating properly. At this time, the rocker switch 72 (FIG. 3) is moved so that its rocker element 74 contacts terminal 78. This turns on the 40 KHz drive signal again, again discharging capacitor C1. 
     When capacitor C1 is discharged, trigger 100 goes high (see FIG. 9), resetting memory latch 106 through capacitor C2. Now, with the 40 KHz drive signal available to the satellites, when the authorized person moves in the area supervised by the satellite 1, a high is produced by trigger 62 and is fed directly to input 142 of NAND gate 114. The other input 112 to NAND gate 114 is also high, since latch 110 was set when the 40 KHz drive was turned off previously. The two high inputs to NAND gate 114 produce a low at its output. This low is applied to input 116 of NOR gate 118, which then removes the inhibit from the oscillator 120. The result is that the speaker 80 sounds during the time when the authorized person is actually moving in the area under supervision. This enables testing of the satellite in question and also facilitates setting of the levels at which it will generate an alarm signal. 
     F -- Return to Supervisory Condition 
     After the walk testing has been completed, and the system is to be placed back into its supervisory condition, the rocker switch 72 is returned to its original condition shown in FIG. 3. By means to be described, this produces a timed 0.4 second low signal on drive conductor 42, followed by a timed 0.4 second high signal, followed by the normal 40 KHz drive signal. The timed 0.4 second high signal is sufficient for capacitor C1 to discharge to its normally discharged condition and is also sufficient time for capacitor C4 to discharge through resistor R5, causing the output of the second trigger 132 to go high. This places a high signal on the reset terminal 134 of latch 110, resetting this latch and thereby disabling any further enunciation of the speaker 80. When the 40 KHz drive signal resumes, after the timed signals, the system is back in supervisory condition. 
     G -- Description of Master Logic Circuit 
     The logic circuit 48 of the master 22, and the wave forms produced thereby, are shown in detail in FIGS. 10 and 11. When the rocker switch 72 is in the position shown the 40 KHz oscillator 46 operates and its signal is fed to input 200 of OR gate 202 to operate driver amplifier 204. Amplifier 204 then feeds the amplified 40 KHz drive signal to the drive terminal 44a. There is no input to the second input 206 of OR gate 202 at this time, because input 206 is fed by AND gate 208, one input 210 of which is a single shot multivibrator 212 which is not operative at this time. 
     When the rocker switch 72 is operated so that its rocker element 74 is in its intermediate position, in which element 74 does not contact either terminal 76, 78, then +6 volts is removed from the second input 214 of NAND gate 216. Gate 216 is a negative logic NAND gate, as indicated by the open circles at its inputs, and produces a high at its output only when both inputs are low, i.e., it functions like a positive logic NOR gate. Both the inputs of NAND gate 216 are now low, thereby producing a high at the input 218 of OR gate 220, which in turn produces a high at the input 222 of AND gate 224. The second input 226 of AND gate 220 is also high at this time, because of inverter 228, the input of which is grounded through resistor R10. The output of AND gate 224 therefore goes high, inhibiting the 40 KHz oscillator 46, which ceases operation. When oscillator 46 turns off, the drive terminal 44a is grounded by means not shown in the driver amplifier 204. 
     The wave forms thus produced at drive terminal 44a are shown in FIG. 11. The 40 KHz drive signal is shown at 230, and the ground signal produced when the 40 KHz oscillator 46 is inhibited is shown at 232. As described, when the 40 KHz oscillator 46 is inhibited, an investigator can walk into the supervised area without causing a further alarm. 
     When the rocker switch 72 is switched to its walk test condition, in which rocker element 74 contacts terminal 78, this supplies a high to the input of inverter 228, causing its output to go low, so that input 226 of AND gate 224 is low. AND gate 224 therefore removes the inhibit or high signal from oscillator 46, and the drive terminal 44a now receives the 40 KHz drive signal again. As previously described, the satellites will now detect motion and the speakers 80 will sound at the time when the motion occurs, so that the system can be walk test. In addition, input 234 of NAND gate 216 goes high and input 214 of this gate goes low causing the output of gate 216 to go low. 
     To return the system back to supervisory position, the rocker switch 72 is returned to its position as shown in FIG. 10. As the rocker element 74 moves, both inputs 214, 234 to NAND gate 216 are low for a brief interval. The output of gate 216 therefore goes high for a brief interval and triggers a single shot multivibrator 236 which produces a 0.4 second high output pulse. This high pulse at input 238 of OR gate 220 produces a 0.4 second high at input 222 of AND gate 224. AND gate 224 now has two high inputs (input 222 from OR gate 220 and input 226 from inverter 228), so the 40 KHz oscillator 46 is inhibited for 0.4 seconds. The 0.4 second off pulse in the drive signal is indicated at 250 in FIG. 11. 
     When the single shot multivibrator 236 times out, and since by this time switch 72 will have reached the position drawn, the output of OR gate 220 goes low again, since it will have lows at both its inputs. The low output of OR gate 220 triggers the second single shot multivibrator 212. Multivibrator 212 produces a 0.4 second high pulse at its output. AND gate 208 now has two high inputs, namely input 210 from multivibrator 212, and the other input 240 supplied directly from terminal 76 and the +6 volts supply. AND gate 208 therefore produces a high output for 0.4 second (the timing duration of multivibrator 212) and this applied to input 206 of OR gate 202, produces a high at its output. The high output of OR gate 202, fed to the driver amplifier 204, produces a high pulse 252 (FIG. 11) at drive terminal 44a for the timing duration of multivibrator 212 (0.4 seconds). 
     As soon as multivibrator 212 times out, the high input to input 206 of OR gate 202 is removed, and the normal 40 KHz drive signal 230 is reapplied to the drive terminal 44a. The system is now back in normal supervisory operation. 
     In the system described, it will be seen that the alarm signal transmitted by the satellites to the master is a high level signal, i.e., it is the removal and subsequent absence of the high level signal produced by oscillator 150. A &#34;high level&#34; alarm signal as here used means a signal which differs by a reasonably substantial amount from the previously prevailing signal, so that even if the signal conductor is unshielded, it will not normally pick up stray signals that would be interpreted as an alarm signal. For example, the difference will usually be at least one volt and preferably higher in a cable of length not exceeding 500 feet. For longer cables, a higher difference will usually be employed. Here, +6 volts has been used for a system in which the cable length is typically up to 1000 feet. 
     It will also be appreciated that certain features of the invention may be used in systems which transmit low level signals over shielded cables containing more than four conductors. For example, the feature of inhibiting the speaker of a satellite which has detected a disturbance, until the drive signal is turned off, the termination of the drive signal causing that speaker (or other alarm indicator) then to enunciate, may be used in other systems, as may the walk test feature.