Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to radio signalling systems
where it is necessary to be able to distinguish between
different individuals.
One application of such a system is where any one of
a number of individuals for example, old people may wish to
summon assistance and it is necessary for someone at the re-
ceiver to be able to identify which person caused the alarm.
Another application is where only certain people are to have
access to a security area and it is necessary for a receiver
to be able to distinguish between those people and other people.
In each case each person using the system can have a
portable pocket token for generating radio signals when
assistance is required or when access to the security area is
required as the case may be.
An object of the invention is to provide a partic-
ularly simple and economical way of distinguishing between
different pocket tokens even when there are a large number of
tokens on the system.
According to the present invention a radio signalling
system includes a number of portable tokens each capable of
transmitting signals in the form of a carrier frequency which
is common to the tokens, with spaced modulation pulses super-
imposed on the carrier and defining a carrier burst between
successive of said pulses, the number of carrier frequency
cycles in a carrier burst between successive modulation pulses
being different for different tokens, and a receiver capable
of distinguishing between received signals from tokens with
different characteristic numbers of cycles in their burst,
wherein for any one token the number of carrier frequency cycles
is the same in each burst of transmitted signals.
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Thus distinction between different individuals or
different tokens is provided merely by different numbers
of carrier frequency cycles ~etween successive modula-tion
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pulses which merely serve to limit the period over which
the cycles are to be counted, and can also operate logic
circuits both in the transmitters and in the receiver.
The system is thus very simple and yet enables
5. very large numbers of people to use the syste~ because
the separation between channels, that is to say between
the numbers of carrier frequency cycles in bursts of similar
tokens can be quite small. In spite of that the equipment
is guite economical in manufacture.
10. For example a permissible range of ~requencies is
50 K~z - 150 KHz and as a 5KHz separation between carrier
frequencies is satisfactory, there are 20 frequencies
available. A 6-bit binary counter has a count of 64, and
a preliminary multi-bit binary counter may be used as a
15. divided of carrier cycles for driving the 6-bit counter.
That would give 20 x 6~ or 1280 'differs' on the system.
A suitable transmitter output power is 5 milli watts,
which would not cause substantial drain on the token
batteries, nor unacceptable interference outside the area
20. where the system is being used.
The invention may be carried into practice in
various ways and three embodiments will now be described by
way of example with reference to the accompanying drawings
in which;
25- FIGURES 1 and 2 are respective logic circuits
for an access system and an alarm system; and
FIGURE 3 is a circuit for an alternatlve access
system.
In both embodiments each user of the system has
30- a portable token containing a simple radio transmitter
working on a common radio frequency channel, and arranged
to modulate a-radio frequency by a 1 millisecond pulse at
regular intervals of a certain number of radio frequency
cycles, the particular number being unique to that user
5. or to the users in a particular group of users. In the
example being described the modulation is amplitude
modulation, but it could equally well be phase or frequency
modulation.
The user may switch his transmitter on manually
10. when he wants to obtain access to a secure area or when
he wants to give an alarm as the case may be, and t~le
modulated radio frequency is transmitted from a'suitable
integral aerial. Alternatively the transmitter could be left
on continuously.
15. Signals are received by an inductive loop signalling
band or by some other convenient means and supplied,to a
réceiver which in the case of the access system is a~r~nged
in accordance with F~GURE 1 to determine if the number of
radio frequency cycles received between successive
20. : modulation pu]ses agrees wlth a pre-set number in which
case an operating circuit releases the entrance to the
prohibited area.
The received signals are amplified with automatic
gain control at 11 attenuated at 12 and fed into a phase-
25. locked loop system 13 having fine and coarse tuning means.
The received radio frequency cycles or a knownsub-multiple or difference frequency from them are amplified
at 15 and used as clock pulses for driving a series of
binary coded decimal counters 16, 17 and 18., and control
30~ of ~he ~irst counter 16 is by means of a monostable circuit
19, a flip-flop 21 and a NAND gate 22. A received modulation
pulse is amplifled at 15' triggers the monostable
circuit 19 and that is arranged to provide an input to the
NAND gate ready for re-setting the first countcr
5. 16 which receives clock pulses and provides control signals
for the remainder of the system.
The first control signal is obtained from the 2
output of the counter 16 and enables a compara'cor 23 which
compares the count achieved on the later coun~ers 17 and
10. 1~3 in the previous burst of carrier radiation between
pulses with the pre-set count as set at 24. The first
control signal is also fed to a decade counter 25 for
counting the number of comparisons made. If the comparator
23 shows coincidence, the count of a counter 26 is
15. increased but if it does not show coincidence, the count
; is decreased and when the count of the counter 26 reaches 4
showing that 4 successful comparisons have been made, a flip~
flop 27 is operated which controls operation of a circuit
; 28 for giving access to the security area.
~0, ~ The second control signal from the number 6 output
of the counter 16 renders the counter 17 receptive to
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clock pulses and an output from that counter 17 in turn
renders the counter 18 receptive, so that the total count
of clock pulses in the interval between two successive
25. modulation pulses is recorded on the two most significant
stages C.D. of the counter 17 and the A.B.C.D. stages
of the counter 18 for subsequrnt comparison with the
pre-set value in the comparator 23. The comparison is
made from the 2 output of the counter 16 before the
30. counters 17 and 18 are re-set from the 6 output of the
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` counter 16 and start to be refilled. The 7 output from
the counter 16 is used to re-set the flip-flop 21 to provide
an input to the NAND ga~e 22 to re-set the counter 16
when it has fulfilled its function.
A re~set button 29 disconnects the circuit 28 and
resets the circuit ready for the next operation.
The logic circuit shown in FIGURE 2 for a recelver
for the alarm system is very similar to the circuit described
with reference to FIGURE 1 and corresponding components
.o have been given the same numerals. In this case however
instead of operating an entrance to a security area, it is
desired to give a warning and a display of the identity of the
individual whose token initiated the alarm. The token
must only be operated when an alarm is required. The
15. alarm circuit corresponds with the operating circuit 2~3
in FIGURE l but the identity display is given on a 3 digit
display unit 31 which receives a count for determining the
display from the counter 17 by way of the comparator
23 at its Al input.
20. : ~ However, the display is only given when a blanking
si~nal is removed in response to operation of the flip-flop
27 after the counter 26 has shown that 4 success~ul comparisons
have been Inade. In this case the comparison is not with a
pre-set number, but with the count in the preceding sample.
25. Thus after each sample of radio frequency, transmission
the count achieved on ~he counters 17 and 18 is transferred
to a store 33 in response to a signal from the number ~ output
of the counter 16. The next count is compared at 23 with the
stored count and if identity is shown, an output signal
increases the count of the counter 26 in the same way as
described with reference to ~GURE l.
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A nu~nber of' refinements may be added to the b~sic
circuit; counts of les.s or more than a present value on
the subsequent counting stages can be deemed present due to
interference and the sample ignored or a decrement applied
5. to the final counter. A tuning indicator can be provicled
consisting of an exclusive or gate connected to the outputs
of the two binary stages immediately prior to the least
significant active stage. If modulating pulses are nicely
centred in their channel an output will be present at this
10. gate and can be used to illuminate a 'tuned' indicator.
Each transmitter can be provided with two modulating
co~lnts, preferably on adjacent c'hannels, so that a final
response a-t the receiver of an odd count represents an
alarm of higher urgency than an even count (or vice versa).
15. In the display the least significant active count bit is not
displayed as count, but as ~degree of urgency'.
The re-set button in the access system of EIGURE 1
co~lld be a timing circuit to automatically reset the
system after a pre~set time, irrespective of whether
20. access had been gained or not.
In th'e simplified version of FIGURE 1 shown in
FIGURE 3 similar numerals have been used for similar components
The radio-f`requency cycles ampli~ied at 42 are used
a control logic arrangement 41 to provide clock pulses for
25. counting by the counter 43 which is a 12 bit binary counter,
the last six units ~16 of which are used to provide
a'count for comparison. Thus 64 cycles are received before
the counter 46 increases its count by 1, and the counter
46 has a total count of 26 or 64.
30. . The f~irst few c~cles at 42 af'ter a modulatlon pulse
at l5 are used as timing sequence control signals from
the control logic 41 and do not affect the count of
the counter 46.
The first control signal enables the comparator
5. 23 which compares the count achieved on the counter 46 in
the previous burst of carrier radiation with the pre~set
count as set at 24. 'i'he f`irsk control signal is also
fed to counters 44 and 45. If comparator 23 shows
coincidence the counk of counter 44 is increased. If the
lO. comparator does not show coincidence the count at counter
45 is increased. When the count on counter 44 reaches
four showing that four successful comparisons have been
made the flip-flop 27 is operated which controls operation
of the circuit 23 for controlling an external mechanism.
15. If the count on counter 45 passes a pre-set value
indicating that several incorrect comparisons have been made
counter 44 will be re-set to zero and also an AGC circuit
at ~F amplifier ll wlll be actuated to desenslti%e the input.
The second control signal from the control logic
B 20. ~ renders the counter 43 receptive to clock pulses so that
the total count of clock pulses in the interval between
two successive modulation pulses is recorded and l/64th
of that count is recorded on the si~ most significant
stages of counter 43 - the counter 46 -
25. for subsequent comparision with the pre--set value at 24 in
the comparator 23.The comparision is made on the first
output of the contro3 logic before the counters are re-set
and start to be refi:lled from the second output of the
control logic.
30. The third output of the control logic is used to
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re-set the control lo~ic to prepare for the reception of
the next modulation pulse. A reset push-button 29 or a
timing circuit disconrlects the circuit 28 and resets the
circuitry ready for the next operation.
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