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Патент USA US3090855

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United States Patent 0" ice
3,96,838
Patented May 21, 1963
1
2
3,09®,838
receiver will respond. At all other receivers, at least one
of these tones will pass through ‘the frequency selective
SELECTIVE SiGNALiNG SYSTEM
Coiin Coates, Oxhey, Watford, and William E. Lardner,
Harrow Weald, England, assignors to Westrex Com
vnetworks thereat to hold the indicators thereof non-opera
tive. There should, of course, be no interval between
the transmission of the holding signal and the other
pany, Limited, London, England
Filed Apr. 15, 1958, Ser. No. 728,673
11 Claims. (Cl. 179—84)
signals.
The invention will be better understood from the fol
lowing detailed description with reference to the accom
panying drawings, in which:
This invention relates to electric selective calling sys
FIG. 1 illustrates diagrammatically one embodiment
of a radio transmitter according to the present invention
and
FIG. 2 illustrates diagrammatically one embodiment
of a radio receiver in accordance with the invention.
receive a message. These selective calling systems may
FIG. 3 illustrates diagrammatically a plurality of radio
be used in a telephone system in which communication 15
receivers arranged according to the present invention to
is over ?xed lines, in a radio communication between a
provide a system wherein it is possible to call simultane
?xed station and a number of vehicles, or an inductive
ously a group of receivers to which one frequency has
system in which a loop antenna surrounds a building and
been alloted in common.
persons within the building carry the receivers. Thus
Referring now to the drawings, in FIG. 1, 10 is a ra
there are known mobile radio telephone systems com 20
dio transmitter of any suitable conventional form coupled
prising a central transmitter and a plurality of mobile
to antenna 11. The output of the transmitter may be
receivers carried by persons or vehicles, each receiver
tems.
10
Selective calling systems are known in which one or
more tones, which may be within the audio range, are
transmitted to select one out of a plurality of receivers to
being provided with a plurality of vibrating reeds tuned
amplitude modulated or frequency modulated, and the
antenna 11 may comprise a dipole of, say, the fixed con
When the central transmitter desires to call a 25 trol station of a mobile vehicle radio system, or the an
to different frequencies, which may be within the audio
range.
particular mobile receiver, it transmits the frequencies
tenna may comprise an inductive loop for communicat
to which the vibrating reeds at that mobile station will ' ing with mobile receivers located within the area bounded
by the loop.
respond, and closure of contacts by these reeds completes
The variable oscillators 12 and 13 may be set by
the circuit of an indicator, optical or acoustic. Verbal
30 switches 14 and '15 respectively to generate oscillations of
messages may then be passed between the two stations.
desired frequencies within the audio range, say 200 c. / s. to
In the known systems the selection of a receiver has
v5 k-c./s. The oscillators ‘12 and 13 may be of any suitable
always been by the transmission of the tones allotted to
form which will permit selection of frequencies. They
that receiver. In accordance with the present inven
may include tuned reeds, or may have preset components
tion the selection of a desired receiver is by absence of
effective calling signals at the receiver and not the pres 35 that are selected by switches 14 and 1-5. Oscillators 12
and 13 may be connected by switches 16 and 17 respec
ence of signals. This enables a number of advantages to
tively to the input to the transmitter 10. Oscillator ‘18
be obtained, any or all of which may be gained depending
generates a holding signal, preferably of a frequency that
vupon the particular circumstances. These advantages are
will not be reproduced by the receiver, say 50 c./ s.
as follows:
(a) The receiver will indicate when the transmitter 40 or 6 kc./s., and is normally coupled by switch 19 to the
input to transmitter .10. Microphone20 is also connected
fails, or ceases to transmit, or when the receiver has been
to the input to transmitter 10.
'
moved out of range of the transmitter. The receiver will
It is important that there he no interval between cessa
indicate continuously in the absence of a signal.
tion of the transmission of the holding signal from oscil
(b) Frequency selective networks used in the receivers
can be made to have as high a selectivity as desired. A 45 lator 1S and the commencement of the audio frequency
tones from oscillators 12 and 13. Either therefore the
large number of calling tones may be obtained within a
switch 19 must be distinct from the switch 16 and the
narrow frequency band.
switch '17 and must be operated manually or automati
(c) The receivers cannot be operated by interfering
cally in such way that there will be a slight overlap in
signals, such as speech, since they are operated by the
50 the transmission of the different signals, or else all these
absence of signals.
switches may comprise different contacts of the one mul
(d) ‘It is possible to send a call simultaneously to all
tiple switch which is arranged so that one contact shall
receivers, say, in emergency.
be made before another is broken.
(6) When two or more calling tones are allocated to
The output of the transmitter '10 will therefore always
select individual receivers, it is possible to call simul
taneously a group of receivers to which one frequency has 55 comprise either the tones generated by oscillators 112 and
13, or the holding signal from oscillator 18. It may
been allotted in common.
also include speech at the same time as the holding signal.
In a preferred embodiment of the invention each re
In the receiver shown in FIG. 2, receiving antenna 25
ceiver is provided with one or more frequency selective
networks each of which is arranged to attenuate a cer
tain audio frequency. There is normally transmitted from
is coupled to the ampli?er and demodulator .26. The de
vice 26 may comprise suitable conventional circuits which
produce an audio frequency outputv from the received
the transmitter a holding signal which is within the pass
waves. The antenna 25 may comprise a dipole ?xed to
range accepted by the receiver but outside the range of all
a vehicle
the case of a mobile vehicle radio system,’ or
the frequency selective networks. Each receiver is pro
a ferrite rod. The output from ampli?er-demodulator
vided with an indicator which is prevented from indicat
ing by receipt of a holding signal or other signal trans 65 26 is applied to the path consisting of the ?rst frequency
selective network 27, ampli?er 23, second frequency selec
mitted through its frequency selective networks. To call
tive network 29, ampli?er 30, voltage sensitive detector
a given receiver, there are transmitted the tones which
31, relay 32 and visual and/ or acoustic indicator 33. The
will be rejected by the frequency responsive networks of
acoustic indicator may include an oscillator as tone gen
that receiver, and the holding signal is interrupted. At
the desired receiver, the frequency selective networks 70 erator. The relay 32 may comprise an electromagnetic
relay, ‘or a thermionic tube or transistor circuit. The ?rst
prevent those tones from reaching the indicator, and in
frequency selective network 27 is arranged to produce an
the absence of the holding signal, the indicator of that
3,090,838
4
in?nite attenuation for one, )3, of the two signals allo
cated to that particular receiver, and the second frequency
selective network 29 is arranged to produce an in?nite at
tenuation of the other, f2, of these two audio frequency
signals. , The relay 32 when operated may close switch
34 to cause the output of the ampli?er-demodulator 2,6 to_
be applied to the reproducer 35, switch 34 having means
for holding it closed until it is reset.
In operation, at the transmitter shown in
1, switch
19 is normally closed and switches 16 and 17 are open. 10
The holding signal from oscillator .18 is now transmitted
to all the ‘receivers. At each of the receivers, the output
of the ampli?er-demodulator 26 will comprise this hold
ing signal. The holding signal is arranged to be of a
frequency, that will not interfere with the reception of
speech, but it is within the pass range of the receiver am
each arranged to attenuate the frequency to which they are
tuned and pass all other frequencies.
According to such a system, receivers A and B may be
called by transmitting frequency f1, while receivers C and
D may be called by transmitting frequency f2. ‘It is thus
possible to call a plurality of receivers comprising one
group of a plurality of groups of receivers by transmit
ting a frequency corresponding to the common frequency
of the receivers in the group. If each receiver includes
in addition a second frequency selective network, such as
networks 29A, 29B, 29C, 29D shown in FIG. 3, each
‘tuned to a di?erent frequency, it is also still possiblerto
call each receiver individually by transmitting its appro
'priate calling frequency.
7
‘The frequency selective networks 27, 29, can be of any
known type having a substantial attenuation’ at one fre
pli?ers 128 and 39. The holding signal is of a different
quency, for example bridges or bridged T circuits. The
frequency to the selecting tones so that it will not be re
selectivity of the receivers will depend upon the gain of
the ampli?ers 23 and 39. By using sui?cient gain, a large
jected by‘ any of the frequency selective networks 27
and 29. When the holding signal is received, therefore, 20 numberrofr selecting tones may be used Within a narrow
frequency band, and therefore for many applications it
it will pass through frequency selecting network 27, am
pli?er 28,1frequency selecting network 29, ampli?er 30
may be su?icient to employ one selecting tone only, with.
to the voltage sensitive detector 31.
one frequency selective circuit in each receiver. Use of
The detector 31 is
two or more selecting tones, however, in addition to
arranged to operate the relay 32 and/or lamp 33 when
the output of the ampli?er 3t! falls below a speci?ed level. 25 increasing the number of receivers that may be used, ‘also
enables group selection,
This level will depend’ upon the distortion and noise trans
The selectivity of the system will‘ also depend upon the
mitted through the frequency selective networks 27 and
amount of harmonic distortion present in the signal, for
29v and the permissible signal level variation from the am
the frequency selective networks will attenuate only the
pli?er-demodulator 26.
‘In order now to call the receiver shown in FIG. 2, oscil 39 fundamental frequency. iIt would of course be possible
lator :12 is set to generate a tone of frequency f1 and oscil
to provide a second rejector circuit to remove the sec
lator 13 is set to generate atone of frequency )3. Switches
ond harmonic, but this will not generally be necessary.
In the event of, say, an emergency arising,’ when all
16 and 117 are now closed and switch 19 opened. Tones
receivers are to be alerted, it is only necessary to cut olf
f1 and f2 are included in the output of the transmitter i10
from antenna 11 and are received by the receiving an 35 the outputs of both, tone generators 12 and :13 and’ also
the holding signal generator v18. ‘Since none of the re~
tenna 25. The output from ampli?er-demodulator 26
will now comprise tones f1 and )‘2 only. Tone f1 Will be
attenuated by frequency selective network 27, and tone
f2 will' be attenuated by frequency selective network 29,
so that no signal will pass-through ampli?er 30, detector
31 and relay 32. Indicator ‘33 will operate and contact
ceivers will receive a signal, their visual vindicators, and
local acoustic signal generators, if provided, will all re
spond. Similarly, if the system is required for a purpose
in which it is necessary that persons at the receivers shall
know whether or not they are in touch with the trans
mitter, as for example in aircraft control, a breakdown at
the transmitter will cause all receivers to be continuously
by reproducer 35.
alerted. Also, in systems, such as inductive systems with
'When the tones f1 and f2 have been transmitted for a
suf?cient time, at the transmitter, 1FIG. 1, switch 19 is 45 in a building, in which a receiver may inadvertently be
moved outside the range of the transmitter, this will be
closed and switches 16 and 17 are opened. The holding
indicated by continuous alerting of the receiver.
signal will again be transmitted. Instead of switch 34
Although it has not been illustrated in the drawings, '
controlled by relay 32, there may be employed the switch
7 hook of the normal telephone set, or another contact re
provision may be made for two-way communication,'
50 and in fact each station may be provided with a trans
quiring manual operation by the person alerted.
When it is desired to call another receiver from the
mitter as shown in FIG. 1 for‘ calling the other stations,
and a receiver as shown in FIG. 2 for receiving calls
transmitter, oscillators '12 and 13 will be set to produce
from the other stations.
another,’ frequency combination, for example tones f1
and f3. In the receiver circuit shown in FIG. 2, frequency
While in the systemdescribed above the selecting tones
selective network 27 will ‘attenuate tone f1, but neither 55 and the holding signal are transmitted as modulation
frequency selective network 27 or 28v will attenuate tone
of a carrier, this is not essential to. the invention, but,
f3, and hence this ‘receiver will not be alerted. IIf, how
in cases such as inductive systems, or in telephony over
34 will close so that a spoken, message can be reproduced
ever, switches 16 and 17 at the transmitter are arranged
so that they maybe operated individually, then bytrans
?xed wires, the selecting tones and'holding signal may
be transmitted over audio signalpaths. Thus, in place
mitting one frequency alone, say tone f1, there will be 60 of the ampli?er-demodulator 26, any other source of
the audio-frequency tones and control signal maybe
' alerted that group of receivers which have a frequency
used. In particular, the invention may. be employed for
selective network that will attenuate this one frequency,
Whereas the remaining receivers will be held silent by
establishing communication between telephone exchanges
this one frequency. ' The holding signal will always be
and/or subscribers, when, if the receiver has its. own
transmitted as an accompaniment of'speech, to holdv all
power supply, an indication will be given of system. fail- .
the receivingv selectors silent, and hence itrwillebe impos
ure. Also,‘ while the invention has been described above
isble’for speech to cause. undesired alerting of a receiver.
Such a system isillustrated in FIG. 3, whereineach of
receiversyA and [B includes. frequencyselective networks
in conjunction with the transmission of messages, it may
in fact be employed for any purpose in which a select;
ing operation is to be performed, such as in the remote
27A and 27B respectively, each tuned to. frequency f1, 70 control of switches. Further, the various’ devices to be
selectively controlled may be diiferent'portions of the
and each of receivers C and D includes frequency selec
tive networks 27C and 27D, each tuned to frequency f2
same apparatus.
different from. frequencyrrfl, Networks 27A, 2713,270
'
"
‘
It is not essential that the selecting tones be within
the audio'frequency range, but‘ in, for, instance, a tele
and. 27D are similar to network 27 of FIGLZ, and are 75 phone system in a mine in which a ?xed transmitter and
3,090,838
6
mobile receivers carried by mine locomotives are induc
tively coupled to a conductor extending through the
mine galleries, the selecting and holding frequencies may
be within the long wave radio band.
The holding signal could, if desired, itself be used for
signaling, for instance, pulses of holding signal could be
transmitted with the selecting tones to cause intermittent
operation of the receiver indicated at the selected station,
which will permit an alerted person to distinguish be
tween an intended call and a transmitter failure.
What is claimed is:
1. A selective signaling system comprising a trans
mitter and a plurality of receivers, means at said trans
mitter for producing a signal of particular frequency,
tioned signal to produce an output signal with said pre
determined level, said ?ltering means having a further
preselected frequency versus attenuation characteristic
for passing said two other signals with substantial at
tenuation thereby rejecting said last-mentioned two other
signals to produce an output signal with a level less than
said predetermined level of said particular signal.
5. The signaling system according to claim 4 in which
said receiver means includes additional means connected
10 to the output of said ?ltering means and controlled by
the predetermined-level output signal thereof for estab
lishing said non-operative condition in said receiver, said
additional means being further controlled by the less
than-predetermined level output signal of said ?ltering
means at said transmitter for producing a plurality of 15 means for changing the non-operative condition of said
receiver into an operative condition.
signals of frequencies different from each other and from
6. The signaling system according to claim 5 in which
said particular frequency for identifying discrete re
said transmitter includes voice-signaling means perma
ceivers, means at said transmitter for alternately render
nently connected thereto, and said receiver means also
ing said particular signal means or said different signal
means effective to transmit the respective particular or 20 includes voice~reproducing means connected to said addi
tional means and rendered non-operative thereby in re
different signals to said receivers, and frequency selective
sponse to the predetermined level output of said ?ltering
means at each of said receivers to pass said particular
means for establishing said non-operative condition in
signal received thereat to produce an output signal with at
said receiver, said voice-reproducing means being changed
least a predetermined level for simultaneously establish
ing non-operative conditions in all said frequency selec 25 from said non-operative condition to said operative con
tive receivers, said means at said receivers identi?ed by
one of said different signals selectively rejecting said one
different signal received thereat to produce an output
dition by said additional means in response to said less
than-predetermined level output of said ?ltering means.
7. The signaling system according to claim 6 in which
said receiver means includes a visual or audible indicator
signal with a level less than said predetermined level of
said particular signal for selectively changing the non 30 also connected to said additional means, said indicator
being non-activated by said additional means in response
operative condition at each of said receivers to an opera
to the predetermined-level output signal of said ?ltering
tive condition, said receivers normally being operative in
means for showing the non-operative condition of said
the absence of signals of said particular frequency.
voice-reproducing means, said indicator being activated
2. A selective signaling system comprising a trans
mitter and a plurality of groups of receivers, means at 35 by said additional means in response to the less-than
said transmitter for producing a signal of particular fre
quency, means at said transmitter for producing a plu
predetermined level output signal of said ‘?ltering means
for showing the operative condition of said voice~repro~
ducing means.
8. A selective signaling system comprising a trans
identifying one of said receiver groups, means at said 40 mitter and a plurality of receivers, means at said trans
rality of signals of frequencies different from each other
and from said particular frequency, each different signal
mitter normally effective to apply a ?rst signal to said
transmitter for transmission to receivers, means at each
signal or one of said different signals, and frequency
of said receivers for passing said ?rst signal received
selective means in each receiver of each of said receiver
thereat to produce an output signal with a predetermined
groups to pass said particular signal to produce an out
put signal at a predetermined level for simultaneously 45 level for effectively disabling all said receivers, means
at said transmitter for disabling said ?rst-mentioned
establishing a non-operative condition in all receivers of
means thereby to prevent the transmission of said ?rst
all said receiver groups, said frequency selective receiver
signal, and means instantaneously operative at said trans—
means of each receiver of one of said receiver groups
mitter when said ?rst-mentioned means is disabled for
selectively rejecting one of said different signals received
thereat to produce an output signal with a level less than 50 selectively applying any one pair of a plurality of pairs
of other signals to said transmitter for calling one of
said predetermined level of said particular signal for
said receivers, said means at one of said receivers selec
changing the non-operative condition of each receiver of
tively rejecting said last-mentioned one pair of said other
one of said receiver groups into an operative condition,
signals to produce an output signal with a level reduced
said receivers normally being operative in the absence
of signals of said particular frequency.
55 below said predetermined level of said ?rst signal for
effectively calling said one receiver by changing said
3. A selective signaling system comprising a trans
last-mentioned receiver from said disabled condition to
mitter and at least one receiver, means at said transmitter
an activated condition, said receivers normally being
for alternately transmitting a signal of a particular fre
operative in the absence of signals of said particular
quency or two other signals having frequencies different
from each other and from said particular frequency, and 60 frequency.
9. The system according to claim 8 in which said
frequency selective means at said receiver to pass said par
means at each of said receivers includes two ?lter net
ticular signal received thereat to produce an output signal
works connected in tandem, each of said ?lter networks
with at least a predetermined level for establishing a
being provided with a preselected frequency versus at
non-operative condition in said receiver, said receiver
means rejecting said two other signals received thereat 65 tenuation characteristic which is di?erent from that of
each of the other ?lter networks, said two ?lter networks
to produce an output signal with a level less than said
at each of said receivers passing said ?rst signal to pro
predetermined level of said particular signal for changing
duce an output signal having substantially said predeter
the non-operative condition of said receiver to an opera
mined level for disabling said receivers, each of said tWo
tive condition, said receiver normally being operative in
the absence of signals of said particular frequency.
70 ?lter networks at said one receiver selectively passing
4. The signaling system according to claim 4 in which
one signal of said one pair of said other signals substan
said frequency selective receiver means includes ?ltering
tially with no attenuation while passing the other signal
means having a preselected frequency versus attenuation
of the same pair of said other signals substantially with
transmitter for alternately transmitting said particular
characteristic for passing said particular signal substan
maximum attenuation thereby rejecting said one pair of
tially without attenuation thereby passing said last-men 75 said other signals to produce an output signal having a
3,090,838
level reduced below said predetermined level of said ?rst
signal thereby calling said one receiver by changing said
ing means and associated indicator being ‘disabled‘by
said predetermined-level output of said tandem-connected
last-mentioned one receiver from the disabled condition
to said activated condition.
10. The system according to claim 9 in which each
?lter networks being applied to said controlled means
connected to said receivers, said voice-reproducing means
of said receiver means includes means connected to the
output of said tandem-connected ?lter networks and con
trolled by said predetermined level output thereof for
simultaneously disabling all said receivers, said last-men
and associated indicator of said one receiver means being
changed from said disabled condition to said activated
condition by said level reduced below said predetermined
output of said tandem-connected ?lter networks being
applied to said controlled means connected thereto in
'
tioned means at said one receiver being further controlled 10 said last-mentioned one receiver.
by said level reduced below said predetermined level out
References Cited in the ?le of this patent
put of said tandem-connected ?lter networks therein for
calling said one receiver by changing said disabled con
,
UNITED STATES PATENTS
2,145,053
Meszar ______________ __ Jan. 24, 1939
connected thereto, and each of said receivers includes a
voice~reproducing means and a visual or audible indi
cator connected to the output of said controlled means
2,293,869
2,513,342
2,658,112
2,677,014
Vaughan ____________ __ Aug. 25,
Marshall ______________ .__ July 4,
Davison _____________ __ Nov. 3,
Moynihan ___________ __ Apr. 27,
2,749,390
Lewinter _____________ __ June 5, 1956
connected to each of said receivers, said voice-reproduc 20
27,817,828
McGuigan ___________ __ Dec. 24, 1957
dition thereof to said activated condition.
11. The system according to claim 10 in which said 15
transmitter includes voice-signaling means permanently
1942
1950
1953
1954
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