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Sept. 17', 1946.
Filed Sept. 18, 1945
a Sheets-Sheet 1
Sept“ 17, 1946..
Filed Sept. '18, 1943
6 Sheets-Sheet 2
D58 D56 DSD
3&4 3&4 3&4
Y cs2
BY§;% Z
Sept. 17, 1946.
Filed Sept. 18, 1945
6 Sheets-Sheet 4
[40 'ms. on
360 "ms. OFF >
'Sept.'17, 1946.v _
Filed Sept. _18, 1943
6 Sheets-Sheet 5
'Sept.‘ 17, 1946.
Fil’e‘d Sept. 18. 1945
e Sheets-Sheet e
Patented Sept. 17, 1946
Charles Gillings and Charles Edmund Beale,
Liverpool, England, assignors, by mesne assign
ments, to Automatic Electric Laboratories, Ihc.,
‘a corporation of Delaware
Application September 18, 1943, Serial No. 502,885
In Great Britain October 30, 1942
11 Claims.
(Cl. 179—18)
‘The present invention relates to telephone or
like signaling systems and is more particularly
difficulty by suitable arrangement of the signal
concerned with the setting and control of auto
According to one feature of the invention it is
arranged to retain the advantages of the prepare
signal and the code method of signaling described
in the ‘prior case but an improved signal code is
vision. From one point of view the invention may
employed such that a maximum of tWo fre
be considered as a development of that disclosed
quencies only are applied to the line at any in
in our application Serial No. 502,884, ?led Sept.
stant for any particular code. This has been
18, 1943, and ?nds advantageous application in 10 achieved by dividing the numerical digit codes
circumstances where a particularly low limit is
into two groups representing the digits 1 to 5 and
set to the ‘power which may be transmitted over
6 to 0 and using a different prepare signal for
the trunk line for dialing and supervisory signals.
each group to distinguish digits for which the
In the arrangement disclosed in the prior speci
code signals may be identical.
?cation the numerical direct current impulses
A typical code on this basis is set out below and
received at the sending end of the trunk were
this is assumed to be employed in the example
converted into Voice frequency coded signals
which follows.
matic telephone switching apparatus over long
trunk routes involving mixed systems of voice
frequency and direct current dialing and super
which were transmitted over the trunk and re
converted at the terminal end into numerical
different frequencies designated W, X, Y, and Z
were used to characterise the code signals and
the arrangement was such that each voice frequency coded numerical signal was preceded by
a pre?x signal which was the same for all nu- -
direct current impulses.
Combinations of four
Prepare Code
merical codes and comprised a pulse of all four
As in the prior ‘arrangement the coded prepare
frequencies W, X, Y, Z, applied simultaneously.
pulse may have a duration of 160 in. s. followed
‘by the coded digit pulse which has a duration of
The pre?x signal was in eliect a prepare signal
which conditioned the V. F. receivers at the dis
> 100 m. s.
tant end of the trunk to receive the V. F. signals 30 The voice frequency supervisory code contem
and also brought about the switching of any echo
plated remains the same as in the prior arrange
suppressor in the route into the correct direction
ment and‘ is quoted below for reference pur
of transmission.
This method of signaling was based on a maxi
mum power limit to line of the order of 2 milli- 35
watts and since when a compound signal was
used, the power was divided in equal proportions
seizing pulse ___________
____________________ __ 100 m, s, X,
between the number of frequencies used in the
compound signal, in the case of a prepare signal
Subscriber answersignal --------------------- -~
comprising all four frequencies W, X, Y, Z, the 40 Aknowle d gment 510ml
power level of each individual frequency was 0.5
milliwatt. Where such signal codes have a duration of 2 or more seconds, there is a danger on
certain trunk routes of overloading the ?lters
peat-ed until ac
"""""""""""""""" " 140 mIsIYf
Called Subscriber clear signal ———————————————— -- {mgggt-ei-fpace (‘9'
operatowcléarsicnal- _
""""""""""""""" " 2081213535:
and other such apparatus if a maximum power 45 0mm“ “18” °“ ‘10mph’ ------------------- -- {300111. 5. Y.‘
level of 2 milliwatts is used and it may be neces
sary to reduce the maximum power level to line
One of the advantages of the earlier arrange
to a value of the order of 0.66 milliwatts. This
ment was that the use of the WXYZ prepare
limitation tends to increase the difficulties of de
signal exercised all tuned circuits so that pro
signing a V. F. receiver which must respond to a 50 vided the prepare signal was received satisfac
code signal comprising, for example, four fre
torily, the following digit code signal comprising
quencies because of the division of the available
power which is thus reduced to a comparatively
small value in each of the four receiving circuits.
a lesser number of frequencies would also be
received satisfactorily and hence the risk of set
ting up Wrong numbers was considerably reduced.
The chief object of the invention is to avoid this
In order that this advantage shall be retained
in the improved signaling arrangement where
or less, relay CNA operates over its left-hand
the prepare signal no longer exercises all tuned
winding and as a result a short interdigital pause
circuits simultaneously, it is arranged according
is provided between successive retransmitted dig
to another feature of the invention that when
the prepare signal WY is used to characterise
the numerical ‘code group 1 to 5, the actual code
its as will appear subsequently. Relay CNA op
erates relay NS which at contacts nsl advances
the, sending control switch SC from its home
position to contact I where relay ST is operated.
Relay ST connects up its low resistance winding
to self-drive the sending control switch to con
tact 4, maintains relay CS and at contacts st3
and std causes a prepare pulse of WY frequency
to be connected to the trunk line to the distant
. signals in this group will be made up of a combi
nation of W and Y frequencies plus the X fre
quency which has already been tested as regards
the correct operation of the receiver by its re
ception as a seizing X pulse when the equipment
is taken into use. Similarly when the W‘Z pre
pare signal is used to characterise digits in the
group 6 to 0, the actual code signals for these
digits will be made up from the W and Z fre-,
quencies plus the X frequency which has already
been proved as regards correct reception durin
the seizing of the equipment.
The invention will be better understood from
the following description of one method of carry
ing it into eifect reference being had to the ac
companying drawings comprising Figs.
exchange.’ When the sending control switch SC
reaches contact 4, relay ST releases after its slow
period to disconnect the WY prepare'pulse and
operate relay S. The duration of the prepare
pulse is measured by the time taken for switch
SC to step to contact 4 plus the slow release
period of relay ST which gives a total ?gure of
160 m. s.
Relay S holds relay CS and connects the digit
code pulse to the line, this pulse being dependent
for its composition on the setting of the digit
Figs. 1-4 of these when arranged in the manner
indicated in Fig. 4a show the circuits of an out
switch DSA and is thus characteristic of the digit '
going relay set, while Figs. 5 and 6 when ar 25 dialled. Relay S short-circuits its high resistance
ranged side by side with Fig. 5 on the left show
winding to self-drive the switch S0 to position 8
where relay Z operates and at contacts 24 dis
the circuits of an incoming relay set, the ar
rangements being suitable for connection at op
connects the digit code pulse from the trunk line.
posite ends of an interexchange V. F. trunk line
The duration of the code pulse is measured by
with operation on a sleeve-controlled basis.
30 the time taken for switch SC to step from con
It will be understood that these circuits are in
tact 4 to contact 8 which is approximately 100
many respects identical with those disclosed in
m. s.
the prior speci?cation and therefore the descrip
tion which follows will only deal in detail with
the modi?cations which have been made to these
lays CNA and CNR and causes switchDSA ,to
self-drive to its home position where relay Z
releases and releases relay CS and causes switch '
circuits to adapt them to function with the re
SC also to drive to its home position.
vised code signaling arrangements which consti
tute the main feature of the present invention.
' Referring now to the outgoing relay set, Fig. 1,
when the operator plugs into the jack shown,
relays M and MM, Figs. 1 and 4,v operate where
upon her supervisory lamp will ?icker due to the
connection of ?icker‘earth over lead 9. Relay
MM in operating brings up relay SR, Fig, 4,
which causes switch TS to advance to position
I where relays CO and FXY are operated. Relay
FXY in operating brings up relay CS and'causes
switch TS to advance to position 3 whereupon
relays FXY and CS release. During this time a
100 m. s. pulse of X voice frequency will be trans
mitted forward over the trunk line to the incom
ing relay set at the distant exchange to bring
about the seizure of an associated ?rst selector.
The operator now operates the dialing key,
whereupon relay RR, Fig. 1, is operated and the
supervisory lamp is extinguished. 'Relay'RR in
operating brings up relay A and this in turn
' brings up relays B, Fig. 3, and BR, Fig, 4, the ‘
latter releasing relay RR.
' When the operator dials the required sub
scriber’s number, the impulses are received on
relay A which repeats them via relay C, Fig‘. 3,
to the driving magnet DSAM of the ?rst digit
Relay Z releases relay NS and holds re
'If in the meantime the operator has dialled
"i6 second digit, say 4, on to switch DSB (not
shown), relay CNR will be maintained operated
in series with relay CNB (not shown) which re
operates relay ‘NS on the release of relay CNA.
Accordingly, when the switch SC comes into its
home position, it will again self-drive to contact
l where relay ST will re-operate to send out the
prepare pulse of WY frequency followed by the '
code pulse KY for digit 5 which is picked up
from the setting of switch DSB.
If, as assumed, the ?rst digit dialled was 5 or, '
less, the interdigital pause between the two code
digits transmitted to line will be measured by 20
self-driving steps of the switch DSA, say 400
m. s., plus 16 self-driving steps of switch S0, say
320 m. s., giving a total of I720 m. s. During this
period ?ve machine-generated-pulses are deliv
ered at the distant exchange to the incoming se
lector after which hunting takes place to select
an idle trunk in the level selected. While the
selector is hunting, the incoming de-coding re
lays are released and re-set to receive the second
prepare and code signals which will require a
‘time period of 160 plus 100 m. s. giving a total
of 260 m. s. which added to the 720 m. 5. already
mentioned gives a total of 980 m. s. for the com
plete interdigital pause.‘
storing‘ switch DSA which steps its wipers to a
characteristic position. Relays B and C hold op 65
If the first digit dialled had been 6 to 0, a
longer interdigital pause would be required to 7
erated during the impulsing and the latter oper
enable the incoming selector to perform its se
ates relay CS, Fig. 4, to prepare the V. F. signal
ing circuit. Relay C also energises the driving
quence of operations and this will be described in
magnet DDM of the digit distributor switch DD
relation‘ to the change of the prepare signal
but the wipers of this switch do not move until
which must also take place to characterise a digit
the magnet is de-energised.
in the second group. Under these conditions re
lay CNA will be operated over its right-hand
At the end of the ?rst series of impulses, relay
Winding in series with relay CNR and at contacts
‘ ‘A holds and relay C releases after its slow period
thus operating relays CNA and CNR, Fig, 3, in
cna3 brings about the operation of relay HS
series over wiper DSAI. If the digit dialled is 5 75 which looks independently of wiper SC2 via its
contacts he I. Relay N8 is inoperative in these
the V. F. receiver VFR, Fig. 2, operates and‘opens
circumstances owing to the shunting effect of the
low resistance common relay CNR. Relay HS
at contacts ‘hsl, Fig. 2, changes over the connec
the circuit for relay BS. On the release of relay
BS, relays MS andMT are operated in .turn .and
hold for the remainder of the Y pulse. When
the Y pulse is ?nished, relay BS re-operates and
further opens the circuit ‘for relay MS. If the
interval between received pulses is of the cor
rect duration, relay MS will release ‘but not relay
MT, so that on reception of the next Y pulse,
relay SA will be operated and at the end of this
pulse relay SB operates in series with relay SA.
tions of the prepare pulse circuit so that when
relay ST operates on the ?rst step of switch SC,
a WZ pulse is connectedto line. This is followed
by the coded digit pulse ‘and when relay Z oper
ates to terminate the pulse, switch DSAdrives to
its home position thus measuring off part of the
interdigital pause. Relay HS is then discon
nected at wiper and bank DSAl but holds for
a short period because of its slug and at contacts
hs2 causes switch .DSA to make another half
During this latter operation relay
HS is released but relay Z .remains held and thus
brings about a substantial increase in the inter
digital pause, For example, if the digit 0 has
been dialled the ‘interdigital-pause would be made
up as follows: 14 steps of the switch DSA plus an ‘
additional half-revolution making 39 steps in all
gives a total period of ‘7180 m. 5. To this‘must be
added 16 steps of switch SC, say 320 m. s., plus a
further260 m. s. representing the transmission
time or the prepare and code pulses for the sec
ond. digit giving a total intercligital pause of
1,360 ‘m. s. The purpose of this extra pause is to
ensure that the distant selector has sufficient
time to perform its level-selecting function plus
its trunk ‘shunting before the next decoded digit
is transmitted.
When all stored digits have been transmitted
in code relay NS or HS ?nally releases so that no
further kick-on circuit is completed for the send
ing control switch SC. All‘ the relays CNA-CND
and relay CNR will also be restored to normal.
Although four digit switches DSA—DSD have
been provided, it is possible in practice that a
lesser number may be found satisfactory depend
ing upon the speed at ‘which the storing and re
transmitting in code can be accomplished. The
digit storing switches DSA to DSD are taken into
use in cyclic repetition, that is to say, when the
fourth digit has been stored on DSD the switch
DSA which will be normal by this time will be
again taken into use to store the ?fth digit and so
on for as many digits as are concerned. Slight
circuit modi?cation, such as is described in the
prior-speci?cation are desirable to ensure that the
cyclic function is operative under all conditions.
When the operator restores her dialling key
at the end of dialling,‘ the current in the sleeve
circuit is increased and the battery applied to the
tip and ring conductors is removed shortly after
Wards.‘ Relay KR, Fig. 1, thereupon operates and
releases relays A, B and BR in turn. Relay B
in releasing homes the digit distributor switch
DD (Fig. 3) to its mid-position preparatory to
the operation of relay AA at a later period, while
relay BR in releasing connects the low resistance
lower Winding of relay RR into the sleeve circuit,
Relay SB‘ operates relay AA, Fig. 3, and relay AA
re-operates relays CS, Fig. 4, and at contacts aal,
Fig. 2, applies X frequency to the outgoing trunk.
The duration of the X frequency acknowledg
ment signal is measured by twelve steps of the
switch DD plus the release time of relay AA which
releases when the DD switch reaches the home
position, and which in turn releases relay CS.
When the X pulse is received at the distant in
coming relay set, transmission of the Y ipulse
ceases and relay BS remains operated, while re
lay MT releases. Relay SA in operating gives
local and through supervision at the outgoing
operator’s position.
Conversation now proceeds, ‘and when the
called party clears, the incoming relay set trans
mits Y pulses of the same type as for the called
party answer signal. At the outgoing end relays
M, MM, KR, CO, FC, BS, SA and SB are held
operated so that the ?rst 140 m. s. of Y frequency
releases relay BS and operates relays MS and
MT as already described. The following 360
m. s. break period operates relay BS which re
leases relay MS and when the next Y pulse is re
ceived relay BS again releases and during the slow
release period of relay MT relay SY now oper
ates. Relay SY locks up to the Y pulse, oper
ates relay SZ and releases relay SA which gives
the necessary supervision at the outgoing oper
ator’s position.
When the operator clears by removing ‘the plug
from the jack, relays M, M and KR release
in turn. Switch TS is thereupon stepped from
position 6 to position 1, whereupon relay BR
operates in series with relay SR. Relay CS now
re-operates whereupon X frequency is applied to
the trunk line. With relay SB operated the switch
TS self-drives from position ‘I to position ~15,
and then steps by means of interrupted earth
on lead E3 to position l9. Relay FXY is then
operated to disconnect the X frequency after a
period of application of two seconds, while. it also
connects up Y frequency to the trunk line. On
the same switch bank TSA, relays ‘SR and BR
are caused to release after their slow periods, and
on the release of relay BR, earth is extended
, over bank TS3 to step the switch from position
19 so that relay FXY releases, and after its
60 slow release period terminates the Y frequency
after a period of‘ application of 300 milli-seconds. “
Switch TS then steps from position 2K! to posi
relay RR being non-operative, but causing the
tion 23 via bank TSB and then slow ‘step-s over
operator’s supervisory lamp to glow. Relays M,
contacts 23 and 24 via bank TS5 to reach its home
MM, KR, 00, FC and BS remain operated and 65 position where relays CO, FC and SB release.
the line condition is now such that the operator
During the application of Y frequency relay
may listen to tones and, if necessary, speak with
BS will have released and relays MS and MT
out the necessity for a called party answer signal,
operated but without useful effect.
but no supervision is given.
The circuit operations under special conditions
When the called party answers, the incoming 70 where the operator clears before receiving the
relay set transmits back 140 m. s. pulses of Y
frequency at spacings of 360 m. s., and this signal
is repeated until acknowledged by the transmis
answering signal, or in the face of any tone ‘or
on a non-metered call, or Where the outgoing
equipment is taken into use from a selector level
sion of an X signal from the outgoing relay set.
or where the operator inadvertently withdraws
Upon reception of the Y frequency, relay Y in 75 the plug during conversational conditions, are
describedin the previously mentioned speci?ca
' Considering now the operations in the incom
ing relay set, Figs. 5 and 6, when the 100m. s. X
seizing pulse is received over the trunk, relay IX
(not shown but assumed to be located in the Valve
of bank SS3 over contacts of the coding relays
XR and YR; while at the same time the impulse
springs 33% M count out a corresponding number
7 receiver IVFR) operates and brings up relay
of impulses to the incoming selector; . When the , ‘
XR. Relay XR at contacts :rr3 operates relay
, K which looks and operates relay PY and extends
earth forwardly over the P conductor to the in
coming selector to busy this switch. At the end
of the X seizing pulse, relay IX releases and
drops relay XR which operates relay NN, Relay
1 '
about the operation of relay IG. Accordingly,
the sender switch SS now steps its wipers until
they encounter the marking earth on contact?
NN further earths the P conductor to the selec
appropriate sender-switch wiper, SS3 in this inq
stance, picks up themarking, relay SZ- operates
to short-circuit the 33% M‘springs to terminate
the impulses to the selector and also release relays
CN and CB. Relay CB drops relay IG which
causes the sending switch SS to home while the
holding circuit for the selector is again trans
tor, operates relay IN over the negative line in 15 ferred to relay IN. Relay CB also drops relays
series with the A relay in the selector, and also
XR and YR and relay C releases after-itsslow
operates relay GX. Relay GX at contacts gxll
period and this is followed by the release of
causes a small current‘ to flow through the vari
relay CC also after its slow period. Relays K,
ous relay contacts in the line circuit which are
thus “wetted.” to prevent coherer trouble. The
selector is now ready to receive the train of
When the outgoing relay set transmits the 160
m.'s. prepare pulse of WY or WZ frequency as
described, relays IW and IY or IW and IZ oper
ate in the V. F. receiver in the incoming relay
set and bring up their relief relays WR and YR or
V WR-and ZR.
Depending upon which of the two
prepare signals is transmitted relay CN or relay
PY, NN, IN, GK and P. remain held.
. ;.
In order'to guard against an inductive kick via
the repeating coil RPCZ to the incoming side of
the circuit when relay IN is transferred to the
incoming selector on the release of relay 1G,,it is
arranged that relay C holds for a short period
because of its slug and maintains the loss pad
in circuit to absorb this inductive surge and pre
vent it interfering with the V. F. receivers on the
incoming side of the circuit.
After a pause long enough to permitthe in
\ CH is operated, either of which is effective to 30 coming selector to hunt over the selected level,
convert, the normal line termination into a loss
which cause is measured in the outgoing relay
pad with considerable attenuation which prevents
set as already described, the next combination of
the passage of signals beyond the incoming relay
prepare and code pulses is transmitted and the
set so that the line is blocked at this point.
operations described are repeated until all the
The main purpose of this arrangement is to pre 35 impulsetrains have been dealt with.
Vent a double transmission of impulses‘ which
The connection is “thus fully set up and the
might take place in connection with tandem
called subscriber isrung. When the called party
calls via a number of exchanges in series as is
answers, battery‘ is reversed'over the forward
fully described, in the prior speci?cation.
trunk so that relay IN releases and relay I oper
The prepare pulse of 160 ms. is followed by the 40 ates. Relay I brings up relay IR which alters the
code digit pulse of 100 m. s. duration.’ In the ex
connections to the loss pad to cut out the line‘
ample in question the ?rst digit is 5 so that relay
CN is operated in response to prepare pulse WY .
andprepares a circuit for relay CB which is at
present short-circuited. When the prepare pulse
is replaced by the code XY for digit 5, relay CN
' is energised via contacts chl , 0111, and :crZ and yrZ
in parallel, in series with relay CB to earth via
contacts s23, ir5, s04 and nnZ. Relay CB in op
erating locksrelaysXR and YR and these relays
over contacts :cr5, 2M and W5 of a suitable pyra
mid chain of contacts connect earth to the sixth
contact in the bank SS3 of the sending switch SS
in order to terminate sending after ?ve impulses
have been transmitted to the incoming selector
as will appear subsequently.
termination and give a straight-through connec
tion. Relay IR also releases relay K which opens
the circuit to relay PY and during the slow release
~r period of the latter, relay‘MA is operated. and
locked. Relay MA connects relay FY to inter
rupted earth on lead I 4 having a periodicity of
140 m. s. on and 360 m. s. 01f. ‘Relay FY there- '
fore pulses to extend backward'over the trunk a
repeated 140 m. s. Y pulse with 360 m. s. spacing.
When this signal is ‘accepted at the outgoing
relay set, the‘ latter returns a 440 m. s. X acknowl
edgement signal which brings up relays X and
KR in the incoming relay set. Relay XR opens
the circuit for relay GX and operates relay 'GY
which disconnects relay FY to terminate the an
swer signal. Relay GX releases after its slow
It will be noted that the contact pyramid of
the code relays WR—ZR connects markings to the
period and drops relay MA whichconnects the
'two banks SS3 and SS4 of the sender switch to
line through for conversation. At the conclusion
control the sending of digits 1 to 5 and 6 to 0 re 60 of the acknowledgment signal, relay XR releases,
»spectively. The particular bank which is to be
relay GX re-operates and relay GY releases.
effective at any time is determined by the selective
The remaining features of operation such as
operation of the relays CN and CH which are re
.normal clear down, clearing in face of tones,
sponsive respectively to the two types of prepare
clearing in face of a short-circuited trunk, non
signals which are used to pre?x the digit signals.
metered calls and tandem connections via a num
Relay CN which is operated at this time at con
ber of exchanges in series are fully described in
tacts cn? operates relays C and P and the latter
the prior speci?cation and will therefore not be
locks up. Relay C operates relay CC which pre
dealt with as they are identical in allrmaterial
pares a circuit for relay IG which operates when
the impulse springs 66% M next open. Relay IG 70 _ A possible alternative code to the one utilised
drops relay IN and transfers the forward holding
in the foregoing description is as follows:
loop extending to the selector to the impulse
springs 33% M which are closed at this time since
they are out of phase with the magnet impulse
springs 66% M which have just opened to bring 75
The use of the two frequency code for each
digit minimises the possibility of wrong numbers
frequencies used for seizure and for characteriz
ing the group, and in utilizing the received signals
which might occur in cases where mixed single
and double frequency codes were used and if the
source of one of the frequencies failed during the
sending out of a double frequency coded digit.
Its use Will of course involve rearrangement of
to produce numerical switching operations corre
sponding to the digit values of said subsequent
'7. In a signaling system including two stations
and a line connected therebetween, a source of
the bank wiring of the digit switches DSA-DSD
a series of impulses at one of said stations, trans
and of the contact pyramid extending to the
lating means controlled thereby adapted to trans
banks SS3 and SS4. Moreover it does not pre
serve the feature previously mentioned, that the 10 mit successively two composite alternating cur
rent signals over ‘the line in response to each seé
numerical code includes only those frequencies
ries of impulses, said two alternating current sig
which have already been used either for the seiz
nals jointly characterizing the number of impulses
ing signal or the prepare signal. In some circum
in a series and the composition of the ?rst sig
stances however the advantages of the stand
nal depending upon which one of a plurality of
ardised two frequency code may be held to out
groups the number of impulses in said series cor
weigh this slight disadvantage.
What we claim as new and desire to secure by
responds to, and means at the other station re
Letters Patent is:
1. In a telephone system having two offices
connected by a trunk line, a plurality of sources
of alternating current of different frequences at
sponsive to said two alternating current signals
for effecting a numerical switching operation cor
responding to the number of impulses in said
one of said offices, means at said one office for
successively transmitting over the trunk line a
8. A signaling system as claimed in claim 7
in which said translating means is’ adapted to
provide a variable pause between successive
groups of signals in accordance with which one
of said plurality of groups the number of im
plurality of alternating current impulses, each
composed of one or more of said frequencies, to -
transmit a digit, each impulse having a part in
determining the digit value, and means at the
other o?ice responsive to said impulses for effect
ing a numerical switching operation in accord
ance with the digit value of said impulses.
2. A telephone system as claimed in claim 1 in
which the digit values are divided into groups
and in which the ?rst of said plurality of im
pulses is characteristic of one of said groups.
3. In a telephone system having two of?ces con
nected by a trunk line, a plurality of sources
of alternating current of different frequencies at
one of said o?ices, means at said one o?ice for
successively transmitting over the trunk line two
alternating current impulses each composed of
not more than two of said frequencies, said two
impulses jointly characterizing a digit value, and
means at the other office jointly controlled by said
two impulses for effecting a numerical switch
ing operation in’ accordance with the digit value
of said impulses.
4. A telephone system as claimed in claim 1 in
which the nature of the ?rst of said plurality of
impulses varies in accordance with the digit value,
and said means at the other office being re- ‘
sponsive to the ?rst of said plurality of impulses
to produce a switching effect which is the same
. for all digit values.
pulses in the previous series corresponds to.
9. In a telephone system including two ex
changes and a line connected therebetween,
means at one of said exchanges for transmit
ting successively two composite alternating cur
rent impulses over the line, means at the other
of said exchanges responsive to the ?rst of said
impulses for preparing to effect a switching op
eration in one of a plurality of numerical groups,
said last means also being responsive to the sec
0nd of said impulses for effecting a switching op
eration in the numerical group prepared by the
?rst impulse.
10. In an automatic telephone exchange in
cluding impulse generating means and automatic
switches controlled thereby, an incoming line over
which groups of composite alternating current
impulses are at times received, means associated
with the line for selecting One of a plurality of
groups of marking circuits in response to the
?rst impulse of a group, and means response to
a second impulse in the group for selecting a
marking circuit in the previously selected group
and for causing said impulsing means to deliver
a series of impulses to the automatic switches in
the exchange, the number of impulses in said se
ries being determined by the selected marking
5. A telephone ‘systemas claimed in claim 1
in which the digit values are divided into groups,
the ?rst of said plurality of impulses being char
11. In a telephone system, a trunk line, trans
mitting equipment associated with one end of
acteristic of one of said groups, and said means
the trunk line and receiving equipment associated
at the other o?ice being responsive to the ?rst of
said plurality of impulses to produce a switch
with the other end thereof, means in the trans
mitting equipment for sending coded signals cor
responding to digits in two numerical groups
ing effect which is the same for all groups.
6. The method of controlling automatic tele
phone switching equipment over a trunk line
which consists in transmitting a ‘seizure signal
composed of a single frequency over the line,
subsequently transmitting signals representing
digit values over the line, each of said subsequent
signals consisting of two impulses, the ?rst of
which characterizes a group of digit values and
is composed of one or more frequencies, and the
second of which characterizes the digit value in 70
a group and is composed of one or more of the
over the line, said signals each comprising a ?rst
impulse varying in accordance with the numeri
cal group to which the digit belongs and a sec
ond impulse varying in accordance with the value
of the’ particular digit in the group, and means
in said receiving equipment responsive to said
coded signals for e?ecting a switching operation
corresponding to the digit value thereof.
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