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Патент USA US2408464

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Get. l, 1946..
R, 0, W|sE
2,408,452
TELEPHONE SYSTEM
Filed June 8, 1945
3 Sheets-Shea?l 1
Oct. 1, 1946'.
R'. o. wlsE
2,408'452
TELEPHONE SYSTEM
Filed June 8, 1945
@iw
:il
_5 Sheets-Sheet 2
Oct. 1, _1946.
R. o. wlsE
' 2,408,462
TELEPHONE SYSTEM
Filed June _8, 1943
3 Sheets-Sheet 3
WMUÜÉO
'
ATTORNEY
2,408,462
Patented Oct. 1, 1946
UNITED sTATEs PATENT OFFICE
21,408,462
TELEPHONE SYSTEMRaymond 0. Wise, Short Hills, N. J., assignor to
Bell Telephone Laboratories, Incorporated, New
York, N. Y.
VApplication June 8, 1943, Serial No. 490,033
10 Claims. (Cl. l79-è-l5)
This invention relates to telephone systems
I and particularly to systems in which connections
between calling and called lines are established
by subscriber controlled automatic selection of
idle channels through multichannel communi
cation media.
Among the objects of the invention are the
provision of a more efûcient and reliable common
2
modulators of the channels, fixed band-pass
ñlters interconnect the ñrst and second modu
lators of the channels, carrier sources of re
quired frequencies are selectively connected to
the second modulators of the channels, and car
rier sources of different frequencies are individ
ually connected to the demodulators.
These and other features of the invention are
embodied inthe systems shown schematically
medium multichannel exchange and the pro
vision therein of the large number of channels 10 in the drawings which form a part of this specifi
cation.
necessary in an automatic telephone system
Referring to the drawings:
i
~
without the use of expensive Variable frequency
Figs. l and lA show a telephone system com
ñlters.
.
prising telephone offices A and B interconnected
This invention is an automatic telephone sys
by
multichannel transmission media VCMII,
tem in which connections between calling and 15
CMIZ, CMI3 and CMM;
called lines are automatically completed under
Fig. 2 shows a telephone system comprising
the control of the calling subscriber over chan
offices
C and D interconnected by multichannel
nels of multichannel transmission media, one
transmission media CM2I, CM22, CMZS and
channel being used for each direction of con
versation. to constitute a conversational connec 20 CM24; and
Fig. Bishows aV telephone system comprising
tion, a feature of the invention being a trunking
telephone oliices E and F interconnected by multi
arrangement wherein a fixed band-pass ñlter is
channel transmission media CM3I, CM32, CM33
provided for the demodulator of each channel,
and CMM.
'
y
carrier sources of required frequency are selec
tively associated with the modulators of the 25 Consider first the system represented in Fig.
1. The subscribers’ stations SI, S2, S3 and S4
channels and a pentode ampliñer is provided
are of the usual type connected to lines termi
for each modulator to prevent interference be
nating in common battery automatic telephone
tween channels.
oñ'ìces and each of the stationsincludes a dial
Another feature of the invention is a- trunk-`
ing arrangement including channels of- a com 30 or other impulse sending device for controlling
the establishment of` desired connections.V The
mon medium wherein carrier sources of required
frequency areselectively connected tothe modu
lators, isolating resistances being provided for
each modulator to prevent cross-talk between
line switches LSI, LS2 andV final switches FSI
and FS2 may be of the crossbar type, the line
switches being operated in responsejto the-ini
channels, the number of modulators connected 35 tiation of calls at subscribers’ stations to con
nect calling lines to district circuits and the final
in parallel to a group or line amplifier being large
enough to prevent interference between channels.
A further feature is av trunking arrangement
switchesbeing directively controlled by ,sender
marker means to select calledA lines. Reference
may be had to the patent to W. W. Carpenter
including the channels of a common medium
wherein carrier Sources of required frequency are 40 2,093,117, granted September 14,~ 1937, for a de
tailed description of the operations `involved in
selectively connectedV to the modulators, isolating
resistances-'are provided for each modulator to
prevent cross-talk between channels, group filters
connecting .calling lines to district circuits and
the operations of final switches in connecting
with called lines in a crossbar system. The line
are provided for groups of channels, a carrier
source of a different frequency is individuallyr 45 switches LSI and L52 may be similar to the line
switch shown in Figs. 14 and l5 of the afore
associated with each demodulator, and a chan
mentioned Carpenter patent, the ñnal switches
nel amplifier is provided for each demodulator
FSI and FSZ‘ may be similar to the final switch
shown in Fig. 61 of said Carpenter patent, the
for establishing connections between -calling and 50 originating senders OSI .and OS2 may be similar
to the> subscriber’s register sender shown in Figs.
called telephone lines under the control of the
22 to 26 of said Carpenter patent, and the termi-` '
calling subscriber in which a double stage of
nating senders TSI and TS2 may be similar to
modulation is employed to obtain single lside
the terminating senders shown in Figs. 46 and
band operation, carrier sources of different fre
quencies are individually connected to :first 55 47 .of said Carpenter patent. lThe districtand t
to obtain satisfactory volume.
4
X A further feature is a carrier switching system
2,408,462
3
4
final circuits IIB, IZB, I3!! and |40 are similar to
the district circuit D-I shown in Fig. 5 of the
fiers being eifective to prevent cross-modulation
patent to Hubbard-Matthies-Myers, No. 2,345,048
between channels.
Assuming now that a call is initiated at the
dated March 2S, 1944 except that each of appli
cant’s modulators Ilil, |24, i3d and Idd has a
variable frequency carrier source associated
subscriber’s station SI, the line LI is extended
through a line switch LSI to a district circuit I Iû
and an originating register sender OSI is con
nected to the district, in the manner set forth in
therewith tunable under the control of anmorigi
nating marker and each of applicant’s demodu
lators H8, E28, I33 and M8 has a fixed frequency
channel frequency. Each of the originating
markers OMI, CM2 and terminating markers
TMI, 'PM2 is similar to the corresponding decoder
markers of the aforementioned Carpenter patent
except that it is arranged to control the tuning of
detail in the aforementioned Carpenter patent
beginning on page 14, line'68, second column and
`ending on page 26, line 3'7, first column. The
number of the subscriber’s station, with which a
conversational connection is desired, is dialed by
the calling subscriber and is registered in the
sender OSI, as described in the Carpenter patent
beginning ,on page 32, line 5, nrst column and
ending on page 36,'1ine 4l, iirst column. An orig
inating marker OMI is associated with the sender
and the number registered in the sender is de
coded, in the manner set forth in the aforemen
tioned Carpenter patent beginning on page 36,
line 44, ñrst column and ending on page 40, line
the variable frequency carrier source of the
38, first column.
carrier source associated therewith; whereas in
the Hubbard-Matthies-Myers patent the trans
mitter (modulator) T--ä of the district `circuit
D-i has a fixed frequency carrier source asso
ciated therewith and the receiver (demodulator)
R-5 of the district D-I is tunable Lunder the
control of an originating marker to a desired
15
modulator of a connected district or ñnal circuit
The marker OMI, thereupon, makes a trunk
in similar manner to that in which the originat
test, in the manner set forth in detail in the
ing marker OMI of the aforementioned patent 25 aforementioned patent to Hubbard-Matthies
to Hubbard-Matthies-Myers controls the tuning
Myers beginning on page 3, line 71, second col
of the receiver R-5 of the district D-I shown
umn and ending on page 5, line 46, second col
in Fig. 5. In the aforementioned Carpenter pat~
umn, to select a trunk or channel through media
ent, the originating decoder marker is shown in
CMI i and CMIZ which' is not in use and the de
Figs. 29 to 40 and the terminating decoder is 30 modulator of which in oiiice B isiassociated With
shown in Figs. 50 to 56. In the Hubbard
a iinal switch having access to the line of the
Matthies-Myers patent the originating marker
called station. The variable frequency carrier
OM is shown in Figs. l2 to 20.
source IIE of the modulator II4, which is asso
Each district circuit and each final circuit is
ciated with the district H8, is tuned to the re
connected, through a low-pass filter, tc a voice
quired frequency under the control of the' marker
frequency hybrid coil; and each hybrid coil is
OM! in similar manner to that in which the re
further connected to a line balancing network,
ceiver Rf--ö of the district D-I in the Hubbard
to a modulator :for one direction of communica
Matthies-Myers patent is tuned to the required
tion, and to a demodulator for the other direc
frequency as described in the Hubbard-Matthies
tion of communication. 'I'h‘e low-pass filter
Myers patent on page 6, beginning in line 54, first
passes frequencies below 3,000 cycles per second
column. A seizure signal is transmitted from the
7,and the modulators are preferably of the double
district circuit IIÜ through the low-pass filter
balanced copper-oxide type to obtain maximum
linearity and thus avoid eXtra band products
lli, hybrid coil H2, modulator H4 and channel
amplifier IIB, through common medium CMII,
which would increase cross-talk. Each demod
ulator has a particular carrier source associated
therewith and is connected to a multichannel
medium through a band-pass filter tuned to pass
a band, the cut-olf frequencies of which are equal
to the carrier frequency of the associated demod
ulator plus and minus the voice frequency band.
For instance, if the carrier frequency of a dernod
line amplifier LAI, common medium CMIZ,
through the particular band-pass filter I lll which
ulator is »100,000 cycles per second, the band-pass
ñlter associated with the demodulator in ques
tion passes a channel band of 97,000 to 103,000
cycles per second.
Thus there are as many
channels through the multichannel media CMI I,
CMIZ, CMI3 and CMM as there are demodu
is tuned to pass the carrier wave and signal band
transmitted by modulator I I4, to the demodulator
»Hi8 Whose carrier source |49 is of the same fre
quency as that of the source H5 connected to
modulator IIfi. The demodulated seizure signal
is thus further transmitted through hybrid coil
142 and low-pass ñlter MI to the ñnal circuit M0.
In response Yto the seizure signal reecived by final
circuit Mû, a terminating >sender TS2 is con
nected to the final circuit |40. Numerical sig
nals are now transmitted by the originating send
er OSH, through district circuit IIII, low-pass
lators connected thereto; and the carrier fre
filter lil, hybrid coil H2, modulator IIli, ampli
quencies have at least an 8 kilocycle spacing. Illi() ñer HS, common medium CMII, line amplifier
Each modulator is supplied with any desired one
LAI , common lmedium CMI'2, and band-pass filter
of a plurality of carrier sources, the connection
Ill?, to demodulator M8; and the demodulated
of a desired carrier being under the control of the
signals are transmitted through hybrid coil |42,
originating or terminating marker which is con
filter yIlil and final circuit M0 to the terminating
trolling the establishment of a desired conversa- A. - sender TS2, in which the identi-ty of the called
tional connection through the associated district
or iinal circuit.
As shown in Fig. l the marker
tunes a variable frequency carrier source to the
desired frequency; 'but this arrangement may be
modified according to Fig. 1A in which a desired „ '
one of a plurality of carrier sources is selected
lunder ,the control of the marker. Each modu
lator is _connected through a pentode amplifier to
one of the multichannel media, the inherent in
eiiiciency of pentodes as plate modulator ampli- '
subscriber’s station is thereby registered. For
description of the sending of the seizure signal
from the district H0, the connection of a ter
minating sender to the final circuit M0, and
'transmission of the numerical digits from the
originating sender to the terminating sender,
reference may be had to the aforementioned
Hubbard-Matthies-Myers patent beginning on
«page 8, line 6, first column. The originating
sender further transmits a signal indicative of
2,408,462
rangement of Fig. 2 is the elimination of` the de
modulator filters shown in Fig. 1, the balance of
the demodulators necessary to prevent cross-talk
is connected to the 'demodulator I I8 which is as
sociated with hybrid coil I|2 and district III);
and this signal' operates register means in Vsender
TS2. A terminating marker TM2 is thereupon
connected to the sender TS2,V the marker is con
nected to the modulator |44 and final switch FSZ
which are associated with the ñnal circuit |40,
the variable frequency lcarrier source Aof modu
lator |44 is tuned to the same frequency as that of
being obtained by the use of double-balanced cop
per-oxide modulators. The most serious cross
talkwill be contributed by the third harmonic of
the carrier modulating with energy approximate
ly three times the frequency of the wanted sig
nal; and this type of cross-talk is prevented by
the source I I9 associated with the demodulator .
I|8 in office A, and the ñnal switch F82 is 00n
6
A' further modification introduced in the ar
the frequency of the carrier source' |I9 which
the group ñlters which pass a non-harmonically
related group of carriers. The remaining serious
distortion products involve the square and higher
orders of the signal energy; and these are reduced
trolled by the marker TM2 to connect with Vthe
line L4 of the called station S4. The called line
is tested and, if the line is idle, a signaling cir 15 by operating the demodulators at low level, single
tube amplifiers being provided to raise the out
cuit is closed in the usual manner. Whenthe
put volume of each demodulator to a satisfactory
call is answered, a two-Way conversational con
nection iscompleted between stations SI and S4,
level.
'
In the arrangement shown in Fig. 3, a double
one way through media >CMI | and CM|2 and the
other way through media CMI3 and CMI4. The 20 stage of modulation is provided with single side
band transmission. Each district circuit 3|0 is
originating marker OMI is released as soon as
connected by a low-pass filter 3|| to a voice fre
the carrier source >I I5 has been tuned to the re
quency hybrid coil 312; and each final circuit
quired frequency; the originating sender OSI is
33D is connected by a low-pass filter 33| to a voice
released as soon as the numerical and channel
frequency indicating signals havey been transmit
ted to th'e called office; and the terminating send
er TS2 and marker TM2 are released as soon as
connection with the called line has been com
25 frequency hybrid coil 332.
The hybrid coil 3I2
is further connected to a line balancing network
3|3, a primary modulator 3I4 and a demodulator
32|; and thehybrid coil 332 is further connected
to a line balancing network 333, a primary modu
in both directions, release of the connection being 30 lator 334 and a demodulator 34|. The primary
modulator 3|4 is connected to fixed-frequency
Veffected when the subscriber at the calling sta
carrier source 3|5’and through a corresponding
tion SI restores the telephone set to normal. The
band-pass filter SI5 >to a secondary modulator
line and ñnal switches LSI and FSZ are there
3I1; and the primary modulator 334 is connected
upon restored to normal; and the carrier sources
I I5 and |45 are disconnected from modulators 35 to a fixed-frequency carrier source 335 and
through a corresponding band-pass filter 336 to
II4 and |44.
a secondary modulator 331. The channels are
'1n the o?ñces C and D shown in Fig. 2, the
divided into groups, the same primary carrier fre
district circuits 2I0 and 23|) and iinal circuits
quency being used for all of the primary modula
22|] and 240 are individually connected through
low-pass filters 2|I, 23|, 22| and 24| to hybrid 40 tors in the group and a different secondary car
pleted. Supervisory signals maybe transmitted
coils 2I2, 232, 222 and 242.
The hybrid coils
rier frequency being used for each secondary
are further connected to line balancing networks
modulator. The particular frequency of the car
2|3, 233, 223 and 243,110 modulators 2|4, 234, 224
vand 244, and through channel amplifiers 2 l 9, 239,
22S and 249 to demodulators 2I1, 231, 221 and
241. Fixed frequency carrier sources 218, 238,v
223 and 248 are connected to the demodulators;
rier source 3|8 which is connected to the sec
and carrier sources of desired frequency are se
lected and connected to the modulators under
control of originating and terminating markers.
The modulators are divided into groups, modula
tors 2I4 and 224 in office C beingconnected
ondary modulator 3|1 on a given call is con
" trolled by the originating marker under whose
control the desired conversational connection is
being established; and the particular frequency of
the carrier source 338, which is connected to the
secondary modulator 331 on a given call, is con
trolled by the terminating marker under whose
control the desired conversational connection is
being established. The frequency of the carrier
connected to each primary modulator of a group
of channels is greater than the band-width passed
and 223` and'through group filter GFI to the mul
tichannel transmission medium CM2 I ; and modu 55 by the group ñlter; and the band-pass ñlter of
each channel selects one of the side-bands of
lators 234 and 244 in oiiice D being individually
the primary carrier and passes it to the second
connected through resistance pads 235 and 24B
ary demodulator. The frequency of the carrier
and through group ñlter GF3 to multichannel
source 3|8 which is selectively'associated with
transmission medium CM23. The resistancepads
and group filters are used instead of pentode 60 the secondary modulator 3 | 1, lies within the band
width of the group iilter 3H! and determines the
amplifiers as in Fig. 1. The demodulators are
position of the modulated single side-band in the
also divided into groups, group filter GF2 con
group band. And the frequency of the carrier
necting a group of demodulators 2I1, 221 to mul
source 338 which is selectively associated with
tichannel transmission medium CM24 and group
Vfilter GF4 connecting a group of demodulatorsV 65 the secondary demodulator 331, lies-within the
band-width of the group iilter 339 and determines
231, 241 to multichannel transmission medium
the position of the modulated single side-band in v
CM22. YSince the energy from one modulator,
the group band. Since the unwanted'side band
which feeds back into another modulator to cause
of each channel is outside of the group band
cross-talk, is inversely proportional to the square
of the number of modulators connected in paral- » 70 and is not transmitted, onli7 a single stage of
demodulation is required. Thus each group'of
lel, the cross-talk due to intermodulation between
secondary modulators is, connected through a
channels will not be substantial with large num
group filter 3|?! or 333 to a multichannel medium
bers of modulators connected to each group filter.
CMSI or CM33, thence through a line ampliñer,
The loss which is introduced by the isolating re
sistance pads is made up in the line ampliñers. ‘f 75 a multichannel medium CM32 or CM34, and an
through individual isolating resistance pads 2|6
2,408,462
8
oi‘?ce filter 32B or 34B to all of the demodulators
32! and 34|, each demodulator having a dif
ferent ñxed carrier frequency. While this ar
rangement requires the use of an additional
cluding control means common to said units for
selectively connecting to the modulator of any
unit connected to a calling subscriber’s line a
carrier source of the same frequency as that oi
modulator and filter for each channel, it greatly
the carrier source connected to the demodulator
reduces the cost of the required carrier supply
of a unit adapted to be connected to the called
system, reduces the delay distortion, and increases
line.
the number of channels which may be provided
through the common transmission media.
The carrier channel equipments may be ar
ranged so that fixed carrier frequencies are as
signed to both the modulator and demodulator
plurality of high frequency carrier channel equip
ment units, means for individually connecting
associated with a ñnal switch in which case the
originating marker selectively controls the con
nection of carrier sources of required frequency
to- both the modulator and demodulator associ
ated with the district through which a call is
initiated. This and other modifications of the
above-described arrangements may be made
Within the scope of the invention.
What is claimed is:
1. In a telephone system, subscribers’ lines, a
4, In a telephone system, subscribers’ lines, a
said units With subscribers’ lines, each of said
units comprising a modulator and a demodulator,
a multichannel transmission medium common to
a plurality of said modulators, resistance pads one
for each of said modulatorsl connecting said
modulators to said medium, a multichannel
transmission medium common to a plurality of
said democlulators, an amplifier interconnecting
said media, a plurality of carrier current sources,
a source of a different frequency for each of said
demodulators,
and
means
including
control
means common to said units for selectively con
plurality of high frequency carrier channel equip
necting to the modulator of any unit adapted to
ment units each comprising a modulator and a
be connected to a called subscriber’s line a car
demodulator, means for individually connecting 25 rier source of the same frequency as that of the
said units with subscribers’ lines, amplifiers, one
source connected to the demodulator of the unit
for each of said modulators, band-pass filters,
connected to the calling subscriber’s line.
one for each demodulator, for passing a difier
5. In a telephone system, subscribers’ lines, a
ent band of frequencies to each demodulator, a
plurality of carrier channel equipment units, each
plurality of carrier current sources, a source of 30 comprising a modulator and a demodulator,
a different frequency for each demodulator, a
means for individually connecting said units
multichannel transmission medium connected to
with subscribers’ lines, a multichannel transmis
a plurality of said amplifiers, a multichannel
sion medium common to a plurality of said mod
transmission medium connected to a plurality of
said filters, an ampliñer interconnecting said
multichannel media, and means including con
trol means common to said units for selectively
ulators, resistance pads one for each of said plu
rality of modulators, each of said pads connect
ing the associated modulator to said medium, a
for each of Said demodulators, and means includ
ing control means common to a plurality of said
the unit connected to the calling subscriber’s line.
6. In a telephone system, subscribers’ lines. a
plurality of high frequency carrier channel equip
ment units, means for individually connecting
said units with subscribers’ lines, each of said
multichannel transmission medium common to
connecting to the modulator of any unit con
a plurality of said demodulators, an amplifier in
nected with a calling line a carrier source of
terconnecting said media, a plurality of carrier
the same frequency as that of the carrier source 40 current sources, a source of a different frequency
connected to the demodulator of a unit adapted
for each of said demodulators, means including
to be connected to the called line.
control means common to said units for selec
2. In a telephone system, subscribers’ lines, a
tively connecting to the modulator of any unit
plurality of high frequency carrier channel
connected to a calling subscriber’s line a carrier
equipment units, means including electromechan 4 Ul source of the same frequency as that of the car
ical switching means for individually connecting
rier source connected to the demodulator of a
said equipment units to subscribers’ lines, each
unit adapted to be connected to the called line
of said units comprising a modulator and a de
and for selectively connecting to the modulator
modulator, multichannel transmission media
of said unit adapted to be connected to a called
common to said units. a plurality of carrier cur 50 subscriber’s line a carrier source of the same fre
rent sources, a source of a different frequency
quency as that of the demodulator connected to
units for selectively effecting the connection of
carrier sources of required frequency to said mod
ulators, thereby to effect the completion of non
interfering two-Way conversational connections
units comprising a modulator and a demodula
through said common media between units con
tor, said modulators and said demodulators being
nected to calling subscribers’ lines and units
divided into groups, group filters, one for each
adapted to be connected to called subscribers’ 60 group ofmodulators and one for each group of
lines.
demodulators, resistance pads, one for each of
said modulators, each of said pads connecting the
plurality
3. In aoftelephone
carrier channel
system,equipment
subscribers’
units,
lines,
each
associated modulator to the associated group fil
comprising a modulator and a demodulator,
means for individually connecting said units With
subscribers’ lines, a multichannel transmission
ter, multichannel transmission media, the group
ñlters of each o-f a plurality of groups of mod
ulators being connected to one of said transmis
medium common to a plurality of saidv modu
sion media and the group filters of each of a
lators, resistance pads one for each of said plu
pluraltiy of groups of demodulators being con
rality of modulators, each of said pads connect
nected to another of said transmission media.
ing the associated modulator to said medium, a 70 '7. In a telephone system, subscribers’ lines, a
multichannel transmission medium common to
plurality of high frequency carrier channel equip
a plurality of said demodulators, an amplifier
ment units, means for individually connecting
interconnecting said media, a plurality of carrier
said units with subscribers’ lines, each of said
current sources, a source of a different frequency
units comprising a modulator and a demodula
for each of said demodulators, and means in 75 tor, said modulators and said demcdulators being
2,408,462
»divided into groups, group filters, one for each
10
for individually connecting said units with sub- f
filter, multichannel transmission media, the
scribers’ lines, each of said units comprising a
primary modulator, a secondary modulator and
a demodulator, multichannel transmission media
common to a plurality of said units, group ñlters,
each of said iilters connecting one of said media
group filters of each of a plurality of groups of
modulators being connected to one of said trans
mission media and the group ñlters of each of a
plurality of groups of demodulators being con
nected to another of said transmission media, a
individually connected to each of said primary
modulators and said demodulators, band-pass ñl
group of modulators and one for each group of
demodulators, resistance pads, one for each of
said modulators, each of said pads connecting
the associated modul’ator tothe associated group
plurality of carrier current sources, a source of a
different frequency for each of said demodula
tors, and means including control means common
to the secondary modulator ofv a different group
of said units, a plurality Aof sources of carrier
current, a source of a different frequency being
ters adapted to pass one of the voice frequency
side-bands from the associated modulator to the
associated secondary modulator, and means in
to said units lfor selectively connecting to the 15 cluding control means common to said units for
selectively connecting to the secondary modu
modulator of any unit connected to a calling sub
scriber’s line a carrier source of the same fre
quency as that of the carrier sourcev connected
to the demodulator of a unit adapted to be con
nected to the called subscriber’s line. '
8. In a telephone system, subscribers’ lines, a
plurality of high frequency carrier channel equip
ment units, means -for individually connecting
said units with subscribers’ lines, each of said
lator of a unit connected to a calling subscriber’s
line a carrier source of required frequency to es
tablish a conversational connection through one
20 of said media with the demodulator of a unit
adapted to be connected with the called sub
scriber’s line.
10. In a telephone system, subscribers’ lines, a
plurality of high frequency carrier channel equip
units comprising a modulator and a demcdula 25 ment units, said units divided into groups, means
for individually connecting said units with sub
tor, said modulators and said demodulators being
scribers’ lines, each of said units comprising a
divided into groups, group filters, one for each
primary modulator, a secondary modulator and
group of modulators yand one for each group of
a demodulator, multichannel transmission media
demodulators, resistance pads, one for each of
said modulators, each of said pads connecting the 30 common to a vplurality of said units, group ñlters,
each of said filters connecting one of said media
associated modulator to the associated group ñl
to the secondary modulator of a different group
ter, multichannel transmission media, the group
of said units,l a plurality of sources of carrier cur
ñlters of each of a plurality of groups of modu
rent, a source of a different frequency being in
lators being connected to one of said transmis
sion media and the group ñlters of each of a 35 dividually connected to each of said primary
modulators and said demodulators, band-pass fil
plurality of groups of demodulators being con
ters adapted to pass one of the voice frequency
nected to another of said transmission media, a
side-bands from the associated modulator to the
plurality of carrier current sources, a source of a
associated secondary modulator, and means in
diiîerent frequency for each of saidY demoduators,
and means including control means common to 40 cluding control means common to said units for
selectively connecting to the secondary modula
said units for selectively connecting to the mod
tor of a unit adapted to be connected to a called
ulator of any unit, adapted to be connected to a
subscriber’s line a carrier source of the required
called subscriber’s line a carrier source of the
frequency to establish a conversational connec
same frequency as that of the source connected
to the demodulator of the unit connected to the 45 tion through one of said media with the de
calling subscriber’s line.
9. In a telephone system, subscribers’ lines, a
plurality of high frequency carrier channel equip
`v
ment units, said units divided into groups, means
modulator of the unit connected to the calling
subscriber’s line.
RAYMOND O. WISE.
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