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Патент USA US2107817

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Feb. 8, 1938.
J, |, DAVls
Filed Jan. 29, 1955
-3 Sheets-Sheet l
Feb. 8, 1938.
Filed Jan. 29, 1956
3 Sheets-Sheet 2
jay Z15;
,Feb. 8, 1938.‘ _
Filed Jan. 29, 1936
s Sheets-Sheet s
Patented Feb. 8, 193a
Jack I. Davis, Chicago, Ill., assignor to Sidney
Ludwig, Chicago, Ill.
Application January 29,1936, Serial No. 61,418
4 Claims. (01. 179-37)
This invention relates to an inter-office com
munication system, and more particularly to a
system wherein a plurality of units in different
parts of the o?ice are connected by a cable.
One feature of this invention is that it results
in better quality of received speaking; another
feature of this invention is that it avoids cross
talk between different lines in the same cable;
yet another feature of this invention is that it
[0 enables a visual indication of the party calling
another, over the same cable wire as the voice
transmission; still another featureof this inven
tion is its simplicity of installation; another fea
ture of this invention is the ease with which addi
The main cable and feeder cables are of low im
pedance, and the amplified output of the micro
phone is transmitted through the cable, the line
impedance being substantially reached at each
end of the transformer.
Referring more particularly to Fig. 1, each unit,
adapted to be placed on a desk, contains a micro
phone III or other means for translating sound
waves into electrical energy, an amplifying sys—
tem for the output of the microphone, indicated 10
here in general by the numeral H, a speaker in
dicated in' general as I2, a power pack iii, an in
dicator l4, and the necessary switches. A feeder
cable l5 connects to the main cable i6, to which
[5 tional units may be connected to those already in , are also connected other desk units l1, I8, and
stalled; a further feature of this invention is that Hi. It is to be understood that the number of
it may be operated entirely from the commercial wires contained in the main cable and feeder
cable are covered by the number of units which
source of alternating current; still another fea
ture of this invention is that it may be used as a it is desired to use, in each case being one more
conference communication system; other features than that number.
The microphone Iii is preferably of the carbon
and advantages of this invention will be apparent
from the following speci?cation and the drawings, button type,__although it may be a ribbon type or
any other in general use. ' Its output is passed
in which
Fig. l is a schematic diagram showing such a through the primary of the transformer 20, power
25 system, one unit being shown in detail; Fig. 2 for it being supplied from the power pack- i3
- illustrates the manner of connecting the various through the line 2i, a ?lter system comprising
units by cable; Fig. 3 is a diagrammatic view of resistors 22 and 23, radio frequency choke coil 24,
one of- the three-way socket-elements used for and condenser 25. The voltage developed in the
connection; and Fig. 4 is a front elevation of a secondary of the transformer 20 is applied
through the volume control 26 to the grid of the
30 preferred form of unit.
Modern inter-office communication systems of triode tube 21. The cathode 28 of this tube is
grounded through the bias resistor 29, shunted by
the type designed for use by executives particu
larly are subject to many disadvantages. The the condenser 30. The heater 3| is supplied with
majority of these systems comprise a unit for a current by the secondary 32 of the power pack.
35 desk including a microphone and‘ a speaker, and . The plate 33 of this tube is connected to the grid
having switch means for connecting it with any 34 of the pentode tube 35 through a resistance
chosen unit in the system. As now designed, most
of these transmit the microphone output through
the cable or other connecting network to the
40 other unit, and shielded connections are necessary
to prevent cross-talk and other interference.
Most of these systems also necessitate additional
wires to transmit current to operate an indicat
ing device. The installations are expensive, and
45 di?icult to change or to provide for the addition
of other units.
The present invention comprises an inter-office
communication system having a plurality of desk
. units each having a microphone,’ a vacuum tube
50" ampli?er for the output of the microphone, a
it speaker, an indicating device, switches, and a
1. power pack ‘to enable operation from commercial
alternating current. A single cable, containing
only one more wire than the number of units is
55 connected to each of the units by a feeder cable.
coupling network comprising resistors 36, 31, 38,
and 39 and coupling'condensers 40, ll and 42.
The cathode 43 of the pentode tube 35 also has
a. biasing resistor 44 in series therewith, shunted 40
by a condenser 45. The heater 56 is also sup
plied with current from the secondary 32 in the
power pack l3. It also has connected thereto the
suppressor or grid 46. The output of the tube
from the plate 46 is passed through the switch 45'
48 and shunted to ground by the condenser 49, and
the primary of the transformer 50. This trans
former is inserted in order to match the output
impedance of the tube substantially with the
line or cable impedance, and the switch 43 is pro- 50
vided so that when it is desired to talk to more
than one of the other units, the switch ‘may be
changed in order to match the output impedance
more nearly to the impedance of several units in
‘ 9,107,817
The output of the line-matching transformer
50 is connected to a bus wire 5| to which are con
_nected the contact points 52 of a number of
switches, of which three are here shownas 53,
54, and 55. Direct current power, supplied by
the power pack I3, is drawn from the lead 2|
through the resistor 58 and the resistors,“ to the
switch points 60 of the switches 53, 54 and 55.
The points ii are the 011 positions of the switches.
10 The resistors 59 would be the same for each of the
switches in a given unit, but would be different in
each-unit. Thus the direct current ?owing from
the unit when any switch is on the point 60 would
means here shown as a relay energizing coil 83
which‘ opens the contacts between 64 and 65; the
voice modulated alternating current from the
ampli?er of'the unit ill, on the other hand,
passes through the operating coil 61 and energizes
the speaker l2. It will be noted that‘ the speaker
in each unit has no connection with the ampli?er
of that unit; on the other hand, the operating
current supplied to the speaker of a unit-qis in
each case developed by the ampli?er in another
unit and transmitted through the low impedance
for each switch.
cable to the speaker.
- ‘
wire or lead 62 which branches it in one case to
the indicator H, in series with the operating coil
20 63 of a relay having an armature 64 making con
tact with the point 65, and in the other case to
the armature 64. This armature is, normally
maintained in contact with the point 55 by a
spring 66. The voice coil 61 of the speaker I 2
25 is connected from 65 to ground. Thus when the
speaker I2 is inserted in the output of the tube 20
69 as a, load, and is shunted ‘to ground by con- >
densers ‘l2 and 13 to make a ?lter network for
the output of the recti?er. A switch 14 is pro
vided to control the alternating current inputto
the transformer 68.
tions between units may be seen more clearly by
v mitted through the main and feeder cables, ener
reference to Fig. 5, which illustrates schemati
electric energy into audible waves.
__The indicator II is essentially an ammeter,
chosen with a range such that each of the re
sistors 59 heretofore mentioned causes the needle
of the indicator to stop at a di?’erent position
on the face thereof, as'may be more readily seen
from Fig. 4. That is, if the operator of the unit
shown in detail in Fig. 1 wishes to speak to the
, unit l8, he would ?rst throw his switch 53 to the
contact 60, which would send a direct current
through the cable to the indicator in the unit I8.
This direct current would energize the relay coil
in that unit and cause the contacts to break, thus
opening the circuit through the speaker‘ voice
coil 61. The indicator needle would swing to a
position governed by the value of the resistor 59,
and would in this case take the ?rst position to
show that unit No. 1 was calling. As soon as the
unit l8 has answered, the operator of unit No. 1
would throw switch 53 on over to point 52, where
50 upon the spring connected to the relay armature
would restore connection to the speakei of. unit
18, and communication would be carried on.
It is also contemplated that a small light or
other indicating device might be used in series
55 with the indicator II to more readily call atten
tion to the fact that one is being called over the
Referring again to Fig. 1, the process willbe
described when the‘ operator of the unit 18 an
60 swers the unit shown diagrammatically in Fig. 1.
When the operator of l8 was signalled by the
closing of the :switch53 the current travelled
along the line or lead 85 between the contact
points 86 and 81 of the plugs 80 (through the_
65 intermediary of the three-way‘ socket element
shown diagrammatically in Fig. 3 but not shown
in ‘Fig. 1) and along the wire 88 of the main
cable IE to the unit It; when the operator of the
unit l8 answers, throwing his number 1 switch
70 would send current through the wire 89 of the
main cable l5, across the contact points 90 and
9| (again through a three-way socket element
not shown here), and through the wire 62 in
the cable l5 to the speaker unit l2. In this con
nection it will be noted that when the switch is
The power pack‘ indicated in general as l3
comprises a transformer 68 and a full switch
rectifying tube‘ 59. The heater ‘Ill is operated
from the secondary 51. The ?eld coil ‘H of the
armature G4 is contacting 65, the ampli?ed out
put of. the microphone of another unit, trans
gizes the speaker and results in translation of the
relay coil 61 through the speaker l2 by blocking
be diiferent for each unit. ‘ BI is the off position
The input to each unit, transmitted through
the cable from some other unit, is through the
set at signalling position and direct current is
used it is prevented from passing through the
The cabling arrangement and inter-connec
cally this invention as embodied in an inter- '
o?ice communication system comprising four
units, each of these units being identical with
that shown in detailin Fig. 1, the cabling corre
sponding to that shown in Figs. 2 and 3. Each
of the fourv units has a feeder cable connecting.
to the main cable at a junction box or three-way
socket illustrated in dotted lines in Fig. 5, shown
in more detail in Figs. 2 and 3. Referring to
Fig. 3 of the drawings, a schematic layout of each
three-way socket shows that it merely. has one
female member 15 for the reception of three. 40
separate plugs. By the use of these three-way
‘sockets each feeder cable from each unit may
be attached to the desired wires in the main cable
without splicing or soldering and similar addi
tional units may be added,~up to the limit of
the number of. wires in the main cable.
Referring again to Fig. 5, the operative con
nections and circuits between units will be more
fully described. Inasmuch as the schematic
illustration in this ?gure shows the device ‘em
bodied only in connection with four units rather
than with the five unit system shown in Fig. 1,
the parts will be indicated by new reference nu
merals. It will be understood,‘ of course, that
each unit is similar to that described in com
plete detail and illustrated in Fig. 1, in that it
contains a microphone, an ampli?er, a speaker,
an indicator, and control switches. The addition '
of more units would merely necessitate the addi
tion of further switches and further wires in the 60
cable, the switches always being one less than
the number of units and the wires in the cable
one more than the number of units.
The left hand unit illustrated in Fig. 5, and
here referred to as llll, is shown connected to
the third unit I03. That is, unit IM has its
switch marked 3 set in the talking position; and
unit I03 has its switch marked I set in the talk
ing position. Thus any sound input to the mi-'
crophone I05 is transmitted through the lead I 06,
the switch 3, the wire I01 and the relay I08 to
the speaker I09 of unit 103. - Similarly, any
sound input to the microphone I I0 is transmitted
through the switch I and the wire III to the
speaker “2 of the ?rst unit. The return cir
cuit in each case is made through the ground
wire H3 common to all of the units. Circuits
are thus formed with a minimum of wires en
nbling units I and 3 to talk to each other, the‘
circuits being distinct so that the users may talk
simultaneously, break in, or in any other way
they desire.
Similarly,v units I02 and I04 are shown with
their switches in such position that they may
necting another unit to the main cable by a
feeder. Fig. 3 is a schematic diagram of one
of the socket-elements ‘ll, and-illustrates how
the sockets are connected. It will be noted that
each socket is provided, in addition to the holes
‘I! into which corresponding pins on the plugs 00
fit, with a recess ll adapted to receive a corre
sponding key on the plugs 00. Thus. the various
plugs, whether on the sections or portions of the
main cable or on the feeder cables, must always 10
'10 talk -to each other at the same time that conver
sation is going on between units iii and I03 with ‘be inserted in the proper way in the socket
no interference. Switch No. 4 of. the second unit members ‘II. 'When it is desired to install an
is set in the talking position, so that a circuit is inter-o?lce system of the type described herein,
completed from its ampli?er through the wire it is not necessary to buy and install at one time
“4 to the speaker of unit 104. This last men
the full number of units which it is contemplated
tioned unit has its second switch in talking posi
using. The main cable is chosen to have a
tion, so that a circuit from its ampli?er is com
number of wires one more than the number of
pleted to the speaker of unit I02 through the units which it is contemplated will eventually be
wire H5.
installed. The system may then be started with
As heretofore described, not only may pairs only two or three units, if that be desired. Addi 20
of units talk independently of each other, but tional units, up to the limit of the capacity of
any number of units may sit in what might‘ be the cable, can be added at any time by merely
called conference arrangement. That is, it purchasing additional sections of. cable and addi
would only be necessary for unit l0l to throw all tional socket elements and plugging them into
three of his switches to the talking position to the already operating portion of the system.
supply the output of. his ampli?er to each of the
As was heretofore mentioned, this system may
other. speakers.
Thus the user of the ?rst unit - beused to speak to one other unit or several other
units at once. When only one unit is being
could speak into microphone I05 and be heard
simultaneously by the users of each of the other
three units. If desired, any of the other units
could throw a multiplicity of their switches in
the talking position and answer the ?rst speaker,
there the last user's voice being reproduced in
such speakers as he chooses by the throwing of
Referring again to Fig. 1, it will be noted that
the operating coil 63 of the relay is in parallel
with the voice coil 51 of the speaker i2. This
condition does not prevent operation of the
40 speaker, however, since the impedance of a voice
coil, usually only a few turns of wire, is very
low compared to the impedance of the relay
winding to currents of.,voice frequency. While
such relay winding has a relatively low current
45 resistance, and thus enables easy operation of
the indicator 14, it has an alternating current
impedance many times that of the voice coil, and
thus the major portion of power of an alternating
current would be developed in the speaker rather
50 than being shunted out through this relay wind
The values of the resistors and condensers
heretofore described are chosen in accordance
' with the particular tubes and circuit employed,
65 and may vary considerably. In the particular
embodiment of this invention disclosed in Fig. 1,
tubes 21 and 35 are models F5 and F6 respectively,
and the rectifier 69 is a Z‘. With these tubes,
and the particular circuit disclosed, the follow
60 ing values were used: the volume control 26 was
200,000 ohms; resistors 22 and 23, 20,000 ohms
and 10,000 ohms respectively; resistor 29, 2,500
‘ ohms; resistor 44, 400 ohms; resistors 36, 31, 38
and 39, 75,000 ohms, 25,000 ohms, 250,000 ohms
65 and 50,000 ohms, respectively. Condensers 25
and 30 were 2 mi. each; condensers 4i and 42,
spoken to, the switch 48 would be in the position
illustrated in Fig. 1; when speaking to several 80
other units at once, the switch would be thrown
to its other position in order to more nearly
match the impedance of the speakers of the other
units in parallel. In this connection it is to be
understood that “speaker” is used to cover the 35
actual sound-producing device and its associated
parts, including, for example, a matching trans
former. Inasmuch as the output of the trans
former 50 is placed on the bus bar or bus wire
5|, any number of switches may have the out
put directed therethrough at once, up to the limit
of the number of switches on the unit. When
the word "number” is hereafter used in the
claims, it will be understood that it means any
number of units from one up to the total num
ber of units in the system.
While I have shown and described certain em
bodiments of my invention, it is to be understood
that it is capable of many modi?cations.
Changes, therefore, in the construction and ar
rangement may be made without departing from
the spirit and scope of the invention as disclosed
in the appended claims, in which it is my inten
tion to claim all novelty inherent in the invention 65
as broadly as permissible, in view of the prior art.
What I claim as new and desire to obtain by
Letters Patent is:
1. An inter-omce communication system of the
character described, including: a plurality of
units, each unit comprising a speaker, a micro
phone, tube means for amplifying the output of
said microphone, and an indicator; a cable con
necting said units, said cable having therein only
one more wire than the number of units; a source 65
of direct current associated with each of said
units; and a‘ relay breaking‘ the circuit to said
£1,- mf. each; condenser 40, .01 mf.; condenser 45,
10 mf.; condensers 12 and 13, 8 mi. each; and
speaker when said indicator is being energized
condenser 49, .02 mf.
by direct current.‘
70 Referring more particularly to Figs. 2 and 3,
the main cable, feeder cables, and three-way
socket-elements 15 will be described. The main
cable l6 comprises a number of sections, as 16, 11,
and 18, between the various socket-elements 15.
75 Each socket-element provides a means of con
2. An inter-office communication system of
the character described, including: a plurality 70
of units, each unit comprising a microphone,
tube means directly connected thereto for ampli
fying the output of said microphone, a trans
former for coupling said tube to a low impedance 75
‘2,107,817 output circuit, a speaker, and switch means; and is adapted to energize the speakers of any num
a cable connecting said units, said_ cable and ber of said other units.
switch means being so constructed and arranged .
4. A unit adapted to be’ connected to any num- ' '
that said cable may be connected to the trans
former of each unit to form a part of thelow
ber‘oi' other similar units by a cable including
only one more wire than the number 01' units
impedance output circuit for transferring energy ‘to form an inter-o?‘ice communication system of
to any number of the speakers ‘01' the other of
said units.
_units, each unit comprising a microphone, tube
3. A unit adapted to be connected to any num-
means directly connected thereto for amplifying
10 ber of other similar units by ‘a cable to form an
inter-office communication system, including‘: a
speaker: a microphone; an amplifier directly
connected‘ thereto and comprising ‘tube >means
for amplifying the output of said microphone;
15 and a transformer‘ for coupling said tube means
to a low impedancecircuit including said cable,
whereby the output of the amplifier of said unit
the character described, including: a plurality of _
the‘output‘of saidmicrophone; switch means; 10
.an indicator; and means permitting energiza
tion or said indicator without energization 0! said
speaker, through the same pair of wires in said
cable effecting energization oi.’ said speaker, as
a result of the operation of the switch means of 1‘
any of said other units.
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