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Патент USA US2115140

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April 26, 1938.
Filed 001;. 29, 1936
Qiou .
I!C.. _w
Patented Apr. 26’, 1938
2,115,140 >
Alton C. Dickieson, Hollis, N. Y., assignor to Bell
Telephone Laboratories, Incorporated, _; New
York, N. Y., a corporation of New York
Application October .29, 1936, Serial No. 108,190
2 Claims.
The invention relates to signal wave transmis;
sion systems and particularly to wave-operated
switching circuits for controlling signal trans
mission in such systems.
The invention has particular'reference to car
rier wave signaling systems of the type in which
carrier energy, or special control energy other
than the carrier, is applied to the transmitter
only during periods of signal transmission, and
10- in which the gain of the receiver is automatically
controlled by the received carrier or other con
trol energy.
In systems of the above type, every time the
carrier or control energy is applied, the receiver
15 gain changes from a maximum value as limited
by the manual control to a value determined by
the carrier amplitude. To allow for correction
for fading, the range of gain needs to be at
least 10 decibels, and may be as much as 50'to
That is, every time the carrier is
20 60 decibels.
applied during the gain adjusting intervaL'the
gain is considerably higher than it should be
?nally, with consequent high noise and speech
distortion. For control current operation where
25 nearly every word means a new application of
(01. 178444)
antennafdue toitheraction of the preceding auto
matic‘ gain‘ control; - This di?iculty‘has jbeen
minimized inthe past 'by the use of a‘ manual
gain control to adda loss inside the circuit, the
gain‘ of which "is being adjusted, to limit- the m.
maximum gain of the receiver, and thus effecé
tively’ to prevent‘the automatic gain control from
operating'in the region Where the static falls.
This method is not particularly suitable at an
attended station because of the wide variations
in conditions requiring lqui-ck adjustments in
gain, and, of course, is not feasible for an unat
tended station.
An object of the present invention is to reduce
in systems of the above type signal distortion at
the receiver’ due to‘ the ‘removal and reapplica
tion of the carrier‘or other control energy.
Another object‘ is 'to'pro'vide-a receiver'gain
of an optimum ‘value when the carrier or control
energy is removed and to quickly readjust the
gain tothe proper value when carrier or control
energy is again applied, if a re-adjustment is
These objects are attained in accordance with
the invention by .so arranging the codan in the‘
receiving circuit that it is controlled by the re
carrier, these transient effects will be quite trou
. ceived carrieror control energy prior to its regu
So-called "Codan” (carrier-operated detector
in level by the automatic gain control and
anti-noise) circuits have heretofore been used lation
to. make the gain regulation neutral, that is, to
. ‘with the receiving circuits in such systems, to
the receiver gain at the last established '30;
prevent the reception of interference (static) maintain
value when the reception of carrier or control
which, during periods of no signal (no carrier)
energy ceases until the carrier is again received
when automatic gain control receivers are at
maximum sensitivity, may be of su?icient ampli
J, tude to annoy the receiving subscriber and to
falsely operate the voiceeoperated‘switching cir
cuits for controlling the application of carrier by
the transmitter at the terminal. The codan cir
cuit accomplishes this by disabling the output
40 of the receiver at each stationduring periods of
no signal transmission and removing'the disabil
ity when signal transmission is taking place."
The fundamental requirements of a codan are
( 1) that it operates on the weakest ‘carrier that
will give a commercial circuit with the static
and set noise present and (2) that it should not
operate on the strongest static through which a
commercial circuit could be given with the cir
cuit elements involved. The attaining of these
50 requirements is made d'i?’icult in present—day
radio receivers because the codan is controlled
from a point in the receiving circuit where the
signal—to-noise ratio (noise considered with car~
rier o?) is made to approach unity, no matter
how large that ratio may be at the receiving
and then to quickly readjust ,. the gain to the
optimum value for signal reception.
Other objects and features of the invention
will be brought out in the following detailed de
scriptionwhen read in connection with theac
companying drawing, the single ?gure of which
shows diagrammatically (and in part schemati
cally). a two-way radio telephonesystem em-- I
bodying the invention.v
The west- terminal of the system in'the draw
ing is shown diagrammatically. This portion is.
not an actual circuit diagramibutrather a single
line layout, each line indicating a two-wire trans 45 "
mission path. A_ break in a path, represented
by separated arrowheads, indicates that the path
is normally disabled at that point,'and an arrow
pointing toward such a break from a control
device (ampli?er-detector) indicates that the
path will be made operative by operation of the 501
control device. A make in the path, represented
by contacting arrowheads, indicates that the
path is operative at that point, and an arrow
pointed towards the contacting arrowheads from 55.
oscillator l at the, east terminal, having its in-:
Va control devicev (ampli?er-detector) indicates put
connected across the receiving circuit RE at a
that’ the path will be disabled at that point by’ r
point between the ?rst detector 6 and the inter
’ operation of the control device. An arrow'point
mediate frequency ampli?er 9, .the. mechanical
~ ving from one box representing acontrol device
relay I3 adapted to be operated by 'detected~cur-> >
.' (automatic gain'control or codan) 'to' another
rent inthe output of the ampli?er-detector’ IZ,Q
and’ the normally’closedishortjcircuiting connec- 1
tion I5 across the'receiving circuit RE; in the ‘out; ; , is‘
put'o'f the audio'ampli?erf Illv adapted to beopened ~ 7 '
representing other ‘apparatus indicates that 5'
‘New , box
the former device controls in some manner the
adjustment or rearrangement of the latter appa-f
* "i
~ 1-7
' The cast terminal of the system employing ap "by; operation of relay I3.
'paratus identical with that at {the west terminal 1* atthe
Also, east
point between
the out-i
RE .
,is shown by a circuit diagram'in which thecom- '
' *ponentparts of the apparatusiand circuits par: putiof the intermediate frequency amplifier 8
and the input of the second detector ,9 is the in
ticularly pertainingto the'invention are: shown ‘put
of the backward§acting portion '14 of ‘an auto 15
’ {is
' w‘
15 in greater detail.
gaincontrol' circuit for the intermediate
- ' The system ‘of
telephone ‘system for’ transmitting telephone sig
halsin opposite directions between- a "west two;
‘ ampli?er '8', comprising the three-electrode vacu- ~
sum tubezdetector il?whaving its'input connected
'acrossrthelreceiving circuit RE by inputtrans- ‘ ,
former IT’; a “gain decrease” relay GD and a 20"’ r
waytelephone line LW' and an east ‘two-way’
“gain- increase disabler’f relay GID controlled'zby
detected carriercurrentin the'output- of del-j; 11'
temicomprises the transmitting circuit
‘ circuits controlled: by; 1
eluding a transmitting'iantenna TAW, and a ‘re-,7 tector “5;. and’ associated
.jThe west terminal of the radio‘ telephone sys-_
these relays.?-The- automatic gain " '
ceiving circuit RW‘VincludingIa receiving antenna» ‘ 'operation'of
control‘ circuit alsoico-r'nprises 1a "forwardé-acting, 25*
RAW','the input, of the circuit 'I'W‘ and the out
25 put
of the circuit RWTbeing coupled in ‘con
portion including the amplifier-detector I Z ' com-I
‘mon also to the codan'circuit II, the-“gain ins"; f
riusate r elation "with‘each other; and in energy
I transmitting relation with the telephone line LW crease” relay GI controlled by the detected car-V
' ‘by the hybrid coil connection Hrandgbalancing , rier current,» and an associated circuit controlled’ a
by operation‘ of- relay GI.
automatic gain?
network N1 in‘ well-known manner.
1 ;_‘Similarly, the eastterminal oftherradio tele-' control'clrcuit is provided for regulating the “gaina ,
phone system comprises’ ' a , transmitting "circuit
of the receiving circuit'RE 'to compensate'for-j
the receivinggcircuit RE including the. receiving
ner which;will'be describedin detaillatergin ‘con: '
'I'Eincluding the transmitting antenna/TAB}! and . variations in'received signal volume, in a'rnané -
nection with the description of ‘operation of the: a5:
; antenna RAE, theinputrof' the circuit TE and the
output of the circuit REbeing ‘coupled incon
Vjugate relationfwith "each other and in energy‘
: complete system;
~ ' The’ transmitting circuittiI‘E‘ at the east‘ termi: 1" I‘ '
transmitting relation“ with the telephone line Vnal of the system contains transmission and con
7' ‘
- LE‘by the hybrid‘ coil Haandassociated balanc-i
4o ing'network N2.’ '7
' The transmitting circuit 'I'W atthe west" ter
trol apparatus similar ‘to that used in the trans
mittingcircuit 'I‘W at’the ‘west terminal ‘of-their
system, as'indicated by the ‘use‘oi-si'mil'ar charac-I ' t
‘minal comprise‘sithe usual transmission appa~
ratus including the radio transmitter I and asso-i.
' ,ciatedv carrier oscillator 2. In the'absence of
ters for identifying corresponding pieces or ap-f “
paratus; eXcept-thatin the circuit TE‘ the char'f' ‘
acters are followed by a prime 1’) mark: The‘
speech wave transmission therein, the‘ circuit
'TW is arranged in any'suitablemanner, asiin-r
frequencies of the carrier’ oscillators, 2, Z’ in the»
‘ to prevent the radiationof carrier oscillations to
RW at the west terminal of the system contains
‘transmitting circuits at the' two terminals'are
V dicated byxthe' normal open circuit 33 between rindicatedras being the same, f1, but may be
transmitter I and transmitting antenna TAW, different irequencies. Also, the'receiving circuit
the east terminal station. ‘ When speech wave ‘7 transmission and control apparatus similarlto
that used in the receiving circuit RE at the east
TW from the line LW, a portion thereof is im-‘ terminal including a’ codan circuit and an auto;
maticfgain control circuit as described aboveras'
1 pressed on the input of the wave-operated 'con
trol device (ampli?er-detector) Lbridged'across ' indicated byjthe use of {the same characters for" 55
signals are received in the transmitting circuit
the circuit TW in front of the radio transmitter
I, and causes theoperation of that device to re
' move'th'e disability in the circuit TW‘ at‘the point
3’, thereby allowing one or'both ofthe' signal side
bands, as ‘well'as theunmodulatedjcarrier com- '
identifying corresponding Vfpieces ofapparatus,
except that in the circuit RW' the characters are
7j -
followed by afp'rime (") ‘mark.
' Forthe purpose ‘of giving exclusive ‘controlof'
the switching apparatus atfboth'terminals toythe '
' particular subscriber associated 'with' the "tele-' 60
V l' 7601 ponent, to be radiated by the transmitting an= phone’ line _LW or LE, who ?rst istartsitol'talkji '
tenna' TAW to theeast terminallof the system.’
, Thereceivingicircuiti-RE in "theveast' terminal‘ 'i additional
as follows:
_ Q
of 'thes‘ystem comprises the usual? transmission
or apparatus "including the radio frequency tuning]
Operation ‘of, the control circuit (ampli?er?
detector)_ 4 at the west terminal in responsejto
' associated carrier'oscillator ‘I, the intermediate telephone‘signals in the circuit 'I‘W, in addition}
rendering the latter circuit operative at the
' frequency wamplifier 8,'the second detector 9 and to
point 3 to? transmit signal side ‘bands and uni?‘
the audio ampli?er I0.
Associated with the receiving fcircuit RE of’ modulated carrier component to the‘transmit
tings‘ antenna to be radiated; also disables: the"
the east terminal isthe codan circuit I I. As in
normally operativeoutput of the codan circuit ‘
dicated, itcomprises the, ampli?er-detector cir
and amplifier means-5, af?rst detector 6 ‘and
V cuit I2, "selective to a beat frequency, i."e.,1the
difference betweenthe two carrier frequencies
f1 and f2, respectively generated by the carrier
oscillator 2" at the 'west terminal'and the carrier
II’ and a portion (gain increase) of the auto?’ "I
'matic gain control circuit I4’ at the points-I8;
as indicated. Similarly, operation of the con
trol circuit .4’ at vthe east‘ terminal in response to 795" '
telephone signals in the transmitting circuit TE,
disables the codan circuit H in its output and the
gain increase action of the automatic gaincontrol
circuit 14, for example, by causing the operation‘
of a relay I9 to break the connection betweenthe
output of the common ampli?er-detector l2 and
the codan relay l3 and the “gain increase” relay
GI at the point 20, as indicated.
Also, operation of the ampli?er-detector l2 at
10 the east terminal, if its output is operative, causes
the operation of the control relay l3 to disable
the ampli?er-detector 4' in its input, for exam
ple, by closing an energizing circuit for a relay 2|
to cause the operation of the latter to produce an
the speech waves in the input of the radio trans~
mitter I controls the device _4 to remove the dis-'
ability at the point 3 in the output of the radio
transmitter l so as to allow the side-band energy
as well as the‘unmodulated carrier component'to
be radiated by the transmitting antenna 'I’AW
to the east terminal of the system, and to disable
the codan II’ and the gain increase portion of
the automatic gain control'lli’ retaining the gain
of the receiving circuit RW at the west terminal
substantially at the last established value while 10
signal transmission from the west terminal con—
tinues in the manner previously described.
At' the east terminal of the system the radiated
open circuit in the input to ampli?er 4’ at the energy
will be picked up by the receiving antenna
points 22. Similarly, operation of the codan cir
RAE and will be selected and ampli?ed by the
cuit H’ at the west terminal, when its output is radio frequency tuning and amplifying means 5
operative, causes the input, of the ampli?er~de~ in the input of the receiving circuit REJThe
tector 4 associated with the transmitting circuit resulting ampli?ed energy is impressed on the
20 TW at the same terminal to be disabled at the
?rst detector 6 in which it will be combined with
point 23, as indicated.
the local carrier f2 supplied from the carrier‘
However, if the frequencies of the carrier oscil
oscillator 1 to produce waves of intermediate fre
lators associated with the transmitting circuit quency. A combination-carrier component in the
at the two terminals of the system are different output of detector 6 will be selected by the ampli
so that different radio frequencies are transmit
?er-detector l2 and will control the operation of
ted for the two directions of transmission, some the codan and the “gain increase” portion of the
or all of the above-described controls may be
automatic gain control circuit in the manner to
be described later.
The system of the invention, as shown in the
The remaining portion of the waves of inter
drawing, operates as follows:
mediate frequency in the output of detector 6
When no speech signals are being received, by will be ampli?ed by the intermediate frequency
the radio telephone system from the west tele_ ampli?er 8, the gain of which is controlled by se
phone line LW associated with the west radio lected carrier waves in the input and output of
terminal or from the east telephone line LE as
the ampli?er by the automatic gain control cir
sociated with the east radio terminal, thereon,
cuit in the manner to be described later, and then
trol circuits at the two radio terminals are in the
condition indicated in the ?gure, with both trans~
mitting circuits TW and TE disabled in their out
puts at the points 3 and 3"respectively, so that
no carrier waves may be radiated to the other
terminal, and with the receiving circuits RE and
will enter the second detector 9 in which the
speech signal waves will be demodulated in well
known manner. The speech waves will then be
ampli?ed by the audio ampli?er‘ “I.
A portion of the waves in the output of the
?rst detector 6, i. e., a combination product of
RW blocked in their outputs at the points 15 and the carrier frequency f1 generated by the carrier
I5’. respectively. The normal blocking of the oscillator 2 at the west terminal and the carrier
outputs of the receiving circuits prevents, when frequency f2 generated by the local oscillator at
signals are not being transmitted, noise currents the east terminal, will be selected in the com
from being transmitted through the associated mon input of the codan circuit H and the for 45
line to the telephone subscriber associated with ward-acting portion of the automatic gain con
each terminal, and being fed back through the trol, connected across the output of the detector,
transmitting circuit at the terminal to the switch
and will be ampli?ed and detected by ampli?er
ing apparatus for controlling the transmission of detector l2. The detected current will energize.
carrier from that terminal. In the absence of the windings of the codan relay I3 and the “gain 50
received carrier, the automatic gain control re
increase” relay GI connected in parallel across
lays at each terminal are unoperated, and the the output of the device l2, causing their opera
gain of each receiving circuit RE, RW is set sub
stantially at the value last established when car
Relay It will operate to open the short-circuit
rier was removed, which value may be sufficient ing connection l5} across the receiving circuit RE 55
to make the noise currents present, if allowed in the output of audio ampli?er l0, thereby al
to go through, of sufficient amplitude to annoy lowing the speech signal waves in' the output
the subscriber associated with the receiving line thereof to be transmitted from the output of the
LE or LW and to cause false operation of the
circuit TE through the hybrid coil Hz to the line'
carrier transmitting apparatus associated with LE over which they will be transmitted to the 60
the transmitting circuit at the same terminal.
east subscriber‘. 'The operation of relay I3 also
Now, let it be supposed that speech waves are
closes an energizing circuit for the winding of
being received in the transmitting circuit TW relay 2! from battery 24, causing the operation
(35 of the west radio terminal from a subscriber as
of the latter relay to open the input circuit of
sociated with telephone line LW and speech ampli?er-detector 4' at the points v22 for the" 65
waves are not being simultaneously received in purpose referred to previously.
the transmitting circuit TE of ‘the east radio
The gain increase relay GI operates simultane
terminal from the telephone line LE associated ously with relay 13, to close its armature ‘and
. with the east subscriber. The main portion of
contact applying a positive voltage from battery 70
the speech waves in the circuit TV! will be im
25, through the charging resistance 26 and the
pressed upon the input of the radio transmitter
normally closed armature and back contact of
l and will modulate therein the carrier frequency the gain increase disabler relay GID, to the reg
from the associated carrier oscillator 2 in well
ulating condenser 2? in the grid circuit of the in
. known manner.
Simultaneously, a portion of termediate frequency ampli?er 8 causing the
carrier was removed, although there'awill ‘bea'
slightdrift to low gain as the chargeslowlyleaks-oif the condenser. - The relay GI and]?
gain of'the latter ampli?er to increase‘ as the
" charge on? condenser .21' increases. , The carrier
waves, as long» as they. continueftof arrive, will
hold- the relay ,GI; operated "and :for. a ‘desired
are arranged to have-a short hangover in opera
tion, provided, for example, by,designing' them~l
‘in hangover interval'after the reception of carrier . to be slow-releasing to the required degree, so ,
that their release will not’ result in signal clipping
The gain of the intermediate-frequency ampli
The release of the codan relay 13 will cause'the‘ '
?er 8 will continue to be. increased by the charg
in'g-of condenser 21 in response to the operation
short-circuiting. connection 15 to be reapplied
of relay GI until the'am'pli?ed waves'in the out» across
By suitable
of ampli?er
of the ‘l0.
, of7 . the
put‘of that ampli?erreach the desired average , circuit associated-with the'gain increase'and de
levelwhen the detected current in the output of ' crease relays, i. 'e., the values of condenser 21, I ‘
detector" l?-connected across the output of am
resistances 26' and 28, and battery 25, the rates ‘
pli?er 8 is su?‘icient to cause operation of the
oflgain increase anddecrease may be‘ made‘fast 15";
gain increase disabler relay YGID in the‘ output, enoughtto follow’ fading, but not :fast enough,
circuit. of the detector.
Theiresultant operation ‘of the relay' on)
' . ;to
a The
,theicodan circuit
~ ‘_ j and
breaks 1 its. armature and back contact disabling
of V the, gain increaser:v circuit‘ operated there‘~v .
the charging’ circuit for condenser'ZTfrom bat through‘ are made exactlysthe same f so {that the)
2o. tery ‘25 through resistance 26, thereby-preventing .above-mentioned fundamental requirements: for
any further increase irithe gain of the interme
. diate: frequency ampli?er 8 due
therideal codan circuit are LQbtalhBdTbOth for vvit g
to " the‘ continued '
operation of’ the gain increase relay‘GI.
and the gain‘ increaserr circuit‘ of vthe automatic "
gain control, i. e., both will operate on the weak;
vIf the output level of ampli?er, 8 remains at
25; the desired value, the gain. thereof will be sta-.
.bilized atra value corresponding to the charge
' est- usable carrier. and will not operate on the; 25 ’
strongest static that'can be over-ridden; It ‘will’ .
be noted also that all of the gain regulation of '
on condenser 21 at‘ the time relay GID operated.
however, the output level of "ampli?er 8. in
' If,
creases about 1 decibel, the marginal gain de
30 crease relay GD will be operated by the detected
the receiving circuit is done beyond the point
where the codan’ circuit is bridged, that is, in the
intermediate frequency ampli?er, sov that the, 30
signal to noise-ratio at‘the input of the codan
is a maximum. Improved operation from‘a noise
' ' current in the output of detector I6to push the
gain of ‘ampli?er 8'down-in the following man
standpoint is thus attained.‘v
Various modi?cations of- theq'ci rcuitisi.
which‘v ' Q
The relay GI’ operates to' connect the rather have been illustrated and described ‘which are
low discharge resistance'28 .in parallel with‘ the within thespirit and scope of the invention will
regulating condenserlr'l in the gridcircuit of the beapparent to persons skil1ed~inthe,~art-; for;
intermediate ampli?er tube 8 causing the gain ‘ example,- as ‘indicated’ above,’ control ‘energy
. .f of ‘the latter-to 'startto decrease as the condenser ' other than the carrier- sent out from 'theutra'nse,
21 discharges’ through resistance 28. 'The level mitting terminal. may be used to ‘control, the
offthe waves’ in the output'of the intermediate codan and automatic gain control‘circuits atthe'
frequency amplifier -8 will decrease in accordance receiver; carrier'energy. may be used for adjust-1
vwith the decrease in gain until thedetected cLu'-'
rentin the'output of detector’ I6 is reduced-to
ingrreceiver gain and control; energy-‘of. another: .
frequency transmitted during'signal transmission‘ 7 ;
the point‘where the gain decrease relay GD re-V
is designed to have little,
45 leases. 'The relay GD
The relays GD
'if any,
forcodanoperation-or vice versa; the codan Oil‘),
may be bridged
. r
at some.
point infrontfof,
. 45
. ‘
the point of gain regulation otherthan. the out~ » "
"and GID are relatively designed so that the level ' put of the first detector, forexample, across the _
at which the gain decrease relay GD releases is receiving antenna; at least a portion ofthe gain 5
not su?iciently low tocause'the release ,of the regulation of the receiver‘. may be ‘made in the;
gain increase disablerrelay GID. When the gain audio ampli?er; the gain increase: relay may be; 50
relay GD releases, the resistance 28is controlled from a different. ampli?er-detector
disconnected from in parallel with’ the regulating than used for controlling the codanrelay, and 5
condenser 21 and the gain of'theintermediate it may be connected to the receiving circuit atia
frequency ampli?er} is stabilized'at the value . different point than’ they codan ampli?er-detecr' ’
corresponding to the charge on regulating cone tor; vor the codanv function. and the gain increase
' denser 21 until such time as the level of_ the waves
function may be accomplished by a single relay 7 1 1'
in the output of ampli?er'l? is reduced to the
What is two
sets is:
of contacts.
point where thegain increase disabler relay GID
' releases. The latter'relay preferably has no hang
1. A terminal station for a'two-way'signaling
over in operation. The release of relay GID re
including a two-way, transmission me-iv
the charging resistance 26 to, regulat-v system
dium, comprising a circuit for transmitting ‘sig
. connects
~ ing-condenser Z'Iso .that the operation of the gain nals to said medium anda circuit for receiving '
GI in response'to the received car
7 increase relay
1 rier will’ cause the gain of the intermediate fre-V
fquency'ampli?er 8 to be increased in the manner
' signals therefrom, transmission apparatus con—;
nected. to the output of said receiving circuit,
means normally disabling said’ receiving circuit
previously described.
near its output, automatic gain'regulating- means 7'
If at any timethe reception of carrier waves
in said receiving circuit-switching; meansrcon~
ceases with cessation in speech transmission at nected to said receiving circuitin front of the’,
the transmitting terminal, each of the gain con
trol relays which may be operated at the receiving 'point of gain regulation therein, 'r'esponsive?t'o
terminal will release so that the gain control
circuits are in the‘condition indicated in the
drawing, and the. gain of ampli?er 8 will remain
impressed'control energy of distinctive frequency‘
when signals are being vreceived at thelterminal
station from said medium to remove the disa
substantially at the value last established corre- ~ bility from. said receiving circuit, and means
sponding to the charge on condenser 21 when
responsive to signal transmission in said trans
mitting circuit to disable said switching means,
and to prevent change in the gain of said re
ceiving circuit until said signal transmission
2. A two-way telephone system comprising two
stations connected by a two-way transmission
medium subject to static, each station compris
ing a transmitting circuit and a receiving cir
cuit connected to said medium, a subscriber’s cir
10 cuit coupled to the output of said receiving cir
cuit and the input of said transmitting circuit,
means normally disabling the output of said re
ceiving circuit to prevent the transmission of
static energy to said subscriber’s circuit and said
15 transmitting circuit in the absence of received
telephone signal energy, means responsive to the
initiation of signals in said transmitting circuit
for sending out over said medium to the other
station said signals and control energy of dis
tinctive frequency, means responsive to the con
trol energy of distinctive frequency‘received from
the other station to automatically regulate the
gain of said receiving circuit in accordance with
the volume level of the received control energy,
switching means connected to said receiving cir
cuit in front of the point of gain regulation
therein, selectively responsive to the received
control energy to remove the disability from the
output of said receiving circuit, means to main 10
tain the gain of the receiving circuit at the last
established value when the reception of said
control energy ceases, and means responsive to
the initiation of signals in said transmitting cir
cuit to disable the switching means connected to 15
said receiving circuit and to substantially pre
vent change in the gain of the receiving circuit
during signal transmission from the station.
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