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Nov. 5, 1946.
Filed Feb. l5, 1945
äÉIl. mmfQ
Patented Nov. 5, 1946
Roland A. Lynn, Teaneck, N. J., assignor to Radio
Corporation of America, a corporation of Dela
Application February 13, 1943, Serial No. 475,728
9 Claims. (Cl. Z50-17)
This invention relates to program change in
dicators and has particularly to do with a methodof automatically monitoring the modulated wave
of a transmitted radio signal and compa-ring it
with the original wave at the point of origin.
The invention also has other uses, as will be ap
parent to those skilled in the art.
In the ñeld of broadcasting the possibility eX
ists, unfortunately, that the wire channel be
tween a studio and the transmitter might be sev
ered by an act of sabotage, and at the same time
an unauthorized program might be substituted to
modulate the transmitter’s carrier. The substi
tution of such a program, if of a subversive na
ture, could easily create a panic. Program cir
a system which departs from practices of the
prior art in the following respects: v
I have arranged for my alarm device to respond
only to unfavorable comparisons between the vol
ume level of the studio program and that of the
same program as received on a monitoring radio.
Heretofore, systems have been tried in which the
alarm would be subject to failure if the trans
mitter modulation output continued during the
absence of studio output. A failure of the alarm
also resulted from the application of a relatively
high level substitute signal to the transmitter
modulator during a period of low volume output'
from the studio. Such “one-way” alarm systems
15 are to be avoided in a dependable monitoring
cuits between studios and associated transmitters
system such as I have disclosed herein.
might be subject to such acts of sabotage with
One of the Weaknesses of the comparison de
out the knowledge of anyone in the studio. Fur
vices used in the past arose from attempts to
thermore, no dependable means are available at
the transmitter whereby a substitute program 20 control the gain in the ampliiiers by half-wave
rectiñcation of the signal components. I have
could be detected, especially if it appeared to be
foundthat full-wave rectification is practically
essential to a successfully operable device of .this
Accordingly, it is an object of my invention to
provide a radio program monitoring system in
cluding a comparison device by which an alarm 25 My invention will now be described in more
detail, reference being made t0 the accompany
may be sounded, or a visual indicator may be
drawing, the sole ligure of which represents
actuated, Whenever a difference occurs between
in diagrammatic form a preferred circuit ar
the modulated waves applied to an outgoing line
and such waves brought in for comparison by a
in any suitable
radio receiver which is tuned to the associated 30
It is another object of my invention to provide
a system of the class described in which an alarm
device may be actuated whenever there is a de
manner to a studio output circuit on which a
modulated audio frequency wave is impressed.
The energy is applied across transformer 2 to
a load impedance 3 having a center-tap 4 which
parture from a normal ratio between the volume 35 connects with a center-tap on the secondary of
transformer 2. The impedance 3 may be used as
of the studio output signal and the volume of the
incoming monitoring signal from the transmit
ter. This means, of course, that the alarm will be
sounded whenever a pause occurs in one modu
a Voltage divider, taps from which are connected
respectively with the control grids in two dis
charge tubes 5. These tubes are connected for
lated wave without a corresponding pause in the 40 push-pull ampliiication. Their cathodes are con
nected through a milliammeter E to a suitable point
other modulated wave. It means, further, that
on a voltage divider l. By this connection, the
an increased Volume in one wave without a si
normal cathode potential is maintained at a suit
multaneous increase in volume in the other wave
able value above ground potential, as indicated
shall likewise be used to actuate the alarm,
According to another object of my invention, 45 atv the left end section of the Voltage divider 1.
The amplifier tubes 5 are preferably of the
means are provided in combination with the afore
pentode type, for the reason that they possess
mentioned comparison device for producing a
a variable mu characteristic which is of value
certain limiting action upon the signal energy
in obtaining a wide latitude of control action.
component which is derived directly from the
studio output circuit and which is applied to the 50 The screen grids are connected through resistor
32 to junction I6 on the voltage divider 1, Each
comparison device. The purpose of the limiter
grid is connected to its cathode. The
is to compensate for a limited modulation which
anodes are connected to the primary terminals
is characteristic of the signal as radiated by the
of a transformer 8. This primary has a center
transmitter. .
to rwhich anode potential is applied from the
In carrying out my invention, I have provided 55
right hand end of the voltage divider 1. »A power
The common cathode of tube 23 is connected
to point A on a balanced bridge circuit consist
ing of four resistors 25, 25, 21, and 28. Resistors
and for other tubes as Well.
25 and 26 are preferably made equal to each
The anodes of tubes 5 are also interconnected 5 other. Resistors 21 and 28 may, if desired, be
by an impedance circuit I0 at the center of which
of the order of one megohm each. These values,
is a connection with the center-tap on the pri
however, are not critical.
mary of transformer 8. One arm of ‘this im
Point C on the bridge circuit is >connected to
pedance circuit is used as a potentiometer for
the mid-tap on the secondary of transformer 8.
ywithdrawing a signal voltage component to be
Point D is grounded. Point B on the bridge cir
used for amplitude limiting. This signal com
cuit is diametrically opposed to point A and re
ponent is impressed through an impedance which
ceives rectified signal potentials from a circuit
includes resistor Il, capacitor I2, and resistor i9,
to be described.
the latter being connected to the voltage divider
Referring now to the lower portion of the cir
1 at point I8. The control grid in a discharge
cuit diagram, I show input leads 4I to which are
tube I3 responds to this signal potential as ap
applied audio frequency output energy from a
plied acrossV resistor I9. This tube is preferably
monitoring radio receiver. The primary winding
of the duplex-diode-triode type. The triode sec
of a transformer 42 is connected to the terminals
tion includes the cathode, control grid, and main
lil. The secondary winding 3d of transformer
anode I4. The diode section includes two anodes
e2 feeds energy to a load impedance 44 which is
I5 adjacent the cathode. The D. C. potential 20 preferably
composed of a potentiometer having
of the cathode is fixed by its connection to point
a tap connected to the control grid of an ampli
I1 on potentiometer 1.
der tube 45. One terminal of the secondary 34
The main anode I4 is supplied with a plus D. C.
is grounded and is also connected through a
potential from a point I5 on the potentiometer 1,
cathode resistor ¿it to the cathode of tube 45.
this point being connected to one terminal of a
Resistor ¿le is preferably shunted by a capaci
primary winding in transformer 33. The other
tor c1.
terminal of this primary is connected directly to
supply unit 9 is used to obtain all the necessary
potentials for the different electrodes of tubes 5
Although the amplifier tube 45 is shown as a
the anode I4.
Transformer 33 has a secondary winding, the
triode, tubes of other types may be used if de
sired. Anode potential is supplied to the tube
terminals of which are connected respectively
45 through the primary winding of a transformer
to the two anodes I5. This secondary also has .a
di! from junction point I6 on the potentiometer 1.
center-tap which is connected to ground through
Transformer 4B is used for the purpose of cou
resistor 22. The control grids in the push-pull
pling the output from tube 45 to lthe anode cir
amplifier tubes 5 are'biased in part by the po 35 cuit of a double-diode full-wave rectifier tube
tential drop across resistor 22.
` e3. The secondary winding of transformer 48
A D. C. potential for normal grid bias is de
is center-tapped andV is connected to point C
rived from point I8 on the potentiometer 1 and
,on the above described bridge circuit. The ter
is impressed upon the control grid of tube i3
minals of the secondary of the transformer 48 are
through grid leak resistor I9.
connected respectively to the anodes in tube 43.
From the above description, it will be seen that
the triode section of tube I3 is enabled t'o am
plify a signal component, and this amplified
component is rectified, causing a negative bias po
tential to be impressed upon the control grids in
tubes 5 in dependence upon the amplitude of the
signal itself. Thus excessive peaks of energy
are limited and only signals within a limited
The common cathode of tube 43 is connected .to
point B on the bridge circuit and also to a control
grid in a discharge tube 43 which is used as a
control value in a relayv circuit.
rI‘wo discharge tubes t9 andäil are suitably
connected to be controlled by differences of po
tential in different arms of the bridge circuit
so that either one of two relays 52 andì53 may
volume range are fed across the transformer 3.
be cie-energized whenever an unbalance of po
The gain reduction is Very rapid. The time con 50 tentials occurs in the bridge circuit. Normally
stant value of the gain recovery is dependent upon
the relays 52 and 53 are energized by a conduc
the values of capacitor 2l and the grid leak re
tive state in the two tubes 49 and 5l). The relays
sistor 22. These values may, for example, be of
52 and 53 are disposed in the anode circuits of
the order of .25 mfd. and 4 megohms, respectively.
tubes 43 and 53 respectively. These anode cir
The limiter tube I3 serves a very useful purpose
cuits also preferably include milliammeters 5l for
under ordinary circumstances. It operates to
convenience in observing the operation of the
rapidly establish a lowered gain characteristic
upon the advent of an audio wave form of ab
The power supply unit 9 furnishes current for
normally high peak value, but does not disturb the ,
operation of the relays 52 and 53 through the
established operating bias of the tubes 5 during
space paths of tubes 49 and 5@ respectively. The
the presence of signals within a normal range 60 relay circuits are Completed through the cath
of amplitude variation. The desired degree of
odes of tubes il@ and 53, through the two arms
the limiting action may be selected by adjustment
of a potentiometer `54%, and through series re
of the tap on potentiometer I0. In certain cases
sistors 55 and 55 to ground. Resistor 56 is pref
the limiter tube might be dispensed with and a
erably made adjustable and the tap on potenti
conventional automatic gain control circuit may
ometer 5d may also be adjusted for balancing
be vconnected to the input circuit of the amplifier
the amplitudes of the currents through tubes 4:9
tubes 5.
and 50.
The limited modulation wave output from
In order to facilitate the starting up or re
tubes 5 is fed across transformer 8 to- the anodes
setting of the apparatus after an alarm and par
of a double-diode full-wave rectifier tube 23. A
ticularly in order to energize relays 52 and 53,
desirable attenuation of the higher audio fre
it is advantageous to momentarily short-circuit
quencies is obtained by the use of capacitors 24
at least a part of the impedance of elements 55
whichi are shunt-connected across the anode
andv 56. This is accomplished by actuating a
leads of tube 23. Grounded by-pass condensers 7.5 key (not shown) which closes contacts ‘51 land
38 may also be provided for the same purpose.
thereby places resistor 58 of relatively low re
sistance value in shunt with resistors 55 and 59.
'I'his operation enables the tubes 49 and 50 to
draw more than a normal now of current and
thus to energize the relays 52 and 53. Under
normal operating conditions, the current flow
through tubes 49 and 55 is sumcient to hold up
the armatures 58 and 19, but may be insufficient
to attract them across their gaps.
Any one or more alarm devices may be actu
ated whenever either of the relays 52 and 53 is
of such movement being dependent upon the de
gree of unbalance between the rectification com
ponents from tubes 23 and 43.
Whenever point A is driven sufficiently nega
tive with respect to ground so that the imped
ance through the space path in tube 50 becomes
too great to permit relay 53 to hold up its arma
ture 10, this armature is released and causes the
alarm indication to be made in such manner as
10 to show an abnormal output from the transmitter
with reference to the studio output. Also when
point B on the bridge circuit is rendered suiiî'
blocking the associated tube 49 or 59 respectively.
ciently negative with respect to ground, then the
The armature 60 of relay 52 is associated with a
impedance of tube 49 becomes too great to allow
back contact 6l, the latter being connected to' 15 armature 60 on the relay 52 to be held up. The
sufliciently de-energized by at least partially
one terminal of an energy source 63. l The other
alarm indication is then made so as to indicate
terminal of this source is connected through a
a failure on the transmitter side of the compari
switch 64 to common return circuit 65 for all of
son device.
the alarm devices.
If the alarm circuits which include pilot lamp
Illustratively, four different types of alarm de 20 1S, buzzer 11, and annunciator 18 are actuated,
vice are indicated in the diagram. These are
then it is known that an abnormal condition has
lamps 66 and 16, buzzers 61 and 11, annuncia
occurred in which the output from the trans
tors 68 and 18, and a monitoring speaker 40.
mitter cannot be accounted for as originating at
The circuits for the different alarm indicators
the studio. The reverse condition is indicated
'66, B1, and 68 are preferably in parallel. The 25 by the actuation of pilot lamp 66, buzzer 61, and
indicator units 16, 11, and 18 are also connected
annunciator 68 when the tube 49 becomes at least
in parallel. Although the individual indicator
partially blocked and relay 52 de-energizes. The
units may be separately cut in and out of service -
amplitude of the output from the loud speaker 40
by individual switches, it is a simpler matter to
will be greatly increased -upon setting off either
provide one switch 64 for opening them all at 30 side of the alarm circuit, provided there is any
once after the alarm has been noted.
signal at all which the monitoring radio receiver
Relay 53 has an armature 1B associated with a
back contact 1l for controlling the parallel con
nected circuits through indicator lamp 15, buzzer
11, and annunciator 18. These alarm circuits are
likewise actuated whenever the armature 10 is re
leased by relay 53, that is to say, when the tube
50 becomes sufficiently blocked.
can pick up.
For the purpose of smoothing the rectified po
tential output from tubes 23 and 43, I preferably
place a delay circuit in shunt with resistors 25
and 26. This delay circuit includes capacitors 29
and resistors 30. The junction between the two
capacitors is connected to point C on the bridge
The winding of a relay 31 may, if desired, be
circuit and also to the anode circuits of the rec
inserted between the common leads 3B and 65 of 40 til-ler tubes 23 and 43.
the alarm indicator cricuits. Relay 31 has conn
It will be seen from the above description that
tacts which are closeable to short-circuit an at
my automatic program change indicator is of
tenuating resistor 39 in the voice circuit of a
value not only in detecting subversive acts which
monitoring loud speaker 4l). Thus the relatively
would interfere with the Iproper operation of a
low-level input to the speaker 49 may be raised ‘ broadcasting system, but it is also of value for
to a high level whenever either of the alarm re
giving an alarm whenever an abnormal trans
lays 52, 53 releases.
mission condition occurs. The loss of normal
The operation of my program change indicator
established levels due to partial or complete fail
is further explained as follows:
ure of any of the equipment in the paths be
The voltage developed across resistor 25 in the
tween the points of origin and reception of the
bridge circuit is dependent upon the amplitude of
the signal component which is rectified by the
tube 23. Likewise, the voltage developed across
resistor 2S is attributable to the amplitude of the
signal component rectified by tube 43. When
the voltage drop in resistor 25 equals that in re
sistor 26, then equal biases are applied to the
grids of tubes 49 and 59 and these biases are of
such value that emission takes place in both tubes,
and they normally draw sunicient current to main
tain the relays 52 and 53 in an energized state.
Under this balanced condition, regardless of the
signal amplitudes, points A and B are both at
substantially ground potential.
When the rectmed signal potential in tube 23
exceeds that in tube 43, then the difference be
tween the potential drop across resistor 25 and
across resistor 25 is such that point A on the
bridge circuit is rendered positive with respect
to ground and point B is rendered negative. This
is true because the values of resistors 21 and 28
are equal and point D is grounded. Therefore,
the ground potential applied to point D is one with
reference to which the voltages of points A and
transmitted signal makes itself evident in the
operation of the alarm system. Failure may oc
cur in the wire lines, at the equipment of the
transmitter, at the transmitter itself, the trans
mitter antenna, the propagation space path, or
in the radio receiver. Abnormally high levels of
signal reception as well as abnormally low levels
on a comparative basis will result in giving an
alarm. Various alarm devices, not only such as
shown, but of other types as well may be placed
at any convenient points and those which are
parallel connected with each other may be dis
posed- at different points remote from one an
other, if desired.
Various modifications of my invention Will,
doubtless, suggest themselves to those skilled in
the art in View of the foregoing description.
Such modifications would, however, be compre
hended within the spirit and scope of the inven
I claim:
l. A program change indicator system for use
in connection with a broadcast » studio output
circuit and with a monitoring receiver, said sys
B\move equally in opposite directions, the extent 75 tem comprising separate full-wave rectiñers, one
being coupled to said studio output circuit and
the other being coupled to the output side of
said receiver, means including a iour-armbridge
circuit for comparing the instantaneousenergy
components delivered by said rectiñers, one arm
of said bridge circuit being in series with the
rectiñer which delivers the studio energy com
ponent, a second arm of said bridge circuit being
in series with the rectiner which delivers the
monitoring receiver energy component, the third
and fourth arms being each grounded at one
end thereof and being connected at the other
ends to the iìrst and second arms at the respec
tive' junctions of the latter with the cathode elec
to the ~junction between the iirst and third im'
pedances, a connection from the cathode elec
trodes of the other said rectifier to the junction
between the second and fourth impedances, cir
cuit means connected to the junction between
the nrst and second impedances and branched to
the anode electrodes of both rectiiiers for pro
ducing potentials across the first and second im
pedances in accordance with the relative ampli
tudes of the waveenergy from the different
sources, a ground connection to the junction be
tween the third and fourth impedances, a pair
of electronic tubes each havingV an input circuit
connected between its ground-connected cathode
and its control-grid and having a load'inits
trodes oi said reotiñers, conductive circuits in
output circuit, a connection from the control grid
terconnecting the anode electrodes of both rec
of one tube to thejunction between the nrst
tiiiers with the junction between the nrst and
and third impedances, and another connection
second bridge arms, a pair of relays each having
from the control grid of the other tube to the
its winding in circuit with the space path of
junction between the second and fourth imped
an individual discharge tube, means for control 20 ances, whereby the unbalancing oi potentials
ling said tubes in opposite senses in response to
across the iirst and second impedances varies
unequal potentials developed across the ñrst and
the output of said tubes in opposite senses.
second bridge arms, and indicating means selec
5. A. circuit arrangement in accordance with
tively actuable by said relays.
claim e in which each said load is constituted
2. In a. program change indicator which com 25 as'the winding of a relay, and said circuit >ar
pares audio-irequency potentials from two dif
rangement includes an alarm indicator operable
ferent sources normally possessing the same wave - upon de-energìZation of one of said relays, and
envelope, the method of actuating a relay-con
a second alarm indicator operable upon de
trolled alarm upon the reception of unlike wave
energization of the other of said relays.
envelopes from said sources, said method com 30
6. A circuit arrangement in accordance with
prising separately amplifying and rectiiying a
full-wave signal component from each source,
claim 4 in which each said load is constituted
as the winding of a relay, and said circuit’ ar
rangement includes a plurality of alarm indi
value in response to the rectiiication of said sig
cators operable upon release of a relay armature,
nal components so long as they follow the same 35 certain of said indicators being individual to each
wave envelope, causing the amplitudes oi said
relay and at least one indicator being common
producing two currents of substantially equal
currents to vary in opposite senses in response to
to both relays.
the reception of unlike wave envelopes,- causing
7. A circuit arrangement in accordance with
a limiting action to be produced with' respect
claim 4 in which each said load is in circuit with
to the ampliñcation gain factor which is applied 40 the winding of a common relay andsaid circuit
to the signal components from at least one of
arrangement includes a load speader the voice
said sources', and causing said alarm to be actu
circuit of which includes an attenuating imped
ated by a relay circuit closure which takes place
ance, and means operable by said relay for short
when the disparity between said current ampli 4. circuiting said attenuating impedance thereby to
tudes reaches a predetermined value.
increase the output of sound from said loud
3. In a program change indicator which com
pares audio-frequency potentials from two differ
ent sources normally possessing the same wave
envelope, the method of actuating a relay-con
trolled alarm upon the reception of unlike wave
envelopes from said. sources, said method corn
8. In a system for comparing the modulation
envelopes in two trains of signals normally hav
ing a common source, one of said trains being
applied to and derived from a space path of
radio propagation and the other or“ said trains
being applied to and derived from an essentially
prising separately amplifying and rectifying a
full-wave signal component from each source,
metallic circuit, separate means for amplifying
producing two voltages oi substantially equal 55 and rectifying the signal energy in each of said
’Value in response to the rectiñcation of said
signal components so» long as they follow the
same wave envelope, causing the values of said
voltages to vary in opposite senses in response to
the reception of unlike wave envelopes, causing
a limiting action to be produced with respect 60
to the ampliiication gain factor which is applied
trains, a four-arm bridge circuit arranged to
be fed with rectiñed energy components from
said separate means, and an alarm circuit respon
sive to a condition of energy unbalance in cor
responding arms of said bridge in the event
that the characteristics of said modulation en
velopes in the two said trains diiier by a predeter
to the signal components from at least one of
mined amount.
said sources, and causing said alarm to be actu
9. A system according to claim 8 and including
ated ‘oy a relay circuit closure which takes place 6 two indicators operatively connected to said alarm
when the disparity between said voltages reaches
circuit, one of said indicators being responsive
a predetermined value.
to an unbalance of potentials in said bridge cir
4. An alarm circuit arrangement comprising
cuit which results from an abnormally high en
two full-wave rectiiiers each transformer-coupled
ergy content of the signal train derived from
to a diiïerent source of audio frequency wave
radio propagation, and the other of said indica
energy, the two said sources normally possessing
tors being responsive to an unbalance of poten
the same wave envelope, a first, a second, a
in the opposite sense.
third and a fourth impedance constituting the
four arms of a Wheatstone bridge, a connection
from the cathode electrodes of one said rectifier
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