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Патент USA US2411853

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' 2,411,853
Patented 'Dec. 3, 1946
3 PATENT orifice; ~;
James L. Finch, East Rockaway; N. Y, assignor'i
to Radio Corporation of America, a. corpora
tion of Delaware
VApplication July 24, 1941, Serial No. 403,821 11 Claims. (01. 178-6)
ception of photo radio subject matter by modu
I propose .to transmit during this period a selected.
frequency ‘corresponding .to some de?nite pic-1
ture ‘shade, preferably black or white or some
de?nite frequency which would be called super
lating the frequency of a carrier wave in direct
black or super-white. "
This application discloses a new and improved
method of and means for transmission and re
proportion to the density or shading of the photo
y '
- In describing my invention in detail reference
radio subject matter being scanned, transmitting
the same and receiving the same.
Figure '1 illustrates somewhat diagrammatically,
matter. Should the receiver adjustment drift,
transmitter ‘of Figure land to be controlled .by
will be made to the attached drawings'wherein
J vIn k'novvn'systemslof photo radio transmission
the essential elements, of a transmitter arranged
of-the type described above any variation in the 10 in accordance with my invention; Figure 2 illus
frequency adjustment of the transmitter circuits
trates a receiver arranged in accordance with my
may result in an error in the received photo'- radio
invention to receive the transmission from the
then the picture may appear darker or lighter
on the average than the original. Similarly, if
the said de?nite frequency transmitted at the
picture or subject, margin; Figure 1a, illustrates
a modi?cation of, a portion of the transmitter-of
the transmitter frequency drifts a like error re
sults. It is conceivable that circuits of high ac
curacy could be arranged for use both at the
transmitting end and receiving end of the circuit
Figure 1; Figure 3 illustratesthe motor, com-i
mutators, etc., of Figure 2 and is used in describ
50 that these errors could be made negligible. 20
This, of course, involves considerable expense.
My invention relates to a new and improved
means for eliminating this change in contrast ‘or
shade or density in the transmitted picture dur
while Figure 4} is a curve illustrating the input and
ing the mannerin which synchronism is obtained;
ing transmission without necessitating the ‘use of
circuits of such high accuracy that the circuits
output of the limiter 14- of Figure 2.
Referring to Figure 1, the circuits and means
at the left of the dotted line comprises a scanner
of a well-known type including a drum I on which
a picture or subject to be scanned is mounted. A
light 3 cooperates with a diaphragm condensing‘
lens etc. 5, which produces light that is concene
trated and re?ected from the subject or image to
a lens vand photo tube arrangement 1. The photo
per se eliminate this error.
In its broadest aspect my invention involves the
use of a de?nite time interval during the trans
mission for the transmission of a' de?nitev fre-- 30 tube has a sensitive electrode 9 and an anode elec
quency used at the receiver to adjust the tune
trode -l I- connected in a circuit across the grid
thereof to prevent changes in the picture or sub
ject weight. ‘In a preferred modi?cation my sys
the grid electrodes of which are also excited by
tem makes use of a portion of the margin of the
carrier waves to be modulated impressed on the
electrodes of a’ pair of ampli?er tubes l8 andg20,
photo radio subject matter and transmits then a
primary winding of a transformer Tl . The photo
de?nite frequency for receiver control purposes. "’
tube ‘controls the current through resistances l4
and I6 and consequently the bias on tubes l8 and
20. The cathodes and grids of tubes [8- and-2i]
are connected to points along the potentiometer
This transmission may take place as stated‘ above
at the picture or subject margin and is applicable
to all scanning systems known today, such as,
for ‘example, those wherein the drum rotates at
a fairly high speed and the scanning element
resistance as shown. I The cam 60 on the drum,
and ‘contacts BI and 62 actuated therefrom and
described more in detail later, controls a circuit
including resistance 64 in series with electrodes
passes along the drum axis once per picture or
the systems wherein the drum rotates at a lower
speed and the scanning element passes back and
< 9 and H. ‘For the ‘present assumethis contact is
' closed and resistance 64 is shorted. The modu
forth along the drum axis a large number of
times per rotation of the drum. In the former
case the margin at which a de?nite ‘frequency is
lator is of the nature of an unbalanced modulator
wherein the carrier tone is applied in push-push
transmitted falls along the axis-of the drum. In
the second type of scanner means is provided for
’ sending the de?nite frequency at one or both ends
of excursions of the scanning element along the
axis of the drum, or vice versa. At the‘ receiv
ing end the said de?nite frequency received dur
ing this margin intervalis used for automatically
correcting the receiveradjustment to follow'any;
frequency variation at the transmitter or receiver;
and the bias potential vas controlled by scanning
is applied to the grids l1 and i9 in push-pull.
The modulated carrier is supplied from the output
electrodes of tubes l8 and :20 to the control grids
of a double triode ampli?er '22 and thence to the
primary winding of a ‘transformer T2. The ar
rangement- includingthe tubes l8 and 20 is similar many respects tqwhat isknown as an un
2,41 1,858
balanced modulator, the grids I1 and I9 being
excited in phase by potentials impressed on the
secondary windings of transformer TI connected
push-push, and in push-pull relation by potentials
the other from resonance at the generated fre
quency the voltage supplied to GI may be made
.to advance or retard substantially 90 degrees
which vary and are impressed on the grids I1 and
with respect to the generated voltage originally
I9 due to the action of the photo tube electrodes
9 and II, the current through which is in turn
controlled by the weight of the subject or pic
grid 4| is varied in accordance with the modulat
ing potentials. As this bias voltage is increased,
ture on the drum. This system and its opera»
tion is well known in the art, and it is believed
assumed to be on GI.
The biasing voltage on
the voltage fed back to GI by way of anode 43
that additional descriptionthereof at this point
tuned circuit L2, C3, C4, etc., is decreased, and
10 vice
versa. This voltage fed back to GI tends to
is unnecessary. A similar arrangement has been
’- ‘change the frequency of oscillator 44 as other
shown on pafe 85 of volume 1, “Radio Facsimile,”v
wise determined by crystal 48.
The amount of
RCA Institutes Technical Press.
this change, or frequency modulation is propor
The amplitude modulated carrier-is supplied by '
secondary winding 30 of transformer T2 to ~the
tional to the amount of voltage thus fed back.
The frequency modulated oscillations are im
pressed on the anode 50 of tube 44 by way of the
electron coupling in the tube and appear in tank
full wave recti?er 36. The recti?er 36 and ap-'
paratus to the right of the dotted line in Figure _
circuit 52. The voltages may then be ampli?ed,
1 comprises the essential elements of a frequency
modulator such as disclosed in Usselman United 20 multiplied and limited as desired in apparatus 54
States application Serial No. 338,838,'?led June
5, 1940, U. 8'. Patent 2,298,437, dated October '13,
1942. The full wave‘recti?er 36 has its cathodes
35 connected together as shown and to ‘one ter
minal of a low pass ?lter 38‘. The other termi
nal of this ?lter is connected to a point'ona re
sistance 3| in shunt to the secondary winding‘ 30
of transformer T2. The low pass ?lter has a pair
and ‘transmitted.
In accordance with applicant’s‘novel systeman
additional de?nite frequency is transmitted for
correcting and synchronizing purposes at the re,
In a preferred modi?cation this is ac
complished by means of a cam 69 on the drum I
which cooperates with, contacts 6| and 62 to in
clude in the circuit across thegrids I1 and I9 of
of output terminals connected together by way of
tubes I8 and 29 an additional: resistance 64 or as
a resistance 39, 'a point on which is connected to 30 much thereof as desired. The modulating volt-,
ground by way of a source of biasing potential’
age on the tubes is thus held at a value _corre,-'
sponding to a super-black picture shade and visv
The potential at the upper terminal of the re
such as to modulate the amplitude of the carrierv
sistances 39 results from recti?cation of the car-'
supplied to TI an additional amount for a short
rier modulated in amplitude‘ in accordance with 35 time interval at the picture margins. Thev ,sys-;
the scanned subject, and therefore the potential
tem is adjusted so that this amplitude modula
at the endof 39 corresponds to the picture den?
tion represents super-black but the circuit may
sity shade or contrast. This potential is supplied
be modi?ed so that the bias represents super
to'the grid 4| of a modulator tube 42 used to mod
white. The additional modulation is outside the
ulate the frequency of the oscillations generated 40 range covered by the picture in either direction.
in the oscillator 44, as will now be, described more
This additional amplitude modulation acts
through the translating means .22, T2, 30, 36, etc.,
The oscillator 44 is of the ‘electron coupled type
to shift the frequency of the oscillator-44
having a plurality of grids GI, G2 and G3,’ of
nite amount, this frequency to which the oscil
which GI and G2 form an oscillation generating 45 lator is deviated being outside the range wherein
circuit wherein G2 serves as the anode; The grid
modulation in accordance with the scanned sub
ject takes place.
I .
GI is connected as shown to a crystal 48,»'and_ by
way of 48 and C5 to ground. The grid GI isalso
At the receiver I propose an arrangementfof,
connected by way of a variable condenser 4‘I and
circuits the equivalent of that shown in block
condenser C6 to ground. The grid GI is also con; 50 diagram in the accompanying ?gure. vIn this
nected to ground by way of a resistance _R|. 'A
?gure 6|’ is any suitable receiving antenna; 62‘
point on this resistance RI is connected “by a
is the radio frequency ampli?er; 63’ is the ?rst
coupling condenser 46 to the grid 4| of tube 42.
detector; 64' is the ?rst oscillator; 65 is the?rst
The anode 43 of tube 42 is connected to a point
intermediate frequency ampli?er and band pass,
on the inductance L2, which forms a tank circuit
connected with the anode 43 and including re
actances C3, C4, C5 and C6. .
As pointed out in more detail in the said Ussel
man application, a ?rst or original voltag'e'of the
?lter; 66 is the automatic gain control; 61 is the
second oscillator; 68 is the second detector; 69
the second IF ampli?er; ‘I0 is the demodulator
suitable for converting frequency modulation to
amplitude modulation; ‘II is atone modulator;
generated frequency appears on the grid GI, A 60 ‘I2 is a tone source and ‘I3 is a control line or
control channel leading from the receiver to the
provided and applied to the grid GI. The ampli
remote central office; ‘I4 is a direct current lim-'
tude of this second voltage is varied in accordance
iting device for cutting out all levels other than
with the modulating potentials to thereby rela
those corresponding to the margin; ‘I5, ‘I6, 11 is
tively vary the amplitude of the phase quadrature
an assembly including the recording drum, two
voltages on the grid GI and as a consequence
commutators and a motor, respectively, and 18
vary the frequency of oscillation of the oscillator
is a device for controlling the speed of the mo-'
44. This second phase quadrature voltage is ob;
tor, the drum, and the commutators; 19 is a
tained by connecting the grid 4| by way of cou-'
suitable for detecting changes in the fre~'
pling condenser 46 to a point on resistance R.‘
quency passed thereby‘ (which may be audible)‘
This radio frequency excitation of grid 4| is am
during the interval corresponding to the picture‘
pli?ed in tube 42, supplied by way of its anode
margin; 80 is a detector which adapts th'e'output'
43 to tank circuit L2, C3, C4, etc., and thence by
of ‘I9 to control the ?rst oscillator 64' through‘ a
way of crystal 48 to the grid GI.- ByLsIi'ghtIy de-L
control device 84; 82'is a cutout device
tuning the'c'ircuit L2, C3, C4 intone direction‘ or 75 suitable
operated by the gain control 66 for preventing
second voltage in phase quadrature therewith is
lnthe voperation of this invention the circuits
and apparatus other than ‘those at the "receiver
will perform ‘as described above. vThe receiver
will perform as follows: the frequency modu
lated received ‘signal is heterodyned to :a vfirst
the transmitter and receiver-are .in
Contact 62 is opened by cam '60 during theitime
a to b in Figures 3 and 4. The margin of the
further control of :64 by 28.1 under certain con
7 subject starts at a but no current ‘can ‘:‘tiow
‘intermediate frequency and passed through a
band pass ‘filter v‘65 in the well known manner.
The automatic :gain control ‘66 is so arranged
that the output of the intermediate ‘frequency
ampli?er T65 is held essentially constant notwith
standing variations in the input signal due to
selective fading, ‘noise, interference or any other“
source, of amplitude ‘modulation. It ‘may
desirable to augment the devices I have shown '
:bystraight limiting circuits for maintaining con
through 1-6 from "M to 1:8 until the commutator
16 reaches
The current flow stops {at-‘vibe
cause 62 'has been closed :by the spring. ‘The
amount of current ?owing under these condi
tions is just sufficient to hold '11 at the correct
Now assume the motor 11 tends to lag. "The
‘timer: to 12 becomes shorter vand current v?ows
through 116 for :a shorter time; This operates
through the speed control device 18 itoiinc'rease
the motor speed by decreasing :a dragfor ‘brake
on the motor. In this manner the definite he:
quency sent out ononargins synchronizes the
stant amplitude out of ‘the IF amplifier '65..
'The commutator 15 closes a circuit ffromii? to
The second oscillator 6.1 and the second de- ’
tector 68 vheterodyne the signal down to some 20 19 and the de?nite frequency sent button the
margins acts through :a vnarrow filter in 1:! hav-'
suitable second intermediate frequency. This in
ing ‘a, sloping characteristic ‘to ‘feed detector 150
termediate frequency is .frequency modulated ‘in
and control the tuning means '84 to reset ‘the
accordance with ‘the original signal. Frequency
receiver tuning each time ‘the switch Eli's closed
demodulator .10 converts this current of ‘varying
to thereby "correct the picture density. : If ‘the
frequency to a direct current ‘which is amplitude
system is in .synchronism :or nearly so_ the lei-re
modulated in proportion'to the frequency modu
cuit "through segment v‘l5’ '(Fig. 3) is completed
lation of the incoming signal. The ‘value of ‘this
at the proper time. The section '15:’... is made
direct current corresponding to the margins of
shorter than the ‘time a to ib ‘toi'iprevent
the picture are assumed vto be either higher or
lower than the said current corresponding to the 30 incorrect correction of ‘the receiver tuning.
It will be necessary atithe start-of aitransmis
remainder of the picture. Limiter 14 is such
toline up this system manual-ly.€?fter it is
that only ‘these extremes will be ‘passed. Po
lined up .it should stay in step. Should thesi‘gnal
tential limiter 14 is "connected through commu
fade to an abnormally low value then‘ cutoff 32
tator ‘T6 to speed control 18. This commutator
revolves at :a speed corresponding to the speed ‘of 35 actuated ‘by the .fast automatic gain control
potentials from 56 will cause :84 ‘to be disconnected
the drum l of the photo radio scanner “and is
from 64' so that '64" will continue to oscillate at
normally held in synchronism with it. When
the same frequency. When the'signal “rises vto
the phase of commutator 16 is correct it con
its ‘normal value again 82 will'again allow 8'4 to
nects limiter ‘I4 to the ' motor speed control
means 18 during the portion of the ‘time corre 40 control the frequency of 6'4’. The output of the demodulator 19 is ‘used'to
sponding to the margin. Should the motor 11
modulate the tone source -72 through tone modu
and commutator "IGtend to lag then "the result
lator ‘H. This modulated tone may befused to
ing effects will readjust ‘the speed control .18
actuate a recorder at the ‘receiving stationor the
so as to hold the motor 11 in step and in proper
recorder may be located at the remote ‘central
phase relation.
o?lce for recording the photo radio subject or it
Commutator 15 also makes connections only
may be recorded at both points. Any ‘other
during ‘the interval ‘corresponding to the margin.
known means can be used for 'usin‘gifth'e vvoutput
If the output of audio ‘frequency amplifier 69 has
of the demodulator H] to control the recorder at
the correct frequency during "the ‘margin interval
then this frequency will be impressed on sloping 50 the central office.
When a bias corresponding to super'éwhite is
?lter ‘T9 and through detector 80 ‘and device 84
will control the ?rst oscillator frequency so "as
to hold it 'at its present value. "The ?lter T19
and detector v80 form a ‘frequency modulation tiemodulation means. Now should the transmitter" .
used at the scanner and a de?nite frequency cor;
responding to super-white is transmitted the cire'
cuit at the scanner is modi?ed as indicate'd'in
Figure La to connect the :resistancev ‘5,4 in' shunt
to the photo tube electrodes 9 and Into impress
modulating voltages on the ‘grids corresponding
to super-white'at the subject ‘margin;
frequency increase the second intermediate fre
quency will, it is assumed, also increase.~ Thus
the ‘frequency of the input to '?lter ‘19 increases
vThe limiter in 14 may comprise ‘an-.telectron dis-‘i
and the output of the sloping ?lter in i‘l-acha-nges
charge ampli?er biased so that 5on]
to change the detector output. The character 60
istics of 19, ‘80 and control or modulator M are
such that it "will now tend "to change ‘the fre
quency of oscillator ~64" to correct the frequency.
The bias applied to the vlimiter in 14 is shown
which exceedthe highest potential pr
the scanning "process overcome the"
extent such that ‘the ‘tube in, ‘H can :p
= ‘
7 1
as indicated ‘at If’ in Figure 4. ‘Invother. words,
the tube in unit 14 is so biased that‘ only poten-e
diagrammatically in‘ Figure 4. The bias result
tials produced by the closing of contacts ‘.61 and
ing from scanning-the picture is represented at
52 are passed thereby. :If the receiver" circuits
Eg and is not sumcient'to cause the :limi-ter .14
are suchv ‘that the bias.- for limiter]?! tis‘ ‘not ‘of
to isupply'out-put current vuntil 62 (Fig. *1) “is
the proper phase the said limiter'wiill include the
opened. Then the bias increases as indicated
at Egm.‘ The output Ip is supplied from 1-4 and 70 proper :number of direct current amplifier stages
to provide :a current pulse “output for either-media
fed through commutator 16 to the speed control
?cation, that ‘is, when the de?nite frequency
means 18 which holds the motor 1'! in step to
synchronize the receiver and transmitter.
represents super-black or superewhit
- A cut-out device 82_.may comprise; magnetic
The mannerin which the synchronizing effect
a winding of which .is connector‘ withrthe
is'obtained will .now :be described. Assume that
automatic gain control circuit 66 as described in
frequency in accordance-with said modulation
beyond said selected range.
4. The method of signaling by proportionally
frequency modulating a radio frequency carrier
my United States application Serial No. 390,228,
?led April 25, 1941.
The control means in unit 18 may be of any
suitable type. For example, this may comprise
through a selected range in accordance with var
a drag, pressure on which is increased when the
motor 11 is running too fast and is reduced as the
speed .of motor 1-‘! approaches synchronism with
iations in subject shade which includes the steps
of modulating the frequency of said carrier be
. yond said selected vrange an arbitrary amount
the scanning drum. Control means of this type
during the time'corresponding to the subject
are well known in the prior art and will not be 10 margin, translating said modulated carrier at a
described in detail here.
receiving point for demodulation purposes, and
. The oscillator control or modulator means M
controlling the frequency of thetranslated carrier
shown at 84 may comprise a motor for driving a
at the receiving point in accordance with said
variable reactance in the oscillator circuit, the
‘modulation beyond said selected range.
motor in turn being controlled as to direction of 15
5.,The method of signaling by modulation-of
rotation and extent of rotation by the output of
a radio frequency carrier in accordance with var
the wave variation‘ detector in unit 80. A pre
iations in subject shade which includes the steps
ferred embodiment for use here has been dis
of producing potentials characteristic of subject
close'd'in my United States application Serial No.
shade, modulating a sub-carrier in accordance
390,228, ?led April 25, 1941. In some cases it 20 with said produced potentials, additionally modu~
maybe‘ preferable to use a modulator or control
lating said sub-carrier an arbitrary amount dur
meansof the reactance tube type, the reactance
ing the time corresponding tothe subject margin,
of which, supplements the reactance of the oscil_
frequency modulating said radio frequency car
lator in unit 64' and is variable in a manner cor
responding to variations in the output of the de
tector 80.
Such a reactance control means has
rier in accordance with the modulations on said
sub-carrier, translating said modulated radio
frequency carrier, heterodyning the same to a
lower frequency, and controlling the heterodyning
frequency in accordance with the frequency mod
been disclosed in Crosby United States applica
tions Serial No. 136.578, ?led April 13, 1937, U. S.
Patent-#2.279,659, dated April 14, 1942, and Serial
ulations on said radio frequency carrier produced
No. 209.919, ?led May 25, 1938, U. S. Patent 30 by said additional modulation of said sub-carrier.
#2250995; dated July 22, 1941.
Modi?cations have been suggested from time
to time in the above description. Many other ob
6. In a photo radio transmission system, means
for proportionally frequency modulating a radio
frequency carrier in accordance with shade var
vious modi?cations will occur to anyone skilled
iations of a scanned subject, means for addition
in the art; For example, in some cases it may 35 ally frequency modulating said carrier an arbi
be desirable to duplicate the system shown to
trary amount during a margin of said scanned
obtain the bene?ts of diversity reception. This
subject, means for transmitting the carrier so
diversity reception can‘be of the space diversity
modulated, a receiver for the carrier so modu
type, the angle of arrival diversity type, the
lated and means for tuning the receiver in ac
polarization of received signal diversity type, etc. 40 cordance with the arbitrary modulations of the
I claim:
transmitted carrier.
1. The method of signaling by photo radio
7.‘ In a photo radio transmission system, means
transmission which includes the following steps,
for proportionally modulating the frequency of a
modulating the frequency of a radio frequency
carrier in accordance with shade variations of a
carrier through a given range in accordance with 45 scanned subject, means for additionally modulat
current derived by scanning a subject, modulating
ing the frequency of said carrier an arbitrary
the frequency of the said carrier current beyond
amount during a margin of said scanned subject,
said givenrange periodically, sending the carrier
a tunable receiver and recording means excited
so modulated to a receiving point, and at the re
celving point amplifying voltages characteristic
of the said carrier while controlling the frequency
of the voltages being ampli?ed in accordance with
said frequency modulation beyond said’ given
by said carrier so modulated and means at said
50. receiver for utilizing said additional modulation
to tune the said receiver and synchronize the
recording means.
8. The method of signaling by ' photo radio
2. The method of signaling by photo radio
transmission which includes the following steps,
modulating the frequency of a radio frequency
carrier in accordance with current derived by
scanning a subject, additionally modulating the
transmission which includes the steps of modu
latirig a radio frequency carrier through a given
modulation range in accordance with current
modulating said carrier an arbitrary amount be
yond said selected range during the time corre
sponding to the subject margin, sending said car
rierxso modulated, heterodyning the. same .to a
lower frequency-and controlling the . heterodyning
selected modulation range during a time‘ in the'
modulation cycle corresponding ‘to the subject
margin,‘ apparatus coupled to said‘ ?rst apparatus
for sending said carrier so modulatecL'a receiver
forv receiving saidfcarri'er, circuits coupled- to said_
derived by scanning a subject, modulating the
said radio frequency carrier beyond said ‘given
modulation range periodically, sending the car-'
frequency of said carrier periodically to an extent 60 rier so modulated to a receiving point and at the
suiiicient to swing it outside the range of modu
receiving point amplifying voltages characteristic
lation by said current derived by scanning, send
of the said carrier while controlling the frequency
ing said‘ carrier so modulated to a receiver in
of the voltages being ampli?ed in accordance with
cluding a recorder and at the receiver using said
said modulation of said radio frequency carrier
additional modulation for tuning the receiver and 65 beyond
said given range.
synchronizing the recorder.
9. In means for signaling by modulation of a
- 3;=-The method of signaling by modulation of
radio frequency carrier through a selected modui
a radio frequency carrier through a selected range
lation range in accordance with variations in the
inaccordance with variations in the scanned sub
scanned subject shade, apparatus for modulating
ject .shade which includes the'following steps, 70 said
carrier an arbitrary amount beyond said»
receiver producing currents characteristic of the
carrier so modulated from which the subject may 7
be recreated, a control‘ circuit coupled to said last
named circuits for controlling the frequency of
the produced currents in accordance with said
modulation beyond said selected modulation
range and other circuits coupled to said control
circuit for inhibiting operation of said control
to said ?rst named circuits for controlling the
tuning thereof.
11. Means for receiving wave energy modulated
through a given range in accordance with signals
‘and in another range in accordance with a con
trol potential, circuits excited by said wave energy
for producing currents characteristic of the mod
ulation on ,said wave energy, a de-modulator
‘coupled to said last mentioned circuits, a utiliza
circuit in the presence of a Weak carrier;
t 10 tion circuit coupled with said de-modulator, a
10. Means for receiving wave energy modulated
circuit coupled with said de-modulator for deriv
through a given range in accordancewith signals
ing potentials characteristic of'the modulation of '
and in another range in accordance with a control
said wave energy through said other range by
potential, circuits for producing currents char- ‘
the control potentials, a control circuit-coupling
acteristic of the ‘modulation on said wave energy,
said last named circuits to said ?rst named cir
a de-modulator coupled to said last mentioned
cuits for controlling the tuning thereof, and
circuits, a utilization circuit coupled with said
means actuated by said wave energy for prevent
de-modulator, a circuit coupled with said de
ing said control circuit from changing the tuning
modulator for deriving potentials characteristic.
of said ?rst named circuits when the wave energy
of the modulation of said wave energy through 20 intensity falls below a usable value.
said other range by the control potentials, and a
control circuit coupling said last named circuits
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