US2488517A

US2488517A - Frequency-modulated picture transmitter

Info

Publication number: US2488517A
Application number: US532252A
Authority: US
Inventors: Louis A Thompson
Original assignee: NEA SERVICE Inc
Current assignee: NEA SERVICE Inc
Priority date: 1944-04-22
Filing date: 1944-04-22
Publication date: 1949-11-15
Anticipated expiration: 1966-11-15

Description

W- 15, 1949 L. A. THOMPSON 2,488,517

FREQUENCY MODULATED PICTURE TRANSMITTER Filed April 22, 1944 2 Sheets-Sheet l TRHNSMITTE /6 awn P475 F74 re? JNVENTOR.

9 Z00/$A.7//0MP50N ATTOiP/VEVS moped Nov. 15.194!

UNITED STATES PATENT. OFFICE Y 2.4mm

FREQUENCY-MODULATED rrc'runn TRANSMITTER ration of Delaware Application April 22, 1944, serial No. 532,252

2 Claims. 1

Y mit pictures by radio and one adapted for radio transmission may not easily be adapted to transmit over a telephone line or the like. v

The present device contemplates a means whereby the standard picture transmitter designed to transmit pictures by wire may be used to transmit pictures by radio. v

It is also well known that pictures in order to be reproduceable with fidelity when transmitted by wire rely to a considerable extent on the perfection of the line connection between the transmitter and the receiver. That is, during transmission, variation in signal, line noises and other disturbances are usually apparent in the final picture and in some instances have such effect thereon as to make the picture illegible. It is particularly desirable in picture transmission for newspaper reproduction that as good a picture as possible be obtained because other losses occur during the reproduction which, if coupled with the losses taken during transmission, may make thepicture so poor that it cannot be used.

By my present invention I have provided an improved method, and apparatus for the transmission of pictures either over the wire or radio whereby the picture is substantially unaffected by line noises, changes in volume and in the case of radio transmission, by static, fading and the' like.

In carrying out the invention I contemplate using a standard picture transmitter and a standard picture receiver, and the apparatus about to be described comprises a converter for use during transmission and an inverter for use In other words, one complete apparatus for the transmission of pictures would include the transmitter embodying the invention in combination with a receiver embodying the invention to provide a complete system.

Then finally there'is the provision of the converter itself as a separate instrument and the inverter as a separate instrument as well as the aforementioned combination of the two.

A brief description of the function of the apparatus will assist in an understanding of the apparatus itself hereinafter described specifically.

To begin with the picture transmitter may be of the character described in my Patent No. 2,284,027 of May 26,1942. As described in the above patent it contemplates scanning a picture to provide a signal in the output of the machine which may be an 1800 cycle signal that varies in strength or amplitude in accordance with the reflection of light from the picture. This may also be considered for the purpose of explanation as an 1800 cycle amplitude modulated signal, the amplitude modulations of which carry the intelligence. Ordinarily this signal is sent from the picture transmitter to the picture receiver over the telephone lines or by means of land line connections. In the receiver the signal is used to reproduce the picture. During the process of transmission an amplitude modulated signal sometimes accumulates undesirable components due to line noises or increases and decreases in signal strength all of which affects the desired modulation component and which results in defects in the finished picture.

Therefore, instead of sending the signal out on the lines in the form of an amplitude modulated wave, as it comes from the transmitter, I pass it through a converter. In the converter I amplify the signal to a predetermined level. Next the signal is demodulated. Then I utilize the demodulated signal to control a variable frequency oscillator. The variable frequency oscillator and a fixed frequency oscillator are each' mitter or may besent by wire if desired without any change. Inthe case a transmitteris modulated it may be either a so-called amplitude modulated type transmitter or it may be a frequency modulated type of transmitter.

Certain improvements in methods and means for providing the signal in the converter will also hereinafter more clearly appear in the more specific description.

The signal after being transmitted by radio is received byany suitable type of receiver depending on the final type of transmitter used. During the transmission it" may be effected in many ways. It may accumulate static, noise and/or fading all of which under ordinary circumstances might be sufilcient to render the ordinary amplitude modulated signal substantially useless for high fidelity reception of picture.

After being received it is amplified together with the above mentioned undesirable defects and then passes through the inverter. In the inverter it passes through a cascade limiter which very greatly limits the signal, taking off all amplitude modulations. Next it is reconverted to an amplitude-frequency modulated hybrid signal, then rectified and passed through a filter. Then this signal is combined with an 1800 cycle carrier. The signal has now become an 1800 cycle constant frequency, amplitude modulated signal which may be supplied to the standard picture receiver with a result, that the picture is substantially the same as it would have been had the facsimile transmitter been directly connected to the facsimile receiver. Thus the signal is handled in such a manner that the usual losses which occur in transmission do not affect it.

It is therefore among the objects of my invention to provide a converter for use in conjunction with a picture transmitter which enables the picture signal to be converted for transmission by to the input. The reactance tube I4 is directly means other than the ordinary telephone line.

Another object of my invention is to provide an apparatus where a picture signal such as is received by radio transmission may be inverted to enable a conventional picture receiver to reproduce the same.

Another object of my invention is to provide an improved method of transmitting pictures or intelligence which is substantially unaffected by static, fading or the like.

Another object of my invention is the provision of an improved frequency modulation system and method 01 operation thereof.

Another object of my invention constitutes the provision of an improved method and apparatus for inverting a signal.

Other objects of the invention are the improvements of circuits embodied in the invention and includes an improved limiter circuit, an improved demodulation circuit and an improved signal reconstruction circuit.

Still other objects of the invention and the invention itself will become more apparent from the following description of a specific embodiment of the apparatus used to transmit and receive a signal which apparatus is illustrated by the accompanying drawings and forms 'a part of this specification.

In the drawings:

Fig. 1 is a schematic drawing showing a complete transmission system constructed according to my invention;

Fig. 2 is a similar view of a complete receiving system for receiving and reproducing the signals from the apparatus of Fig. 1,'and

Fig. 3 is a simplified view of a portion of the circuit of Fig. 2.

One aspect of the invention includes the combination of the elements of Figs. 1 and 2 to provide a complete system for the transmission of intelligence. Referring now to the drawings throughout which like parts have been designated by like reference characters. In Fig. 1 I have shown at ID a picture transmitter which may be of the character such as is illustrated and described in my patent herelnbefore mentioned. The invention is by no means limited to this precise form of transmitter, however, this type being shown merely for the purpose of explanation. The output from the transmitter II is connected directly to the input of an amplifier II where the amplitude of the signal is increased to the desired value, the amplifier being provided with a gain control H. The signal in the putput of the amplifier II, in case the transmitter of my previous patent is used, will be an 1800 cycle signal having amplitude modulations which vary in accordance with the variations in reflective properties of the picture being transmitted. In other words the intelligence is contained in the modulation envelope of the signal.

A full wave diode detector I2 is connected in the output of the amplifier II. The carrier wave is rejected and the modulation component is passed through a filter ii. The filter l3 removes any of the carrier which may appear in the output of the detector l2; an output, which now comprises the clean modulation component, is applied to the input circuit of the tube II which includes at least a grid and a cathode. This tube acts as a direct current amplifier and is commonly known as a reactance tube. The output from the tube is in substantially direct proportion coupled to and feeds the signal to an electron coupled oscillator IS.

The signal is used to control the oscillator Ii, the output of which is connected to the input circuit of a mixer tube It. The frequency of the oscillator I5 is determined by the signal and is therefore of variable frequency when a signal of varying amplitude is fed into it by the reactance tube. Also connected to the mixer tube I6 is a fixed frequency oscillator H. The two signals are mixed in the mixer and in the output there is a composite signal comprising the variable frequency signal, the fixed frequency signal, the diflerence between the two signals, and the addition of thetwo signals. In this case I am interested in the difference between the two signals.

Referring back to the variable frequency oscillator, the manner of varying the frequency is of particular interest. It should be noted that the anode supply voltage for the reactance tube I4 is fed through the resistance 20 to the junction of the split inductances Zl-Z! which are the inductive components of the resonant circuit which comprises the two inductances' and the capacity 23. It will be noted that the capacity II includes a main condenser and a trimmer as is common practice. From the inductance 2| the anode supply is through the resistor 24 to the anode of the tube M.

It will also be noted that in the oscillator circuit, this being an electroncoupled oscillator, the second grid 26 serves as the anode, and that the anode supply voltage is through the lower inductance 22. Therefore, it will appear that the reactance tube l4. and the anode of the oscillator tube l5 both get their anode supply voltages through the voltage dropping resistor 20. when a signal appears on the grid of the tube II, that tube starts to draw current in the plate circuit. Since this current is through the inductance II it causes the reactance of that inductance to vary according to the current drawn reducing the reactance and increasing the frequency. Therefore, if the frequency of the oscillator circult of tube I5'is adjusted to 100 kc. with no signal on the reactance tube and the signal is applied to the reactance tube It, the eflfect of the reactance tube on the oscillator circuit is such as to cause the frequency in this circuit to increase. Another important feature of this part of the circuit resides in the fact that the voltage -on anode 26 of the oscillator tube is supplied through the dropping resistor 20. When the reactance tube is operating with a signal on the grid, the current drawn in the plate circuit of the tube It is through the resistance 20. This causes a voltage drop across the resistor which varies with the plate current and since the volt- Y mitter ill to transmit black and then adjust the oscillator I6 by means of-condenser 23 to 100 kc. Then if I transmit white, and turn the gain of the amplifier up, I can thus adjust the intensity of the signal in the reactance tube so that the frequency in the variable frequency oscillator I6 is increased by 1500 cycles, causing the output from that oscillator to be 101.5 kc. An indicator is provided at 42 to enable the determination of age on anode 26 of the oscillator tube i6 is supplied through this same resistor, the plate voltage on 26 will vary directly as the plate current drawn in the plate circuit of the reactance tube and this will also cause the frequency of the oscillator circuit to vary. Therefore, I have several conditions in the oscillator circuit acting in conjunction to vary its frequency over a considerable range. One being the change in the reactance of the inductive component and the other being the change in the plate voltage. The fact that these two effects are in the proper phase to increase the frequency swing and that the reactance tube is directly coupled to the oscillator materially increases the swing over that which could be obtained with ordinary reactance modulation.

The fixed frequency oscillator I1 is also an electron coupled oscillator and includes the LC circuit comprising the inductances 3l32 and the condenser 33. In this case as in the other the grid 36 acts as the anode, the oscillator section includes the grid 36 and the first grid and cathode, this circuit being conventional.

The two oscillators I5 and I1 feed their signals to the grids of the twin triode tube l6 which acts as a mixer tube, the two plates of which are connected together. As previously stated the signals in the output of this tube, according to accepted theories, are the sum of the two frequencies from oscillators l5 and H, the difference between the two frequencies and the individual frequencies.

For one particular mode of operation I assume that a frequency swing of 1500 cycles is desired, this being in the audible range. Then the fixed frequency oscillator i'l may be set for 102.5 kc. and the variable frequency oscillator set for 100 kc.

If then, the variable frequency oscillator I5 is adjusted to provide a 100 kc. signal with no signal or picture black on the reactance tube I,

this signal can be raised to 101.5 kc. when pic- .ture white or maximum signal is applied to the tube It. This enables me to obtain a frequency swing of 1500 cycles or a signal in the output of the mixer tube which is the difference between the signals of oscillators I5 and IT. This represents a changeof 2500 to 1000 cycles. I am therefore enabled to obtain a much greater frequency swing utilizing but a single tube, than has ordinarily been practical in this particular frequency band.

With no signal in the rea'ctance tube l4, the output of the mixer tube i6 there would appear 100 kc., 102.5 kc., 2.5 kc. and 202.5 kc. Then if this adjustment. I now have a condition where the frequency will swing between and 101.5 kc. or 1500 cycles. And this will cause the low frequency output in the mixer to vary between 2500 and 1000 cycles with black being 2500 cycles and white being'1000 cycles and any frequency between the two being diiferent degrees of blackness and whiteness.

Since I am only interested in a varying audible frequency I next pass the signal from the tube l6 through a band pass filter 40; here all signals but the 1000 to 2500 cycle varyin frequency signal are filtered out and the output from the'filter is a frequency modulated signal which varies between 1000.cycles and 2500 cycles depending upon the reflective properties of the picture. This signal may then be used to modulate any type of transmitter indicated at 4 I It will be noted that I have designated the radio transmitter as being relatively close to the converter and picture transmitter. However, it should be particularly noted thatthis signal is of such a frequency that it can be readily transmitted from the converter at one point such as a newspaper ofllce to the transmitter at a remote point over ordinary telephone or land wires.

The transmitter can be a frequency modulated or an amplitude modulated transmitter and the :ignal from the converter used to modulate either As I previously stated the converter could also be built integral with the modulation part of the picture transmitter. As will appear from my aforementioned patent, the transmitter there includes a-photo cell. The output of the photo cell is substantially the same, except for magnitude, as the signal from the filter l3. Therefore, if desired, the 1800 cycle oscillator of the transmitter could be eliminated and the signal from the phOtO cell amplified and applied directly to the reactance tube It. I would then have a straight frequency modulated apparatus and the demodulation step would be eliminated.

The receiving end of the system is best shown in Fig. 2. Here the signals, if the transmission is by radio, are received over a suitable receiver 50. Connected to the receiver 50 is an amplifier in which the signal is raised to ahigh value. At the output to the receiver the signal may contain certain undesirable components such as noise and static that may have been picked up during transmission, and in addition the signal may be subject to severe fading. In other respects, however, the signal may be the same as it was when it was used to modulate the transmitter ll; 1. e. a

frequency modulated signal having a swing between 1000 and 2500 cycles.

Next the signal is put through what I prefer to term a push-pull-cascade limiter circuit. This is effected .ly feeding the signal into the primary of a push-pull transformer 52, the secondary of which has its opposite ends connected through the resistances 53 and 58' to the grids 54 and 54' separated. Asssuming that a fairly strong sig-' nal is being received, this signal is amplified to a high level and is applied to the grid 54 of the first triode section of the tube 55 through a series resistance 53. The grid 54 is driven positive on the positive half of the signal which causes grid current to flow in the grid-cathode circuit. This grid current causes an IR drop across the resistor 53 with the result that the signal into the tube is limited.

The anode voltage for the first triode section is supplied by the manner in which the second triode section is connected. The anode 51' of the second triode is connected to the positive plate voltage supply and normally this tube draws current from the plate circuit to the cathode 59 which is connected through a resistor 58 to the grid 60 of the second section and the anode 51 of the first triode section. The circuit then for the cathode of the second section is through the first triode section to the cathode 58 and thence to ground. The voltage drop across resistor 58 provides bias for the grid 60. Since the cathode 55 of the first section is at ground potential the plate voltage at 51 is above ground potential due to the current flow in 58.

When the positive signal appears on the grid 54 the plate draws current which is through the resistor 58, thus increasing the negative bias on the grid 80 which action decreases the normal plate current in the second section thus reducing the plate voltage at 51.

, This still further increases the limiting action of the circuit. This limiting-action is relatively large, depending, of course, On the amplitude of the signal. The final result is that the resultant wave form is devoid of all amplitude modulations caused by static, noise or other amplitude picked up previously in the system and is substantially a square wave. Of course, on the other half of the wave the other limiter tube 55 is operating in a like manner.

The maximum limiting of the signal voltage may be as much as from 2000 to 2. In actual practice I have found that a desirable method of operation is to raise the signal to a relatively high value before it is limited thus providing sufiicient signal to always provide limiting action. In practice I have found it desirable for the limiter to start functioning on voltages as low as 2 volts.

As a result of the limiting action, the output from the limiter is substantially a constant voltage of varying frequency.

The output from the two limiter circuits are connected in push-pull to the primary of the transformer iii the secondary of which feeds into a sloping filter 82. This filter, as the name implies, has a 45 sloping characteristic. A signal having a frequency of 1000 cycles, which may be picture white, will pass through the filter without any substantial loss. As the frequency of the signal increases the loss becomes greater, hence a 2500 cycle signal suffers a very substantial loss. Likewise, the signals between 1000 and 2500 cycles suffer a loss substantially in direct proportion to their frequencies. The result is that the signal in the output of the sloping filter 52 comprises a signal that varies in amplitude in a manner corresponding to the original amplitude modulation component. The signal at this point comprises a hybrid signal which consists of frequency and amplitude modulations.

The signal is then fed through the transformer 53 the secondary of which is connected to the amplifier tube 64 which may beanother twin triode with the two sections connected in parallel, and in which the signal is amplified.

The output from the tube 84 is connected to the transformer 55, the secondary of which is connected to the anodes of the demodulator tube 61. This tube rectifies both halves of the signal which then appears on the cathodes of the tube which are connected together, and is a pulsating direct current the amplitude of which varies according to the modulation envelope. The pulsations in the signal are removed in the filter comprising the input condenser 58, the choke 58 and followed by the series choke and condenser 'I0'II connected between the conductor 12 and ground. Thus at 13 there remains only a signal which corresponds exactly to the modulation envelope of. the original signal.

The next step in the procedure is to use this signal energy to reconstruct an 1800 cycle amplitude modulated signal in order that it may be supplied to a conventional facsimile receiver and there used to reproduce the picture.

For generating the 1800- cycle carrier signal, I provide an 1800 cycle audio oscillator which may be of any standard typ ving a good sine wave and having a constant frequency output of 1800 cycles. The output from the'oscillator is connected in push-pull to the grids 8i of the tube 82, which may be a twin triode. The anode voltage for this tube is obtained from the signal (at 13) after it leaves the filter, the signal being supplied to the anodes through resonant filters each of which comprises an inductance 84 and paralleled by capacities 85 and 81 respectively. The two inductances are connected end to end and as stated each has a condenser across it. The signal is fed to the junction of the inductances and capacities. nect to the anodes of the tube 82. The two resonant filters 84-85 and 88-81 are tuned to the frequency of the oscillator 80. Tube 82, which may be termed a restorer tube, only operates to pass a signal when plate voltage is supplied to the anodes. Because the plate voltages on this tube are being supplied by the signal which is substantially a replica of the original signal used to modulate the carrier at the facsimile transmitter, the signal in the plate circuit of the tube varies with the plate voltage. The signal as it appears across the parallel resonant circuits is an 1800 cycle amplitude modulated carrier which is supplied through the coupling condensers'90 to the potentiometers 9! which are connected in series and grounded at their junction. The sliders 92 of the potentiometers are connected to the control grids 83 of the twin 75 triode tube 84 which "operates in push-pull. The

The leads 88 and 89 con-' cathodes of the tube are resistor 95. The plate circuit of the tube is connected to the output transformer 96. The tube 94 amplifies the signal which is thus supplied to the transformer 96. The level of the signal may be determined by a meter M indicator disposed in the output. The output from the inverter may be connected directly to a standard picture receiver for reproduction or it may be sent over land wires to the place where the picture receiver is located.

It should particularly be noted that the filter grounded through the I network ahead of the point 13 and comprising the inductance 69 and capacity 68, and the inductance l6 and capacity?! are effective to prevent anything but the D. C. component of the desired signal from getting through to the point 13, and that the resonant circuits 84-85 and 8687 are resonant to 1800 and therefore present a high impedance to the 1800 cycle carrier and therefore readily transfer the 1800 frequency through the coupling condensers 90 to the input circuit of the tube 93. However, these resonant circuits readily admit the D. C. energy from 13 to the anodes of tube 82.

Thus the signal in the restorer circuit is a clean cut signal without any spurious modulations due to the undesirable elements in the incoming signal.

The two taps shown for the output meter provides means whereby when a greater resistance is in series, the gain can be raised and the proper level determined for sending the picture by land wire. If the picture is fed directly to the picture receiver. the gain may be lowered and the proper level determined by shifting the meter to the tap where the lesser resistance is in the circuit. In either case the meter enables the proper output to be determined.

Having thus described my invention, I am aware that numerous and extensive departures may be made therefrom without departing from the spirit or scope of the invention.

I claim:

1. A system of frequency modulation including a variable frequency oscillator circuit, a fixed frequency oscillator circuit, a mixed circuit, said oscillator circuits having their outputs connected to the mixer circuit, said variable frequency oscillator circuit including a vacuum tube having at least two grids, a cathode and a plate, a resonant circuit comprising a split inductance and a tuning capacity disposed across said inductance, one end of said resonant circuit being connected directly to one of said grids to act as an anode and to the other grid acting as a control grid, a reactance tube having at least a grid, a cathode and an anode and connected to the resonant circuit to control the frequency of the resonant circuit and means for supplying a signal to said reactance tube grid, anode voltage supply means for the reactance tube and oscillator tube serially connected through a resistance to the junction of said split inductance.

2. A system of frequency modulation including a variable frequency oscillator circuit, a fixed frequency oscillator circuit, a mixer circuit connected to said oscillator circuits, said variable frequency oscillator circuit including a vacuum tube having at least two grids, a cathode and a plate, a resonant circuit comprising an inductance and means for tuning said resonant inductance, one end of said resonant circuit being connected directly to one of said rids, said grid adapted to act as an anode and the other grid functioning as a control grid, a reactance tube having at least a grid, a cathode and an anode connected to control the frequency of said resonant circuit and means for supplying a signal to the input of said reactance tube, anode voltage supply means for said reactance and oscillator tubes serially connected and voltage dropping resistance means in said supply means connected to vary the anode voltage on said variable oscillator with varying current demands in the resactance tube.

LOUIS A. THOMPSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,629,685 Ditcham May 24, 1927 1,964,375 Wright et al June 26, 1934 2,060,778 Finch Nov. 10, 1936 2,298,657 Smith et a1. -,Oct. 13, 1942 2,305,842 Case Dec. 22, 1942 2,342,943 Kell Feb. 29, 1944 2,403,358 Gerhard et a1 July 2, 1946