US2155479A - Means and method for transmitting synchronizing pulses in television - Google Patents

Description

April 25, 1939. RT. FARNSWORTH MEANS AND METHOD FOR TRANSMITTINQ SYNCHRONIZING PULSES IN TELEVISION Filed July 11, 1936 3 Sheets-Sheet l I 3 Z 1"ig. 1.

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SIGNALS BY TO REPRODUCER 7 I ATT RNEYS.

April 25, 1939. P. T FARNSWORTH 2,155,479 MEANS AND METHOD FOR TRANSMITTING SYNCHRONIZING PULSES IN TELEVISION Filed July 11, 1936 3 Sheets-Sheet 3 TO SIGNAL AMR INVENTOR PH/LG 71 FARMS'WORTH.

ATTORNEYS.

Patented Apr. 25, 1939 t MEANS AND METHOD FOR. TRANSIVHTTING SYNCHRONIZING PULSES IN TELEVISION Philo T. Farnsworth, San Francisco, Calif., as-

signor, by mesne assignments, to Farnsworth Television & Radio Corporation, Dover, Del., a corporation of Delaware Application July 11, 1936, Serial No. 90,173

13 Claims. (C1. 2178-69-5) My invention relates to television, and more or synchronizing energy in one form or another particularly to a means and method of transmismay be transmitted to the receiving station there sion of synchronizing pulses, both high and low to be utilized in maintaining the spatial relafrequencies. tionship of the received picture. It is believed Among the objects of my invention are: to prothat wide reference to these many methods need 5 vide a train of unidirectional synchronizing not be made here. One of them, however, has pulses; to provide a train of unidirectional syn-- been described and claimed in my Patent No. chronizing pulses having a lower frequency pulse 1,773,981, wherein the low scanning frequency is modulated thereon; to provide a means and modulated on the high scanning frequency and method whereby such a train of unidirectional the modulated scanning frequency is again modu- 10 synchronizing pulses is regularly reduced in lated on the carrier together with the signal, for amplitude to provide a lower frequency pulse; to transmission. The general details of this system provide a means and method of separate transare also disclosed in a prior patent, No. 1,773,980, mission of synchronizing pulses and related teleof which the one just referred to is a division. vision signals; and to provide a means and It should'be noted, however, that in the above 16 method of modulating a series of relatively high cases the synchronizing frequencies are sine frequency synchronizing pulses with a lower frewaves, and in that respect the present applicaquency synchronizing pulse. tion differs, in that it broadly concerns the use Other objects of my invention will be apparent v of unidirectional synchronizing pulses upon which or will be specifically pointed out in the descripa lower frequency pulse is modulated, preferably 20 'tion forming a part of this specification, but I do by reduction in amplitude of the higher frequency not limit myself to the embodiment of the invenpulses during the lower frequency pulse period. tion herein described, as various forms may be The pulses are preferably derived from the energy adopted within the scope of the claims. used for the scansion used to create picture sig- Referring to the drawings: nals. 26

Figure 1 is a diagrammatic representation of I have also found that ity is preferable to transa television transmitting station wherein low fremit synchronizing pulses of this kind over a sepaquency synchronizing pulses are modulated onto rate carrier, preferably identical with, or which high frequency synchronizing pulses, the comdiffers from the carrier propagating the signal bined pulses then being modulated together with impulses by not more than plus or minus the 0 the signal onto a carrier for simultaneous propasignal impulse frequency. It should, however, be gation. understood that both pulses and signals can be Figure 2 is a similar diagrammatic representapropagated by a single carrier, and the reasons tion of a transmitting station, wherein the low for my preference for two carriers will be apfrequency pulses are modulated onto the high freparent later in this description. 35 quency pulses, these pulses being modulated onto In the broad statement above, I have used the a carrier for propagation. Television signals are word propagate with respect to the carriers, as modulated on a separate carrier and separately it should be understood that these carriers may propagated. be radiated into space or may be propagated in Figure 3 is a diagrammatic representation of other ways well known in the art such as, for 40 a single low frequency pulse as preferably moduexample, carrier transmission on wires, or translated on a high frequency pulse train. mission lines, and I do not wish to be limited in Figure 4 is a diagrammatic representation of a the scope of my invention to the use of carriers television receiver adapted to receive signals radiated through space alone. either.from the carrier of the system as shown These and other broad aspects of my invention 45 may be more fully understood by reference to the drawings:

In Figurei, I have shown a television image dissector I, provided with a conventionalized lens 2 for focusing an image of an object 3 upon the interior of the dissector. I prefer to utilize the cathode ray beam dissector disclosed in the patents above referred to, although any of the modifications thereof can be used, or cathode ray image dissectors of other kinds are equally satisin Figure 1, or simultaneously from two carriers, as shown in Figure 2.

Figure 5 is a circuit diagram of a thermionic tube adapted to demodulate and separate the high and low frequency synchronizing pulses.

Figure 6 is a schematic diagram indicating the essential connections within certain of the equipment shown as blocks in the diagrams in Fig. 1.

In television systems, there are a large number of different ways in which synchronizing pulses factory. In the dissector referred to, there is a photoelectric cathode upon which an optical image is directed. The emitted electrons are maintained in substantially parallel paths and are accelerated toward an apertured anode. The entire electron beam is acted on by deflecting means to scan the cross section of the beam and thus allow electrons from successive elementary areas of the cathode to enter the aperture to produce a train of television signals, each corresponding inintensity to the intensity of illumination of the elementary area scanned to produce the signal. My invention is also applicable to other scanning systems as will be noted later.

The cathode ray beam in the dissector is acted upon by the field of two deflection coils 4 and 5 so positioned as to deflect the beam, when energized, in two directions, preferably at right angles. One of these coils is supplied with energy from a high frequency oscillator 6. The entire picture field is scanned, preferably line by line, by the high frequency oscillator, and then returned to repeat the cycle by the low frequency oscillator I. I prefer that both of these oscillators deliver their energy to the scanning coils in such a way that there is a saw tooth wave form in the scanning coils. Such oscillators are known in the art and their detailed construction is no part of the instant invention. Suitable examples, however, are shown 2,051,372, issued August 18, 1936, and 2,059,683, issued November 3, 1936, and in the patent to Farnsworth and Lubcke, No. 2,059,219, issued November 3, 1936.

The net result of the two saw tooth scanning components is that the object is slowly scanned across a line to produce signals, and the return made at high speed to begin the next line, This means that signal amplitudes will be relatively large during the scansionof the line itself but will be relatively small during the return of the beam due to the high return speed, with the reception of a lesser number of electrons.

Synchronizing pulses are desired between lines and between pictures. In manufacturing these pulses use is made of the fact that the derivative of the saw tooth wave form used for scanning has approximately the desired pulse wave form, as is disclosed in detail in my copending application, Serial No. 449,984, filed May 5, 1930. As there pointed out, the back E. M. F. across any inductive circuit carrying current of saw tooth wave form, such as the deflecting coils themselves, comprises a wave of this pulse type, the pulses occurring during the "fly-back time; i. e., during the negative or steep slope of the saw tooth wave. The application cited further shows the use of this pulse completely to interrupt the signal during the fly-back time, and in the further exposition of this invention it will be assumed that this is done. Voltages of rectangular pulse wave form exist in various points in the generating circuits, or it may be generated by the use of a small transformer IOI, I02 in series with the deflection coils 4 and 5 respectively, the transformer preferably having a negligibly small primary inductance in comparison with the impedance of the scanning coils, the secondary being effectively open-circuited; in other words, feeding mere y the grid of a vacuum tube.

The showing of Fig. 6 indlcates schematically a typical connection for the purpose. The tubes I04 and I05 represent the output tubes of typical saw tooth generators, correspondingfor example, to the tube 27 of Patent No. 2,059,219. The

in my prior patents, Nos.

be, for all intents and secondaries of transformers I02 and IOI are connected through the lines 9 and I0 respectively to the modulator I I.

I then prefer to modulate the high frequency pulse with the low frequency pulse by applying both pulses to a. tube I03 operating at its point of negative cut off. I apply the high frequency pulse positively to the grid of the tube and then apply the low frequency pulse negatively, this being accomplished in the instant case' by proper poling of the transformers I0! and I02. The polarity of the pulses may also be regulated by using an amplification stage to reverse the polarity of one set of pulses, as is well known in the art. The resultant output is as shown in Figure 3, where the high frequency pulses I2 have been reduced in amplitude during the entire low frequency pulse period, as indicated by the labeled arrows.

Figure 3 is somewhat conventional in that the tops of the high frequency pulses are usually not exactly square, as shown in the drawings, but may have a slight curvature. This curvature, however, does not in any way affect the operation of the system but merely affects the mean amplitude to a slight extent.

In the meantime, the output of the scanning oscillators is acting on the beam of the dissector tube to create a series of trains of signals into the signal amplifier. As has been pointed out above, the current is not completely interrupted between lines but falls to such a low amplitude during the return'of the beam that the effect may purposes, considered as a complete interruption in the signal train, particularly as the variations occurring at that time do not enter into the operation of the system, nor can they be construed as any part of the picture signal.

Referring again to Figures 1 and 6, the modulated high frequency pulses may'then be passed through a circuit I4 into a pulse modulator I5 where the synchronizing pulses may be combined in any customary manner with the signals as, for example, by connecting the output circuit- I4 across the input resistor of an amplifier tube I05 in parallel with the output of the signal amplifier, and then the combined signal and pulses passed into a signal impulse modulator I5 where they are all modulated in the usual manner on a carrier, thence to be radiated through an antenna system I'I.. As the signals and pulses are out of phase, the pulses will take place during the interruptions in thesignal current.

As it is preferable, however, to make the amplitude of the pulse frequencies in relation to the picture frequencies as large as possible, preferably not less than three or four to one, and more preferably five to ten to one, it can readily be seen that the output of the transmitter must be sufliciently large to handle the energy of the pulses without in any way disturbing the eflicient and undistorted radiation of the signal impulses occurring between the synchronizing pulses. This means that the transmitter must be much larger than would be necessary for the picture frequencies alone. It should be noted that the synchronizing pulses are enormously larger than the minute variations occurring in the signal interruptions, and these variations, although superimposed on the pulses, are too small to be noticed and later are eliminated, as will be described. 2

I prefer to split the work between two transmitters, as shown in Figure 2. Here the high frequency pulses, modulated by the low frequency pulses, pass through a circuit is into a special transmitter where the synchronizing pulses alone are modulated in the usual way onto a carrier for transmission by a synchronizing antenna system 2|. In-as-much-as in this transmitter we are not concerned with the periods between the synchronizing pulses, and as the synchronizing pulses are of a transient nature, this transmitter may be highly overloaded as far as the pulse amplitudes are concerned without employing a tube of excessively high power and without excessive heating. Asthis tube is preferably modulated to saturation, the small variations above referred to totally disappear at this point.

The output of the dissector tube is then passed directly through the signal amplifier into a signal modulator to modulate a separate carrierwhich may be radiated by the signal antenna system 22. The two carriers, one for the pulses and the other for the signals, are simultaneously radiated, with the pulses 180 out of phase with the signal groups, 1. e.,the pulses are so phased in the modulator as to cause increasing amplitude of radiation with increasing pulse, while the picture modulation amplitude decreases with increasing light. In other words, the pulses correspond to black, or blacker than black signals. I prefer that the carriers be of identical frequency; and it is quite convenient, in order to maintain these car-.

riers at the same frequency, that/both carriers be controlled, for example, by crystal control.- using a crystal common to both circuits.- Other' means, however, will suggest themselves to those skilled in the'art.

It is obvious, however, that the carriers need not be at'ide'ntical frequencies, all that is necessary forreception ofxboth carriers by a single receiver" being that the two carriers bewithin the side bands covered by the signal frequencies;

that is, that they shall not differ by more than It will be noted,jtherefore, that-in my preferred] system, using either a, single carrierv or two separate carriers at:identical orcloselyadjacent frequencies, that a composite television-signal will be received'by asingle-receiyer, thissignal comprising groupsof television signals having therebetween large amplitude synchronizing impulses,

these synchronizing impulses being reduced inamplitude at regular intervals corresponding to the low scanning frequen'cy.,-

A preferred receiving circuit is shown in extreme lowest terms in Figure 4. Here the entire train of signals together with the'pulses,,is received by a signal and pulse receiver 24, rectified and passed to the grid of the cathode ray tube 25.

I This grid will, therefore, have impressed'upon it signal frequencies and pulse frequencies. I prefer to apply the composite current to the grid with the pulse frequencies applied negatively. This maybe accomplished by regulating the number of amplification stages in the receiver.

The cathode ray tube has also associated with .it, receiving scanning coils 26 and 2'! energized by a receiving-high frequency oscillator 28 and a receiving low frequency oscillator 29, these oscillators also being preferably so regulated as to cause a saw tooth wave form in the scanning coils similar to that used in the transmitter.

I lators.

I prefer to utilize a portion of the sawtooth output of these oscillators to throw the grid of the cathode ray tube highly negative during the return of the beam to start the line of scanning, as described in the application of Farnsworth, Serial No. 449,984, filed May 5, 1930, for a Television scanning and synchronizing system". There a portion of the output of the scanning generator passes through a transformer, and the sharp field collapse, due' to the descending portion of the wave, creates an intense negative pulse which is applied to the cathode ray grid, extinguishing or greatly reducing the brilliancy of the spot. Thus what occurs in the remainder of the circuit during this period makes no direct impression onthe received picture, and as the synchronizing pulses are also negative, theefiect is merely enhanced. In fact, it is quite possible to utilize the negative high frequency synchronizing pulse on the grid to extinguish or dim the spot between lines without recourse to a special pulse for that purpose. This efiect, however, is no part of my present invention, and will be elsewhere 7 described and claimed.

The composite current comprising pulse and signal is also passed into a pulse demodulator 3|.

As I desire to apply the high frequency pulses to this demodulator positively, I may either take off the synchronizing pulses from the receiver amplifier at some stage ahead of the television signals where the phase will be correct, or I may add another stage to reverse the phase of the pulses. I have found it convenient to so arrange my receiver circuits that I take the pulses off positively after the receiver detector, and continue' the amplification of the signals through the proper number of stages so they will be applied to the grid negatively. I have sometimes found it convenient to adjust my amplification stages to reduce the pulse energy in the circuits leading positivel yafter the receiver detector, and conto the cathode ray tube after having taken off the pulses from the detector.

' I then provide an over biased tube 32, the tube being preferably self biased by the introduction of a leak resistor 33 and condenser 34 in a cathode lead. I also prefer to place a grid resistor 35 from grid to ground. Proper regulation of values in the resistor 33 and condenser 34 will provide a strong bias which will persist during the low frequency pulse period. As these values will be different with different 'scanningofrequencies no values will be given, but such regulation "is well known to those skilled in the art.

The anode 36 is in series with a high frequency resistor 31 and a low frequency resistor 38, and of course the usual anode supply. The low frequency resistor 38 is also shunted by an integrating condenser 39. A high frequency pulse lead 40 is taken directly off the anode through a high frequency blocking condenser M and a low frequency pulse lead 42 taken off between the two resistors 31 and 38 through a similar low frequency blocking condenser 44. The leads 40 and 4'42 go directly to the synchronizing grids of the respective high and low frequency scanning oscil- Here again, the relative values of re sistance and capacity will vary in accordance with frequency.

As the high frequency pulses are applied positively to the grid, they will appear negatively in the pulse lead 40. The low frequency interruption of these pulses will cause a decrease in the current and they will therefore appear as positive pulses in the low frequency lead 42. These polarities are exactly those desired for application to the respective synchronizing grids of the scann ng oscillators.

When applied to these grids the pulses serve to hold the receiving scanning oscillators in step with the scanning oscillators at the transmitter, from which the pulses were derived.

Thus it will be seen that the picture formed by the cathode ray beam impinging on the fluorescent screen of the receiving tube will be held in step with the picture scanned at the transmitter, the synchronizing pulses having been transmitted between the signal groups representing the lines of the picture.

It will be obvious that my invention may be applied to systems other than those using cathode ray dissector tubes, In case a Nipkow disc or similar scanning means is used synchronizing pulses may be generated by the use of synchronizing apertures in the disc located between lines, and the energy thus produced may be handled as aseries of unidirectional pulses modulated by a low-frequency pulse between pictures. After generation, the synchronizing pulses may be handled as above described in carrying out my invention as applied to a dissector tube.

It'should also be noted that electrostatic deflection of the dissector beam may be used, a derivative of the deflection energy being used for the synchronizing pulses, such adaptation being obvious. Furthermore, while my preferred dissector comprises the use of a broad cathode ray beam scanned past an aperture, my invention is equally applicable to those cathode ray dissectors wherein a photo-sensitive mosaic is scanned, directly or indirectly by a narrow cathode ray beam to produce a train of television signals. In fact the invention is applicable to any of the known scansion devices used to vision signals.

The advantages of my invention are numerous. The use of unidirectional pulses allows amplitude reduction modulation to provide low frequency pulse periods. The unidirectional pulses being transient in nature allow the use of tubesmodulated to saturation without excessive heating. The pulses, being unidirectional are available for application to vacuum tubes either positively or negatively as desired, for either action thereon or for other purposes, brilliancy reduction for example, as in the receiving tube. The pulses may be passed, or stopped, as desired, by the use of produce a train of telean over-biased tube, or separated as in the case of the demodulator at the receiving end. By reduction modulation of the pulses as described,

I synchronizing pulses may be obtained, of opposite polarity, this opposition being valuable in the use of the pulses at the receiving end, for synchronization.

Furthermore, the use of separate carriers, one for the picture signals and one for the synchronizing pulses, greatly reduces the total amount of power necessary at the transmitter, 1

does in fact reduce the complexity of the transmitter as a whole, and reduces interference between signals and synchronizing pulses at the transmitter, thus reducing picture distortion.

I claim:

1. In television, the method of synchronizing which comprises the steps of generating a train of relatively high frequency unidirectional pulses, interrupting said train at regular relatively low frequency intervals, and modulating a carrier withrthe interrupted train for transmission as a synchronizing component.

2. In television, the method of synchronizing which comprises the steps of generating a train of relatively high frequency unidirectional pulses, reducing the pulse amplitude of said train at regular relatively low frequency intervals, and modulating a carrier with the interrupted train for transmission as a synchronizing component.

3. In television, the method of synchronizing which comprises the steps of scanning an object to produce an interrupted series of signal impulses generating a train of relatively high frequency unidirectional pulses, interrupting said train at regular low frequency intervals, modulating a carrier with said signal impulses, modulating the, same carrier with' said interrupted train, and interlocking said train and said series with said pulses positioned in the interruptions of said signal train.

4. In television, the method .of synchronizing which comprises the stepsof scanning an object to produce an interrupted series of signal impulses generating a train of relatively high frequency unidirectional pulses, interrupting said train at regular low frequency intervals, modulating a carrier with said signal impulses, modulating a second carrier of identical frequency with said interrupted train, simultaneously radiating both carriers, and interlocking said train and said series to radiate said pulses in the interruption of said signal train.

5. In television, the method of synchronizingv which comprises the steps of scanning an object to produce an interrupted series of signal im-, pulses, generating a train of synchronizing pulses at the frequency of signal'interruption, generating a carrier and modulating both said signals and said synchronizing pulses on said carrier, said pulse modulation being of opposite sign to the modulation produced by light in said signal impulses.

6. In television, the method of synchronizing which comprises the steps of scanning an object to produce an interrupted series of signal impulses, generating a train of synchronizing pulses at the frequency of signal interruption, modulating separate carriers of identical frequency with said signal series and said synchronizing pulses respectively, and simultaneously radiating said modulated carriers, said pulse modulation being of opposite sign to the modulation produced by light in said signal impulses.

7. In television, the method of synchronizing which comprises the steps of generating a series of unidirectional synchronizing pulses, and reducing the amplitude of a group of said pulses at regular intervals.

8. In television, the method of synchronizing which comprises the steps of generating a scanning current, scanning an object therewith to produce an interrupted series of signal impulses, deriving a train of unidirectional synchronizing pulses from said scanning current, said pulses being of the order of from three to ten times the amplitude of said signal impulses, modulating a carrier with said signal impulses, modulating a second carrier having a frequency equal to said first carrier plus or minus the frequency of said signal impulses with said synchronizing pulses, and simultaneously propagating said carriers.

9. In television, the method of synchronizing which comprises the steps of generating a scanning current, scanning an object therewith to produce an interrupted series of signal impulses, deriving a train of unidirectional synchronizing int pulses from said scanning current of opposite sign to the signals produced by illumination of said object, said pulses being of the order of from three to ten times the amplitude of said signal impulses, modulating a carrier with said signal impulses, modulating a second carrier having a frequency equal to said first carrier plus or minus the frequency of said signal impulses with said synchronizing pulses, and simultaneously propagating said carriers.

10. In television, the method of modulating a high frequency train of unidirectional synchronizing pulses with a low frequency pulse which comprises biasing the control electrode of a three electrode tube to cut-off, applying the high frequency pulses to said grid positively, and apply-' ing said low frequency pulse to said grid negatively thereby reducing the amplitude of the resultant output pulses during the period of said low frequency pulses. I

11. In television, a cathode ray image dissector, a. high frequency scanning oscillator, a low frequency scanning oscillator, means for defleeting said beam with energy generated by said oscillators, a thermionic tube having a grid biased to cut-off, means for applying energy from said high frequency oscillator to said grid positively, and means for applying energy from said low frequency oscillator to said grid nega tively. the output of said tube being used as a synchronizing component in a television system.

12. In televidon, a cathode ray image dissector, a high frequency scanning oscillator, a low frequency scanning oscillator, means for deflecting said beam with energy generated by said oscillators, a thermionic tube having a grid biased to cut-0E, means for applying energy from said high frequency oscillator to said grid positively, and means for applying energy from said low frequency oscillator to said grid negatively, the output of said tube being transmitted as a synchronizing component, and the output of said dissector being transmitted as a signal component.

13. In television, a cathode ray image dissector, a high frequency scanning oscillator, a low frequency scanning oscillator, means for deflecting said beam with the energy generated by said oscillators, a thermionic tube having a grid biased to cut-off, a means for applying energy from said high frequency oscillator to said grid positively, means for applying energy from said low frequency oscillator to said grid negatively, means for modulating the output of said tube on a carrier, and means for modulating the output of said dissector on a second carrier, said carriers being simultaneously rad-P ated PHILO T. FARNSWORTH.

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