US2186634A - Method and system for television communication - Google Patents

Description

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Patented Jan. 9, 1940 UNITED STATES 5 METHOD AND SYSTEM FOR TELEVISION CODIMUNICATION Allen B. Du Mont, Upper Montclair, N. 1., assignor to Allen B. Du Mont Laboratories, Inc., Passalc, N. 1., a corporation of Delaware Application January 26, 1938, Serial No. 188,981

- 4 Claims.

My invention relates to improvements in methods and systems for television communication.

In the more successful methods and systems proposed heretofore for television communication, and employing a cathode-ray pick-up tube at the transmitter and a cathode-ray viewing tube at the receiver, it has been required .that there be at each station two circuits for deflecting the scanning ray at the line and field fre-' quencies, respectively. Also, it has been required that line and field pulses be transmitted on the same carrier wave as the video signals, and these pulses have been used at the receiver for synchronization. In these prior methods and systems, not only has difficulty been encountered in .holding the deflecting circuits at the receiver locked in step with those at the transmitter, but a substantial portion of the transmitted energy has been expended in transmitting the synchronizing pulses.

With the foregoing in mind, it is one of the objects of my invention to provide an improved method and system for television communication employing cathode-ray tubes at both stations, and in which no synchronizing pulses are required to be transmitted to the receiver,-and in which either the deflection circuit at the line frequency or both deflection circuits at the linewaves at therespective line and field frequencies are generated at the transmitter and transmitted either over wire or radio to the receiver. These some waves are applied to the deflection plates or coils of the pick-up tube at the transmitter and the viewing tube at the receiver, so that the electron ray in the tubes must of necessity be always in step during the scanning action. When a common power supply at a given frequency of cycles, for example, is available, only the voltage wave at the line frequency is transmitted, and voltage waves at the field frequency are generated locally at the two stations and locked in step by the common power supply. Because of the impossibility of this system getting out of synchronization as to both line and frame frequencies, a much higher interlace ratio is possible. This allows a given detail picture to be transmitted over a much narrower frequency band, thereby allowing more transmitters to operate in a given frequency band. Likewise, the transmission of much higher definition pictures over the same frequency band can alsobe accomplished. j

Another advantage of this system is that it will receive pictures of transmitters operating at any desired line and frame frequencies. With the present systems it is necessary that all transmitters operate with exactly the same respective scanning frequencies if all receivers are to be capable of receiving from any one of the transmitters.

My invention resides in the improved method and system of the character hereinafter described and claimed.

For the purpose of illustrating my invention, an embodiment thereof is shown in the drawings,

wherein Figures 1 and 1a. are simplified, diagrammatic views of a television transmitting station and a television receiving station, respectively, constructed and operating in accordance with my invention;

Figs. 2 and 2a are views similar to Figs. 1 and la, respectively, showing a modification;

Figs. 3 and 3a are views similar to Figs. 2 and 2a, respectively, showing another modification;

Figs'. 4 and 4a are simplified views, in block diagram, of a complete television system operating in accordance with my invention;

Figs. 5 and 5a are views similar to Figs. 4 and 4a, showing a. modification;

Fig. 6 is a schematic view showing an interl laced pattern possible .to obtain with my improved method and system; and

Fig. 7 is a detail, elevational view of a part which may be used in connection with my invention.

With reference to Fig. l, the-numeral l0 designates a scanning device in theform of a cathoderay pick-up tube of any suitable, conventional construction, and comprising a photosensitive screen II and an electron gun for developing a ray l2 of electrons directed at and focused on the adjacent surface of th'egscreen.

In operation, a light image of the view or object l3 for transmission is projected by a lens system l4 onto the photosensitive screen I l, and the electron ray I2 is deflected horizontally at a suitable line-frequency by the plates I 5 and is simultaneously deflected vertically at a suitable held frequency by .the plates l5 so that the screen I I is scanned.

Since the specific form of construction of the pick-up tube l8 does not have any important bearing on my invention, and since the principle and manner of operation of cathode-ray pick-up tubes of the general type shown are well known to those skilled in the art, no detail explanation is believed to be necessary. The picture or video signals developed by the tube l0 may be taken from a collector electrode ll, fed to an amplifier tube l8, and thence transmitted over a line [3 to the receiving station.

At the transmitting station are deflecting circuits common with respect to the pick-up tube In and the cathode-ray receiver tube 20 at the receiving station. One of these circuits comprises a gas-discharge tube 2| and an amplifier tube 22, and generates a saw-tooth voltage wave at the desired field frequency. In operation, the condenser 23 is charged at a uniform rate until the plate of tube 2i reaches a potential whereat ionization takes place to cause practically instantaneous discharge of this condenser. Resistances 24, 25 and 26 are used to regulate the frequency of the saw-tooth voltage wave generated, and a cathode-bias resistor 2! is effective to regulate the amplitude of the voltage wave. The sawtooth voltage wave is applied to the amplifier tube 22 through a coupling condenser 28 and an input resistance 23. The reference numerals 30 and 3! designate, respectively, a cathode-bias resistance and a plate resistance for the tube 22.

The amplified saw-tooth voltage wave at the field frequency is fed through a coupling condenser 32 to the transmission line 33. This voltage wave is impressed across the deflecting plates I5 through ground and a connection 34 from the line 33. Also, this same voltage wave is impressed across the corresponding deflecting plates Hit of the receiver tube 20 through ground and a connection 34a from the transmission line 33.

The construction and principle of operation of the scanning device or cathode-ray tube 20 at the receiving station are similar to that of the transmitter tube In, the main difference residing in the fact that the screen of the receiver or viewing tube 20 is a fluorescent screen I la which may be formed on the inside surface of the end wall of this tube.

The deflecting circuit for generating a sawtooth voltage wave at the desired line frequency comprises a gas-discharge tube 35 and an amplifier tube 36. This circuit is the same as the frame-deflection circuit except that the capacity of the condenser 31 is smaller than that of the corresponding condenser 23 since the frequency of the voltage wave generated is higher. The amplified saw-tooth voltage wave at the line frequency is fed through a coupling condenser 38 to the transmission line 39, from whence it is impressed across the deflecting plates l5 through ground and a connection 40 from the line 39. Also, this same voltage wave is impressed across the corresponding deflecting plates Ilia of the receiver tube 20 through ground and a connection 40a from the transmission line 33.

At the receiving station, a connection 4| from the transmission line l8 applies the amplified picture or video signals to the control grid or electrode 42 of the electron gun in the tube 28, whereby the intensity of the ray I20, of electrons is modulated or made to vary in accordance with the lights and shadows at the respective elemental areas of the object I3 being televised. Since the electron rays l2 and l2a are scanning the respective screens H and Na in exactly the same manner and in synchronism, a visible image of the .object l3 will be produced on the fluorescent screen Ila.

At the transmitting station, a transformer 43 supplies the necessary heater voltages, and also has high voltage windings for the rectifier tubes 44 and 45. The rectifier tube 44 supplies voltage to the amplifier tubes I8, 22 and 38 and the gasdischarge tubes 2| and 35. The rectifier tube 45 supplies the necessary voltages for the pick-up tube l8.

Condensers 46, 41, 48 and 49 are filter condensers, and 50 designates a filter choke. Resistances 5i, 52, 53 and 54 are bleeder resistances for rectifier tube 44, and resistances 55,55, 51, 58 and 59 form the bleeder for rectifier tube 45.

When the two deflecting circuits are properly locked in at the respective desired frequencies, a resistance 80 may be used to provide coupling between these circuits.

If it is desired to have and maintain the line frequency an even multiple of the field frequency, this may be accomplished by a connection 8| and a coupling condenser 62 between the plate of the amplifier tube 36 operating at line frequency and the grid of the gas-discharge tube operating at the field frequency. Otherwise, this connection and condenser are omitted.

Resistances 63 and 54 are connected across the bleeder and allow for positioning of the beam pattern on the photosensitive screen ll. Resistances 65 and 88 connect the position controls to the deflection plates, as shown.

"Ihe condensers 61, 68, 69 and 10 are by-pass condensers.

The picture or video signals taken from the collector electrode I! are fed to the amplifier tube l8 through a coupling condenser H and an input resistance 12. A resistance 13 supplies plate voltage to the tube l8, and a resistance 14 applies the proper voltage to the collector elec trode I I.

When the distance between stations is great, or in cases where the signals are transmitted at low level, amplifiers may be used locally at the receiving station, and supplied respectively from the connections 34a, 40a and 4| to amplify the two saw-tooth voltages and the incoming video signals to sufiicient levels.

At the transmitter, a screen of the secondaryemissive type may be used in lieu of a photosensitive screen.

For the purpose of transmitting sound simultaneously with the picture, a microphone I5 and amplifier I6 are provided at the transmitter, the sound signals being sent over an independent transmission line 11 to a loudspeaker I8 at the receiving station.

At the receiver, a transformer 78 supplies any necessary heater voltages, and also high voltage to a rectifier tube 80. The proper voltages are supplied to the receiver tube 20 by the circuit shown, comprising filter condensers 8| and 82, bleeder resistances 83, 84, 85 and 88, and a bypass condenser 81. The correct bias is applied to the control or modulating electrode 42 by a resistance 88, and the video signals are applied to this electrode through a coupling condenser 88.

In the system shown in Figs. 2 and 2a, the construction and manner of operation of the pick-up and viewing tubes are the same as in Figs. 1 and 1a.

Also, with the following exceptions, the two systems are the same, and for simplicity of comparison, corresponding parts and connections in the two systems have been designated by the same respective reference characters.

In Figs. 2 and 2a there is the common linedefiection circuit at the transmitting station,

comprising the gas-discharge tube and the amplifier tube 36, for generating and amplifying the saw-tooth voltage wave at the line fre- ,quency, this wave being impressed across the deflection plates I6 of the pick-up tube I0 and transmitted as before over the line 39 and connection 40a and impressed across the deflection plates l6a of the viewing tube 20. However, the receiving station has'its own field-deflection circuit '''comprising a gas-discharge tube 2Ia and an amplifier tube 22a corresponding to and operating in the same manner as the tubes 2| and 22, respectively, in Fig. 2.

That is, in Fig. 2, the tubes 2| and 22 operate as in Fig. 1 to generate a saw-tooth voltage wave at the field frequency which is impressed across deflection plates I5 by the line 33 and connection 34, but the wave is not transmitted as in Fig. 1. At the receiver, the tubes Zia and 22a operate to generate asaw-tooth voltage wave at the'field frequency which is impressed across deflection plates I5a by the line 33a.

For, the purpose of maintaining operation of the two field-deflection circuits at the respective stations at the same desired field frequency, a common and local source of 60-cycle alternating current is utilized. Asshown, the GO-cycle wave is applied to the grid of both gas-discharge tubes 2| and 2Ia.

In Fig. 2a there is, in addition, the rectifier tube 9| and associated bleeder circuit for supplying voltage to the tubes 2la and 22a.

In the system shown in Figs. Band 3a, the construction and manner of operation of the pick-up and viewing tubes are the same as in Figs. 1, la, 2 and 2a.

Also, with the following exceptions, the system in Figs. 3 and 3a is the same as that in Figs. 2 and 2a, and for simplicity of comparison, corresponding parts and connections in these two systems have been designated by the same reference characters. In Fig. 3, the saw-tooth voltage wave at the field frequency is generated by a half-wave rectifier tube 92 and the associated filter arrangement comprising a filter condenser 93 and the resistance 94 connected across this condenser. With each positive halfcycle of the 60-cycle power "frequency, the condenser 93 is charged, and this charge leaks out by way of the resistance 94, thereby generating a 60-cycle saw-tooth voltage wave with a somewhat longer return trace than by the method in Figs. 1 and 2. In. the systems described, this is not objectionable because there is effective scanning during the return trace as well as during the initial trace.

In Fig. 3a, the difference over Fig. 2a resides also in the deflecting circuit for generating the saw-tooth voltage wave at the field frequency. That is, as in Fig. 3, a half-wave rectifier 92a and a filter condenser 93a and resistance 94a are used for this purpose.

In the transmitting station shown in Fig. 4, the output from the cathode-ray pick-up tube 95 is fed through a preamplifier 96 and a main amplifier 91 to the transmitter 98. A generator 99 operates to develop the saw-tooth voltage wave.

at line frequency which is fed to the sound amplifier I00 and transmitter III. whereby this wave is transmitted on the sound carrier. A generator I02 operates to develop asaw-tooth voltage wave at the field frequency of 60 cycles, and is held at this frequency by a local (SO-cycle power supply I93.

From the foregoing it will be understood that in Fig. 4 the video signals are transmitted on one carrier wave and the saw-tooth voltage wave for' deflecting the cathode-ray at line frequency is transmitted with the sound signals on the sound carrier wave.

At the receiving station, shown in'Fig. 4a, the signals are amplified by a radio-frequency amplifier I04. A locally generated oscillation is mixed with the signals, as represented, and fed to the video intermediate-frequency amplifier I95 and the audio intermediate-frequency amplifier I06. The video signals are then demodulated, amplified, and applied to the control grid of the viewing tube I01 to modulate the intensity of the electron rat.

The audio signals and the saw-tooth voltage v wave at line frequency being on a difierent carrier wave, do not interfere with the video signals, and are separated and pass through their own intermediate-freque'ncy amplifier I06 and are then demodulated. The voltage wave passes through a selective filter I08, and after being amplified by an amplifier Ill9is impressedacross the deflection plates of the viewing tube III! to cause deflection of the electron ray at the line frequency.

The sound signals are amplified by an amplifier I I0 and fed to the loudspeaker I.

As in Figs. 2a and 3a,a local generator H3 operates to develop the saw-tooth voltage wave at the field frequency, andis held at this frequency by the local 60-cycle power supply Ill common to both stations, as in Figs. 2, 2a and Figs. 3, 3a.,

In Figs-5 and 5a no sound is transmitted, and

, both saw-tooth waves, at the line and field frequencies respectively, are transmitted on the sound carrier. That is, at the transmitter, the outputs from both deflection circuits 99 and III! are fed to the audioamplifier I00 and then pass to the transmitter Ill]v for transmission on the sound carrier, as represented in Fig. 5. The operation, otherwise, is the same as in Fig. 4. At the receiver, no local field generator, such as the generator H3 in Fig. 4a, is necessary, since the saw-tooth voltage wave at the field frequency is received from the transmitter, along with the saw-tooth voltage wave at.'the line frequency. These waves are demodulated, and separated by the respective filters I08 and H6, and then amplified by the respective amplifiers I99 and H1 before being impressed across the deflection plates.

In cases where a secondary-emissive screen is used at any of the transmitting stations in lieu of a photosensitive screen, one method for this purpose might be to employ an aluminum plate with letters, numerals, or any desired figure or view printed, with a carbon ink, on the surface to be scanned. As the printed surface is scanned by the electron ray, the voltage output is different when'the ray is directed at the aluminum surface than when it is directed at the carbon printing, whereby the picture signals are devel-.

oped in accordancewith the printing scanned by the electron'ray. It will be understood that in this method different transmitter tubes are required for different messages, since the message transmitted from any one tube is fixed by the particular carbon printing on the aluminum plate. However, if for example letters of the alphabet are printed on the aluminum plate and arranged in some such manner as in Fig. 7, and at the transmitter the electron ray is positioned so that it scans only one letter or figure at a time and under control of the operator, only such letter or figure will appear on the fluorescent screen of the viewing tube. In this way, messages can be sent, such as by spelling out code words.

Code messages can also be transmitted by using a conventional type of viewing tube at the receiving station and placing in front of the fluorescent screen a transparent card with the letters of the alphabet thereon and arranged in some such manner as in Fig. 7. At the transmitter, no picture signals are developed, but only the two sawtooth voltage waves at the respective line and field frequencies are transmitted to the receiver, as in Figs. 1 and 1a and Figs. 5 and 50. By controlling the relation of these waves at the transmitter, the scanned area at the viewing tube can be positioned or selected to embrace any one letter or numeral on the card.

By actually transmitting to the receiver the saw-tooth wave at the line frequency, and either transmitting also the saw-tooth wave at the field frequency or using local field generators at the two stations synchronized by a common power supply of a given frequency, as explained, it follows that the two electron rays will always scan the respective screens in the same manner and in synchronism. For this reason, it is possible in my improved system and method to obtain the advantage of using lin and field frequencies to give an interlace ratio greater than 2 and even as high as 4.

For example, in the various systems used heretofore, in which cathode-raytubes are employed at both stations, and line and field impulses are transmitted for synchronization, some success has been had in obtaining an interlace ratio of 2. But even at this lowest interlace ratio, the line and field impulses sometimes fail to hold the interlaced pattern at the viewing tube.

Assuming a line frequency of 6615 and a field frequency of 60 in my improved system, as shown in Figs. 1 to 511', there will be 110.25 lines per field, and accordingly a frame frequency of 15 and an interlace ratio of 4. That is, at each station, and

with reference to Fig. 6, the electron ray during the first field of any one frame will scan the effective screen area ABCD along the solid lines 1 to 110.25. During the second field of this frame, the electron rays will scan the dot lines 111 to 220.50. During the third field of this frame, the electron rays will scan the dash lines 221 to 330.75. During the fourth field of this frame, the electron rays will scan the dash-dot lines 331 to 441, and then be at the initial starting point for the next frame of four fields.

From all the foregoing it will be seen that in my improved method and system, practically one hundred percent of the transmitted energy for the video signals is used for the same, since the saw-tooth voltage wave or waves are transmitted on the sound carrier. Also, no synchronizing pulses are required, and when both voltage waves are transmitted no deflection circuits are required at the receiver.. When there is available a common power supply at a given frequency such as 60 cycles, it is only necessary to transmit the saw-tooth wave at the line frequency since the saw-tooth voltage wave at the line frequency can ,be generated locally at the two stations in a number of difierent waysand must of necessity be in step. Any difference in phase at the two stations can be readily corrected for by a suitable phase-shifting arrangement.

In the specification and claims, the following meaning is intended for the various terms used. A frame is a single complete picture. Frame frequency is the number of times per second the frame or entire effective screen area is completely scanned in interlaced scanning. Field frequency is the number of times per second the frame or entire effective screen area is fractionally scanned in interlaced scanning. The interlace ratio is the ratio of field frequency to frame frequency. For other definitions, reference may be made to the publication Electronics, issue of August, 1937.

It will be understood that various embodiments of my invention other than those disclosed, are

possible without departing from the spirit of my invention or the scope of the claims.

I claim as my invention:

1. In the art of simultaneous television and sound communication betwen two stations wherein cathode-ray tubes are ultilized at the respecsound signals on another carrier wave, de-

veloping a continuous electrical wave at a line frequency for deflecting the scanning my at said first-named station at said line frequency, transmitting said continuous electrical wave on the sound-carrier to said other station from said firstnamed station, demodulating at said other station to obtain thereat said electrical wave, and utilizing the electrical wave so obtained to deflect the scanning ray at said other station at said line frequency.

2. In the art of simultaneous television and sound communication between two stations wherein from one of said stations to the other station picture signals are transmitted on a carrier wave and sound signals are simultaneously transmitted on another carrier wave, the method of operation which comprises utilizing a ray-deflecting wave to control scanning action at said firstnamed station, transmitting said ray-deflecting wave on the sound carrier to said other station from said first-named station, demodulating at said other station to reproduce thereat said rayrefiecting wave, and utilizing the ray-deflecting wave so obtained at said other station to control scanning action at said other station.

3. In the art of simultaneous television and sound communication between two stations wherein cathode-ray tubes are utilized at the respective stations for scanning, the method of operation which comprises transmiting from one of said stations to the other station picture signals on a carrier wave and sound signals simultaneously on another carrier wave, utilizing a raydeflecting wave to cause deflection at a given line frequency of the scanning ray at said first-named station, transmitting said ray-deflecting waveon the sound carrier to said other station from said first-named station, demodulating at said other station to reproduce thereat said ray-deflecting wave, and utilizing the ray-deflecting wave so obtained at said other station to cause deflection at said line frequency of the scanning ray at said other station.

4. In a system for television communication between two stations, scanning devices in the greases 5 form or! cathode-ray tubes located respectively at said stations, means common with respect to said tubes and .located at one of said stations and operating to develop a continuous electrical wave at a given line frequency, means for reproducing said continuous electrical wave at the other station, means associated with the cathode-ray tube at said flrst-named station and having impressed thereon said electrical wave and operating to deflect the scanning ray at said line frequency, means associated with the cathode-ray tube at said other station and having impressed thereon said electrical wave and operating to'deflect the scanning ray at said line frequency, a deflecting.

circuit associated with the cathode-ray tube at said flrst-namedstatlon for deflecting the scanning ray at a given field frequency, a deflecting circuit associated with the cathode-ray tube at 5 said other station for reflecting the scanning ray at said field frequency, a source of alternatingcurrent common with respect to and local to said stations, and connections between said common source of alternating-current and said deflecting circuits for maintaining operating action of each of the latter at said field frequency.

' ALLEN B. DU MONT.

CERTIFICATE OF CQRRECTION. Patent no. 2,186, 6514.. January' 9, 19140.

ALLEN B. 1311- MONT.

It is hereby certified that error appeais in the printed specification of theabeve numbered patent requiring"-correction as follows: Page 1;, sec-- ond eolumn, line 55, claim 2, and page 5 second column, line 6, claim 11., for the word "reflecting read -d.ef1ecting--; and that the said Letters Patent should be read ,with this correction'therein that the same may conform to: the record pf the'case in the Patent Office.

Signed and sealed this 18th day of June, A. D.'19L .O

I Henry Van Arsdale (Sea1) Acting Commissioner of Patents.

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