US2124478A

US2124478A - Television system

Info

Publication number: US2124478A
Application number: US750794A
Authority: US
Inventors: William A Tolson
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1934-10-31
Filing date: 1934-10-31
Publication date: 1938-07-19
Anticipated expiration: 1955-07-19
Also published as: FR793766A

Description

JiillQ, 1938. w. A. ToLsoN TELEVISION SYSTEM Filed Oct. 5l, 1934 5 Sheets-Sheet l UW@ IQQQN N @www am INVEN''R mlliam H. Tolsan www a'r 'ron/VEY NDP werkten L July 19, 1938. w. A. ToLsoN TELEVISION SYSTEM Filed 0G13. 3l, 1934 s shets-sheet 2 INVEN''R VVllamH. Tolgson,

July 19, 1938. w. A. ToLsoN 2,124,478

TELEVIS ION SYSTEM Filed Oct. 31, 1934 5 Sheets-Sheet 5 H'I'TOHNEY Patented July l19, 1938 UNITI-.jo STATES PATENT OFT-lcs TELEVISION SYSTEM William A. Toison, Westmont, N. J., assignor, by

o Corporati meme assignments, to Badi on of vAmerica, New York, N. Y., a corporation of -Delaware limitationA ombel- 31, 1934, semi No. 150,394

2 claims (ci. 17a-1.5)

, tube and the vacuum tubes at the receiver from an alternating current power line, such as a 60 cycle line. This requires that the alternating current be stepped up to the desired voltage, rectiiled and ltered. It has been foundthat in a receiver where the voltage supply unit must be placed reasonably close to the cathode-ray tube there is an alternating current eld surrounding this unit whichextends to the cathoderay tube and affects the deflection ofthe electron beam. In addition to this alternating current eld which is superimposed upon the deecting field of the cathode-ray tube, there is an alternating current component in the output of the filter, because of imperfect filtering, which modulates the outputs of the deilecting circuits and which appears on the cathode-ray tube electrodes.

The effect of the alternating current field and alternating current hum component of the filter output, prior to my invention, was to impair or destroy the detail of the picture. An additional effect was a noticeable movement of the edge of the picture. Shielding the voltage supplyunit failed to eliminate the effect of the alternating current leld while the alternating current component in the lter, output could be reduced to a negligible value only by employing a filter so expensive as to be prohibitive.v

The .alternating current eld and the hum or ripple component in the lter output were found to be especially objectionable in a system employing the so-called @interlaced scanning. If they were of suillcient magnitude they destroyed the interlacing, In all cases they caused the scanning lines to pair with the result that the line structure of the picture became prominent. In addition to these effects, there were the previously mentioned effects tending to ruin the picture.

At the transmitting station, also, it isl desirable to obtain all unidirectional voltages from an alternating current supply. Just as in the case of the receiver, prior to my invention any stray fields from the power supply which reached the cathode-ray transmitter tube caused a disturbance in the electron beam deilection `such that interlaclng and picture detail were adversely aifected. Likewise, any hum or ripple voltage which reached the deflecting circuit and cathode-ray tube caused a loss in picture detail.

Another diiliculty was encountered prior to my invention whenever an attempt was made to use incandescent lamps lighted by alternating 5 current for illuminating a scene to be transmitted. That the light` from such lamps varies in intensity is well understood, there being no visible ilicker eifect if cycle current is used only because of the high frequency of the varia- 10 tion in light intensity. However, this variation in the light striking the mosaic of a transmitter tube causes an alternating current component to be superimposed upon the picture current with the result that the picture appearing at the re ceiver is unevenly lighted from top to bottom. This uneven brightness or shading of the picture could not be tolerated, largely because the shading did, not remain stationary. Instead, the uneven shadin`g drifted across the picture. Because 20 of this phenomenon, direct current lighting was utilized in television studios until my invention made it unnecessary.

An object of my invention is to provide a television system, and a method of operating the same, in which the magnetic or electrostatic fields and the alternating current hum or ripple from `power supply units will have substantially no detrimental eiect upon the picture.

A further object of my invention is to provide a television system employing interlaced scanning in which filtered direct current derived from an alternating current source may be utilized without impairing the interlacing.

A further object of my invention is to provide a television receiver of moderate cost which may be operated from an alternating current power line.

A still further object ofAmy invention is to provide an improved television transmitter in which the effects of stray electric fields and of voltage ripples fromk an alternating current power supply are minimized.

It is a still further object of my invention to provide a television transmitter of the type utilizing a cathode-ray transmitter tube which will satisfactorily transmit a scene illuminated by incandescent lamps or the like lighted' by alternating current. v

I'he above-described diillculties in obtaining a good picture without the use of batteries in the transmitter and/or receiver were encountered, for example, in systems which employed a frame frequencyof 48 pictures per second, the voltages being derived from a 60 cycle line. It had been 55 found that a frame frequency much-lowerl than this caused flicker but that there appeared to n be no reason for going to a frequency above 48 pictures per second as at this frequency there was no flicker.

In accordance withthe preferred em, ent

of my invention I increase the frame frequency to the frequency of the power line; namely, to 60 pictures per second in the example being considered. As will appear from the follcwingde- Vscription, this is done in a system vin which the synchronizing of the transmitter and receiver is accomplished by means of synchronizing impulses vtransmitted over the same channel as the picture signals. The resulto! changing the Y frame frequency to the same frequency as that of the power supply is that the magnetic and electrostatic fields and the alternating current ripple from the power supply unit no longer affect the picture to any appreciable extent.

Other objects,ffeatures and advantages of my invention-will appearfrom the 'following descripn tion taken in connection with the accompanying drawings in which Figures 1 and 2 are circuit diagrams of a transmitter and a receiver, respectuely. embodying my invention', and

lii'igs. 3 to 8 are diagrams which are referred to in explaining my invention.

.Iss

Referring to Fig. l, a television transmitter system is illustrated which includes a cathoderay transnLitter tube I of the type described in an article by Dr. V. K.'Zworykin which appears in the January, 19.34 issue of The Journal of vided for deilecting the electron beam horizontally and vertically, respectively, in order to scan the mosaic I3 with the electron beam.

An image of the object IQ to be transmitted is projected upon the mosaic Il by means of a suitable optical system indicated at 2|. If an indoor .scene is to be transmitted. it will usually be desirable to illuminate the scene by incandescent lamps, indicated at Il, which are lighted from the same cycle power line 25 which supplies power to the transmitting system.

The electrodes of the transmitter tube supplied with unidirectional voltages 'of the proper value by means of a power supply 4unit connected to an alternating current supply which, in the example being described, is the 80 cycle power line Il., The power supply unit is of ccnventlonal design and comprises a step-up transdeiiectingl plates II, respectively, an electric dis- -charge tube impulse generator is provided. This former 21, a full wave rectifier 20 and a filter 3|. The electrodes of the tube I are connected to suitable points on a voltage divider 33, which is connected across the output terminals of the Illter II.' Y

To produce thedesired saw-tooth current and saw-tooth voltage, for the deecting coils I1 and generator consists of a main` `oscillator ll from which are derived electrical impulses of the desired frequency for horizontal deflection and also electrical impulses of the ldesired frequency for vertical deection. The main oscillator 35 may I arev be a blocking oscillator consisting of an electrical discharge tube 31 having a cathode Il. a controlv grid" and an anode 43. The control grid 4I is connected through a variable grid leak resistor ll and through a resistor s1 in the rcontrol circuit (which will be'k described hereinafter) to ground an'd through ground to the cathode 39.'

I'he anode I3 is maintained-at a positive potential 'oy means of a suitable voltage-source such as a battery 4l which has its negative terminal connected to the cathode ls and its positive terminal connectedl through a resistor 5I .and the primary winding II of a transformer E5 to the anode 4l. A condenser 51 Ais connected in series with the secondary I9 of the transformer .755, the condenser l1 and secondary 59 being connected between the cont-rol grid Il and the cathode 39. y

The transformer windings I3 and 59 are so phased that aniincrease .in plate current in the tube SI causes the control grid 4I to become more lpositive whereby grid current flows into the grid condenser. tu charge it -in -sach a direction as to apply ainegative ybias to the. control grid 4I. The plate current finally approaches its saturation value so that its rate of increase becomes less, the voltage fed. back to the grid decreases whereby the, plate current decreases', thus reversing the phase of the voltage applied to the control grid Il through the trans# former 55 and biasing the tube 31 beyond cutoi'f. During the time the grid has been positive, the condenser 5l has been charged suillciently to hold the tube biased beyond cut-ofi and no further oscillation tairesl place until the4 charge has leaked ofi the condenser 51 through the grid leak resistor' 45 sufficiently to lower the grid voltage to a value which again permits flow of plate current.

The voltage impulses produced by the oscillator 3l are impressed upon a chain of frequency dividers 6.', 61 and Si, each of which may be a blocking oscillator operating the same as the main oscillator II but at a lower frequency.

'Each oscillator is locked in step with the oscillation of next highest frequency;

The frequency of the main oscillator 35 is divided by the frequency divider chain in three steps of seven, yas indicated on the drawings, to produce -voltage impulses occurring at the rate of 60 per second, this being the frame frequency desired for practicing my invention in the specific example being described. Thev 60 cycle impulses are supplied to the input circuit of a saw-tooth wave generator or4 deilecting circuit 61 which` converts them into voltage impulses of the nec-- essary shape for causing a flow of current hav-- ing a saw-tooth wave shape through the de- *n fleeting coils I'I. The horizontal deflecting impulses are obtained by impressing a portion of the output energy of the main oscillator 35 upon theinput circuit of 7 y a frequency divider I., such as another blocking oscillator, which divides the frequency ofthe main oscillator output by two to produce 10,290 impulses per second. The output.; of the frequency divider I! is impressed upon'a saw-toothwave generator or deilecting circuitll which.

has its output connected across the horizontal defiectlng plates II for impressing a saw-tooth voltage thereon.

It will be noted that the'vertical deflect-.ing and horizontal deilecting frequencies are such as to ".power line 25. The control circuit 13 includes an electric discharge tube 15 having a cathode 11, a control grid 19 and an anode 0|. The tube 15 is so adjusted-that it functions as a distorting amplifier or detector. In one specific embodiment this is accomplished by' maintaining the control grid 19 at a high negative bias by means of a'biasing source, such as a battery ,the negative bias being high enough to bias the tube beyond the cut-oil point. The anode 8| is maintained at a positive potential by means of a source of potential, such as a battery 85, having its negative terminal connected to the cathode 11 and its positive terminal connected to the anode 8| through the resistor 41. 'I'he resistor `41l is shunted by a lter condenser 81.

It will be noted that the resistor 41 is connected in series with'the grid leak resistor 45 of the blocking oscillator 35 whereby any change in voltage drop across the resistor I1 causes a change in the bias on the control grid 4| of the blocking oscillator. 'Ihe filter condenser 81 has suillcient capacity to integrate current impulses and cause a substantially steady flow of current through the resistor 41. Y

The current flow through the control circuit resistor 41 is caused to vary in response to Aany phase shift between the 60 cycle current from the power line 25 and the output of the frequency divider 65 by impressing the impulses from both the power line and the frequency divider upon the input circuit of the control tube 15.

The 60 cycle impulses appearing in the output circuit ofthe frequency divider 65 are impressed through a coupling condenser 89 and the secondary 4of a transformer 9| upon the control grid 19 of the tube 15. A 60 cycle voltage from the line -25 is also impressed upon the control grid 19 through the transformer 9| in such phase that it adds with the voltage impulses supplied by the frequency divider 65.

'I'he effect of the control tube 15 and associated control circuit isto hold the output of the frequency divider 55 at the same frequency as the power line frequency and in a substantially fixed phase relation thereto. It will be understood that the frequency of the power line 25 is maintained substantially constant. However, slight frequency variations in the line frequency willoccur and the control circuit 13 will cause the impulse generator to follow such frequency variations. f

The above described control circuit is described and claimed in application Serial No. 729,730, filed June 9, 1934, in the name of A. V. Bedford, and .assigned to the 4Radio Corporation of America.

During the transmission of a picture, the picture signals appear across a resistor 93 connected between the mosaic I3 and the second anode and are impressed upon a picture signal ampliner 95 which amplifies them and impresses them upon a radio transmitter 91 or, if preferred, upon a transmission line (not shown). y

In order to maintain the scanning at the receiver synchronized with the scanning at the transmitter, the vertical and horizontal syn-'" chronizing impulses are impressed through wave shaping ampliers 99 and lli; respectively. and

through an amplifier upon a 'suitable point in the picture signal ampliiler 95. When impressed upon the amplifier 94, the synchronizing impulses having the characteristics indicated by the curve |03, the horizontal synchronizing impulses .n being represented by the reference letter :l: and the vertical synchronizing impulses being representedby the reference letter il.

Referring specifically tothe manner in which the square top impulses :l: and 1l are derived from the blocking oscillator outputs, the wave shaping am'pliilzs 99 and |0| may be resistance coupled amplifiers-having successive vacuum tubes adjusted to clip oil' the lower and top portions of the wave impressed thereon.l A wave shaping amplier of this character is shown inthe abovementioned application Serial No. 729,730.

The vertical defiecting or framing impulse y is made wider than the horizontal deflecting impulses :r by properly adjusting the blocking

oscillators

65 and 59, respectively. The width of an impulse may be made the desired value kby shunting thev primary winding of the blocking oscillator transformer with a suitable amount of capacity, this capacity usually being distributed capacity. In general, also, the width of an impulse produced by the lower frequency oscillator tends to be greater than the width of an impulse produced by the higher frequency oscillator.

The ampliiier 94 reverses the phase of the impulses :c and y, as shown by the curve |03'a, whereby they have a negative polarity. The synchronizing impulses may be mixed with the picture signals in the manner described and claimed in application Serial No. 653,976, filed January f 28, 1933, in the name of R. L. Campbell, and assigned to the Radio Corporation of America. As described in the Campbell application, the negative synchronizing impulses are impressed upon an amplifier stage in the amplifier where the polarity of the picture signals representing black is negative. This removes undesired transients or the like from the region of the picture signals so that they may be clipped or saturated olf in a succeeding amplifier stage.

Referring to Fig. 2', the receiver comprises a cathode-ray tube |05 of the type employingelectrostatic focusing. It consists of an evacuated envelope |01 having an electron gun at one end and a fluorescent screen |09 at the opposite end. The electron gun includes an 'indirectly heated cathode Iii, a control electrode i|3 and a rst anode ||5. A second anode ||1 consisting of a silver coating on the inner surface of the envelope |01 is provided for aidingl in the electrostatic -focusing of the electron beam and foraccelerating the electrons in the beam. 'I'he cathode-ray tube |05 is provided with deflecting cois I9 and |2| for deilecting the electron beam vertically and horizontally, respectively, whereby the fluorescent screen |09 is scanned by the electron beam to produce a picture.

The picture and synchronizing signals which have been radiated from the transmitter are received by a suitable radio receiver |23 and supplied to a picture amplifier |25. 'Ihe signals impressed upon the picture amplifier are represented by the curve V|20 Where the synchronizing viinpulses are shown at :n and 1l, and the picture signals at z. The signals appearing in the output circuit of the amplifier |25 are impressed through a coupling lcondenser upon control electrode 3 of'the cathode-ray. tube whereby the electron beam is modulated in accordance with the. picture Theplctureandsynchronisingsignalsarealso impressed upon a separating circuit |31 in which the synchronizing signals are separated from the picture signals, and in which the horizontal synchronising impulseaare separated from the framing impulses. The vertical synchronizing or framing impulses are supplied to a suitable vertical dehecting circuit |33 which converts the synchronizing impulses" into voltage impulses of the proper wave shape for causing a how of saw-tooth current through the vertical dehecting coils Ill.

The horizontal synchronizing impulses are supplied toa horizontal deflecting` circuit |32 which converts them into voltage impulses of the proper wave shape for causing a how of saw-tooth current through the horizontal dehecting coils |2|. The horizontal and vertical dehecting circuits may be `oi' any suitable design, such as the one illustrated as the hcrizontal dehecting circuit. 'I'his circuit, which is known as asingleftube dehecting circui isl described and claimed in application Serial No. 718,353, hled March 31, 1934, in the name of A. W. Vance, and assigned to the Radio Corporation of America.

LThe horizontal dehecting circuit comprises a *vacuum tube |33 having a cathode |34, a controll grid |35 and aplate |33. The plate |423 is connected to a suitable positive voltage supply through an inductance coil |31. 'I'he control grid |3l is connected through aninductance coil |33 to a point ona voltage divider |4I which is negative with respect to the cathode |34. The coils A|31 and |33 are inductively coupled in such phase relation that lan increase in plate current causes a voltage to be induced in the grid circuit which drives the control grid |35 more positive. As described in the above-mentioned Vance application, this causes a saw-tooth current to appear in the plate circuit of the vacuum tube.v By ad- Justing the natural period of oscillation of the tube |33 it may be made to produce saw-tooth current impulses in synchronization with the horizontal synchronizing'impulses which are impressed upon its control grid |33.

In the receiver illustrated, two voltage supply units are employed, one unit |43 for supplying the comparatively high voltages to be first and second anodes of the cathode-ray tube |05, and the other unit |43 for supplying the lower voltages to the dehec'ting circuits and other parts of the television receiver.

'Ihe high voltage supply unit |43 comprises a step-up transformer |41, a double wave rectifier tube |43, and a hlter ill. The hrst and second anodes of the cathode-ray tube |33 are connected to suitable .points on a voltage divider |33 which is connected across the outputfterminals of the filter III, suitable by-pass condensers |34 and |33 being provided for the two anodes. v

tube |33 also is connected through a resistor |33 to a point on the voltagefdivider |4| which is' negative with respect lto ground. Suitable bypass condensers are provided for reducing to a. minimum the amount of cycle hum which may The plate |33 and control grid appear upon the grid and plate ofvacuum tube |33 and upon the control grid of the cathoderay tube ill. 'Die remaining units ofthe rev ceiver, also, preferably are supplied with voltage from the powerk supply unit |33 as indicated byl the conductors |33 andill. '1

Preferably all the apparatus shown in Fig. 2 is installed in a single cabinet-similar to the usual radio console, the power units |43 and |45 being located in the bottom of the cabinet and the cathode-ray tube |03 being located near the top of the cabinet. This places the power units so close to the tube |03 that it is practically im-- possible to prevent the stray held of the power units from influencing the dehection.

` The two voltage supply units are supplied wit alternating current'from a 60 cycle power line 23a which preferably is the same power line as the one supplying power to the transmitter. If the same power'line or powerfnetwork supplies the transmitter and receiver, the picture appearing on the cathode-ray tube screen |09 will have no movement caused by any small hum or stray held in the receiver. Ii' the transmitter and receiver are supplied from separate 60 cycle power lines, there will be a very slow movement in the picture due to the stray held and hum, but, where the power line ,frequencies are held within close limits, this movement of the picture will be so slowl that it will not be objectionable. In either case,

by making the frame frequency equal to the frequency of the lpower line an impairment of picture detail due to stray alternating current held and hum is avoided.

The eifect of the ripple and stray field on "a picture appearing at a receiver in a television sysamplitude than the saw-tooth curve |61 although the vertical and horizontal dehecting currents may have substantially the same amplitude.. A

Also, only a small number of horizontal lines (10%) have been indicated for each picture frame although, in the system illustrated in Fig. l, the actual number of lines per picture frame is 171%.

It will be understood that because of interl'acingv the apparent number of horizontal lines is double the actual number of lines.

'Ihe stray held from a power supply unit is inl dicated by the sine wave curve |39.. It is super-Ik imposed upon the vertical dehecting heldto produce a resultant dehecting held indicated by the dotted line curve |1I. It will be understood'that the eh'ect of the stray held has been greatly ex;

receivertube. The solid lines |13're'present the 70.

scanning lines produced during the hrst ,picture frame while the dotted lines |15 Arepresent the. scanning lines produced during the succeeding. picture frame. The construction lines |16 and.

|11 indicate how Fig. 4 has been built up fromA the curves in Fig. 3.

An inspection of Fig. 4 will show that in spite of the large stray field component in the vertical deflecting field, the horizontal scanning lines of two successive pictures are perfectly interlaced,

the only effect of the hum being that in the middle of the picture the lines are crowded together somewhat, while at the bottom and at the top of the picture they are spread apart. In an actual picture, the crowding and spreading of the lines are not noticeable.

It can readily be demonstrated that if the phase of the stray field component shifts appreciably with respect to the vertical defiectin'g field during the transmission of a picture, the interlacing of the picture will be destroyed either because of a pairing of the lines or because of the scanning lines of one picture frame being displaced by one or more lines with respect to those of the preceding picture frame.

It can also be shown that there must be an even number of half sine waves of the stray eld component per picture frame in order t avoid destruction of the interlacing. In otherl words, the frame frequency should either be equal to the power supply frequency or be a whole number sub-multiple thereof. For example, with a 60 cycle power supply, the frame frequency may be 30, 20, 15, or 10 pictures per second providing a system is employed which will reduce flicker at these lower frequencies.

The effect of voltage ripple and stray eld on the horizontal deflection will now be considered with reference to Figs. 5, 6 and 7. In Fig. 5, as in Fig. 3, the vertical deecting current and field is represented by the saw-tooth curve |65 while the horizontal deilecting current and eld is represented by the other saw-tooth curve 151. Also, the 60 cycle stray eld and hum component is represented by the sine wave curve |69. The stray field is superimposed upon the horizontal defiecting field to produce a resultant horizontal deilecting field indicated by the dotted saw-tooth curve |10.

The character of the picture produced by the composite horizontal deflecting field is indicated at Fig. 6 where the lines III represent the scanning lines. It will be seen that the eifect of the stray ileld is to cause a bend in the edges of the picture. It will be apparent from an inspection of Fig. 5 that this bend in the picture edge is the same for each succeeding picture frame whereby the picture remams perfectly stationary so far as hum eifects are concerned. Inpractice the bend in the edge of the picture is so slight that it is hardly noticeable, due to the fact that the pattern remains stationary.

The horizontal deflection may be affected also by the 60 cycle hum modulating either the horizontal deiiecting circuit or the electrodes of the cathode-ray tube, or both. No curves have been shown to represent this modulated condition but it will be evident that the modulation will give the picture the characteristic indicated in Fig. '7 where the scanning lines are shown at lll. As in the case of the superimposed field, the bend in the edge of the picture will remain constant for succeeding picture frames. It will be noticed that in Figs. 6 and 7, the eil'ect of hum on the vertical denection has been ignored.

Obviously there is an advantage in preventing a continuous movement of the edge of the picture as any such movement in itself would be objectionable. However, the greatest advantage in preventing this movement is that if only a slight 5 movement of the picture edge occurs, the picture detail is impaired. It is obvious that because of the very small size of a picture element, a very slight deflection of the electron beam produced by the hum or stray field would cause a blurring of the picture.

While the effects of ripple and stray field have been described specifically with reference to the television receiver, the same effects are present at the transmitter even though it is feasible to employ better filters and shielding there than at the receiver.

If alternating current lighting is employed for illuminating the scene to be transmitted, as indicated in Fig. l at 23, the picture at the receiver is shaded substantially as shown in Fig. 8. My improved transmitter causes this shading pattern to remain stationary on the picture. 'Ihe stationary shading pattern is far less objectionable than a shading pattern which is moving across the picture. Furthermore, the stationary shading pattern can be corrected for, by means of a suitable circuit, to produce a picture of uniform shading.

In the claims, the reference to a sub-multiple of a number includes the number itself.

It will be understood that various modifications may be made on my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and are set forth in the appended claims.

I claim as my invention:

1. The method of operating a television system of the type comprising a receiver adapted to be energized from an alternating current power line, the receiver includingn a cathode ray tube on an element of which the received picture appears, which comprises deriving unidirectional potentials from said power line, supplying said potentials to said receiver, distantly producing and transmitting picture and synchronizing signals, intercepting said signals and supplying them to the receiver, and supplying to the receiver framing impulses recurring at substantially the same frequency as that of the said power line.

2. The method of operating a television system of the type comprising a receiver adapted to be energized from an alternating current power line, the receiver including a cathode ray tube on an element of which the received picture appears, which comprises deriving unidirectional potentials from said power line, supplying said potentials to said receiver, distantly producing and transmitting picture and synchronizing signals, intercepting said signals and supplying them to the receiver, and supplying to the receiver framing impulses recurring at a frequency substantially the same as that of a sub-harmonic of the frequency of the said power line.

WILLIAM A. TOLSON.