US1955320A - Television synchronization method - Google Patents

Description

April 17, 1934. J. c. BATCHELOR ET Al. 1,955,320

TELEVISION SYNCHRONIZATION METHOD Filed NOV. 16. 1929 2 Sheets-Sheet l ATTORNEY April 17, 1934- J. c. BATcHr-:LOR Er AL 1,955,320

TELEVISION SYNCHRONIZATION METHOD Filed Nov. 16, 1929 2 Sheets-Sheet 2 ATTORNEY Patented Apr. 17, 1934 UNITED STATES PATENT OFFICE TELEVISION SYNCHRONIZATION IVIETHOD Pennsylvania Application November 16, 1929, Serial No.407,699

2 Claims.

Our invention relates to facsimile-transmission systems, and it has particular relation to systems especially adapted for the reproduction, at a distance of moving-picture films with, or without, sound accompaniment.

In the copending application, Serial No. 376,- 117, filed July 5, 1929, in the name of Vladimir K. Zworykin and assigned to the Westinghouse Electric and Manufacturing Company, there is disclosed an improved transmission system wherein synchronism between a scanning device at a sending station and an analogous device, comprising a cathode-ray tube at a receiving station is automatically maintained.

In the system described in the said copending application pictures are transmitted and synchronism between the scanning devices is maintained by the utilization of only a single radio or Wire channel.

The object of our invention is to provide an improved system for maintaining the scanning devices at the transmitting and receiving stations in synchronism in a system of the abovementioned type.

In practicing our invention, we prefer, at the sending station, to cause a motion picture lm, or the like, to travel at a fixed rate of speed between an oscillating, curved mirror and a photoelectric cell. Light from a fixed source is focused by the mirror to a ne point that moves from side to side transversely of the moving lm and, after traversing the said film to be modulated thereby, falls upon the photoelectric cell cathode.

The lens system is so designed that, in the absence of the moving film, an image of the mirror is focused upon the cathode of the photoelectric cell, which image does not move during the oscillation of the mirror.

At the sending station, the mirror is oscillated. by means of a portion of the output current from an alternating current generator. Another portion of this output current is introduced into the input of the transmitting station so that it appears in the final output of the transmitter for transmission to the scanning device at the receiving stations.

At each receiving station, we provide a cathode-ray tube having a plurality of cathode-ray controlling devices associated therewith. One of the controlling devices functions to modulate the intensity of the ray in accordance with the received frequencies. Another controlling device functions, under the control of the frequency generated by the alternating current generator at the transmitter, to determine the speed of horizontal movement of the cathode ray. Another of the said control devicesv is periodically deenergized in response to an impulse sent out by the sending station each time the space between two pictures on the film being transmitted passes before the scanning beam. The last named controlling device controls the periodicity and speed of the vertical movement of the cathode ray and automatically adjusts the framing of the received picture.

The novel features that we consider characteristie of our invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof will best be understood from the following description of a specic embodiment when read in connection with the accompanying drawings, in which:

Figure 1 of the drawings is a view, partly in schematic perspective, and partly diagrammatic, of the essential elements constituting a film-'facsimile sending station comprising a` preferred embodiment of our invention;

Fig. 2 is a diagrammatic view of the essential elements of a receiving station comprising a preferred embodiment of our invention;

Fig. 3 is a view of a portion of a sound-andpicture film in its passage through the transmitting apparatus;

Fig. 4 is a diagram showing the mannerin which scanning frequencies are utilized in the system, and

Fig. 5 is a diagram to which reference will hereinafter be made in describing the manner in which synchronism is secured. l

The apparatus illustrated in Fig. 1 comprises lm drums and 11 over which a motion-picture film 12 is drawn by means of a driving sprocket 13. Other incidental apparatus, such as filmguiding rollers, film-magazines, lm-gates, sources of motive power, belting or gearing, etc., forming no part of our invention and, further, being well known to those skilled in the art, have not been illustrated.

Light from a fixed source 14 is reflected, as a fine point, onto the film 12 from a curved mirror 15, the focus of which lies in the plane of the film. The mirror 15 is supported for movement about a vertical axis parallel to the film 12 and a suitable magnetic structure 16, capable of being energized from a source 17 of alternating current is provided for causing the mirror 15 to so oscillate that the reflected ray 18 rapidly moves from side to side. transversely of the film. In the example illustratedy the output of the source 17 has a frequency of 750 cycles per second.

The magnetic structure 16 comprises a magnetic core 19 having n winding 20 thereon connected to a battery 21 to form an electromagnet. The core 19 has four pole pieces 22 projecting therefrom. These pole pieces have windings 23 thereon which are connected in series and supplied with current from the alternating current generator 17 through the conductors 24. A vertical rod 25 is rigidly secured at its upper` end to a supporting member 26 which is attached to the core 19 while the lower end of the rod 25 is pivoted in a supporting member 27. The curved mirror 15-is rigidly attached to the rod 25 in any suitable manner.

A magnetic vane 28 is attached to the lower end of the rod 25 and positioned between the pole pieces 22 to vibrate the rod in accordance with current variation in the coils 23 surrounding the pole pieces. It will be noted that the coils 23 are so wound that the pole pieces act simultaneously to turn the vant` 28 in a given direction.

It also lies within the scope of our invention to afiix the mirror to a tuned reed, or to one of the tines of a tuning fork, and to excite the reed or fork from an oscillation generator of any desired type.

A photoelectric cell 29 is disposed behind the lm 12 to receive the light which is reflected therethrough from the oscillating mirror 15. A spherical lens 30, or a lens system, is interposed between the film 12 and the photoelectric cell 29, the characteristics of the lens 30 being so chosen that in the absence of the film, an image of the entire mirror 15 is focused on the cathode 31 of the cell. Movement of the mirror 15, therefore, does not cause movement of the light spot on the cathode, from side to side and, consequently, the most sensitive portion of the cathode may be continuously utilized.

In order to prevent the light ray 18 from overshooting at the transparent margins of the picture, whereby bright lines would appear on the receiving screen, and to prevent a sound-record, if present on one of the edges of the film, from interfering with the transmission of the pictures, we preferably interpose an opaque mask 32 between the mirror 15 and each of the said edges, as also indicated in Fig. 3.

The photoelectric cell 29 is connected to the input of an amplifier 33, the output of which is connected to the input of a second amplier 34 through the transformer windings 35 and 36 and condensers 37 and 38 and through the filter 39.

The lter 39 is of a well known type comprising inductance coils and condensers and designed to prevent the transmission of the 750 cycle current therethrough and to pass all other frequencies. The transformer windings 35 and 36 are by-passed by the condensers 37 and 38, respectively.

The amplifier 34 is followed by a modulator 40 which, in turn, is so connected to a carrierwave oscillation-generator 41 as to modulate the output thereof. The oscillation generator 41 is operatively associated with a radiating system which may comprise an antenna 42, a tuning condenser 43, a coupling inductor 44 and a ground connection 45 or it may be coupled to a power line (not shown) through coupling devices of types well known to those skilled in the art.

One terminal of the primary winding 46 of the rst transformer is connected to one of the output terminals of an alternating current generator 47 which, in the example illustrated, generates a current having a frequency of 4000 cycles per second. The other terminal of the primary winding is connected to a fixed contact point 48. The fixed contact point 48 is normally disassociated from a movable contact point 49 carried by a pivotally supported arm 50. One end of the arm 50 is slightly curved upwardly and is disposed in the path of travel of a plurality of camsurfaces carried by the disc 51 which is mounted upon a rotatably supported shaft 53. The shaft 53 is connected through a conductor 54 to the other output terminal of the 4000 cycle generator 47.

In addition to the disc 51, the shaft 53 carries the sprocket-wheel 13 having teeth which are spaced to correspond with marginal openings in the film, the said sprocket-wheel being so disposed with respect to the lm that when it is rotated the film is driven past the scanning ray 18.

The number of teeth in the sprocket-wheel 13 is so correlated to the number of cam surfaces that, at each instant when the space between any two picture-frames passes through the path of travel of the scanning ray 18, the pivoted arm 50 is raised to complete a circuit permitting potentials at the 4000 cycle frequency of the alternating potential source 47 to be impressed across the primary winding 46 of the transformer and, consequently, to induce corresponding potentials across the input circuit of the amplifier 34.

In order that framing of the picture may be expeditiously accomplished at the transmitter, in the event that the sprocket holes are not properly spaced with respect to the picture-elements, it is desirable to provide the arm 50 with supporting means (not shown) whereby it may be so adjusted, circumferentially of the disc 51, that the framing impulses occur earlier, or later, in the travel of the lm 12.

In the operation of the sending station just described, the film 12, in its travel from the storage-reel to the take-up reel, is subjected to the oscillating scanning ray 18 from the mirror 15, which ray, after its passage through the lm, falls upon the photoelectric cell 29 to modulate the output therefrom in accordance with the density of the lm 12.

When the transparent space between two frames passes through the path of the scanningway 18, the circuit including the 4000 cycle source 47 is closed by the movement of the pivoted arm 50 and the output of the transmitter, accordingly, is modulated at the frequency of the said source. The manner in which the periodic modulation of the carrier-wave is utilized, at the receiving stations, will be explained, in detail, later.

'Ihe horizontal scanning at the receiver is accomplished by inserting in the input of the amplier 34 a portion of the output from the 750 cycle generator 17. The 750 cycle current is supplied to the input of amplifier 34 through the primary winding 55 and the line 56.

Since the synchronizing or scanning frequency of '750 cycles modulates the same carrier-wave as the picture frequency, it is necessary to prevent any 750 cycle frequency which might be in the picture frequency from appearing in the transmitter output. This particular picture frequency is suppressed by the lter 39. It has been found L' that one frequency, or a very narrow band of frequencies, may be removed from the picture frequency band without causing noticeable distortion at the receiver.

The character of the modulated carrier-wave y which is transmitted is indicated in Fig. 4. The

curveA represents the wide band of picture fre- '..mitted onlyl during the period that the picture frequencies represented by curve A are not being transmitted. This follows from the fact that the framing frequency of 4000 cycles is transmitted only during those instances when the opaque space between two frames passes through the path of the scanning ray 18. The synchronizing frequency of 750 cycles is transmitted continuously.

The apparatus at a receiving station as shown in Fig. 2 comprises an energy receptor'constituted by an antenna 57, a tuning condenser 58, an inductor 59 and a ground connection. The energy receiving circuit is coupled to a radiofrequency amplifier 60, of any suitable type, which amplifier is followed by a detector-amplifier 61. The output from the detector-amplifier 61 is impressed upon a filter 62 which may be substantially a duplicate of filter 39. The filter 62 suppresses the 750 cycle scanning frequency and passes the remaining frequencies. The output of the filter 62 is connected to a transformer 63.

The receiving apparatus further comprises a cathode-ray tube 64 having a thermionic cathode 65, a control-electrode 66, an anode 67, magnetic pole pieces 68 for defiecting the electronstream in a direction indicated by the arrows A-B, and a pair of plates 69 for defiecting the electron stream in a direction C--D at right angles to the direction A-B. The control electrode 66 of the cathode-ray tube is connected to the cathode through a source 70 of biasing potential and the secondary winding 71 of the transformer 63.

The pole pieces 68 extend from a closed core 72 which has windings 73 on the legs thereof. It has been found that this form of deflecting structure will deflect the electron stream the required amount with less amplification of the '750 cycle current than is required with other structures.

Obviously, any other well known arrangement for deiiecting the electron stream in two directions at 'right angles to each other may be utilized.

The particular arrangement illustrated is preferred when the tube contains an accelerating and focusing anode as described in application Ser. No. 407,652, filed Nov. 16, 1929, in the name of Vladimir K. Zworykin and assigned to the Westinghouse YElectric & Manufacturing Coni,- pany.

The potential gradient between the control electrode 66 of the' cathode-ray tube and the cathode 65 thereof comprises two components, a direct current biasing-potential from the source and an alternating potential representative of the picture-modulation of the incoming signal from the transformer 63. Since the magnitude of the electron stream between the cathode 65 and the anode 67 is a function of the potential between the control electrode 66 and the cathode, and since the brilliancy of the fluorescence of the screen 74 at the end of the tube is at every instant a function of the magnitude of the electron stream, or cathode ray, passing through the anode, the said brilliancy is always proportional to the picture modulation.

The windings 73 of the deiiecting magnet are connected to the output of an amplifierv 75 which is sharply tuned to the scanning ,frequency of 750 cycles. 'I'he input of the amplifier 75 is coupled to the input (preferably the coil 76) of the filter 62 by means of a winding 77. 'I'hus it is seen that the deecting coils 73 are supplied with a 750 cycle current which is transmitted from the sending station to deflect the electron stream in the direction of the arrows A-B in synchronism with the deflection of the scanning-ray 18 at the transmitter.

One of the defiecting plates 69' is connected to ground. The opposite plate 69 is connected by a conductor 78 to one plate of a condenser 79 which plate is connected to the anode 80 of a thermionic rectifying device 8l. The cathode 82 of the rectifier 81 is connected to ground through a source of potential 83. The condenser 79, preferably shunted by an adjusting-condenser 84, is connected in shunt to the condenser constituted by the deflecting plates 69 and, therefore, in shunt to the cathode and anode of the rectifying device 81.

The function of the rectifying device 81 is to build up a charge upon the condensers 79 and 84 and upon the deflecting plates 69 11i order that the Cathode ray shall be deflected in the direction of the arrows C-D proportionally thereto. The deflection of the said ray in the direction C-D corresponds to that of the scanning operation at the transmitting station which is introduced by the motion of the film longitudinally thereof. The size of the adjusting condensers '79 and 84 and the potential of the source 83 are so chosen that the charge upon the deflecting plates 69 in the cathode-ray tube will build' from zero to the desired maximum during the time required for the film at the transmitter to travel a distance corresponding to the advance of a single picture frame past the path of the scanning-ray.

The charging current is limited by the emission of the cathode of the tube 8l which may be adjusted by the filament rheostat 85. Therefore, the condensers are charged at straight linear chargle as shown in Fig. 5.

Accordingly, if at the instant the light from the oscillating mirror 15 starts to traverse a given picture frame, the charge upon the defiecting plates 69 is zero, or substantially zero, the successive portions of the path traced by the scanning ray 18 on a single picture-frame of the film will be represented by successive shifts of the electron stream in the cathode-ray tube in the direction of the arrows C-D.

In order, therefore, that the deflecting plates 69 shall be com'pletely discharged at the beginning of the scanning operation of each individual picture-frame at the transmitter, means are provided whereby their charge is controlled by the framing-frequency impulses sent out by the transmitting station each time the framingfrequency generator 47 is permitted to modulate the carrier-wave through the action of the camdisc 51 and the pivoted contact device associated therewith.

The charge-control means for the deflecting plates 69, preferably, comprises a short-circuiting device 86 connected in shunt thereto and responsive to the framing-frequency impulses. For this purpose we preferably employ a threeelectrode thermionic tube 87. having an anode 88, a control electrode 89 and a cathode 90 heated by a battery 91.

The input circuit of the thermionic tube` 87 comprises a resistor 92 and a source 93 of p0- tential for.so negatively biasing the grid of the tube 87 that normally there is no flow of current therethrough. The resistor 92 is coupled through a condenser 94 to a tuned circuit comprising an inductance coil 95 shunted by a variable condenser 96.

The circuit which supplies the input of the short-circuiting tube 87 with the framing frequency impulses includes an amplifier 97 which is connected to the output of the filter 62 through conductors 98 and 99. the conductor 99 containing an inductance coil 100 and a condenser 101 in series tuned to the 4000 cycle frequency.

The output of the amplier 97 is connected to a filter 102 designed to pass a frequency of 4000 cycles only. The output of the filter 102 is connected to the input of a double-wave rectifier 103, the output of which, in turn, is connected to an inductance coil 104 coupled with coil 95. In this way, a frequency of 8000 cycles is impressed on the input of the vacuum tube 87. 'I'he frequency doubler circuit is employed because filters can be more easily designed for the lower frequencies while, at the same time, the pulses which make the tube 87 conductive should be spaced apart by a very small time interval in order to discharge the condenser 79 at substantially the instant that the 4000 cycle current is transmitted from the sending station.

Each incoming impulse at double the framingfrequency, impressed across the resistor 92, overcomes the bias from the source 93, causes the thermionic tube 87 to assume the conductive State and to, accordingly, deprive the deflecting plates 69 of the charge they have acquired during the scanning of a single picture-frame at the transmitter.

The periodic charge and discharge of the deflecting plates 69 is illustrated graphically in Fig. 5, wherein distances on the Yaxis represent potentials existing across the said plates, and distances across the X axis are representative of the time.

From an inspectfdj'l of Fig. 5, it will be noted that the potential across the plates 69 is indicated as rising gradually from zero to its maximum in a period of one-twentieth of a second,

i and then dropping quickly to zero to again repeat. The charging period indicated corresponds to a film speed of substantially 20 pictures per second. which speed is satisfactory for the transmission of film-facsimiles with the illusion of motion. If it is desired to transmit a combined picture and sound lm, the lm speed may be increased and the condensers may be reduced in capacity proportionally in order that and description, modifications thereof will be apparent to those skilled in the art. Our invention, therefore, is not to be restricted except insofar as is necessitated by the prior art or by the spirit of the appended claims.

We claim as our invention:

1. In facsimile-transmission system, a moving picture lm, means for scanning the individual frames on said film, means under the control of said lm for generating a framing frequency during the interval between the scanning of successive frames, means for generating electrical energy which is modified in accordance with the characteristics of the object to be transmitted and the framing frequency, said energy including a wide band of frequencies, a source of alternating 'current of a frequency within said bank of frequencies, means for suppressing from said band of frequencies a frequency which is substantially equal to that generated by said source, and means for modulating a carrier wave by the current from said source and by the unsuppressed frequencies of said band, a receiving station including means for generating a beam of energy` means for varying the intensity of said beam of energy in accordance with the modulation frequencies due to the scanning of said frames, and means for vibrating said beam of energy in accordance with the combined effect of the frequency of current from said source and said framing frequency.

2. A facsimile-transmission system comprising means for transmitting a carrier current. means for generating a band of frequencies in accordance with an object to be transmitted, means for suppressing one ofthe frequencies within said band, means for modulating said carrier current by a scanning frequency substantially equal to said suppressed frequency, means for modulating said carrier current by the unsuppressed frequencies of said band, a cathode ray tube including control means for controlling the intensity of the cathode ray and deecting means for deecting the cathode ray, means for receiving said modulated carrier current, means for suppressing said scanning frequency and impressing the unsuppressed frequencies upon said control means, and means for impressing said scanning frequency upon said deecting means.

JOHN C. BATCHELOR. ARTHUR W. VANCE.

Images