US2122974A

US2122974A - Television transmitter

Info

Publication number: US2122974A
Application number: US714001A
Authority: US
Inventors: Browne Cecil Oswald
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1933-03-07
Filing date: 1934-03-05
Publication date: 1938-07-05
Anticipated expiration: 1955-07-05
Also published as: GB419811A

Description

July 5; 1938. c. o. BROWNE 2,122,974

TELEVISION TRANSMITTER Filed March 5, 1934 fLscrequ4a/vsr INVENTOR 6. 0. .5 omvs BY M ATTORNEY Patented July 5, 1938 UNITED STATES PATENT OFFICE TELEVISION TRANSMITTER Great Britain Application March 5, 1934, Serial No. 714,001 In Great Britain March 7, 1933 17 Claims.

The present invention relates to television and the like transmitting systems and is more particularly concerned with a system for transmitting motion picture films.

A film transmitting system is already known in which a motion picture film, whilst being moved uniformly past a scanning slit, is illuminated by light from an are, images of successive strips of the film which appear in the scanning slit being translated over a suitably placed scanning aperture by means of a scanning member such as a mirror drum. Behind the scanning aperture is disposed a light-sensitive device such as a photo-electric cell and electric impulses developed in the cell, which may be called the picture signals, are amplified and transmitted to a receiver.

Synchronizing signals are transmitted in conjunction With the picture signals but the nature of the synchronizing signals depends upon the type of receiver with which images of the motion picture film are to be reconstituted.

If the receiver be of. the cathode ray tube type two synchronizing signals are usually'transmitted.

The frequency of one of these synchronizing signals is equal to the frequency with which lines of a picture are scanned, either at the transmitter or receiver, and therefore may be appropriately called the line synchronizing signal. The other synchronizing signal is of frequency equal to that with which complete pictures are scanned and may therefore be called the frame synchronizing signal.

It has been found that in a cathode ray tube receiver a longer time is required in which to return the ray over a fluorescent screen associated with the tube from its position at the end of a picture to its position at the commencement of a picture than to return the ray from its position at the end of a line to its position at'the commencement of a line, in other words the back stroke in picture scanning takes a longer time than the back stroke in line scanning, and it is an object of the present invention to ensure that, during the comparatively long interval between the scanning of successive complete pictures, no signal is transmitted which might cause an illumination of the fluorescent screen of the cathode ray tube.

According to the present invention, in a film television transmission system in which scanning is effected in one direction by the motion of the film and in a perpendicular direction by the motion of a scanning member such as a mirror drum, there are provided means for suppressing the picture signals during the generation of frame synchronizing signals.

A constructional embodiment of the invention will now be described by way of example, reference being made to the accompanying diagrammatic drawing, in which Fig. 1 is a plan view of television transmitting apparatus constructed and arranged in accordance with the present invention,

Fig. 2 is a view, in side elevation and on a smaller scale, of a shutter M which is shown in Fig. l, and

Fig. 3 is a front elevational View of a screen I which is shown in Fig. 1.

It will be assumed that reception of the transmitted pictures is to be effected with the aid of a cathode ray tube and that the deflections of the cathode ray are to be controlled by means of two synchronizing signals sent from the transmitter.

Referring now to the figures, at a transmitter a motion picture film I is moved uniformly and vertically downwards past a horizontally disposed slit 2, the length of which is equal to the width of the pictures on the film. This slit 2 serves to analyze each picture into strips and will therefore be called a first scanning aperture. The strip of film which at any instant lies in the slit 2 is illuminated by light from a suitably disposed are 3 with the aid of a lens system 4 and light which passes through the film l and first scanning aperture 2 is reflected, from one of ninety plane mirrors 5 disposed upon a rotating mirror drum 6, on to a plane vertical scanning screen 1 situated about six feet from the mirror drum 6. The mirrors are all mounted tangentially about the surface ofv the drum and the latter rotates about a vertical axis. Close to the drum is placed a spherical lens 8 adapted to focus an image of the film strip in the first scanning aperture 2 on to the scanning screen I. In the screen is a second, slit-like, scanning aperture 9 behind which is disposed a photo-electric cell l0 and, as the mirror drum 6 rotates, images (one of which is shown at H) of successive strips of the film which app-ear in the first scanning aperture 2 are translated over this second scanning aperture 9. Corresponding electric picture signals are generated in the photo-electric cell I0 and these are amplified and transmitted to a receiver.

Synchronizing signals are also generated at the transmitter and transmitted over the same channel as the picture signals. The synchronizing signals are generated in the following way:-

Parallel with the first scanning aperture 2, but slightly out of line therewith, is disposed a second aperture I2 (in the form of a slit) which is illuminated uniformly by light from a source I3. A portion of. this aperture I2 is normally obscured by a rotatable shutter I4 and another portion l5 (shown cross hatched in Fig. 2) is at all times unobscured by the shutter 14. The unobscured portion l5 of this aperture l2 will be called the line aperture and the whole of the aperture I2 will be called the frame aperture.

Images of the line aperture l5 are translated, by means of the rotating mirror drum 6, over a line slit I6 disposed in the scanning screen 1 above and slightly to one side of the second scanning aperture 9. One of such line images is indicated at I! in Fig. 3. Behind the line aperture is disposed a photocell (not shown) and its associated amplifier. As the image 11 passes over the line slit IS a very short line synchronizing signal is generated.

The line aperture I5 is so disposed, relatively to the mirrors of the mirror drum 6 and the line slit it, that the images I! cross the line slit 16 during the time that images of the film strip are passing over the second scanning aperture 9. Line synchronizing signals are thus generated simultaneously with the picture signals, but subsequent treatment of the signals is such that there occurs a delay of the synchronizing signals relative to the picture signals in such manner that by the time the line synchronizing signals are effective in controlling the scanning at the receiver, the line signals occur in the intervals be tween picture signals.

Geared to the film sprocket (not shown) which draws the film l past the first scanning aperture 2 is the rotating opaque shutter M, the axis of which is disposed perpendicularly to the plane of the film I. The gearing between the film sprocket and shutter is such that while one complete picture on the film moves past the first scanning aperture 2, the shutter l4 performs half a revolution.

The shutter I4 comprises a central disc l8 and an outer annular ring IS, the ring being spaced apart from the disc by two arms 20 and 2! which are symmetrically disposed with respect to the centre of the disc IS. The edge of the disc l8 bears two symmetrically disposed

notches

22 and 23 and the outer edge of the ring l9 also bears two symmetrically disposed

notches

24 and 25. The gearing between the film sprocket and the shutter is such that, for every half revolution of the shutter M, an arm 20 or 2| obscures the first scanning aperture (and therefore of course the second scanning aperture 9) whilst a

notch

24 or 25 uncovers the whole of the frame aperture l2. The covering of the scanning aperture 2 and the uncovering of the frame aperture 12 commence simultaneously and last for the same periods of time. a

The effect of covering the scanning aperture 2 is, of course, to suppress all picture signals.

The uncovering of the frame aperture lasts for a time about equal to that necessary to scan five lines and during this time five images of the frame aperture are swept over the line slit I6 at the screen 1'. One such image is shown at 2B in Fig. 3 and the line image ll forms a part of the image 26. In this manner five electrical pulses with short intervals between them are generated in the synchronizing cell Ii] at the end of the scanning of each complete picture on the film and each of these five pulses is of longer dura tion than one line synchronizing pulse although of the same amplitude. The five separated broad pulses together form what will be called a frame synchronizing pulse.

The operation of the transmitter as described so far is thus as follows:

Trains of picture signals are developed in the photo-cell [0, each train corresponding to a line on a picture of'the film. Simultaneously with each train there is generated in a second, or synchronizing, photocell a line synchronizing signal, this signal occurring when its corresponding picture train is about half complete. In this same synchronizing photo-cell there are generated frame signals, these occurring in the intervals between the scanning of successive whole pictures. Picture signals are suppressed during the generation of frame signals by means of the arms 20 and 2| of the rotating shutter I l.

The effect of suppressing the picture signals at the end of the scanning of each complete picture is the same as giving to the picture signals (dur ing this time) a wave form corresponding to black or nearly black. The picture signals and both sets of synchronizing signals are transmitted to a receiver over one channel and the synchronizing signals are inserted into the channel in the opposite, or what may be called the blacker than black direction, from the picture signals. In other words, the picture and synchonizing signals are inserted into the transmission channel on opposite sides of a datum line corresponding approximately to black in the picture.

The advantages of this transmitting system can be appreciated from a contemplation of the cathode ray tube receiving system.

The three received sets of signals are amplified l and applied between the cathode and a modulating electrode of the cathode ray tube.

The picture signals modulate the intensity of the ray above and below a certain average value, whilst the frame and line signals, being applied between the cathode and modulating electrode in the dark direction, tend to reduce the intensity of the ray towards zero.

In addition, the line and frame signals are separated in any known or suitable manner and the line signals are used to control the frequency of a time base device which in turn controls the scanning of lines on a fluorescent screen associated with the tube whilst the frame signals are used to control the frequency of a second time base device which in turn controls the scanning of complete pictures on the fluorescent screen.

At the end of the scanning, or reconstitution, of each picture the ray moves diagonally across the fluorescent screen to the starting point for a succeeding picture under the control of a frame signal.

During this time the wave form of the picture signals corresponds to black or nearly to black. Each of the five pulses which comprise a single frame synchronizing pulse reduces the intensity of the ray completely to zero but in between each of the five pulses the screen may possibly be not quite black and in order to ensure absolute blackness of the screen in between pictures an auxiliaryblack0utsignal is generated in between suc cessive whole pictures and the frame signals are superimposed on this blackout signal. The blackout signal is of course inserted into the transmission channel in the blacker than black direction and is a broad. pulse lasting for the whole time between pictures.

The effect of superimposing a frame pulse on a blackout pulse is that at the receiver, the intensity of the ray remains at zero even between the separate pulses which make up one frame synchronizing pulse.

The blackout pulses are generated at the transmitter in the following way:

The two

notches

22 and 23, cut on the central disc [8 of the shutter 54 co-operate with a beam of light passing through an aperture (or an image of an aperture) H. The aperture is covered during the generation of picture signals and is covered in between the scanning of successive complete pictures and during the generation of frame synchronizing signals. Light passing through the aperture is reflected from a stationary reflector 4| into the synchronizing cell 43 so that, in effect, the frame signals are superimposed upon a broad blackout signal, the frame and blackout signals both being inserted into the transmission channel in the blacker than black direction.

The light passing through the film l and first scanning aperture 2 is very intense and precautions are therefore taken to ensure that if the film drive fails the film does not become overheated.

The shutter M is driven by gear 28, which is loose on the shutter shaft 29, through a spring 30. A safety shutter 3! is also loosely mounted on the shaft 29. The shape of this safety shutter is as shown by the dotted lines in Fig. 2. Two

arms

32 and 33 of the safety shutter are adapted to cover or uncover the first scanning aperture 2 and the blackout aperture 21 respectively. Angular movement of the safety shutter is limited suitably by stops 34 and 35. The safety shutter is counterweighted so that, normally, it covers the apertures 2 and 2'3. The safety shutter bears a friction pad 36 which is held in engagement with shutter It by means of the collar 31 slidably mounted on shaft 29. The collar 37 is operated by a system of levers controlled by an electromagnet 38. The electromagnet 38 is run in parallel with the light source l3 which is utilized to generate the frame and line synchronizing signals.

Assuming the film to be stationary and the source l3 to be out the operation of the device, on commencing to scan a film, is as follows:

On starting up the film drive, the shutter M commences to rotate, but the safety shutter 3!, not being pressed into frictional engagement with shutter l4, obscures both

apertures

2 and 2?. 0n lighting up the lamp is, however, electromagnet 38 is energized and collar 31 is moved in such manner that the pad 35 is forced against shutter I4 and the safety shutter 3| is rotated so as to uncover the

apertures

2 and 21. Thus light can only reach the film provided simultaneously the lamp i3 is on and the film drive is in operation.

The spring 3!! operates as a mechanical filter between a driving motor for the system and shutter l4 and the friction pad 36 serves to damp the mechanical filter.

The safety shutter is designed so as not to cover the frame and line apertures l2 and I5 when the driving mechanism stops so that the generation of synchronizing signals continues and, in the case where a cathode ray tube receiver is used, the ray is prevented from coming to rest and so burning a hole in the fluorescent screen during intervals between the transmission of say, complete motion picture films.

I claim:

1. Film television transmitting apparatus comprising means for moving a film at substantially uniform speed past a member having ascanning aperture, a light sensitive device, a scanning device adapted to sweep light representatives of the optical values of said film over the light sensitive device, means having a frame aperture, means having a black-out aperture, means for projecting separate beams of light through said frame and black-out apertures respectively once for every complete scanning of the object, and a rotatable shutter geared to the film moving means having means to obscure said scanning aperture and uncover said frame and black-out apertures substantially simultaneously and for substantially equal periods of time.

2. Apparatus as claimed in claim 1, wherein said shutter comprises a central disc and an outer ring spaced apart from said disc by two arms disposed symmetrically with respect to the centre of the disc, the edge of said disc and the outer edge of said ring each being provided with a notch, and, during rotation of the shutter, the notch on said disc being adapted to cover and uncover alternately the black-out aperture, the notch on said ring being adapted to cover and. uncover alternately the frame aperture and each of said arms being adapted to cover and uncover alternately the scanning aperture.

3. Apparatus as claimed in claim 1, wherein there is provided an angularly movable safety shutter which is adapted normally to cover the scanning aperture and means for holding said safety shutter in frictional engagement with said rotatable shutter, the arrangement being such that on rotation of said rotatable shutter the safety shutter is moved so as to uncover said scanning aperture.

4. The apparatus as claimed in claim 1 comprising, in addition, an angularly movable safety shutter adapted normally to cover said scanning aperture, electromagnetic means for holding said safety shutter in frictional engagement with said rotatable shutter, said arrangement being so arranged that on rotation of said rotatable shutter the safety shutter is moved so as to uncover said scanning aperture, and means for controlling the electromagnetic means from the same source of current as supplies the light utilized for producing synchronizing signals.

5. A television transmitting device comprising an optically-slotted means for scanning an optical view by dividing the view into a series of linear portions, means for projecting individual linear portions one at a time past the optical slot, a rotating cylindrical member in the path of the light issuing beyond the optical slot, a plurality of mirrors supported on the periphery of the rotating cylindrical member, photo-electric means arranged to receive the reflected light from the mirrors, means for simultaneously developing a set of line synchronizing impulses, means for developing a set of frame synchronizing impulses, means for blacking-out said optical view while said frame synchronizing signals are being developed, means for developing an auxiliary signal corresponding to black in the optical view, and means for transmitting the picture and frame signals sequentially.

6. A television transmitting device comprising an optically-slotted means for scanning an optical view by dividing the view into a series of linear portions, means for projecting individual lin ear portions one at a time past the optical slot, a rotating cylindrical member in the path of the light issuing beyond the optical slot, a plurality of mirrors supported on the periphery of the rotating cylindrical member, photo-electric means arranged to receive the reflected light from the mirrors, means for simultaneously developing a set of line synchronizing impulses, means for developing a set of frame synchronizing impulses, means for blacking-out said optical View while said frame synchronizing signals are being developed, means for developing an auxiliary signal corresponding to black in the optical view, means for combining the auxiliary signals and the frame synchronizing signals to form signals corresponding to blacker-than-black picture signals, and transmitting the synchronizing signals and the picture signals sequentially.

7. In a television transmitter for transmitting optical views recorded on a pliant medium, opti cally-slotted means for scanning an optical view by dividing the view into a series of linear portions, means for projecting individual linear portions one at a time past the optical slot, a rotating cylindrical member in the path of the light issuing beyond the optical slot, a plurality of mirrors supported on the periphery of the rotating cylindrical member, photo-electric means arranged to receive the reflected light from the mirrors, means for simultaneously developing a set of line synchronizing impulses, means for developing a set of frame synchronizing impulses, means for blacking-out said optical view while said frame synchronizing signals are being developed, means for developing an auxiliary signal corresponding to black in the optical view, safety means for shielding the pliant medium adapted to shield the medium upon non-operativeness of the driving means for the medium, and means for transmitting the picture signals and synchronizing signals sequentially. 8. In a transmitter for an electro-optical picture transmission system in which scanning of the image or object to be transmitted is accomplished by a process involving two scanning motions of different frequencies, a first apertured diaphragm and a second apertured diaphragm, means for illuminating said diaphragms, a third apertured diaphragm associated with a photo-cell, means for translating images of the apertures in said first and second diaphragms over the aperture in said third diaphragm, a rotatable shutter arranged to cover the aperture in said second diaphragm with respect to said photo-cell over a portion of each revolution, and means for transmitting electric impulses generated in said photocell to a receiver.

9. In a transmitter for an electro-optical picture transmission system in which scanning of the image or object to be transmitted is accomplished by a process involving two scanning motions of different frequencies, a first apertured diaphragm, means for illuminating said diaphragm, a second apertured diaphragm associated with a photocell, means for translating images of the aperture in said first diaphragm over the aperture in said second diaphragm, a rotatable shutter arranged to cover a part of the aperture in said first diaphragm with respect to said photo-cell over a portion of each revolution, and means for transmitting electric impulses generated in said photo-cell to a receiver.

10. In a transmitter for an electro-optical picture transmission system in which scanning of the image or object to be transmitted is accomplished by a process involving two scanning motions of different frequencies, an apertured picture diaphragm, a first photo-cell, apertured first and second synchronizing diaphragms, a third apertured synchronizing diaphragm associated with a second photo-cell, means for illuminating said picture diaphragm and first and second synchronizing diaphragms, means for translating images of the apertures in said picture diaphragm and said first and second synchronizing diaphragms over said first photo-cell and the aperture in said third synchronizing diaphragm respectively, a rotatable shutter arranged to cover the aperture in said second synchronizing diaphragm with respect to said second photo-cell over a portion of each revolution and, over another portion of each revolution, to cover the aperture in said picture diaphragm with respect to said first photo-ceil, and means for transmitting electric impulses generated in said photo-cells to a receiver.

11. In a transmitter for an electro-optical picture transmission system in which scanning of the image or object to be transmitted is accomplished by a process involving two scanning motions of diiferent frequencies, an apertured blackout diaphragm, a first photo-cell, apertured first and second synchronizing diaphragms, a third apertured synchronizing diaphragm associated with a second photo-cell, means for illuminating said black-out diaphragm and first and second synchronizing diaphragms, means for imaging the aperture in said black-out diaphragm on said first photocell, means for translating an image of the apertures in said first and second synchronizing diaphragms over the aperture in said third synchronizing diaphragm, a rotatable shutter arranged to cover the aperture in said black-out diaphragm and the aperture in said second synchronizing diaphragm with respect to said photocells over a portion of each revolution and to uncover the apertures in said black-out and said synchronizing diaphragms over another portion of each revolution, and means for transmitting electric impulses generated in said photo-cells to a receiver.

12, In a transmitter for an electro-optical picture transmission system in which scanning of the image or object to be transmitted is accomplished by a process involving two scanning motions of different frequencies, an apertured blackout diaphragm, means for illuminating said black-out diaphragm, first light-responsive means, means for imaging the aperture in said black-out diaphragm on said light responsive means for generating black-out impulses, a first apertured picture diaphragm, means for illuminating said first picture diaphragm, second lightresponsive means, a second apertured picture diaphragm associated with said second light-responsive means, means for translating images of the aperture in said first picture diaphragm over the aperture in said second picture diaphragm for generating picture signals in said second lightresponsive means, apertured first and second synchronizing diaphragms, means for illuminating said first and second synchronizing diaphragms, a third synchronizing diaphragm associated with said first light-responsive means, means for translating images of the apertures in said first and second synchronizing diaphragms respectively over the aperture in said third synchronizing diaphragm for generating synchronizing impulses in said first light-responsive means, a rotatable shutter arranged to cover the aperture in said black-out diaphragm and the aperture in said second synchronizing diaphragm with respect to said first light-responsive means over a portion of each revolution and to uncover the apertures in said black-out diaphragm and said second synchronizing diaphragm while covering the aperture in said picture diaphragm with respect to said second light-responsive means over another portion of each revolution, and means for transmitting electric signals generated in said first and second light-responsive means toa receiver.

13. In a transmitter for an electro-optical picture transmission system, as claimed in claim 12 wherein the means for translating images of the aperture in said first picture diaphragm over the aperture in said second picture diaphragm includes a rotatable mirror drum.

14. In a transmitter for an electro-optical picture transmission system in which scanning of the image or object to be transmitted is accomplished by a process involving two scanning motions of different frequencies, means for generating a series of synchronizing impulses comprising means for producing identically steep initial portions of said impulses at the rate of the higher of said frequencies, means for prolonging the duration of predetermined groups of impulses in the series, and means for limiting the prolonged duration to less than the period of recurrence of the impulses, and means for transmitting said impulses to a receiver.

15. In a transmitter for an electro-optical picture transmission system in which scanning of the image or object to be transmitted is accomplished by a process involving two scanning motions of diflerent frequencies, means for generating a series of synchronizing impulses comprising means for producing identically steep initial,

portions of said impulses at the rate of the higher of said frequencies, means for prolonging the duration of predetermined groups of impulses in the series, and means for limiting the prolonged duration to less than the period of recurrence of the impulses, means for repeating the prolonged impulses at a rate equal to the lower frequency,

and means for transmitting said impulses to a receiver.

16. In a transmitter for an electro-optical picture transmission system in which scanning of the image or object to be transmitted is accomplished by a process involving two scanning motions of different frequencies, means for generating a series of synchronizing impulses comprising means for producing identically steep initial portions of said impulses at the rate of the higher of said frequencies, means for prolonging the duration of predetermined groups of impulses in the series, and means for limiting the prolonged duration to less than the period of recurrence of the impulses, means for generating black-out impulses each of which has a duration at least equal to that of one of said groups, means for combining the black-out impulses with synchronizing impulses in the same sense and in such phase that said groups of synchronizing impulses of longer duration are superimposed upon the black-out impulses, and means for transmitting the synchronizing impulses together with the black-out impulses to a receiver.

17. In a transmitter for an electro-optical picture transmission system in which scanning of the image or object to be transmitted is accomplished by a process involving two scanning frequencies, means for generating synchronizing impulses, comprising means for producing identically steep initial portions of said impulses at the rate of the higher of said frequencies, means for prolonging the duration of predetermined groups of impulses in the series, means for providing at least two successive prolonged impulses in said groups, and means for limiting the pro longed duration to less than the period of recurrence of the impulses, and means for transmitting said impulses to a receiver.

CECIL OSWALD BROWNE'.