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Publication
_________________________________________________________________
Number GB404020A
i»?
Title
_________________________________________________________________
EN Title Improvements in or relating to television
Abstract
_________________________________________________________________
19156.
[Class 40 (iii).] To compensate for differences in mean density of a
kinematograph film, the size of a fixed scanning aperture or the
intensity of illumination is controlled so that the average values of
the direct and alternating components of the picture signals may be
kept constant.
At a transmitter the light-source 3 and amplifier 8 operate at their
maxima, and the width of the stationary aperture 6, which scans the
film 1 in co-operation with the mirror drum 4, is varied either
manually or automatically by movement of the shutter 9.
To this end portion of the picture current, after rectification at 13,
may be applied to an obturating device which comprises a moving-coil
galvanometer with a vane 17, Fig.
2 counterbalanced by a weight 19 and shaped so as to vary the width of
the aperture 6.
A potentiometer 15, Fig.
1, for adjusting the zero
sensitivity are provided.
and
a
resistance 16 for adjusting the
The natural frequency of the system is about 8 c.
/sec.
Alternatively a moving-coil loud-speaker may control the shutter, the
diaphragm providing a damping effect ; or a light-valve comprising a
current-carrying ribbon in a magnetic field may be used.
Two sides of the rectangular aperture may be thus controlled, one in
accordance with the amplitude of the alternating component and the
other in accordance with the peak value of the illumination.
Or the size of the aperture may be controlled by an auxiliary cell
illuminated by the total light from the picture or by a pencil of
light passing through a marginal strip of the film.
The rectifier may be biassed so that the device only comes into
operation when the signal-amplitude passes a predetermined limit.
At the receiver a similar arrangement may be adopted ; or the
intensity
of the light-source or an obturator controlling the
light-beam may be varied.
The alternating
and the direct
auxiliary beam.
component
component
of the signals may be controlled as above
of the signals compensated by means of an
Description
_________________________________________________________________
AMENDED SPECIFICATION
Reprinted as amended in accordance with the decision -of the Assistant
Comptroller acting for the Comptroller-General, dated the twentysixth
day-of April, 1935, under Section 21 of the -Patents and Designs Acts,
1907 to 1932.
(The
Amendments
SPECIFICATION
are
shown
Application
Date:
July
6,
Specification Left: Aug 4, 1933.
in
erased
1932
No
and
italic
19156/32
type)
404,30
PATENT
Complete
Mi Accepted: Jan8, 1934.
PROVISIONAL SPECIFICATION
Improvements in or We-, ELECTRIC & MUSICAL INDUSTRIES LIMITED, a
British Company, of l Eayes, Middlesex, and CECIL OSWALD Bnow NE, a
British Subject, of 29, Monk's Drive, West Acton, London, W 3, do,
hereby declare the nature of this invention to be as follows The
present invention relates to television and is more especially
concerned with those systems in which scanning is effected by
translating images of the object to be transmitted over a stationary
scanning aperture.
In most systems of television it is desirable that the optical-images
which are reconstituted at the receiving end should possess as good
definition, or detail, as the eve can appreciate Assuming the speed of
scanning to remain constant, for any given available light intensity,
the greater the definition the less the average brightness of the
reproduced image since an increase in definition is -effected by a
reduction
in the size of-the scanning aperture The degree of
definition which the eye can appreciate in any object is, however, to
a certain extent dependent upon the average brightness of that object
(the brighter the object the greater the amount of detail which is
appreciable)
and therefore if variations occur in the average
brightness of the object -at the transmitter, it is desirable either
to, vary the definition of the transmitted electrical images or to
provide auxiliary means for compensating the natural or inherent
variations in brightness of the object, because it is uneconomic to
transmit a greater degree of definition than the eye can appreciate at
the expense of average brightness.
Variations in average brightness of the object also give rise to the
difficulty that, in the case of the transmission of picture films, for
example, when the average brightness of the pictures is low, inlPrice
1 A¦-l % 43 d relating to Television sufficient picture signal
amplitude is obtained for-a given intensity of illuminant and when the
brightness is high, under the same transmitting conditions -the
picture signal amplitude is too great and, ifthe picture signals and
synchronising signals are transmitted over the same channel, may give
rise to deformation of the reconstituted images owing to inadequate
difference
in
amplitude
between the picture signals and the
synchronising signalsl.
Compensation for variations in average brightness of the object can be
obtained either by varying the intensity of the illuminant under which
the object is scanned, or by varying the amplification produced by the
amplifiers associated with the scanning photo-electric device.
Neither of these methods have been found entirely satisfactory,
however, since in most systems of television it is difficult to,
obtain adequate illumination of the object and adequate amplification
of the picture signals; any diminution of either of these factors is
therefore to be avoided if possible.
According to the present invention television apparatus for the
transmission or reception of images comprises a scanning aperture and
means whereby the size of said aperture can be varied at will or
automatically, in accordance with the average brightness of the image
being scanned.
According to a further feature of the present invention a television
system
comprises
a
scanning
aperture
and
an
electrically
photo-sensitive' cell associated with an amplifier, wherein the
amplifier is such as to give substantially the maximum amplification
of the currents developed in said device which is possible without
introducing a parasitic background effect which is appreciable at
minimum or zero signal strength, and 7,5 so 404,020 wherein means are
provided for varying the size of said aperture.
In carrying the invention into effect we may proceed as follows:It
will be assumed, for convenience, that two picture films are to be
transmitted, the first of comparatively high average transparency, or
average brightness, and the second of comparatively low 1.0 average
transparency.
Both films are scanned by means of the same apparatus Thus the
comparatively transparent film is first passed through a scanning gate
at a uniform speed whilst it is illuminated by transmitted ligbt from
a source of constant intensity By means of a uniformly rotating drum
upon
the
surface
of
which
are arranged, symmetrically and
tangentially, a plurality of plane mirrors, images of each pictlre on
the film are translated over a single rectangular aperture behind
which is disposed a photo-electric cell associated with an amplifier.
The light soufce is adjusted to give its maximum output and the
amplifier is aljusted to give maximum amplification without there
arising an appreciable parasitic background effect due either to noise
generated by valves or to the wellknown " Johnson noise " encountered
in audio-frequency amplifiers and which is due to the resistances
incorporated in thc amnlifying circuit.
The picture signals generated in this way are transmitted, together
with synebronising impulses either by wires or wirelessly and at the
receiving station the picture signals are reformed into images of the
picture film in any known or suitable manner.
Images of the film are also reconstituted at the transmitting station;
this may be achieved by means of scanning apparatus similar to that
above described hut working in the inverse manner The definition of
the transmitted images is judged, bv eye, from this reconstituted
image and alterations of the definition are effected by altering the
size of the scanning aperture.
Alterations in the area of the scanning aperture in the transmitting
apparatus affects the number of elemental areas of each picture which
are transmitted If this number is too, few the definition of the
reconstituted
image is poor although its average intensity is
relatively high:
if the number is too great
reconstituted image than the
there is a greater detail in the
eye can appreciate at the expense of
average brightness, and the probability is that the transmission
channel cannot handle such high detail without serious distortion.
In order, therefore, to secure the best possible definition the size
of the scanning aperture in the transmitting apparatus is reduced
until, as estimated by eye, the reconstituted image shows t Ee best 70
definition.
Tluder these conditions the picture is being transmitted with the
maximum available illumination of the object, maximum permissible
amplification omf 75 the picture signals and with the best definition.
If now the second comparatively opaque film be passed through the
scanning apparatus compensation for its 80 opacity cannot be achieved
bv altering the light source or the amplifier since these are already
giving their maximum outputs, but in accordance with the present
invention compensation is achieved 85 by increasing the area of the
scanning aperture in accordance with the lower average brightness of
the second film.
The increase in area serves to decrease the definition and to increase
the average 90 brightness and is adjusted until the reconstituted
images just attain their best definition for the available brightness.
Uinder these conditions the average amu Iitude of the picture signals
developed 95 in the photo-electric cell is roughly the same as for the
first film.
The second film is thus, also transmitted under the best possible
conditions of maximum brightness maximum amplifi 100 cation and best
definition.
In order that the variation in size of the scanning aperture may be
effected automatically advantage may be taken of the fact that the
average brightness of 105 the image being scanned determines the
average amplitude of the developed picture signals.
The picture signals derived from a picture of average brightness are
there 110 fore divided into two channels One of the two, sets of
signals is fed to a mixing amplifier where the picture signals are
mixed with synchronising signals prior to transmission over a single
channel, and 115 the second set is fed, if necessary after further
amplification, into a rectifier.
The rectified impulses are then arranged to operate, for example by an
electromagnetic device which will be described 120 later, a, shutter
which obscures a part of the scanning aperture In order that the
shutter shall obscure correctly an increasing proportion of the
scanning aperture with increasing picture signal amplitude 125 two
preliminary adjustments are necessary In the first place the correct
size of aperture for a given average amplitude of picture signals' is
obtained by adjusting the zero of the electromagnetically 130 404,020
operated shutter, and in the second place the sensitivity of the
movement of the shutter is adjusted by varying a shunt across coils of
the electromagnetic device operating the shutter until a given change
in picture signal amplitude produces a correct change in aperture
area.
Having once adjusted the apparatus in this way variations in average
brightness of two, films, or in different parts of one film, or in any
object which may be scanned, are automatically allowed for and the
object is at all times transmitted
possible conditions mentioned above.
under
approximately the best
The electromagnetically operated shutter above referred to comprises a
moving coil Galvanometer with a vane substituted for the usual
indicating pointer the vane being arranged to obscure the scanning
aperture in accordance with the movement of the coil The restoring
force of the coil, usually provided by means of a pair of springs, is
adjusted so that the shutter has a period equal to several picture
frames, say, for example, one eighth of a second Damping means may
also be Provided in order to prevent trains of oscillations being set
up.
An alternative form of shutter comprises a moving coil loutdsrneaker
movem ent in which the coil operates the shutter In this case the
dianhragm of the sneaker mav be conveniently retained for damping
nulirposes and if desiredl may be arranged to workl in a dashpot in
order to increase the damping effect.
As an alternative to the moving coil type of shutter two of the
boundaries of the scanning aperture' may be obscured by means of a
light valve comprising a loo-n of conducting ribbon carrving the
rectified picture current impulses' and situated in a magnetic field
Variations in size of the scanning anerture may then be effected by
nassing direct current of strength depending upon the amnlit ude of
the picture impulses, through the conducting loop' of ribbon.
If desired, instead of varying the area of the scanning aperture in
accordance with the variations in picture signal amplitude, the area
may be varied in accordance with, the variations in opacity of the
background of the film, that is to say, in accordance with those
variations in opacity of the film due to, the developing and fixing
processes which the film has undergone This is effected by passing a
beam of light of constant intensity through an edge portion of the
film lying alongside the pictures, into a photo-electric cell.
Alternatively, an auxiliary cell may be disposed so as, to receive
light from the whole of the scanned picture and thus register the
changes in average brightness of the whole picture.
In either case the currents developed in the cell, if necessary after
amplification, are utilised to control an electromagnetically operated
shutter as described above.
The variations in area of the aperture may be effected, in the case
when the aperture is rectangular, by altering any one or more of its
boundaries, but preferably by altering either one or both of those
boundaries lying perpendicular to the direction of scanning thus'
varying the size of the aperture in the diredtion of scanning.
Although the invention has been described as applied to a television
transmitting station, it will be apparent that the average brightness
of the reconstituted image may be varied by varying the area of a
scanning aperture at the receiving station, a nd in those cases where
scanning is effected by translating images of the object over more
than one aperture, all of the apertures may be varied in size.
Dated this 6th day of July, 1932.
REDDIE & GROSE,
London, E C 4.
Agents
for
the
Applicants, 6, Bream's Buildings,
COMPLETE SPECIFICATION (AMENDED)
Improvements in or relating to Television We, ELECTRIC & MUSICAL
INDUSTRIES LIMITED, a British Company, of Hayes, Middlesex, and CECIL
OSWAII) Buow NE, a British Subject, of 29, Monk's Drive, West Acton,
London, W 3, do hereby declare the nature of this invention and in
what manner the same is to be performed, to be particularly described
and ascertained in and by the following statement: 105 The present
invention relates to television.
A fundamental
production of
aperture.
feature of all
relative motion
television scanning systems is the
between an object and a 110 scanning
The terms " object " and " scanning aperture " should be given broad
mean40410204, ings In the ease of transmitting systems, for example,
the object may oe the actual object of which electrical images are to
be transmitted, a motion picture film or an optical image of a real
object, whilst in the case of reception the object is usually a
viewing screen Similarly the aperture may be either a real aperture or
an optical image of a real aperture, such as is used in so-called spot
scanning.
In most known systems, light which passes through the scanning
aperture at the transmitter is utilised to generate picture signals in
a photo-electric device, the signals so generated being amplified and
transmitted to a distant receiver where they are again amplified and
used to control the amount of light which passes through a receiver
scanning aperture on to a screen.
The fineness with which the object is scanned at either end is
determined by the ratio of the size of the object to the size of the
scanning aperture, and it is clearly desirable that this ratio should
be maintained as high as possible.
If, however, it be desired to make any change in this ratio it is
clearly more convenient both at the transmitter and the receiver to
alter the area of the scanning aperture rather than to alter the area
of the object, a diminution of the aperture size effecting an increase
in fineness and an enlargement of the aperture effecting a decrease in
fineness.
A difficulty which is common to most systems of television is that of
passing smicient light through the scanning aperture to give rise to
an adequate picture signal in the scanning photo-electric, cell, a-ld
on account of this difficulty it has been found desirable to set the
amplifier at a certain fixed maximum level of amplification and to
ensure that the picture signal supplied to the amplifier is always of
sufficient strength to override any parasitic background effect
analogous to valve noise or to the wellknown Johnson noise encountered
in audio-frequency amplifiers and which is due to the resistances
incorporated in the amplifying circuit.
During scanning
generated.
the picture signai supplied to the amplifier is that
in the scanning photo-electric cell and this in turn is a true
representation of the fluctuations in the light falling on the cell
Since the light falling on the photo-cell is always of the same sign,
that is to say, varies from zero, corresponding to complete opacity or
blackness, up to some value corresponding to complete transparency,
the waveform of the light signal, and also the corresponding waveform
of
the
electrical
-signal
generated
by the photo-cell, is
unidirectional and fluetuating This waveform may be regarded as the
sum of a direct component and an alternating component The 70 direct
component will have a value equal to the average value of the waveform
over a given time interval, and the alternating component will have an
amplitude equal to the mean departure of the waveform 75 from its
average value To avoid confusion , the term amplitude will only be
used in describing the alternating component of the picture signals,
and the magnitude of the total picture signals 80 will be described by
the term peak value.
If therefore the amplifier associated with the scanning photo-cell is
capable of amplifying alternating current only, then, since the direct
component of the 85 picture signal is not transmitted through the
amplifier, in order to override the background effect, it is,
necessary to control only the alternating component of the picture
signal supplied to the ampli 90 fler so that it remains at a
sufficient amplitude On the other Tiand if the amplifier is capable of
amplifying both alternating and direct current then the picture signal
supplied to the amplifier 95 must be controlled in such manner that
the
peak
value
of the picture signal current is maintained
sufficiently high.
In practice, the scene td be transmitted is generallyr scanned about
twenty times 100 per second, and, since the scene does not usually
change very rapidly, the picture signals corresponding to successive
scannings do, not differ very markedly from one another Thus veriv
little change 105 occurs in the mean amplitude or mean peak value of
the picture signals during a single complete scanning It is therefore
possible to control the mean amplitude or mean peak value of the
picture 110 signbals to a substantially constant level sufficiently
high to override the background noise without seriously distorting the
waveform of the alternating component of -the picture signals, since
this 115 component comprises frequencies considerably higher than the
frequency at which control is necessary.
A change in the average amplitude of the picture signal, or of the
alternating 120 component of the light falling upon the scanning cell,
is, due to a change in the amplitude of black to white in the object,
that is to say, to a change in the difference of intensity of the
light passing 125 through the darkest and lightest portions of the
object (which of course may be due to a change in the intensity of the
light illuminating the object) and a change in the average peak value
of the picture sig 130 4-T 404,020; nal or of the light falling upon
the cell is due to a change in the average peak value of the object,
that is to say to a chance in either the average brightness or the
average amplitude of black to white in the picture or in both A change
in the average peak value of the object occurs when the intensity of
the light projected upon the object for the purposes of scanning
changes in intensity and also when the average opacity of the film
changes.
In
a known picture telegraph transinitter, the picture to be
transmitted, which is disposed upon a film, is illuminated with light
from a source of substantially constant intensity and light passing
through the picture is collected by a photo-electric cell which forms
the first member of a transmission channel Light also passes from the
source into a second photo-electric cell without passing through the
picture The two cells are inserted in two arms of a Wheatstone Bridge
in such manner that fluctuations in the intensity with which the
picture is illuminated are automatically compensated and the balance
of the Bridge is disturbed to an extent dependent upon the " tone
value " of the picture In this arrangement the compensation for
fluctuations of the light source is effected electrically.
According to the present invention, however, there is provided a
method of transmitting images of an object to a distance in which
light modulated in accordance with the tone value of the object is
passed into a light sensitive device, wherein changes in average
amplitude or of average peak value of the light falling upon the
device, due to changes in the nature of the object, are diminished.
Other features of the invention will
description and the appended claims.
appear
from
the following
A constructional embodiment of the invention will be described by way
of example, reference being made to the accompanying drawings in which
Fig 1 illustrates diagrammatically a scanning system arranged in
accordance with the present invention and adapted for use at a
transmitting station, and Fig 2 illustrates, also diagrammatically, a
scanning aperture of adjustable size such as may be used in the system
illustrated in Fig 1.
It will be assumed for convenience 6 ho that two motion picture films
are to, be transmitted, the first being of high average transparency
or brightness and high average amplitude of black to white, that is to
say, of high average peak value and the second film being of
comparatively low average brightness and low average amplitude of
black to white It will be assumed also that in both cases the picture
signals are to be amplified by means of an amplifier capable of
amplifying 70 both alternating and direct current.
A conception of the difference between the two films may be obtained
by supposing that in place of the two films mentioned above, one film
is scanned twice in 75 succession, the first time by means of a
scanning beam of high intensity and the second time by means of a beam
of low intensity In the first case the object scanned is of high
average brightness 80 and high average amplitude of black to white and
in the second case the object is of comparatively low average
brightness and low average amplitude of black to white 85 The two
scannings, whether of the same or of different films, are effected by
means of the same apparatus.
The film 1 of high average peak value is first moved, in a direction
perpendicu 90 lar to the planel of the paper, past a scanning gate 2
at uniform speed whilst it is illuminated by light from an arc 3 The
arc is adapted to emit light of constant intensity The gate 2 is in
the form of a 95 slit disposed perpendicularly to the direction of
motion of the film A drum 4, which is adapted to rotate about an axis
parallel to the direction of motion of a film 1, carries a plurality
of plane mirrors 100 all of which are arranged symmetrically and
tangentially upon its surface.
Each mirror 5 on the drum 4 reflects an image: of a strip, of the film
in the gate 2 over a single rectangular scanning 105 aperture 6 behind
which is disposed a photo-electric cell 7 associated with an amplifier
8 As the film 1 moves past the gate 2 and the drum rotates, images of
successive strips of the film are trans 110 lated over the aperture 6
and corresponding electric picture signals are generated in the cell
7.
The arc 3 is adjusted to give its maximum output of light and the
amplifier 8 115 is adjusted to give maximum amplification without
there arising an appreciable parasitic background effect due either to
noise generated by valves or to the Johnson noise referred to above
120 The picture signals generated in this way are transmitted,
together with synchronising impulses, by means of apparatus which will
be described later, and at the receiving station the picture signals
125 are reformed into images of the picture film in any known or
suitable manner.
Images of the film are also reconstituted at the transmitting station;
this is achieved by means of scanning appara 10 404,020 tus which is
not shown but which may be similar to that above described working in
the inverse manner The fineness and average peak value of the
transmitted images may be judged by eye or may be measured from this
reconstituted image and alterations of these features of the image are
effected by altering the size of the scanning aperture 6 by means of a
shutter 9 which forms one boundary of the aperture 6.
-Measurements of the average peak value of the transmitted images may
be
made by applying the picture signals to a peak voltmeter
Alternatively the picture signals may be analysed by applying them to,
one pair of deflecting plates of a cathode ray oscillograph, a
comparatively low frequency saw-tooth wave being applied to the usual
second pair of deflecting plates of the oscillograph.
Alterations in the area of the scanning aperture in the transmitting
apparatus vary, in effect, the number of elemental areas into which
each picture is divided for transmision If this number is too few the
fineness
of
scanning is too, low and the definition of the
reconstituted image is poor, although the amplitude of the signals
generated in the photo-cell is high; if the number is too great there
is a greater detail in the reconstituted image than the eye can
appreciate, and the probability is that the transmission channel
cannot handle such high detail without serious distortion.
Furthermore if the fineness of scanning is increased unduly and the
scanning aperture made too small, the picture signal supplied to the
amplifier would be too weak to override the parasitic background
effect mentioned above In general, therefore, the detail and average
peak value of the picture provide a good indication of the nature of
the signal supplied to the amplifier Initial setting of the picture
signal amplitude can thus be made from an inspection of the picture
and adjustments subsequently made by reference to the peak voltmeter
or cathode ray oscillograph The film of comparatively low average peak
value is' now passed through the scanning apparatus Compensation for
its low average peak value cannot be achieved by altering the arc 3 or
the amplifier 8 since these are already giving their maximum outputs,
but is achieved by increasing the area of the scanning aperture 6 The
increase in area serves to decrease the fineness of scanning and also
to increase the average brightness and amplitude 6 f the light falling
upon the cell 7 and is adjusted until the picture signals developed in
the photo-electric cell 7 are roughly the same as for the first film.
The second film is thus also transmitted under the best possible
conditions of average brightness and fineness of scan 70 ning, whilst
the amplifiers have for the whole transmission been working at their
maximum amplification The frequency and phase characteristics of the
amplifiers thus remain constant although 75 variations of both average
brightness and fineness of scanning are achieved.
If the amplifier 8 is' capable of amplifying both alternating and
direct currents, as has been assumed so far, then 80 the amplified
signal provides a true representation of the average peak value of the
light falling upon the cell 7; instead, therefore, of varying the
aperture size manually, the amplified current may be 85 used to
control automatically the amount of light falling upon the cell so
that its average peak value is, maintained substantially constant.
This may also be done if the amplifier 90 8 is capable of amplifying
alternating current only, because in this case it is only the
alternating component of the light falling upon the cell 7 which need
be maintained constant 95 In either case, therefore, the amplified
picture signal may be used to control automatically the size of the'
scanning aperture.
The picture signals derived from either 100 of the films are therefore
divided into two channels One of the two sets of signals is fed to a
mixing
amplifier 10 where the picture signals are mixed with
synchronising signals prior to amplification 105 in the amplifier 11
and transmission over a single channel and the second set is fed, if
necessary after further amplification in the amplifier 12, into a
rectifier 13 The rectified impulses are then 110 arranged to operate,
for example by means of an electromagnetic device 14, the shutter 9
which constitutes one boundary of the scanning aperture 6.
The electromagnetically operated 115 shutter 9 is illustrated in
greater detail in Fig 2 It comprises a moving coil galvanometer with a
vane 17 substituted for the usual indicating pointer, the vane being
arranged to obscure the scan 120 ning aperture 6 in accordance with
themovement of the coil 18 The vane 17 is counterbalanced by a weight
19 and the restoring force of the coil, usually provided by means of a
pair of springs, is 125 adjusted so that the shutter has a natural
frequency somewhat lower than the frequency with which successive
complete pictures on the film are scanned The natural frequency may
be, for example, 130 404,020 about 8 cycles per second Changes in
average amplitude or peak value of successive whole pictures of the
film
are thus faithfully reproduced in the photoelectric cell
currents, because the shutter does not respond to, changes of
frequency as high as the frequency with which successive complete
scans are effected, but all changes in average amplitude or peakl
value of the light falling upon the cell, which are due to changes in
the nature of the object, of frequency as low as or lower than the
naturdl frequency of the shutter are automatically eliminated or at
least diminished Differences in average opacity of two films are for
example compensated for in this way.
Damping
means may be provided
oscillations being set up.
in
order
to
prevent
trains
of
If desired the shutter may comprise a moving coil loudspeaker movement
in which the coil operates the shutter In this case the diaphragm of
the speaker may be conveniently retained for damping purposes and if
desired may be adapted to form part of a dash-pot in order still
further to increase the damping effect.
Alternatively two of the boundaries of the scanning aperture may be
obscured by means of a light valve comprising a loop of conuducting
ribbon carrying the rectified picture current impulses and situated in
a magnetic field Variations in size of the scanning aperture may then
be effected by passing direct current of strength depending upon the
amplitude
ribbon.
of
the
picture
impulses,
through the condueting loop of
In order that the vane 17 may obscure correctly an increasing
proportion of the scanning aperture 6 with increasing picture signal
amplitude
or
peak value, two 4.5 preliminary adjustments are
necessary.
In the first place the correct size of aperture for a given average
amplitude or average peak value of the amplified picture signals is
obtained by adjusting the zero of the electro-magnetically operated
shutter by means of the potentiometer 1.;, and in the second place the
sensitivity of the movement of the vane 17 is adjusted by varying the
resistance 16 in shunt with the coils of the electromagnetic device
operating the vane, until a given change in the amplified picture
signal amplitude or peak value produces a correct change in aperture
area.
Having once adjusted the apparatus in this way variations in the
average amplitude of black to white of two films or in different parts
of one film, or in any object which may be scanned, are automatically
allowed for, and the amount of light reaching the photo-electric cell
is varied, at frequencies lower than the frequency with which the
object to be transmitted is completely scanned, in a manner inversely
dependent upon the changes 70 in average amplitude of the object ache
relationship between the amount of light and average amplitude is not
necessarily a linear one.
As shown in Fig 1, as the drum 4 75 rotates about its axis, images of
strips of the film are translated over the aperture 6 in the direction
indicated by the arrow A The shutter 9 is thus controlled in motion in
the direction of scanning onlv, SO and the width of each line scanned
therefore remains constant If desired, however, two perpendicular
sides of the aperture 6 may be constituted by shutters, the motion of
one of them being con 8 e 5 trolled in the direction of scanning and
the motion of the other being controlled in a direction perpendicular
thereto.
This is of particular advantage where the changes occurring in the
object are very 90 considerable Alternatively the rectifier 13 shown
in Fig 1 may be biassed so that the shutter is operated only if the
picture signals exceed a predetermined average amplitude or average
peak value This 95 method of working is convenient if it is -required
to control the picture signal amplitude to, load to any desired
extent, but not overload, the radio transmitter.
In other cases it may be preferable to 100 vary the area of the
aperture by controlling two or more boundaries independently One
boundary may be controlled by the mean picture signal amplitude, for
example, and another by the peak picture 105 signal amplitude.
If desired, in the case of film transmission, instead of varying the
area of the scanning aperture in accordance with the variations in
amplitude of black to 110 white in the film, the area may be varied in
accordance with the variations in opacity of the background of the
film, that is to say, in accordance with those variations in opacity
of the film due to 115 the developing and fixing processes which the
film has undergone This may be effected by passing a beam of light of
constant intensity through an edge portion of the film lying alongside
the pie 120 tures, into a photo-electric cell Alternatively, an
auxiliary cell may be disposed so, as to receive light from the whole
of the scanned picture and thus
brightness of the 125 whole picture.
register
the changes in average
In either case the currents developed in the cell, if necessary after
amplification, can be utilised to control an electromagnetically
operated shutter as described 130 404,020 ' abovee.
The invention may be embodied in a television receiving station In
this case the light source and amplifiers are preferably adjusted to,
give' their maximum output and amplification respectively Any fall in
amplitude or average peak value of the received picture signals, due
for example to fading, which would produce a decrease in average
amplitude of black to white or average brightness of the pictures
reformed on the screen, is compensated by increasing the size of the
scanning aperture This has the effect of increasing the average
amplitude or brightness, of the reformed picture and correspondingly
decreasing the fineness of scanning.
It will be apparent, however, that a change either in the average
brightness or in the amplitude of black to, white of the object at the
transmitter or a change in either of these properties of the received
picture currents at the receiver, may be compensated by varying the
mean intensity of the scanning light source or by varying the size of
any obturator inserted between the' scanning light source and the cell
at the transmitter, or between the scanning light source and the
viewing screen at the receiver An electromagnetic device operated in
the manner described above may be used for effecting the variations in
size of such an obturator It should be noted, however, that if either
of these methods be adopted, although compensation for changes in
average amplitude or peak value is achieved the optimum relationship
between the average brightness and the fineness of scanning is not
maintained Compensation is therefore preferably effected by varying
the size of the scanning aperture.
It will be clear that varying the size of the scanning aperture or any
other obturator in the path of the light between the object and the
scanning cell has the effect of altering both the direct and
alternating components of the light falling upon the cell If,
therefore, it be desired to compensate for changes; in the alternating
component only it is necessary to offset the change produced in the
direct component by the change in aperture size This may be done, for
example, by projecting an auxiliary light beam upon the cell The
scanning aperture or other obturator is then varied in size so as to
compensate for changes in the alternating component and the intensity
of the auxiliary beam is varied so as to, compensate for the changes
in the direct component due to the change in aperture or obuturator
size.
If it be desired to compensate for changes in the direct component
only, the size of the scanning aperture or obturator is maintained
constant and the intensity of the auxiliary light beam varied so 70 as
to compensate for changes in the direct component.
The changes in
values of the
falling upon the
connected by any
the nature of the object may be such that the average
alternating and direct components of 75 the light
cell change in opposite senses, the changes not being
simple relationship.
In this case it may be desirable to, compensate for the two changes
separately 80 This may be effected by using the scanning aperture or
obturator to compensate for changes in the alternating component and
using the auxiliary beam to, compensate for changes in the direct
component 85 which are due both to changes in the nature of the object
and to changes in aperture or obturator size 1 In practice the
aperture or obturator may be controlled in size by means of 90
rectified currents derived from the alternating component of the
picture signal and the auxiliary beam controlled in intensity by means
of currents derived from the direct component of the picture 95
signal.
Having now particularly described and ascertained the nature of our
said invention and in what manner the same is to be performed, we
declare that what we 100
Claims
_________________________________________________________________
claim is:1 A method of transmitting images of an object to a distance in which
light modulated in accordance with the tone value of the object is
passed into a light 105 sensitive device, wherein changes in average
amplitude or of average peak value of the light falling upon the
device, due to changes in the nature of the object, are diminished 110
2 A method of receiving pictures in which a beam of light modulated in
accordance with received electric picture signals is projected upon a
screen, wherein changes in averag 3 amplituude or of 115 average peakl
value of the light falling upon the stream, due to changes in the
nature of the received signals serving to modulate the said beam of
light, are diminished by means of currents depen 120 dent upon the
average amplitude or average pealk value of said picture signals.
3 A method according to claim 1 or 2, wherein the light is obturated
to' compensate, at least partially, for the changes 125 in nature of
the object or received signal.
4 A method according to claim 3, wherein obturation of the modulated
light is effected by varying the size of a scanning aperture inserted
in the path of the 130 404,020 light.
A method according to claim 3, wherein obturation of the modulated
light is effected by varying the size of an auxiliary obturator such
as a lens stop for example inserted in the path of the light.
6 A method according te-ny of claims 3- te-5 ,-erein-the variations in
obturation aro effected automatioally.
A A method aeeording to-olaim 6,-wheein-eurrcnts derived from the
picture signals are utilised in-effecting the automatic veriations in
obturation.
6 8 A method according to claim 1 or 2, wherein compensation is
effected
by projection upon the light-sensitive device at the
transmitter or the screen at the receiver of an auxiliary beam of
light or controllable intensity.
7 9 A method according to claim 6 8, wherein currents derived from the
amplified picture signals are utilised in effecting changes in the
intensity of the auxiliary light beam.
8 40 Apparatus for transmitting images of an object to a distance
comprising a light-sensitive device adapted to generate picture
signals and means adapted automatically to diminish changes in the
average amplitude or average peak value of the light falling upon said
light-sensitive device which would otherwise occur as a result of
changes in the nature of the object.
9 44 Apparatus for receiving and reconstituting pictures upon a screen
comprising a light source the intensity of which is adapted to be
controlled by received picture signals and means adapted to be
actuated by currents varying with the average amplitude or average
peak value of said pictitre signals for automatically diminishing aute
a Geally to diminish changes in the average amplitude or average peak
value of the light projected upon said screen from said source which
would otherwise occur as a result of changes in the received picture
signals serving to modulate the said beam So of light.
42 Apparatus according to claim 8 or 9 44, wherein said means comprise
an obturator, such as a scanning aperture for example, of adjustable
size.
43 Apparatus according to claim Ad, comprising means responsive to the
picture signals which are adapted automatically te vary the size of
said obturator.
11 44 Apparatus according to claim 10 43, wherein the sai means wielie
are provided for varying adapted to vary the size of the obturator in
accordance with the average amplitude or average peak value of said
picture signals, the said means being ae responsive only to picture
signals which exceed a certain predetermined amplitude.
12 46 Apparatus according to claim 10, 12-er-43, wherein a boundary of
said obturator is constituted by a shutter 70 which is adapted to be
controlled in motion by currents derived from the picture signals.
13 46 Apparatus according to claim 10, a 4, wherein two boundaries of
said 75 obturator are constituted by shutters both of which are
adapted to be controlled in motion independently in accordance with
changes in currents derived from the picture signals 80 14 hi
Apparatus according to claim 13 46, wherein one of said shutters is
controlled in accordance with changes in the average peak value of the
picture signals and the other of said shutters is controlled 85 in
accordance with changes in the direct component of the picture
signals.
48 Apparatus according to claim 12 46 or 13 46, wherein said shutter,
or one of said shutters, is adapted to be con 90 trolled in motion in
a direction parallel to the direction of scanning.
16 49 Apparatus according to any of claims 10 42 to 13 46, wherein at
least one boundary of said obturator is constituted 95 by a shutter
which is adapted to be controlled in motion in the direction of
scanning and at least one other boundary of said scanning aperture is
constituted by a second shutter which is adapted to be 100 controlled
in motion in a direction perpendicular to the direction of scanning.
17 32 Apparatus according to any of claims 12 46 to 16 1, wherein the
natural frequency of said shutter (or each of said 105 shutters) is
less than the frequency at which complete scans of the object at the
transmitter, or screen at the receiver, are effected.
18 21 Television transmitting appar 110 atus constructed, arranged and
adapted to operate substantially as described herein or as shown in
the accompanying drawings.
19 22 Television receiving apparatus 115 according to claim 9,
constructed,
arranged
and adapted to operate substantially as
described herein or as shown in the accompanying drawings.
23 A method of transmitting 120
substantially as described herein.
images
of objects to a distance
21
24
A method of receiving pictures according to claim 2,
substantially as described herein 125 Dated this 4th day of August,
1933.
REDDIJE & GROSE,
London, E C 4.
Agents
for
the Applicants, 6, Bream's Buildings,
Lteamington Spa: Printed for His Majesty's Stationery Office, by the
Courier Press -1935.
4.-1.
1191;.
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