US2240534A - Television receiving system - Google Patents

Description

Patented May 6, 1941 2,240,534 TELEVISION RECEIVING SYSTEM John Charles Wilson, Bayside, N. Y.,

Hazeltine Corporation, a corporat asslgnor to on or Dela- Application October 13, 1938, Serial No. 234,741 14 Claims. (C1. 178-7.3)

ceivers.

In accordance with present television practice, a transmitted signal comprises a carrier, modulated during successive intervals or trace periods by high-frequency and uni-directional components representative of light variations in an image being transmitted and of its average background illumination, respectively, and modulated between the trace periods, that is during retrace intervals, by synchronizing components which correspond to initiation of successive lines and fields in the scanning of the image.

At the receiver, a beam is so deflected as to scan and illuminate a target in series of fields of parallel lines. The synchronizing components of the received signal are separated from the other modulation-signal components and utilized to control the scanning apparatus of the receiver so as to synchronize its operation with that of similar apparatus utilized at the transmitter in developing the signal. The intensity of the beam is controlled by the light-modulation components, thereby to reconstruct the image.

An important requirement in a properly op-- erating television receiver is adjustment of the system so that the modulation signal, whether derived from a positively or negatively modulated carrier, is maintained constant with respect to a particular shade value or level. More particularly, the system must be so controlled that any given level or amplitude of the modulation signal which corresponds to black or any particular shade value must be established and held fixed with respect to the signal input-brightness characteristic of the reproducing device of the system. in order that signal components representing any given shade will appear as the same shade in the reproduced image. That is, the system must be controlled so that the light gradations or shades from black to white represented by the various light-modulation components are all faithfully represented in the reconstructed image. V

Further, as is well known, it is frequently desirable to provide automatic control of certain operating characteristics of a television receiving system in accordance with the average received carrier amplitude and independent of light such average intensity modulation, for example, automatic amplification control. Since the average intensity of the carrier is varied at the transmitter in accordance with background light-modulation components,

cannot be utilized to effect satisfactory automatic amplification control in the conventional manner. Some controlling eflect is required which is dependent solely upon the average carrier intensity, as determined by the power of the transmitting station, its distance, fading, etc., and independent of .the degree of modulation. The amplitude of thesynchronizing pulses can be utilized for this purpose only in the case of negative modulation, in which these pulses are transmitted on outward peaks of modulation at a relatively fixed amplitude. In positive modulation systems, the synchronizing pulses are ordinarily transmitted on inward peaks of modulation, corresponding substantially to zero amplitude of the carrier wave. Special arrangements, therefore, are usually required for providing automatic control of an operating characteristic of a television receiver in accordance with the average amplitude of the received carrier wave and independent of light-modulation components.

It is an object of the present invention, therefore, to provide an improved signal shade-leveladjusting system for a television receiver.

It is a further object of the invention to provide a television receiver including improved means for automatically controlling a plurality of operating characteristics of the receiver in accordance with the average intensity of the received modulated-carrier signals and independent of the light-modulation components.

It is a particular object of the invention to provide a combined automatic amplification control and signal shade-level-adjusting system for a television receiver.

' In accordance with the present invention, a television signal receiver adapted to reproduce a television signal-carrier wave having lightmodulation component-s including a component corresponding to a predetermined shade level which tends to vary during reproduction and line-synchronizing modulation components outside the amplitude range of said light-modulation components comprises means for deriving from a received carrier its modulation signal. Asynchronous means are provided, responsive to a predetermined level whenever such level may occur in the signal of duration greater than that of a line-scanning pulse, such means including a time-constant circuit for substantially eliminating the effect of said line-synchronizing components on the output thereof, for developing a control eifect variable in accordance with the intensity of the carrier wave and independent of the light-modulation components and for utilizing this control effect for maintaining at a constant value a predetermined intermediate si nal level of said-modulation signal corresponding to a predetermined shade value. Suitable means are arranged to utilize the last-mentioned modulation signal for reproducing the light-modulation components.

Further in accordance with the present invention, the receiver may comprise a main signaltransiating channel including signal-detecting and reproducing means and a separate control signal-translating channel. Means are included in said control channel for deriving a control effect variable in accordance with the intensity of said carrier and independent of its light-moduiation components. Individual means are provided for utilizing the control efi'ect to control an operating characteristic of the system, for example, to maintain the intensity of the signal input to the signal-detecting means within a relatively narrow range for a wide range of received signal intensities and for utilizing the control efiect for maintaining at a constant value a predetermined intermediate signal level of the signal with respect to a predetermined shade level thereof.

In accordance with another feature of the present invention, there is provided a single means responsive to a predetermined shade level of the modulation envelope of a received television signal for developing a plurality of control effects and individual means are provided for utilizing each oi the control effects forydjustin a separate characteristic of the receiver.

I For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawin Fi 1 is a circuit diagram, partially schematic, of a cathoderay tube television receiving system including circuits embodying the present invention; Fig. 2 is a graph of the envelope of a positively modulated television carrier wave to aid in the understanding of the invention; while Figs. 3-7, inclusive, are groups of wave forms representing the derived signal developed at different points in the receiver of Fig. 1.

Referring now more particularly to the drawing, the system there illustrated comprises a re ceiver of the superheterodyne type including an antenna system Ill-H connected to radio-frequency amplifier l2 to which is connected in cascade, in the order named, an oscillator-modulator l3, an intermediate-frequency amplifier ll, a detector l5,.a video-frequency amplifier l6, and a cathode-ray signal-reproducing tube l'l constituting the main signal-translating channel of the receiver. The cathode-ray tube. ll comprises the usual envelope I8 containing, in the order named, a cathode l9 having a heater 20, a signal-input or control grid'2l, a screen 22, a first anode 23, a second anode 24, and the usual fluorescent screen 25 at the end of the tube. Linefrequency and field-frequency scanning coils 26 and 21 are disposed about the tube for deflecting the beam in two directions normal to each other. The output of the video-frequency amplifier is.

connected to the control grid 2|. Operating potentials are supplied from a suitable source, such as the battery 30. A line-frequency generator ii and a field-frequency generator 32 are also cou- I pied to the output circuit of the detector I! and are connected to the scanning coils 22 and 21, respectively, of the cathode-ray tube in the conventional manner. Suitable means are included in the generators 3| and 32 for separating the synchronizing pulses from the other modulation components and from each other, in the usual manner. The stages or units Just described may all be of conventional well-known construction so that detailed illustrations and descriptions thereof are deemed unnecessary herein.

Referring briefly, however, to the operation of the system described above, television signals intercepted by the antenna circuit lO-ll are selected and amplified in the radio-frequency amplifier l2 and supplied to the oscillator-modulator l3, wherein they are converted to intermediatefrequency signals which, in turn, are selectively amplified in the intermediate-frequency amplifier I4 and delivered to the detector I5. The modulation components of the signal are derived by the detector l5 and are supplied to the videofrequency amplifier l6 wherein they are amplified and from which they are supplied to the control grid 2| of the cathode-ray tube IT. The detected signal components are also supplied to the generators 3| and 32' to synchronize the operation thereof. With proper operating potentials supplied from the battery 30 to the electrodes of the tube l1, an electron beam is emitted from the cathode 20 and its intensity is controlled by the grid 2| in accordance with the video-frequency voltages impressed thereon. The screen 22 serves to control the average beam intensity while the first and second anodes 23 and 2d cooperate to accelerate and focus the beam. It will be appreciated that the tube I! has a substantially linear response or signal input-brightness characteristic, as distinguished from the curved characteristics of conventional amplifier tubes, so that variations in its grid-bias voltage serve simply to adjust the point on this characteristic at which the tube ll operates, substantially without affecting its response, so as to determine the average brightness of the reproduced image for a given input signal level. I

The intensity of the electron beam of the tube I1 is modulated or controlled in accordance with the video-frequency voltages impressed upon the control grid of the tube in the usual manner. Saw-tooth current waves are generated in the line-frequency and field-frequency generators SI and 32, which are controlled by detected synchronizing pulses, and applied to the scanning elements of the cathode-ray tube H to produce electric-scanning fields, thereby to deflect the ray in two directions normal to each other so as to trace a rectilinear scanning pattern on the screen of the tube and thereby to reconstruct the transmitted picture.

Referring now more particularly to the portion of the system of Fig. 1 embodying the present invention, for the purpose of developing an automatic amplification control-bias voltage there is provided an auxiliary asynchronous control means including a control signal-translating channel at least in part separate from the main signal-translating channel and comprising a vacuum-tube stabilizer repeater 33, a rectifier 34 and a direct-current reversing amplifier 25, connected as shown in the order mentioned. The

' and leak resistor 31, connected to the output circuit of the detector ii. The load circuit of the tube 33 comprises a resistor 33 and a condenser 30 in parallel having a predetermined time constant which is long compared to the duration of the line-synchronizing pulses but short compared to the field-blanking intervals. For conventional systems utilizing 30 frames (60 fields) of 441 lines per second interlaced, the resistor 38 may be of the order of 100,000 ohms and the condenser 33 of the order of 0.006 microiarad, providing a time constant 01 0.0006 second.

The diode rectifier 34 is coupled across the load circuit of the tube 33 and is'provided with a load circuit comprising a parallel-connected resistor 40 and condenser 4| having a large time constant. This time constant should be considerably greater than the field-scanning period, for example, of the order of 5; second. The input electrodes of the reversing amplifier are connected across the load circuit 01' rectifier 34 while its anode circuit includes a load resistor 42. Positive potentials are applied to the anodes oi the tubes 33, 34 and 35 by way of their respective resistors 38, 40 and 42 from suitable sources, indicated at +3. The same positive potential is applied to the cathode oi' the diode 34 as to the anode of tube 33 since the diode cathode is directly connected to the. anode of tube 33 and hence has the operating potential of tube 33 applied thereto. A suitable biasing battery 35a is included in the cathode circuit oi the tube 35 and a higher potential is applied to its anode than to the anodes of tubes 33 and 34, since the grid of tube 35 is connected to the anode of tube 34 and its operating poten tials must therefore be adjusted accordingly. The anode of the tube 35 is connected through isolating resistor 49 and a lead 47 to the control grid 2| of the tube l1 and by way of a suitable filter, including series resistors 43 and shunt condensers 44, and conductors 45 to the control electrodes of one or more vacuum tubes included in the amplifier l2, oscillator-modulator I 3, and amplifier l4, as shown. The leads 45 arid 41 may include negative bias batteries 40 and 48, respectively, to compensate for the high potential 01' the anode of the tube 35from which the automatic amplification control and signal-level-adjusting potential is derived. However, if the grid of the tube l1 and cathodes of the tubes in the stages 02, i3 and I4 are properly biased, the batteries 46 and 48 I may be omitted.

The operation and results obtained by the sys- Item of Fig. 1 may best be understood by reference to Figs. 2-7, inclusive. In Fig. 2 there are illustrated the wave forms of portions of the complete carrier envelopes of television modulated-carrier waves of the positively modulated type including a component corresponding to a predetermined shade level which tends to vary durin reproduction and line-synchronizing modulation components outside the amplitude range of said lightmodulation components. The portion shown at A represents a wave of relatively large amplitude while that at A represents a similar wave differing only in that it is of lesser amplitude, as might be caused by occasional fading. In Figs. 15-? corresponding wave portions are shown, illustrating the form of the signal derived at various points of the system for the assumed signal inputs of difl'erent amplitudes A and A.

The portions of the wave of Fig. 2 at which the carrier amplitude is reduced to zero, certain of 3 which are indicated at L, represent line-synchro. nizing pulses which occur during line-retrace periods. The portions between the synchronizing pulses L represent trace portions of successive lines, certain of these being modulated as indi cated at M1 and M2 by the high-frequency and background light components. During the fieldretrace periods, part of one of which is indicated at Y-Y, the amplitude of the wave is independent of light-modulation components. The fieldretrace periods ordinarily include the line-synchronizing pulses, as well as additional broad pulses, not shown. which latter constitute collectively a frame-synchronizing pulse.

It is well known that, with the positive type of modulation such as is represented in Fig. 2, transitions of the scanning beam from a dark to a lighter part of the images are represented by increases in the carrier amplitude while the ave age amplitude of the carrier wave is varied in accordance with the low-frequency or unidirectional background illumination components of the image. Thus, in the particular portions of the wave shown, during the line-trace modulationperiods represented at M1, the amplitude of the carrier is such that it reaches the white level, whereas this does not occur during the modulation periods represented at Ma. Since the average intensity of the carrier is thus varied in accordance with background illumination variations, these carrier variations cannot be used for the purpose of automatic amplification control. Nor, as stated above, are there in this type of wave any synchronizing pulses which could be utilized directly for the purpose in question, since the signal is reduced to zero during the duration of the synchronizing pulses. However, the modulation envelope includes a certain representative level, for example, the blanking level at recurring intervals or portions, at which the signal may represent black or blacker than black and which appears in the signal not only for black pictures but also during the frame-retrace periods. This level, indicated at B in Fig. 2, affords a measure of the intensity of the carrier wave which is independent of light-modulation components and of other signal characteristics, such, for example, as its periodicity or synchronization. This level may, therefore, be utilized automatically to control certain characteristics of the receiver, such as its amplification and the setting of the black level in the signal to be reproduced.

In the operation of the present invention, therefore, the modulated-carrier wave is detected by the detector IE to derive a voltage wave corresponding to one-half of the modulation envelope of Fig. 2, which voltage is impressed upon both the input circuit of the amplifier iii in the main signal-translating channel of the receiver as well as on the input electrode of the stabilizing tube 33 bv way of the coupling condenser 36 and leak resistor 31. The grid condenser and leak serve antomatically to vary the bias of the control grid which. in the absence of a signal. is provided with zero bias, so that the signal is stabilized; that is, the inward peaks of modulation representing the synchronizing pulses are established at a substantially fixed level as they appear on the control grid. Where the connection between the grid circuit of tube 35 and the detector is substantially a direct one, so that the unidirectional component of the signal is not lost, the signal may be sufli ciently stabilized without the use of the stabilizing tube 33. However, where a non-conductive coupling intervenes between the detector and the unidirectional component is suppressed and the modulation signal tends to center itself about the zero axis and to appear as illustrated by the curves of Fig. 3. In this case, therefore, it must be stabilized as explained. Moreover, the circuit constants and operating voltages of the tube 33 are such that the tube operates beyond cutoff for signal levels above a predetermined value so that the signal is eilectively limited.

The wave forms of the current in the anode circuit oi tube 33, stabilized with respect to the peaks of the synchronizing pulses and limited so that parts of the video-frequency portions of the signal are cut oil, are illustrated in Fig. 4.

Due to the time constant provided by the anode load circuit comprising resistor 38 and condenser 39, which is long compared with the duration of the line-synchronizing pulses but short compared with the duration field-blanking intervals, the voltage developed between the anode of the tube 33 and ground is not of the same form as the anode current, but has the wave forms shown in Fig. 5. In other words, the output circuit time constant may be said to cause the system to ignore the synchronizing pulses because of their short duration; that is, the condenser 39 may be considered as by-passing the synchronizing pulses or components or as included in a time-constant circuit for substantially eliminating the effect of the line-synchronizing pulses. The portions m, m of the wave 2,240,534 control circuit, as the condenser. of Fig. 1, the' v forms of Fig. 5 are representative of and vary with the signal level during the field-blanking intervals. However, these portions are minima in the waves of Fig. 5 so that they cannot be measured directly.

In order to measure these voltage minima', there is obtained the difference voltage between the anode voltage shown in Fig. 5 and the operating or +3 voltage, which difference voltage is developed across the resistor 38 with wave forms such as shown in Fig. 6, which, it is noted, have peak amplitudes p, p corresponding to the minima m, m, and occurring during and dependent solely upon the blanking level during the fieldblanking intervals. This voltage, therefore, constitutes a control signal which has peak amplitudes corresponding to the average intensity of the received carrier and ind pendent of its lightmodulation components, such amplitudes being indicated at p and p in Fig.6 for signals of different intensities. Moreover, the control signal is derived in response to a predetermined shade level of the modulation signal, in the present case, the blanking or black level indicated at B in Fig. 1.

The control signal developed across the resistor 38 is thereupon rectified by the peak diode rectifier 34, thereby developing across its load resistor 40 a control-bias voltage dependent solely upon the portions 9 and p oi the waves of Fig. 6. This control-bias voltage is applied negatively to the grid of tube 35, wherein it is amplified with a reversal in the polarity of its variations. The rectified and amplified control-bias of one or more of the tubes in the stages I2, l3 and I4. Since this voltage varies inversely in accordance with variations of the average carrier intensity and independently of light-modulation components, the amplification in the stage; l2, l3 and I4 is controlled accordingly. Thus, the output signal intensity of the channel including the stages 12, i3 and i4 is maintained within a relatively narrowrange for a wide range of received signal intensities.

It willbe noted that where the expression "wide range of signal-input intensities is employed herein and in the appended claims, it refers to such intensity variations of the received carrier as are due to fading, to the different field strengths of different signals and the like, and are independent of light-modulation components. and not to the relatively smaller average carrier intensity variations which are caused by variation of the average background illumination.

As just explained, the amplifier 35 simply serves to reverse the polarity of the variations of the unidirectional-bias voltage developed by the rectifier 34 to develop a voltage which increases negatively with increasing. carrier amplitude and independent of light-modulation components. This is, of course, the required polarity for eflecting the automatic controls described. Various other embodiments of the invention will be readily apparent to those skilled in the art, in some of which a reversing amplifier will be unnecessary. For example, the tubes 33 and 34 may be so connected that the voltage developed in the output circuit of the tube 34 increases negatively with increasing carrier amplitude, as by including a resistor equivalent to resistor 33 in the cathode circuit instead of the anode circuit of tube 33.

The bias voltage is applied to the control grid 2| of cathode-ray tube I! to adjust the point on its signal input-brightness characteristic corresponding to a predetermined shade value, to the voltage is developed across the load resistor 42 end that all the various shade values which the diflerent components of the signal input represent appear as the proper corresponding shade values in the reconstructed image. In particular, the tone of black is represented by a point at or near the cathode-ray tube cut-oil point under varying'conditions of synchronizing-pulse amplitude. That is, the control voltage supplements the fixed grid-bias voltage applied from the battery 30 and serves to establish the point of operation representing black at the bottom of the linear response characteristic of the tube il.

As stated above, the curves of Fig. 3 indicate the wave form of the signal when unstabilized, while the curves of Fig. 4 show the same wave limited and stabilized with respect to the peaks of the synchronizing pulses. Such a wave stabilized, but not limited, is developed in the output circuit of the detector l5 and, provided the amplifier l6 does'not include any non-conductive connections, a similar wave is developed in the output circuit of this amplifier. As explained above in connection with the tube 33, if nonconductive connections are included so that the direct current component is lost, suitable means are provided for reinserting the direct current component, that is, stabilizing the signal with respect to the peaks of the synchronizing pulses.

It will be seen from the curve of Fig. 4, however, that even if the signal is stabilized with respect to the peaks of the synchronizing pulses, if, by reason of fading or the like the average signal intensity independent of light modulation is varied, the level of amplitude in the signal which represents black, or any given shade value, is correspondingly varied, so that signal components representing the same shade value have din'erent amplitudes at diflferent parts of the signal. As stated above, the unidirectional voltage developed across the resistor 42 increases negatively in accordance with increases in the amplitude of the blanking or black level of the signal. Hence, this bias voltage is of proper polarity to provide a variable bias on the control grid 2| which serves to adjust the point of operation of the signal input, on the linear response characteristic of the tube I1 and thereby maintain the signal level corresponding to black fixed with respect to the tube characteristic at all times. Thus, the signal as effectively applied to the control grid 2| has a wave form such as illustrated in Fig. 7, where, it will be seen, the blanking level B is the same regardless of variations in the signal intensity. Preferably this level is at the cut-off point on the input signalbrightness characteristic of the reproducing tube. Thus, not only are the efiects of fading and the like on the signal intensity compensated for by the automatic amplification control but the image is reconstructed with proper shade values corresponding to the various signal components representative of such shade values, from black to white, by the automatic shade-level control action, in accordance with the present invention.

The arrangement of the invention thus comprises an asynchronous means including tubes 33, 34, and 35 responsive to a predetermined level, such as the blanking level, whenever such level may occur in the signal of duration greater than that of a line-scanning pulse. This asynchronous means includes a time-constant circuit including condenser 39 for substantially eliminating the eflect of the line-synchronizing components on the output thereof and is utilized for deriving a control effect in the load circuit of tube 35 which is variable in accordance with the intensity of the received carrier wave and independent of lightmodulation components. Means are provided for applying the derived control effect through resistor 49 to the signal-reproducing device I! to maintain at a constant value a predetermined intermediate level of the modulation signal output thereto, the level corresponding to a predetermined shade value of the transmitted picture. It will be appreciated that the present invention may also be utilized automatically to control various operating characteristics of a television receiver other than or in addition to its amplification and signal shade-level adjustment, for example, its selectivity or its tuning.

It is further to be noted that while the invention has been described above in connection with a receiver adapted for a positively modulated signal, the invention is equally applicable in con nection with a. receiver adapted for a negatively modulated signal, as will be readily appreciated by those skilled in the art. In this case the control voltage may of course be derived by simply peak-rectifying the signal carrier since here the synchronizing pulses extend outward beyond the amplitude range oi the light-modulation components and directly afiord a measure of the average carrier intensity independent of its lightmodulation components.

While there has been described what is at present considered to be the preferred embodiment "of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention. i

What is claimed is:

1. A television receiver adapted to reproduce a television signal-carrier wave having light-modulation components including a component corresponding to a predetermined shade level which tends to vary during reproduction and line-synchronizing modulation components outside the amplitude range of said light-modulation components comprising, means for deriving from a received carrier its modulation signal, asynchronous means responsive to said predetermined level whenever such level may occur in the signal of duration greater than that of a line-scanning pulse, said means including a time-constant circuit for substantially eliminating the efiect of said line--synchronizing components on the output therer 1, for deriving a control effect variable 'in accordance with the intensity of said carrier wave and independent of light-modulation components, means for utilizing said control efiect for maintaining at a constant value a predetermined intermediate signal level of said modulation signal corresponding to said predetermined shade value, and means for utilizing said last-mentioned modulation signal for reproducing the light-modulation components.

2. A television receiver adapted to reproduce a television signal-carrier wave having light-modulation components and line-synchronizing modulation components outside the amplitude range of said light-modulation components and having a predetermined blanking level corresponding to black in the transmitted image which tends to vary during reproduction comprising, means for detecting said signal to derive its modulation signal, asynchronous means responsive to said predetermined level whenever such level may occur in the signal of duration greater than that of a line-scanning pulse, said means including a timeconstant circuit for substantially eliminating the effect of said line-synchronizing components on the output thereof, for developing a control effect variable in accordance with variations in said blanking level, means for utilizing said control efiect for maintaining at a constant value a predetermined intermediate signal level of said modulation signal corresponding to said blanking level, and means for utilizing said last-mentioned modulation signal for reproducing the light-modulation components.

'3. A television receiver adapted to reproduce a television signal-carrier wave having lightmodulation components including a component corresponding to a predetermined shade level which tends to vary during reproduction and synchronizing-modulation components outside the amplitude range of said light-modulation components comprising, means fordetecting said signal to derive the modulation signal stabilized with respect to the peaks of said synchronizingmodulation components, asynchronous means responsive to said predetermined level whenever such level may occur in the signal of duration greater than that of a line-scanning pulse, said means including a time-constant circuit for substantially eliminating the eflect of said line-synchronizing components on the output thereof, for developing a control signal from said stabilized signal variable in amplitude in accordance with variations in a predetermined shade level thereof from a given level, means for developing a control eii'ect proportional to the amplitude of said control signal, means for utilizing said control eilect for maintaining at a constant value a predetermined intermediate-signal level of said modulation signal corresponding to said predetermined shade value, and means for utilizing said last-mentioned modulation signal for reproducing said light-modulation components.

4. A television receiver adapted to reproduce a television signal-carrier wave having light-modulation components including a component corresponding to a predetermined shade level which tends to vary during reproduction and line-synchronizing modulation components outside the range of said light-modulation components comprising, means for detecting said signal to derive the modulation signal stabilized with respect to the peaks of said line-synchronizing-modulation components, means for rectifying the modulation signal including a time-constant circuit for eliminating the eflect of said line-synchronizing components to develop a first voltage in' which said predetermined shade level of said modulation signal is represented by amplitude minima values, means for rectifying the difference voltage between said first voltage and a voltage of fixed value to derive a control eifect, means for utilizing said control effect for maintaining at a constant value a predetermined intermediate signal level of said modulation signal corresponding to said predetermined shade value, and means for utilizing said last-mentioned modulation signal stabilized with respect to said level for reproducing said light-modulation components.

5. A television receiver adapted to reproduce a television signal including a component corresponding to a predetermined shade level which tends to vary during reproduction and line-synchronizing modulation components outside the amplitude range of said light-modulation components comprising, a signal-reproducing device having a predetermined input signal-brightness characteristic, means for applying said signal to said device, asynchronous means responsive to said predetermined level whenever such level may occur in the signal of duration greater than that of a line-scanning pulse, said means including a time-constant circuit for substantially eliminating the effect of said line-synchronizing components on the output thereof, for developing a control effect, and means for utilizing said control effect for controlling said reproducing device to adjust the point on its said characteristic corresponding to said predetermined shade value.

6. A television receiver adapted to reproduce a television signal including a component corresponding to a predetermined shade level which tends to vary during reproduction and line-synchronizing modulation components outside the amplitude range of said light-modulation components comprising, a signal-reproducing device having a predetermined input signal-brightness characteristic, means for applying said signal to said device, asynchronous means, responsive to said predetermined level in the signal during recurring portions thereof of duration greater than that of a line-scanning pulse, said means including a time-constant circuit for substantially eliminating the effect of said line-synchronizing components on the output thereof, for developing a controlling efliect, and means for utilizing said control efiect for controlling said reproducing device to adjust the point on its said characteristic corresponding to said predetermined shade value.

'7. A television receiver adapted to reproduce a television signal having a predetermined blanking level including a component corresponding to a predetermined shade level which tendstovary during reproduction and line-synchronizing modulation components outside the amplitude range of said light-modulation components comprising a signal-reproducing device having a predetermined input signal-brightness characteristic, means for applying said signal to said device, asynchronous means, responsive solely to variations of the blanking level of the modulation envelope of said signal whenever such level may occur in the signal ofduration greater than that of the linescanning pulse, said means including a time-constant circuit for substantially eliminating the effect of said line-synchronizing components on the output thereof, for developing a control eflect, and means for utilizing said control effect for controlling said reproducing device to adjust the point on its said characteristic corresponding to said predetermined shade value.

;8. A television receiver adapted to reproduce a television signal including a component corresponding to a predetermined shade level which tends to vary during reproduction and 'line-synchronizing modulation components outside the amplitude range of said light-modulation components comprising, a ynchronous means, responsive to said predetermined level whenever such level may occur in the signal of greater duration than that of a line-synchronizing pulse, said means including a time-constant circuit for substantially eliminating the efl'ect of said line-synchronizing components on the output thereof, for developing a control effect, and individual means for utilizing said control effect foradjusting a plurality of separate operating characteristics of the receiver.

9. A television receiver adapted to reproduce a television signal-carrier wave modulated by light-modulation components including a component corresponding to a predetermined shade level which tends to vary during reproduction and line-synchronizing modulation components outside the amplitude range of said light-modulation components comprising, asynchronous means, responsive to said predetermined level in the signal during recurring portions thereof of duration greater than that of a line-scanning pulse, said means including a time-constant circuit for substantially eliminating the effect of said line-synchronizing components on the output thereof, for developing a control efiect, and individual means for utilizing said control effect for adjusting a plurality of separate operating characteristics of the receiver.

10. A television receiver adapted to reproduce a television signal-carrier wave modulated by light-modulation components including a component corresponding to a predetermined shade level which tends to vary during reproduction and line-synchronizing modulation components outside the amplitude range of said light-modulation components and having a predetermined blanking level comprising asynchronous means, responsive solely to variations of the blanking level of the modulation envelope of said signal whenever such level may occur in the signal of duration greater than that of the line-scanning pulse, said means including a time-constant circuit for substantially eliminating the effect of said line-synchronizing components on the output thereof, for developing a control effect, and individual means for utilizing said control effect for adjusting a plurality of separate operating characteristics of the receiver.

11. A television receiver adapted to reproduce a television signal-carrier wave having lightmodulation components'including a component corresponding to a predetermined shade level which tends to vary during reproduction and line-synchronizing modulation components outside the amplitude range of said light-modulation components comprising, a main signal-translating channel, a separate control signal-translating channel, asynchronous means responsive to said predetermined level whenever such level may occur in the signal of duration greater than that of a line-scanning pulse, said means including a time-constant circuit for substantially eliminating the effect of said line-synchronizing components on the output thereof, included in said control channel for deriving a control efiect variable in accordance with the intensity of said carrier wave and independent of its light-modulation components, means for utilizing said control effect for maintaining at a constant value a predetermined intermediate signal level of said signal in said main channel with respect to said predetermined shade level thereof, means for utilizing said last-mentioned signal for reproducing the light-modulation components, and separate means for utilizing said control effect for controlling an operating characteristic of said main channel.

12. A television receiver adapted to reproduce a television signal-carrier wave having light-modulation components and having a predetermined blanking level which tends to vary during reproduction and line-synchronizing modulation components outside the amplitude range of said lightmodulation components comprising, a main signal-translating channel including signal-reproducing means, a control signal-translating channel, said control channel including asynchronous means responsive to said predetermined level whenever such level may occur in the signal of duration greater than that of a line-scanning pulse, said means including a time-constant circult for substantially eliminating the-effect of said line-synchronizing components on the output thereof, for developing a control efiect varisynchronizing modulation components outside the amplitude range of said light-modulation components and having a wide range of signal-input intensities comprising, a main signal-translating channel including signal-reproducing means, a separate control signal-translating channel,- asynchronous means responsive to said predetermined level whenever such level may occur in the able in accordance with variations in said blanking level from a predetermined signal value, and individual means for utilizing said control effect to adjust a plurality of separate operating characteristics of said main signal-translating channel.

13. A television receiver adapted to reproduce a television signal-carrier wave having lightmodulation components including a component corresponding to a predetermined shade level which tends to vary during reproduction and linesignal of duration greater than that of a linescanning pulse, said means including a time-constant circuit for substantially eliminating the effect of said line-synchronizing components on the output thereof, included in said control channel for deriving a control effect variable in accordance with the intensity of the signal and independent of light-modulation components, means for utilizing said control efiect to maintain the intensity of the signal output of said main channel within a relatively narrow range,- means for utilizing said effect for maintaining at a constant value a predetermined intermediate signal level of said signal in said main channel with respect to said predetermined shade level thereof, and means for utilizing said last-memtioned signal for reproducing the light-modulation components.

14. A television receiver adapted to reproduce a television signal carrier modulated during trace periods by background and video-frequency lightmodulation components including a component corresponding to a predetermined shade level which tends to vary during reproduction unmodulated by light-modulation components during retrace periods, having a predetermined blanking level, and modulated by synchronizing-modulation components during retrace periods comprising, means for detecting said signal to derive the modulation signal stabilized with respect to the peaks of said synchronizing components, a main signal-translating channel including signal-reproducing means, a control channel, means for applying said derived modulation signal to both said channels, said control channel including asynchronous means responsive to said predetermined level whenever such level may occur in the signal of duration greater than that of a line-scanning pulse, said means including a timeconstant circuit for substantially eliminating the effect of said line-synchronizing components on the output thereof, for developing a control signal from said stabilized detected signal having its peak amplitude variable in amplitude in accordance with variations in the blanking level thereof from a fixed level, means for developing a unidirectional-bias voltage in accordance with the peak amplitude of said control signal, and individual means for utilizing said bias voltage to adjust a plurality of separate operating characteristics of the main signal-translating channel.

JOHN o. wnsom

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