US2217948A

US2217948A - Automatic amplification control

Info

Publication number: US2217948A
Application number: US170556A
Authority: US
Inventors: Daniel E Harnett; Harold A Wheeler
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1937-10-23
Filing date: 1937-10-23
Publication date: 1940-10-15
Anticipated expiration: 1957-10-15
Also published as: BE430728A; CH213354A; FR844956A

Description

' D. E. HARNETT ETAL Filed Oct. 23. 1937 AUTOMATIC AMPLIFICATION CNTROL- Oct. l5, 1940.

ATTORNEY Patented Oct. l5, 1940 v UNITED STATES AU'roMA'ric Wheeler, tine Corporation,

ABIPLIFICATION CONTROL Daniel E. Harnett, Tuckahoe,

Great Neck, Ni

and Harold A. Y., assigner; to Hazela corporation of Delaware Application October 23, 1937, Serial No. 170,556

4 Claims.

This invention relates to television-receiving systems and, more particularly, to the automatic control of one or more operating characteristics of such systems in accordance with the ampli- 5 tude characteristics of received signals.

The invention is especially directed to the provision oi an improved automatic amplification control system for television receivers.

In accordance with present television practice,

a transmitted signal comprises a carrier which is modulated during successive intervals or trace periods by high-frequency and direct (average) components representative of an image being transmitted and of its average background illumination, and is modulated between the trace periods or during retrace intervals by synchronizing components which correspond to initiations of successive lines and frames in the scanning of the image.

At the receiver, a beam scan and illuminate a target in a s eries of frames of parallel lines. The synchronizing components of the received signal are utilized to control the scanning apparatus of the receiver so as to syn- 2.3 chronize its operation with that of similar apparatus utilized ing the signal. The intensity of the beam is controlled by the light modulation components, thereby to reproduce the image.

In certain proposed television systems, the video-frequency modulation caused by the image is so impressed on the carrier that increases in the carrier amplitude correspond to increases in Vbrightness of the image, this being termed posi- 3; tive modulation; the background illumination is transmitted directly, that is, by variation ofthe average carrier intensity during the trace intervals, directly in accordance with the low-frequency light` variations or steady background; 49 and the synchronizing impulses are impressed on the carrier in the opposite sense, ordinarily periodically reducing the carrier to zero. While systems utilizing this type oi' signal have, in general, given satisfactory results, they have been objec- 45 tionable in that it has been difllcult or impossible heretofore to provide satisfactory automatic ampliiication control. More particularly, since the average intensity of the carrier is varied in accordance with background light-modulation com- 59 ponents, the average intensity cannot be utilized to eilect satisfactory automatic amplification control in the conventional manner which requires some controlling efl'ect dependent solely upon the average carrier intensity as determined by the 55 power of the transmitting station, its distance, fading, etc., and independent of the modulation. Nor can the amplitude of the synchronizing impulses oi such a signal be made readily available for this purpose, as they can in the case of nega- 00 tive modulation where these impulses are transis so deected as to at the transmitter in develop-t mitted on outward peaks of modulation at a relatively xed amplitud since, in positive modulation systems, the synchronizing impulses are ordiiarily transmitted on inward peaks of modala lon.

It is an object oi' the present invention, therefore, to provide in a television receiver improved means for automatically controlling one or more y operating characteristics of the receiver in accordance with an amplitude characteristic oi' the received modulated-carrier signals and independently of the video-frequency modulation componente. l

It is a further object o! the invention to provide an improved automatic amplification control system iortelevlsion receivers adapted for the reception of positively modulated television signals including background and high-frequency light-modulation components.

In accordance with the invention, a television receiver adapted for the reception of a television signal carrier positively modulated during trace periods by background and high-frequency light components and unmodulated by light components during retrace periods comprises a carrierfrequency signal-translating channel in the receiver including means for rectii'ying the. carrier signal translated by the `channel only during retrace periods for developing a control-bias voltage, together with means for utilizing the developedvoltage to adjust an operating characteristic of the receiver. s

In a preferred embodiment of the invention,

the control means comprises a signal repeater having an input circuit adapted to -receive the modulated-carrier signal and having an output circuit. A rectifier is coupled to the output circuit and means are provided for developing a periodic wave' having `impulses occurring during the eld retrace periods of the signal and for utilizing this wave to control the repeater so as to prevent rectication oi' the carrier rexcept during the eld retrace periods. The scanning wave generator of the receiver may be used as the source of the periodic impulse wave. 'I'he unidirectional voltage developed by the rectier is utilized-to control the amplification in various stages of the system, thereby to maintain the signal output intensity Within a relatively Vnarrow range fora lwide range of signal-input intensities. y 2

For a better understanding of the present invention, together with other and vfurther objects thereof, reference is had to the following description taken in connectionwith the accompanying drawing, and'its'scope will be pointed eut in the appended claims.l i

In the accompanying drawing, Fig. l'is a circuit diagram, partially schematic, of al cathoderay tube television-receiving. system including 6U circuits embodying the present invention; Fig. 2 is a group of curves representing certain operating characteristics of the receiver of Fig. 1; and Fig. 3 is a diagrammatic illustration of a positively modulated television carrier wave, to aid in the understanding of the invention.

Referring now more particularly to the drawing, the system there llustrated comprises a receiver of the superheterodyne type including an antenna and ground system IIi-II connected to a radio-frequency amplifier I2 to which is connected in cascade. in the order named, an oscillator-modulator I3, an intermediate-frequency amplifier I4, a detector I5, a video-frequency amplifier I6, and a cathode-ray signal-reproducing tube I'I. A line-frequency generator I8 and a frame-frequency generator I9 are also coupled to the video-frequency amplifier and connected to the scanning elements of the cathode-ray tube in the conventional manner. The stages or units Ill-I9, inclusive, may all be of conventional wellknown 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 IIi-II are selected and ampliiied in the radio-frequency ampliiier I2 and supplied to the oscillator-modulator i3; where they are converted to intermediate-frequency signals which, in turn, are sethe,intermediate-frequency amplifier I4 and delivered to the detector I5. The modulation components of the signal are derived by the detector I5 and are supplied to the video-frequency amplifier I6 wherein they are amplied and from which they are supplied in the usual manner to a brilliancy-control electrode of the cathode-ray tube I1 and to the synchronizing-control elements of the generators IB and I9. The intensity of the electron beam of the tube I5 is thus modulated or controlled in accordance with the video-frequency voltages impressed upon the control electrode of the tube in the usual manner. Saw-tooth current or voltage-scanning waves are generated in the lineand frame-frequency generators I8 and I9, which are controlled by synchronizing-voltage impulses supplied from the video-frequency amplifier I6 and applied to the scanning elements of the cathode-ray tube I1 to produce electricscanning iields, thereby to deflect the ray vertically and horizontally so as to `trace a rectilinear scanning pattern on the screen of the tube and thereby to reproduce the transmitted picture.

Referring now more particularly to the portion of the system of Fig. 1 embodying thepresent invention, for the purpose of developing a control-bias voltage there is provided an intermediate carrier-frequency signal-translating channel including a repeater and a rectifier 2I as shown. The repeater 20 may comprise a vacuum tube of the pentode type having an input circuit coupled to the intermediate-frequency arn liiier through a coupling transformer 22, 24 and an intermediate-frequency by-pass condenser 23. A fixed negative bias is provided for the tube 20 from a suitable source, for example, a battery 25 connected'between its cathode and ground. Operating potent' als are supplied to the anode and screen of the tube from a suitable source, indicated as +B and +Sc, respectively. The diode rectifier is coupled to the output circuit of the repeater by means of a

transformer

21, 29 and is provided with a load circuit `modulated as indicated at M1, Mz,

comprising a resistor 3| and by-pass condenser 3U having a large time constant. This time constant should be considerably greater than the frame period which may, for example, be m second, in which event the time constant may be of the order of 116 second. The negative end of the resistor 3| is connected, by way of a suitable lowpass illter including a series resistor 32 and shunt condenser 33 and conductor 34, to the control electrodes of one or more vacuum tubes included in the amplifier I2, oscillator-modulator I3, and amplifier Il, as shown.

The frame-frequency generator I9, as stated above, may be of conventional design and adapted to develop a suitable saw-tooth frame-frequency scanning wave. As is well known, however, such a generator also generally develops at certain points of its circuit a periodic-impulse wave having impulses at the frame frequency, and this wave may be taken from a suitable point in the generator. For the purpose of controlling the amplifier 20 and rectifier 2| to limit rectification of the carrier signal translated thereby to the signal-retrace periods, therefore, an output circuit of the generator I9, in which a frame-frequency periodic-impulse wave is developed, is connected by way of a conductor 2B and the winding 22 to the control grid of the tube 20. It is so connected that the impulse voltage is applied with positive polarity to the grid of the tube 20.

The operation and results obtained by the system just described may best be understood by reference to Figs. 2 and 3. The graphs of Fig. 2 represent the operating characteristics of the amplifier 20,' the abscissae representing time and the ordinates voltage. The line A represents the fixed negative-grid voltage determined by the battery 25. while the line B represents the resultant grid-bias voltage comprising the fixed bias, line A, with the frame-frequency impulse Wave superimposed thereon. The cutoff bias voltage of the tube is indicated at C and it will be noted that the normal grid-bias voltage represented by the portions of graph B between the impulses is below cut-off, while the impulses carry the tube well above cutoff, permitting the translation of signals by the tube 20 to the rectier 2l during these periods.

In Fig. 3 there is illustrated the wave form of the modulation envelope of a television carrier wave of the positively modulated type. The time Y-Yi corresponds to, or at least includes, the duration of one i the impulses of line B, Fig. 2.

The portions of the wave where the carrier is reduced to zero, certain of which are indicated at L. represent line retrace periods or impulses. The portions between the impulses L correspond to the trace portions of successive lines and are and Ms by the high-frequency and background light components, except during the frame retrace periods,

.one off which is indicated at Y-Y1, during which periods the amplitude of the wave is independent of light modulation. The frame retrace periods include the line impulses as well as additional broad impulses, as indicated at F, which latter constitute collectively a frame-synchronizing impulse.

It is well known that, with the positive type of modulation. such as is impressed on the signal shown in Fig. 3, increases in light intensity are represented by increases in the amplitude of the carrier and the average intensity of the signal is nents of the image. Thus,

varied in accordance with the low-frequency or direct current background illumination `coxn in the particular portion of the wave shown, at one point during the line-trace modulation period 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 Mz and M11. 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 amplication control. in this type of wave any synchronizing impulses which could be utilized for the purpose in question, since the signal is reduced to zero during the line retrace or line-synchronizing impulse periods. However, since those portions of the wave occurring during the frame retrace period, indicated at YY1, and between the line retrace periods have substantial amplitude and are not modulated by light components, they afford a measure of the intensity of the carrier wave independent of light modulation which may be utilized automatically to control a characteristic of the receiversuch as its amplification.

In the operation of the present invention, therefore, a part of the modulated-carrier output of the intermediate-frequency amplifier Il is impressed upon the control grid of the tube by way of the transformer 24, 22. While, as stated above, the battery serves normally to bias this tube below cutoi, as shown in Fig. 2, the impulses of ythe periodic frame-frequency wave, which are shown in line B of Fig. 2 and each of which occurs during a frame retrace period. are impressed upon the control grid of the ampliiier and iserve to reduce the bias and unblock the tube during at least part of the duration of the frame retrace periods of the wave.' Hence, only those portions of the wave which are independent of light modulation are translated to the rectier 2| and rectified therein. A control-bias voltage is thus developed across the resistor 3l and condenser 30 which is'fairly steady and is proportional to the average intensity of the carrier, independent of light modulation. 'I'he lter, comprising the resistor 3l and condenser 30, supplemented by resistor 32 and condenser 33, serves to remove uctuations from the rectied voltage andthe resultant unidirectional-bias voltage is impressed on the control electrodes o1' one or more of the tubes in the stages I2, I3, and I4 to control the gain oi' these stages inversely in accordance with variations of the average carrier intensity during the frame retrace peri Thus, the output-signal intensity of the channel, including the stages I2, I3, and Il, -is maintained within a relatively narrow range for a wide range of signal-input intensities. It will be noted that where the expression wide range oi' signal-input intensities is employed herein and in the appended claims. it refers to the intensity variations of the received carrier such as occur during the frame retrace periods. independent of light modulation, and not to the relatively smaller average carrier intensity variations which are caused by variation oi.' the average background illumination.

It will be appreciated that the .present invention may be utilized automatically to control various characteristics ot a television receiver other than its amplification, for example, its selectivity or its timing.

Nor, as stated above, are there.

Thus, while therehas been described what is at present considered to be the preferred embodiment of the invention, it will be apparent to those skilled in the art that various changes and modications ymay be made without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A television receiver adapted for the reception of a television signal carrier positivelymodulated during trace periods by background and high-frequency light components and unmodulated by light vcomponents during retrace periods comprising, -a carrier-frequency signal-translating channel in said receiver including means for rectiiying the carrier signal translated by said channel only during retrace periods for developing a. control-bias voltage, and means for utilizing said voltage to adjust an operating characteristic of the receive i 2. A television receiver adapted for the reception of a television signal carrier positively modulated. during trace periods by Ibackground and high-frequency light components and unmodulated by light`components during retrace periods comprising, means for developing-a .periodic wave having impulses occurring during said"retrace\` periods, a carrier-frequency signal-translating channel in said receiver including means for rectifying the signal translated by said channel,

means controlled by said impulse wave for preventing re'ctiilcation of said translated carrier except during said retrace periods, thereby to develop a control-bias voltage, and means for utilizing said bias voltage to adjust an operating characteristic of vthe receiver.

3. A television receiver adapted for the reception of a television signal carrier positively modulated during trace periods by background and high-frequency light components and unmodulated by light components during retrace periods comprising, a carrier-frequency repeater having an output circuit, a rectier coupled to said output circuit, means for controlling said repeater for preventing translation of a signal carricr thereby except during said retrace periods, and means coupled to said rectiiler for utilizing` the products of rectication to control an operating characteristic oi said receiver.

4. A television receiver adapted i'or the rei ception of a television signal carrier positively modulated during trace periods by background and high-frequency light components and unmodulated by light components during retrace periods comprising. acarrier-frequency ampliiier having an output circuit,a rectiiler coupled to said output circui a scanning-wave generator for developing periodic impulses during said retrace periods, .means for controlling said ampli- 'iler including means coupling said generator to said amplliler for preventing translation of la carrier signal by said ampliiier except during said retrace periods, and means coupled to said rectiiier for utilizing the products o! rectiiication to control an. operating characteristic of said receiver.