US2198983A - Scanning-wave generator synchronizing system - Google Patents

Description

April 30, 1 940.

J. c. WILSON SCANNING-WAVE GENERATOR SYNCHRONIZING SYSTEM Filed Aug. 26, 1938 LJ FIGJ.

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SYNOHRORIZING siih'ron ATTORN EY Patented Apr. 30, 1940 SCANNING-WAVE GENERATOR SYNCHRO NIZING SYSTEM Application August 2c, 1938, Serial No. 226,872

9 Claims. (Cl. 178-695) UNITED STATES PATENT OFFICE This invention relates to television systems and more particularly to the synchronization of scanhing-wave generators embodied in such systems.

In television systems of the scanning beam type, in order to effect scanning of the target of the reproducing device by the beam, saw-tooth current or voltage waves are generated and utilized to produce, respectively, electromagnetic or electrostatic fields of saw-tooth wave forms which deflect the beam in two directions normal to each other for line and field scanning, thereby to trace the Well-known rectilinear scanning pattern upon the target of the tube. Synchronizing pulses are generally utilized to control the operation of the scanning-wave generators, thereby to synchronize the scanning action with other related actions in the system. For example, synchronizingpulses are usually transmitted as modulation components impressed upon the same television carrier wave as the video-frequency components and these pulses are detected and utilized at the receiver to synchronize its scanning action with the corresponding scanning at the transmitter.

It is important that the synchronization be ac complished with precision in order that proper scanning may be obtained. This is especially true in scanning systems of *the interlaced type, where the field-scanningfrequency is not an integral submultiple of the line-scanning frequency so that the lines of one field interlace or fall between the lines of a preceding field and a plurality of successive fields thus constitute a single frame or a single complete image. To obtain a precisely interlaced relation between the successive fields, it is especiallyimportant that the field trace scansions be initiated at precisely related intervals.

' Heretofore it has been the practice to synchronize scanning-wave generators at the termination of each trace-scanning period or, in other words, at the initiation of each retrace period. This has been accomplished by applying synchronizing pulses to the scanning generators so as to trigger their operation at these points.

However, it has been found that, with such synchronization, it is still diflicult to initiate the trace-scanning actions at precisely the required points in the scanning cycles.

It is an object of the present invention, therefore, to provide an improved television scanning system embodying scanning-wave generating means and synchronizing means whereby there may be developed scanning waves having their trace periods initiated at precisely predetermined system comprises a generator for developing scanning waves having trace and retrace periods. There are provided also means responsive to the synchronizing components in the signal translated 'by the system for developing synchronizing impulses. Means are further provided for utilizing these impulses to synchronize the'generator at the initiation of each trace-scanning period.

In a preferred embodiment of the invention, the system is adapted for the translation of a. signal which includes synchronizing pulses which have leading: edges. occurring during the recurring scanning intervals and includes meanstresponsive to such leading edges of the synchronizing pulses in the signal input to the system for developing synchronizing impulses representing the initiation of each retrace-scanning period as well as additional synchronizing impulses delayed for predetermined periods with respect to the main synchronizing impulses and representing initiation of each trace-scanning period. These impulses are utilized to synchronize the generator at the initiations of both the traceand pulses may, of course, represent information or be utilized to efiect a controlling action separately, if desired.

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 drawing, Fig. 1 is a circuit diagram, partially schematic, of a. complete television receiving system including scanningwave generating and synchronizing apparatus.

embodying the present invention, while Fig. 2 is a group of curves illustrating the forms of the signal waves utilized and developed in accordance with the present invention, to aid-in the understanding thereof.

Referring now more particularly to Fig. 1 of the drawing, there is illustrated a television receiving system which may be of either the tuned radio-frequency or superheterodyne type and including, in cascade, an antenna system Ill-I I, a carrier-frequency translator I2, a detector I3, a video-frequency amplifier I4, and an image-reproducing device I5, such as a cathode-ray reproducing tube. Where the receiver is of the superheterodyne type, the conventional frequency changer and intermediate-frequency amplifier are included in the "carrier-frequency translator I2. A line-frequency scanning-wave generator I6 and a field-frequency scanning-wave generator I! are coupled to the video-frequency amplifier I4 by way of a suitable synchronizing signal separator I8 and are connected to the scanning elements of the device I5 in a conventional manner. The generator I! includes synchronizing means constructed in accordance with the present invention. This generator is shown in detail and will be hereinafter further described. The parts of the system indicated in block form may be of any conventional construction and operation.

Since the operation of the system of Fig. 1, as thus far described, is, in general, well understood in the art, a detailed explanation of its operation is deemed unnecessary. Briefly, however, vision-modulated carrier waves are interdevice I5.

cepted by the antenna III-I I and and selectively amplified in the carrier-frequency translator I2. Where the receiver is of the superheterodyne type, the signals are impressed on a frequency changer in the translator I2 where they are converted into an intermediate-frequency signal and thereupon iurther selectively amplified. The amplified signal from the translator I2 is delivered to the detector I3, in which there are derived the modulation-frequency components including the video-frequency and synchronizing components, The modulation-frequency components are supplied to the video-frequency amplifier I4, wherein they are amplified and supplied in the usual manner to a brilliancy control element of the The modulation-frequency components are also supplied to the synchronizing signal separator I8, wherein the line-synchronizing and field-synchronizing components are separated from the video-frequency components and from each other and applied to the scanningwave generators I6 and I1, respectively.

' The intensity of the scanning beam of the device I 5 is thus modulated or controlled in accordance with the video-frequency voltages impressed on its control element in the usual manner. Sawtooth scanning Waves developed by the generators I6 and I! are controlled by the synchronizing components supplied from the separator I8, as

will be hereinafter further described, and are utilized in the conventional manner'to produce electric fields of saw-tooth wave form to deflect the scanning beam in two directions normal to each other so as to trace the usual scanning pattern upon the screen of the device, thereby to reconstruct the transmitted picture.

Referring now more particularly to the portion of the system of Fig. 1 embodying the present invention, the generator I'I comprises two shown. For the purpose of adjusting the rate of flow of charging current through thetube 22, thereby to control the amplitude of the wave developed by the generator, there is provided a source of adjustable grid-bias voltage, for example, a battery 23 with a voltage-divider resistor 24 connected thereacross and having an adjustable tap. The condenser 2| is also provided with a discharging circuit comprising the space current path of a vacuum tube 25 connected thereacross and normally biased beyond cutofi. A constant current thus flows through the tube 22 to charge the condenser 2I linearly over a relatively long period. The tube 25 serves periodically to discharge the condenser 2 I, thereby to develop thereacross a scanning voltage wave of saw-tooth wave form.

The relaxation oscillator 20 comprises a vac-- uum tube 26, the anode circuit of which includes an inductance element 21 connected in series with a source of operating potential, indicated at +3. The control grid-cathode circuit of the tube 26 includes an inductance element 28 coupled to the inductance element 21 and connected to a source of grid-biasing potential indicated at C. An adjustable resistor 29 is included in the cathode circuit of the tube 26 for adjusting the bias on the tube, thereby to determine the periodicity of the oscillator. The anode of the tube 26 is coupled to the control grid of the tube 25 by way of a coupling condenser 30 and leak resistor 3|. For the purpose of applying field-frequency synchronizing pulses to the oscillator 20, an output circuit of the separator I8 includes an inductance element '32 which is inductively coupled to the inductance element 28, as shown,

Considering the operation of the oscillator 2|], at the beginning of each cycle current flows from the source +B through the inductance element -21, the. tube 26 and resistor 29 gradually rising in value approximately linearly. The, change in the current in the inductance element 27 induces a voltage in the inductance element 28 of such po1arity as to increase the potential the conductance'of the tube during this part of the cycle to a maximum value. When, however,

the rate of change of current through the element 21 decreases, the voltage induced in the element 28 and, hence, the voltage on the-grid of the tube 26 also decreases. This continues until the resultant grid voltage and, hence, the conductance of the tube 26 are decreased to such values that the rate of change of'anode current reverses, that is, the current begins to decrease. This point represents the endof the trace period. The induced voltage, therefore, reverses and the grid potential becomes more negative, efiecting a regenerative action rapidly cutting off the a ode current and rendering the tube 26 inoperative. The grid potential thereafter. assumes its initial value, so that current again begins to flow through the tube and the cycle begins denovo. The current through the inductance element 21 is thus of saw-tooth wave form while the voltage wave form of the field-synchronizing signal delivered from the separator l8 and impressed upon the grid of the tube 26 by way of the inductances32, 28. The synchronizing pulses j occur at the field-scanning frequency, their leading edges, indicated by the arrows, constituting impulses which are precisely timed to correspond to the initiation of successive field-scanning retrace periods, in accordance with the usual practice. Their durations, however, due to various actions and characteristics of the system, are ordinarily not uniform at the point in the system where they are utilized.

The synchronizing pulses ,f are applied positively to the control grid of the tube 26 so that their leading leading edges initiate its firing. The oscillator 20 thus develops across the inductance element 21 very large voltage pulses, the amplitudes' and durations of which are substantially independent of the synchronizing signal, as indicated by curve B. Where the invention is employed in connection with certain types of scanning-wave generators it may be desirable to limit the amplitude of the constant deviation control pulses. This, of course, may be readily accomplished by suitably designing the constantduration pulse oscillator in accordance with wellknown practice. Each of the pulses p ofthe signal developed by the oscillator 20 thus includes a first impulse, that is, a leading edge, and a second impulse, that is, a trailing edge. The duration of these pulses is-independent of the synchronizing signal and of a precise predetermined value a dependent only upon the circuit constants of the oscillator; thus, the first impulses are made to occur at the initiations of the retrace-scanning periods and the second impulses determine the initiations of the trace-scanning periods. In other words, the pulses have a duration substan tially equal to the retrace-scanning periods and the oscillator 20 may be said to constitute a timedelay synchronizing means, since the trailing edge impulses of the constant-duration pulse are delayed forpredetermined periods with respect to the leading edges of the synchronizing pulses in the signal translated by the system.

The signal thus developed by the oscillator 20 is applied positively to the control grid of the tube of generator I!) and is effective to render the impedance of this tube gxtremely low for a period long enough substantially completely to discharge the condenser 2| and to maintain the tube 25 conductive for an interval thereafter. The voltage developed across the condenser 2 I, therefore, is of a wave form such as is illustrated by curve C of Fig. 2, the trace portions indicated at t corresponding to the periods during which the condenser 2! is charged linearly and the retrace portions 1' corresponding to the periods when the pulses p from the oscillator 20 are applied to' the tube 25 so that the condenser 2| is discharged. Each trace portion t is initiated precisely at thetrailing edge of a pulse 1) when the discharge tube 25 is suddenly biased beyond cutoff. It will be seen, therefore, that not only is the generator l9 synchronized at the initiation of the retrace period S1 by the leading edges or first impulses of the pulses supplied from the oscillator 20, but

I also the trailing edges S2 of these pulses provide a synchronizing impulse which synchronizes the operation of the generator l9 to initiate each trace-scanning period. Moreover, since the trace synchronizing impulses are precisely timed, due

to the independence of operation of-the oscillavention and that the essential principles of the present invention, including the synchronizing of an oscillator at the initiation of each trace- .scanning period and each retrace-scanning period and the provision of individual synchronizing impulses for accomplishing each of these actions, may be obtained with various other arrangements. Moreover, While the preferred embodiment of the invention which has been illustrated and described in detail comprises a system in which a scanning wave is developed which includes a quiescent period between the termination of the discharge or retrace portion of the scanning wave and the initiation of the trace portion thereof, it will be appreciated that the present invention is readily adaptable for use in connection with a system wherein a scanning wave is developed in which the trace periods initiate immediately upon termination of the discharge or retrace portions of the scanning wave. The system described, however, is deemed preferable to the last-mentioned system in that the trace periods always initiate from a reference level, whereas this is not necessarily true in the case of the wave which does not include the quiescent period.

While there has been described what is at present considered to be a 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.

What is claimed is:

1. A television system adapted for the translation of a signal including synchronizing components v occurring during predetermined recurring scanning intervals, comprising a generator for developing scanning waves having trace and retrace periods, means responsive to said. components in the signal translated by the system for developing synchronizingimpulses, and

tion of a signal including synchronizing pulses having leading edges occurring during predetermined recurring scanning intervals, comprising a generator for developing scanning waves having trace and retrace periods, and time-delay synchronizing. means responsive to said leading edges of said pulses in the signal translated by the system for synchronizing said generator at the initiation of each trace-scanning period.

3. A television system adapted for the translation'of a signal including synchronizing pulses having leading edges occurring during predetermined recurring scanning intervals, comprising a generator for developing scanning waves having trace and retrace periods, means responsive to said leading edges of said pulses in the signal I translated by the system for developing impulses delayed for predetermined periods with respect thereto, and means for utilizing said impulses to synchronize said generator at the initiation of each trace-scanning period.

4. A television system adapted for the translation of a signal including synchronizing pulses having leading edges occurring during predetermined recurring scanning intervals comprising a generator for developing scanning waves having trace and retrace periods, meansfiresponsive to said leading edges of said pulses in the signal translated by the system for developing pulses of predetermined durations, and means for utilizing the trailing edges of said last-mentioned pulses for synchronizing said generator at the initiation of each trace-scanning period.

5. A television system adapted for the translation of a signal including synchronizing pulses having leading edges occurring during predetermined recurring scanning intervals, comprising a generator for developing scanning waves having trace and retrace periods, a constant-duration pulse generator, means for utilizing said leading edges of said pulses in said signal translated by the system for synchronizing said constantduration generator at the initiation of each constant-duration pulse, and means for applying saidconstant-duration pulses to said scanningwave generator to synchronize its operation at the initiation of each trace-scanning period.

6. A television system adapted for the translation of a signal including synchronizing components occurring during predetermined recurring scanning intervals, comprising a generaairwaves tor for developing scanning waves having trace and retrace periodspmeans responsive to said components in the signal translated by the system for developing synchronizing impulses corresponding to the initiations of trace and retrace-scanning periods during said scanning intervals, and means for utilizing said impulses to synchronize said generator at the initiation of each trace-scanning period and each retracescanning period.

8. A television system adapted for the translation of a signal including synchronizing components occurring during predetermined re curring scanning intervals, comprising a scanning generator including capacitance means, charging each of said retraceand trace-scanning periods.

9. A television system adapted for the translation of a signal including synchronizing components occurring during predetermined recurring scanning intervals, comprising ascanning generator including capacitance means, charging and discharging circuits for said capacitance means, a vacuum tube for controlling said circuits, said tube being normally nonconductiveto efiect charging of said capacitance means and being periodically rendered conductive to effect discharge of said capacitance means, thereby to developacross said capacitance means a sawtooth scanning wave including trace and retrace periods, means for deriving from-said synchronizing components in the signal translated by the system pulses having durations corresponding to the durations of the retrace-scanning periods, and means for applying said last-mentioned pulses positively to said vacuum tube to render said tube conductive only during said retrace periods. r

' JOHN 0. WILSON.

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