US2240422A

US2240422A - Synchronizing circuit

Info

Publication number: US2240422A
Application number: US271778A
Authority: US
Inventors: Jr Newland F Smith
Original assignee: Philco Radio and Television Corp
Current assignee: Philco Radio and Television Corp
Priority date: 1939-05-04
Filing date: 1939-05-04
Publication date: 1941-04-29
Anticipated expiration: 1958-04-29

Description

April 29, 1941. N. F. SMITH, JR 2,240,422

SYNCHRONIZING CIRCUIT Filed May 4, 1959 3 Sheets-Sheet l W mm \ K (\P i JJ mm April 29,1941. N. F. SMITH, JR

SYNCHRONIZING CIRCUIT Fiied May 4, 1939' 3 Sheets-Sheet 2 April 29, 1941. N. F. SMITH, JR

SYNCHRONIZING CIRCUIT 3 Sheets-Sheet 3 Filed May 4, 1959 Patented Apr. 29, 1941 azsanz UNITED STATES PATENT ()FFICE SYNCHRONIZIN G CIRCUIT Newland F. Smith, Jr., Wyndmoor, Pa., assignor,

by mesne assignments, to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware 7 Application May 4, 1939, Serial No. 271,778

10 Claims.

This invention relates to improvements in synchronizing circuits such, for example, as those now employed in television receiving systems.

More specifically it relates to systems in which local means are employed at the television re ceiver to generate a deflecting signal in response to synchronizing signals transmitted through the medium of modulated carrier waves or by other means and which may comprise impulse signals or signals of other nature. However, although the invention will be described as applied to a conventional television receiver, it will appear.

picture to be televised is analyzed into a large number of elemental areas and the amplitude of.

an electrical signal is caused to vary from point to point along a time axis in response to the variations of light intensity from point to point on the surface of the picture. The signal 'thus formed is termed the video signal and the means whereby it may be produced are familiar to those skilled in the art, so that no detailed account need be given of this process. Suffice it to say that the usual method comprises causing the said video signal to vary in accordance with the light intensity variations of the elemental areas taken successively in a systematic manner, which process is commonly termed scanning the picture. The usual method is to scan from side to side in lines taken successively from top to bottom. For example, the pick-up system for producing the Video signal may comprise an electron discharge device in which an electron beam is deflected in such a manner as to produce scanning of this type, which is well known. Such deflection may be produced by applying the proper voltage or current wave-forms to deflecting plates or coils associated with the electron discharge device.

In order to obtain a satisfactory reconstitution of the picture at the receiver itis essential that a scanning action be carried on there which is substantially the same as that at the transmitter. One way of achieving this result is to transmit, along with the video signal, a signal corresponding in form to the deflecting signal used at the transmitter which may be employed to actuate the deflecting meansin a reconstituting electron discharge device at thereceiver.

a signal, it is more usual to generate at the transmitter a series of time-spaced synchronizing pulses of periodicity corresponding to that of the deflecting signal. These pulses may be combined with the video signal to form a composite signal which may be caused to modulate a single carrier. This may be done in such a manner as to facilitate separation of the synchronizing pulses from the video component at the receiver by methods which are known. Ihe pulses nay be employed at the receiver to actuate means for generating deflecting signals which are of substantially the same form as those at the transmitter.

In certain cases it is found that the energy content of the received synchronizing pulses is not sufiicient to permit them to be used directly to actuate the deflecting voltage generator which may comprise a circuit including a condenser, a

resistor, and a space discharge device, the condenser being periodically charged by the pulse signals and subsequently discharging through the resistance until it is again recharged. Since it is not desirable to employ an ordinary vacuum tube amplifier in order to obtain pulses of sufficient energy content to charge the condenser, it is common to employ a synchronizable oscillator, such as one of the blocking-tube variety. Such a device may be operated at a frequency appreciably different from that of the occurrence of the synchronizing pulses. However by virtue of the introduction of synchonizing pulses it may be caused to generate pulse signals of considerably greater energy content than the synchronizing pulses but isochronous in time with these pulses. These generated pulses may be supplied to the discharge circuit and are more effective than the received synchronizing pulses to actuate the device.

One disadvantage of such a system arises from the fact that the free period of the blocking-tube or similar oscillator may tend to vary during operation to such an extent that the device is no longer capable of being synchronized or lockedin with the synchronizing pulses so as to generate a pulse signal corresponding to each and every. one of them. Alternatively the received synchronizing pulses may be so small that they are ineffective to initiate action of the oscillator. Thenormal chain of time-spaced pulses will then be broken by gaps in which the output voltage from the oscillator remains constant at a particular level. During these gaps. there will be no pulses operative to actuate the discharge tube, no deflecting signal will be generated after'the condenser in the discharge circuit'has once discharged and, in the case where an ordinary cathode ray tube is employed as a reconstituting meana-the beam will tend to remain stationary although its intensity will continue to vary in response to the video signal. This failure of the beam to scan the screen of the tube may have several undesirable efiects among the most obnoxious of which are the tendencies to spoil a considerable portion of the reconstituted picture and also permanently to impair the fluorescent screen by burning it due to the continued action of the beam upon a relatively small portion of its surface area.

It is the principal object of the present in vention to provide an improved method and means whereby the above-mentioned difficulties may be obviated. According to the method of invention means are provided whereby, so long as the deflecting system remains locked in with the received synchronizing pulses, the control voltage applied to the control grid of the electron discharge device which is used as a reconstituting means in a television receiver, is caused to vary in such a sense with respect to the cutoff voltage of the tube that the beam is permitted to impinge upon the screen so as to produce a luminescent spot which is capable of being moved about the screen and varied in intensity so as to produce the picture. Further means are provided which, when the synchronization .fails by virtue of the failure of the local pulse-generating oscillator to respond to incoming synchronizing pulses, the video signal variations applied to the picture tube are caused to occur in such a sense with respect to the tube cut-ofi that no electrons are caused to impinge upon the screen.

Reference may now be had to the accompanying drawings wherein:

Figs. 1A, 1B and 1C illustrate the method of the invention;

Fig. 2 is a diagrammatic illustration of one embodiment of the invention; and

Fig. 3 is a similar diagram of a modification.

The method of the invention is particularly adapted for use with systems employing a composite signal in which the synchronizing component manifests its-elf as a series of pulse'signals, as shown in Fig. 1B, of such amplitude that they extend in a sense corresponding to decreasing light intensity and appreciably beyond that level of video signal which corresponds to zero light intensity or, in other words, in the direction of blacker than black as it is frequently referred to. Fundamentally, the method consists in combining the video signal (see Fig. 13) with the signal produced by the aforementioned pulse generator (see Fig. 1A) in the receiver at some point in the video system prior to the control grid of the picture tube. Thus the signals of Figs. 1A and 1B are combined to obtain the signal of Fig. 10. So long as the pulse generator continues to function in a satisfactory manner the signal obtained from it will consist of pulses corresponding in time to the synchronizing pulses which appear in the composite signal and which, when combined therewith, will add to the amplitude of the said synchronizing pulses. When the synchronization fails, due to the failure of the pulse generating oscillator to respond to an incoming synchronizing pulse, no pulses will be supplied by the local oscillator and the efiectof combining with the composite signal will be to give no efiective increase in the amplitudes of the synchronizing pulses. The signal resulting from this combining operation is next leveled on the tips of the synchronizing pulses, as indicated in Fig-1C. This may be achieved, for example, by using a peak detector. In most cases this-will not require the introduction of any additional equipment, it being customary to include such a diode leveler in television receivers at a point just prior to the control grid of the picture tube. Finally the transmission characteristic of the means used to supply the combined signal to the control grid of the picture tube is so modified that only that portion of the signal in excess of a particular amplitude level is transferred. This level is so established that the video signal is supplied intact to the grid of the picture tube so as to vary the spot intensity in the desired manner so long as the deflecting system is in synchronism but so as to prevent the transfer during the time when the system is unsynchronized. This obtains by virtue of the greater effective amplitude of the synchronizing portions of the video signal when augmented by pulses from a local oscillator, as compared with their effective amplitude when not so augmented, the efiect of which is to raise the video variations above the transmission level when pulses are added and to drop the video variations below the said level when pulses are not added.

Figs. 1A, 1B and 1C illustrate this by showing at the left the conditions obtaining when the pulses are added and showing at the right the conditions obtaining when the pulses are not added. The dotted pulse representations indicate missing pulses due to failure to synchronize. The cut-off level may be established with respect to the signal as shown in Fig. 1C. It will be noted that, in order to permit of the desired amplitude selection so as to pass the entire video signal when the receiver is synchronized and to exclude all video when it is'not, it is essential that the amplitude of the pulses shown in Fig. 1A which are to be combined with the composite signal of Fig. 1B should be at least equal in amplitude to the difference between that value of signal which corresponds to maximum light and that which corresponds to no light. This difference is indicated at X in Figs. 1A and 13. It will be understood that the transition from the condition in which the entire video signal is transferred to the picture tube to the condition in which no video is transferred is not instantaneous when the synchronization fails. Rather there is a gradual transition, the time required for the complete change being dependent upon the magnia pick-01f tube V1 which is so biased by means of the resistor I that it can pass only the synchronizing portion of the composite signal supplied to it from the video detector and which has the wave form corresponding to that shown in Fig. 1B but reversed in phase. The grid circuit may also be made to function so as to level the incoming signal as is familiar to those skilled in the art. The output of this tube is amplified and reversed in phase in a conventional manner in V2 from the output circuit of which it is supplied to the grid circuit of the blocking tube oscillator which comprises the electron discharge tube V3,

the transformer 2, the condenser 4, and the resistance 3. This oscillator serves as the local generator of pulse signals and operates in a familiar manner which is set forth in a copending application of Frank J. Bingley, Serial No. 224,646, it of course being understood that any other convenient type of oscillator may be. employed if desired. The output of the oscillator actuates the discharge circuit comprising the tube V4, the resistor 5, the condenser 6, and the peaking resistor 1, all of which are well known. The deflecting Voltage produced is a combination of a sawtooth voltage and an impulse voltage and is supplied to the deflecting coils 8 of a reconstituting device by means of the output tube V5.

The other branch of the circuit comprises three stages of video frequency amplification including the tubes V6, V7 and V8 followed by a diode leveler V9, which functions in conjunction with the resistor 9 and the condenser H) to level the combined signal as hereinbefore described. The actual manner in which leveling obtains is set forth in a copending application of Alexander R.- Applegarth, Serial No. 219,643.

According to the method of the invention, pulse signals are fed from a resistor inthe plate circuit of the blocking tube oscillator by means of a connection which comprises blocking condenser H and isolating resistance l2, to the plate of the amplifier tube Vs which serves to combine this signal with the video signal already there, as illustrated in Fig. 1C. This composite signal is leveled and is supplied to the control grid I3 of the cathode ray tube M which may be so biased by the voltage tapped from the divider l6 at the tap l that only the signal above a particular level will be effective to operate the tube.

Fig. 3 shows a further embodiment of the invention in which the circuit is substantially entirely the same as that of Fig. 2. The significant difierence is that here impulses in a positive sense which are present in the grid circuit of the blocking tube oscillator are derived from the horizontal synchronizing circuit and are applied across a resistor H included in the plate lead of the leveling diode V9. The result is the same in either case the sole difference lying in the mode of deriving the synchronizing pulse and the mode of applying it in order to augment the synchronizing pulse amplitudes.

It will of course be understood that there are other points. in the horizontal deflecting circuit from which either positive or negative pulse signals might be obtained and other points to which they might be supplied for combination with the video signal. For example, positive pulses may be derived from a resistor placed in the cathode circuit of the discharge tube V4, from a resistor in the cathode circuit of the blocking-tube oscillator, or from a single resistor in series with both cathodes, as well as by the method shown. Negative pulses might alternatively be derived from the pea-king resistor in the discharge tube Vs. It will also be seen that the choice of a place at which to apply the said pulses for the purpose described is principally a matter of convenience and of obtaining the proper polarity. For example, in Fig. 3, the connection through the, condenser H and the resistor I2 might be applied to the plate of the tube V1. in which case, however, it wouldbe desirable to employ a. smaller amplitude of pulse signal which might be obtained by usinga larger value of isolating resistor I2 or by any other convenient means.

It will be understood,.therefore, that the invention is not limited to the specific embodiments herein illustrated and described but contemplates the use of any suitable means for carrying out the methods herein disclosed.

I claim:

1. In a television system, a source of a composite signal including video and synchronizing components, said synchronizing component comprising a series of time-spaced pulse. signals superimposed upon said video signal during intervals in which said video component is established at a predetermined level; a deflecting circuit supplied with signal from said source and normally adapted to be synchronized in response to said synchronizing pulses; means adapted to generate pulse signals of duration comparable to said intervals in response to said synchronizing component only when said deflecting circuit is properly synchronized; signal combining means, coupled to said composite signal source and to said pulse generating means, for producing a signal which is the sum of said generated pulse signals and said composite signal, said last-named means comprising an amplifier including a plurality of electron discharge devices each having a cathode, an anode and control grid, the grid of one of said devices being coupled to said composite signal source and the plate of one of said devices being coupled to said pulse generating means; means for leveling said combined signal on signal peaks corresponding to said pulse signals; electron discharge means for converting the variations in said video signal into variations in light intensity; means coupling said leveling means to said electron discharge means; and

means for permitting said electron discharge means to respond to only that portion of said leveled sigznal above a predetermined amplitude level.

2. In a television system, a source of composite signal comprising video and synchronizingcomponents established on opposite sides of a blanking level, said synchronizing component comprising a series of time-spaced pulse signals occurring during intervals in which said video component is established at said blanking level, a picture reproducing device adapt-ed to be controlled in response to a signal supplied thereto, means for rendering said reproducing device inoperative to signals not exceeding a predetermined level, a deflecting circuit operatively associated with said reproducing device and supplied with at least the synchronizing component of the signal from said source'and normally adapted to be synchronized in response to the individual pulse signals comprising said synchronizing component, means adapted to generate a pulse signal whenever said deflecting circuit responds to a pulse of said synchronizing component, said. generated pulse signals having an amplitude at least as great as the amplitude of said video component, means for utilizing said composite signal and said generated pulse signals to produce a modified signal including said composite signal and said generated pulse signals and having its peaks leveled, and means for supplying said modified signal to said reproducing device, whereby in the presence of said generated pulse signals said video component will be caused to rise above said predetermined level and said reproducing device will respond thereto, while in the absence of said generated pulse signals, due to the failure said deflecting circuit to respond to a pulse of said synchronizing component, said video component will fail to exceed said predetermined level and said reproducing device will fail to respond to any portion thereof.

3. In a television system, a'source of composite signal comprising video and synchronizing components established on opposite sides of a blanking level, said synchronizing component comprising a series of time-spaced pulse signals occurring during intervals in which said video component is established at said blanking level, a picture reproducing device adapted to be controlled in response to a signal supplied thereto, means for rendering said reproducing device inoperative to signals not exceeding a predetermined level, a deflecting circuit operatively associated with said reproducing device and supplied with at least the synchronizing component of the signal from said source and normally adapted to be synchronized in response to the individual pulse signals comprising said synchronizing component, means adapted to generate a ulse signal whenever said deflecting circuit responds to a pulse of said synchronizing component, said generated pulse sig nals having an amplitude at least as great as the amplitude of said video component, means for utilizing said composite signal and said generated pulse signals to produce a modified signal which is the result of combining said composite signal and said generated pulse signals and bringing the peaks thereof to a common level, and means for supplying said modified signal to said reproducing device, whereby in the presence of said generated pulse signals said video component will be caused to rise above said predetermined level and said reproducing device will respond thereto, while in the absence of said generated pulse signals, due to the failure of said deflecting circuit to respond to a pulse of said synchronizing component, said video component will fail to exceed said predetermined level and said reproducing device will fail to respond to any portion thereof.

4. In a television system, a source of composite signal comprising video and synchronizing components established on opposite sides of a blanking level, said synchronizing component comprising a series of time-spaced pulse signals occurring during intervals in which said video component is established at said blanking level, a picture reproducing device adapted to be controlled in response to a signal supplied thereto, means for rendering said reproducing device inoperative to signals not exceeding a predetermined level, a deflecting circuit operatively associated with said reproducing device and supplied with at least the synchronizing component of the signal from said source and normally adapted to be synchronized in response to the individual pulse signals comprising said synchronizing component, means adapted to generate a pulse signal when- .ever said deflecting circuit responds to a pulse of said synchronizing component, said generated pulse signals having an amplitude at least as great as the amplitude of said video component, means for combining said generated pulse signals with said composite signal, means for modifying said combined signal to bring its peaks to a common level, and means for supplying said combined and leveled signal to said reproducing device, whereby in the presence of said generated pulse signals, said Video component will be caused to rise above said predetermined level and said reproducing device will respond thereto, while in the absence of said generated pulse signals, due to the failure of said deflecting circuit to respond to a pulse of said synchronizing component, said video component will fail to exceed said predetermined level and said reproducing device will fail to respond toany portion thereof.

5. In a television system, a source of composite signal comprising video and synchronizing components established on opposite sides of a blanking level, said synchronizing component comprising a series of time-spaced pulse signals occurring during intervals in which said video component is established at said blanking level, a picture reproducing device adapted to be controlled in response to a signal supplied thereto, a deflecting circuit operatively associated with said reproducing device and supplied with at least the synchronizing component of the signal from said source and normally adapted to be synchronized in response to the individual pulse signals comprising said synchronizing component, means for generating a pulse signal whenever said deflecting circuit responds to a pulse of said synchronizing component, means for combining said generated pulse signals with said composite signal, means vfor modifying the combined signal to bring its peaks to a common level, means for supplying the combined and leveled signal to said reproducing device, and means for rendering said reproducing device operative in response to the combined and leveled signal and for rendering the reproducing device inoperative in response to the composite signal alone, whereby the reproducing device responds to said video component only as long as the deflecting circuit responds to the syn I chrom'zing pulses.

6. In a television system, a source of a composite signal comprising video and synchronizing components established on opposite sides of a blanking level, said synchronizing component comprising a series of time-spaced pulse signals occurring during intervals in which said video component-is established at said blanking level, a

picture reproducing device adapted to be con- 1 .signal from said source and normally adapted to by synchronized in response to the individual pulse signals comprising said synchronizing component, means forming a part of said deflecting circuit and adapted to generate a pulse signal whenever said deflectin circuit responds to a pulse of said synchronizing component, said generated pulse signals having an amplitude at least as great as the amplitude of said video component, means for utilizing said composite signal and said generated pulse signals to produce a modified signal including said composite signal and said generated pulse signals and having its peaks leveled, and means for supplying said modified signal to said reproducing device, whereby in the presence of said generatedpulse signals said video component will be caused to rise above said predetermined level and said reproducing device will respond thereto, while in the absence of said generated pulse signals, due to the failure 5, of said deflecting circuit to respond to a pulse of said synchronizing component, said video component will fail to exceed said predetermined level and said reproducing device will fail to respond to any portion thereof.

= f 7. In a television system, a source of a composite signal comprising video and synchroniz "occurring during blanking intervals. in which said video component is established at said blanking level, a picture reproducing device adapted to be controlled in response to a signal supplied thereto, means for rendering said reproducing device inoperative to signals not exceeding a predetermined level, a deflecting circuit operatively associated with said reproducing device and supplied with at least the synchronizing component of the signal from said source and normally adapted to be synchronized in response to the individual pulse signals comprising said synchronizing component, means adapted to generate a pulse signal of duration comparable to the duration of said blanking intervals whenever said deflecting circuit responds to a pulse of said synchronizing component, said generated pulse signals having an amplitude at least as great as the amplitude of said video component, means for utilizing said composite signal and said generated pulse signals to produce a modified signal including said composite signal and said generated pulse signals and having its peaks leveled, and means for supplying said modified signal to said reproducing device, whereby in the presence of said generated pulse signals said video component will be caused to rise above said predetermined level and said reproducing device will respond thereto, while in the absence of said generated pulse signals, due to the failure of said deflecting circuit to respond to a pulse of said synchronizing component, said video component will fail to exceed said predetermined level and said reproducing device will fail to respond to any portion thereof.

8. In a television system, a source of a composite signal comprising video and synchronizing components established on opposite sides of a blanking level, said synchronizing component comprising a series of time-spaced pulse signals occurring during blanking intervals in which said video component is established at said blanking level, a picture reproducing device adapted to be controlled in response to a signal supplied thereto, means for rendering said reproducing device inoperative to signals not exceeding a predetermined level, a deflecting circuit operatively associated with said reproducing device and supplied with at least the synchronizing component of the signal from said source and normally adapted to be synchronized in response to the individual pulse signals comprising said synchronizing component, means adapted to generate a pulse signal of duration comparable to the duration of said blanking intervals whenever said deflecting circuit responds to a pulse of said synchronizing component, said generated pulse signals being at least partially concurrent with said synchronizing pulses and having an amplitude at least as great as the amplitude of said video component, means for utilizing said composite signal and said generated pulse signals to produce a modified signal including said composite signal and said generated pulse signals and having its peaks leveled, and means for supplying said modified signal to said reproducing device, whereby in the presence of said generated pulse signals said video component will be caused to rise above said predetermined level and said reproducing device will respond thereto, while in the absence of said generated pulse signals, due to the failure of said deflecting circuit to respond to a pulse of said synchronizing component, said video component will fail to exceed said predetermined level and said reproducing device will fail to respond to any portion thereof.

9. In a television system, a source of a composite signal comprising video and synchronizing components established on opposite sides of a blanking level, said synchronizing component comprising a series of time-spaced pulse signals occurring during intervals in which said video component is established at said blanking level, a picture reproducing device adapted to be controlled in response to a signal supplied thereto, means for rendering said reproducing device inoperative to signals not exceeding a predetermined level, a deflecting circuit operatively associated with said reproducing device and supplied with at least the synchronizing component of the signal from said source and normally adapted to be synchronized in response to the individual pulse signals comprising said synchronizing component, means adapted to generate a pulse signal whenever said deflecting circuit responds to a pulse of said synchronizing com-,

ponent, said generated pulse signals having an amplitude at least as great as the amplitude of said video component, means for combining said generated pulse signals with said composite signal, means for modifying said combined signal to bring its peaks to a common level, said lastnamed means comprising a condenser, a resistor and a diode, constructed and arranged to function as a peak detector of the combined signal, and means for supplying said combined and leveled signal to said reproducing device, whereby in the presence of said generated pulse signals said video component will be caused to rise above said predetermined level and said reproducing device will respond thereto, while in the absence of said generated pulse signals, due to the failure of said deflecting circuit to respond to a pulse of said synchronizing component, said video component will fail to exceed said predetermined level and said reproducing device will fail to respond to any portion thereof.

10. In a television system, a source of a composite signal comprising video and synchronizing components established on opposite sides of a blanking level, said synchronizing component comprising a series of time-spaced pulse signals occurring during intervals in which said video component is established at said blanking level, a picture reproducing device adapted to be controlled in response to a signal supplied thereto, a deflecting circuit operatively associated with said reproducing device and supplied with at least the synchronizing component of the signal from said source and normally adapted to be synchronized in response to the individual pulse signals comprising said synchronizing component, means for generating a pulse signal whenever said deflecting circuit responds to a pulse of said synchronizing component, means for combining said generated pulse signals with said composite signal, means for modifying the combined signal to bring its peaks to a common level, means for supplying the combined and leveled signal to said reproducing device, and means for biasing said reproducing device so as to render it operative in response to the combined and leveled signal and to render the reproducing device inoperative in response to the composite signal alone, whereby the reproducing device responds to said video component only as long as the deflecting circuit responds to the synchronizing pulses.

NEWLAND F. SIVHTH, JR.