US2339536A - Television system - Google Patents

Description

Jan. 18, 1944. K R WENDT 2,339,536

TELEVISION SYSTEM Filed June 28, 1941 i 2 Sheets-Sheet 1 VERT/ML DEF'L Jan. 18, 1.944. K, R WENDT 2,339,536

TELEVISION SYSTEM Filed June 28, 1941 2 Sheets-Sheet 2 Em. 5f @y M 56 57 Patented Jan. 18, 1944 UNITA TELEVISION SYSTEM Karl E. Wendt, Audubon, N. J., assigner to Radio Corporation ofAmerica, a corporation of Delaware Application June l'2.8, 1941, Serial No. 400,177 13 Claims. (Cl. Pls-69.5)

My invention relates to television receivers or the like and particularly to a lmethod of and means for synchronizing cathoderay deflection circuits. l j

It has been the usual practice to synchronize the cathode ray deflection or scanning at the receiver with the scanning at the transmitter by transmitting to the receiver a mixture of picture signal and synchronizing pulses and by apply-l Also, a circuit so synchronized will fall out ofl synchronism immediately upon the absence of synchronizing pulses.

An object of the present invention is to provide an improved method of and means for synchronizing a cathode ray deecting circuit.

A further object of the invention is to provide a cathode ray deflecting circuit which will not fall out of synchronism immediately upon the failure of synchronizing pulses.

A further object of the invention is to provide a cathode ray deflecting circuit which is not very susceptible to noise signals.

A still further object of the invention is to provide a cathode ray deecting circuit in which the return line period is made to start at or before the front edge of a synchronizing pulse.

In practicing one particular embodiment of the invention, I utilize a circuit for producing an oscillator frequency control voltage which changes in value with any change in the phase relation of the incoming synchronizing pulses with respect to the oscillator output. In a specific example, a balanced diode bridge circuit has applied thereto both incoming synchronizing pulses and pulses from the deflecting circuit oscillator whereby the bridge circuit output or control voltage varies with changes in the phase relation of the said pulses. The said control voltage is applied to the oscillator to control its rate of oscillation whereby its output pulses are held in a fixed phase relation to the synchronizing pulses.

circuit at the time a sloping side of a synchronizing lpulse is impressed upon it. q

Other objects, features and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawings in which y e Fig. 1 is a block diagram of a television receiver embodying my invention. Fig. 2 is a circuit diagram ofthe horizonta deection circuit of Fig. 1 designed in'accordance with my invention, Figs. 3a, 3b, 3c and 3d are curves which are referred to in explaining the invention, and

Figs. 4. and 5 are circuit diagrams of other embodiments of the invention.

Corresponding partsin the several` figures are indicated by like reference characters.

Referring to Fig. 1, there is shown a television receiver of the usual superheterodyne type comprising a first detector and tuning oscillator in-` dicated at Ill, an I. F. amplifier H, a second detector I2, a video amplifier i3 and a cathode ray receiver tubey I 4.

The vertical deflecting circuit I5 and the horizontal deflecting circuit (indicated by the bracket and legend) are synchronized by the usual vertical and horizontal synchronizing pulses which have been separated from the picture signal and from each other by suitable separating and ltering circuits indicated at I8 as is well known in the art.

The horizontal deecting circuit, to which my invention is applied in this particular embodiment, comprises an oscillator I9, a sawtooth genv u, Thus, the oscillator I9 is held in a xed phase re- In this embodiment, the oscillator pulses may lation to the synchronizing pulses, and the horizontal deection of the cathode ray of tube I4, likewise, is made to occur in the necessary iixed phase relation with respect to the horizontal deflection at the transmitter.

Referring to the embodiment of the invention shown in detail in Fig. 2, thehorizontal synchronizing pulses are applied with positive polarity to an amplifier 26 which may be of the cathode follower type. The positive synchronizing pulses appearing across the output resistor 28 are impressed across one diagonal of the diode bridge transformer.

circuit comprising the A. F. C. circuit 22 as will be described hereinafter.

Referring now to the oscillator I9, it is preferably lof the well known blocking oscillator design comprising an amplifier tube 32 having an iron core feedback transformer 33, a grid condenser 34 and a variable grid leak resistor 36.

The blocking rate or frequency of oscillation of a blocking oscillator may be varied or controlled by changing the rate at which the'blocking charge leaks oil' the grid condenser 34 assuming the cut-ofi point of the oscillator tube remains unchanged. Li the present circuit, however, it is controlled principally by causing the discharge to reach the said cut-off point at a variable time by a D. C. control voltage from the A. F. C. circuit.

The blocking oscillator pulse, similar to that shown in Fig. 3c but of opposite polarity (and containing a sawtooth component), appears on the grid of the oscillator tube 32; This pulse, the portion 58a being positive at this point, is impressed upon'the grid of a discharge tube 35 of the sawtooth generating circuit 2|. Thus the usual sawtooth condenser 31 is discharged periodically to produce a sawtooth voltage wave thereacross as is well known in the art.

This sawtooth voltage is impressed upon an output tube 40 to produce a sawtooth current through the deilecting coils 45 in the usual manner. This sawtooth current is shown in Fig. 3d.

Referring to the A. F. C. circuit 22, it may comprise four diodes 38, 39, 4| and 42 connected in the form of a Wheatstone bridge as illustrated. The synchronizing pulse from cathode resistor 28 isv applied across the diagonal terminals indicated at a and b, these being the junction points, respectively, of the cathode and anode of diodes 38 and 4| and of the cathode and anode of diodes 39 and 42. the synchronizing pulses may be traced from the terminal a through resistor 28, through ground, and through a control voltage storage condenser 43 to the terminal b.

Voltage pulses from the blocking 'oscillator |9 are supplied through an amplifier tube 46 to a transformer 41 and impressed as keying pulses across the other diagonal terminals of the bridge indicated at c and d by means of this Since the bridge is balanced, these pulses will not appear across the diagonal terminals a and b.

In series with the, transformer secondary there is a resistor 48 shunted by a condenser 49. During the circuit operation, a bias voltage is built up across the R. C. circuit 48-49 having such polarity as to make the bridge nonconducting unless an oscillator pulse is being applied through the transformer 41. Thus, the oscil lator pulses act as keying pulses for the bridge circuit' and the condenser i3v will store or integrate current pulses whereby its charge depends upon the instantaneous voltage of a synchronizing pulse at the instant the bridge circuitis keyed. With the bridge properly balanced, any changes in the amplitude of the oscillator pulses will not aiect the charge on condenser 43.

The basic diode bridge control circuit above described is claimed in my Patent 2,250,284, issued July 22, 1941, and entitled Frequency control circuits.

The control voltage of condenser 43 is impressed upon the grid circuit of the oscillator tube 32 by means of a D. C. amplier 5| of the The circuit for applyingcathode follower type, its cathode resistor 52 being in series with the oscillator grid leak resistor 36. It is evident that a change in the said control voltage will change the point at which the discharge reaches the vcut-oil? point whereby the instant at which the oscillator can and will unblock is changed.

The operation of Fig. 2 will be better understood by referring to Figs. 3a to 3d.

Fig. 3a shows a horizontal synchronizing pulse 54 set on a blanking pulse 56 which is preceded and followed by picture signals 51. The synchronizing pulse 54, after separation from the picture signal, may have the wave shape shown in Fig. 3b and it is this pulse that is applied across the bridge terminals a and b in Fig. 2.

Fig. 3c shows the blocking oscillator pulse with its negative portion 53a and its positive portion 58h as it is applied to the anodes of the diodes 38 and 39. The amplitude of the blocking oscillator pulse is several times that of the synchronizing pulse, this being indicated in Figs. 3b and 3c in volts by way of example.

As shown by Figs. 3b and 3c, the blocking oscillator pulse 58h and the synchronizing pulse 54 occur in such time relation that the pulse 58h keys the bridge circuit to make the four diodes conducting during the occurrence of the sloping front of pulse 54. It follows that the charge received by condenser 43 depends upon the phase relation of the two pulses 58h and 54. If pulse 58h occurs later than shown on the drawing as a result of the oscillator I9 tending .to slow down, the effective amplitude of the bridge output pulses will increase and the control voltage across condenser 43 will be increased to make the grid of amplifier tube 5I less negative whereby the' grid of the blocking oscillator tube 32 is also made less negative. This permits unblocking of the oscillator tube an instant sooner than before and the oscillator is prevented from slowing down. The opposite control action takes place if the oscillator I9 tends to speed up.

The phrase effective amplitude includes p0- larity changes as well as amplitude changes. For example, if the charge on condenser 43 goes from a positive value to an equal but opposite negative value, the eiective amplitude has been changed.

It has been found that the above described control action locks the oscillator I9 with the synchronizing pulses so tightly that no speed control knob is required on the television receiver panel as one of the controls. The only speed control adjustment may be a screw driver adjustment for initially adjusting the grid leak resistor 36.

While the invention has been described as applied to the horizontal deflection, it is equally applicable to the vertical or frame frequency deflection.

An additional advantage of the invention is that the return line period of the deflection circuit can be made to start at, or even before the beginning of the blanking pulse. This is especially desirable in the horizontal deection circuit since it is a difficult problem to make the horizontal return line period as short as the blanking period without increasing the cost too much. The present invention may even permit shortening of the blanking period (shortening of pulse 56, Fig. 3a) so that a'n increased amount of picture detail may be transmitted.

assasse Y is positive at the grid of the discharge tube 35.

the return line period of both the sawtooth voltage across condenser 31 and of the sawtooth current through the deiiecting coils 65 starts with this pulse portion 58a which occurs ahead of the pulse portion 58h that is being utilized as a keying pulse in the A. F. C. circuit.

' From the foregoing it is apparent that with v my improved method of deection control the return line :c does not extend into the picture signal region 51 (Fig. 3a) as it would if it began in accordance with th'e-usual method at a later time such as indicated by the arrow y in Fig. 3a.

` Figure 4 shows another embodiment of the in vention which also comprises a balanced bridge circuit of four diodes, but which avoids the use of a transfomer for applying pulses thereto. I'he transformer is replaced by two coupling condensers 6| and 62 which connect the diagonal terminals c and d to the oscillator plate and grid circuits, respectively, where the oscillator pulses appear with opposite polarity.

A resistor 60 connected between the terminalsc and d functions in combination with lthe condensers 6I and 62 to hold the diodes nonconducting between keying pulses, the action being slmilar to that ofthe R-C circuit IIIB- lill in Fig. 2. A The synchronizing pulses, which may have the characteristics shown in Fig. 3b, are applied through a condenser 66. Theyare impressed across the diagonal terminals a and b with negative polarity at the terminal a through a circuit that may be traced from terminal a. through a resistor 61, through ground, and through a control voltage storage condenser 68 to the terminal b.

Just as in the circuit `of Fig. 2. there is produced across the storage condenser 68 a control voltage which changes with any change in the phase relation of synchronizing pulses and oscillator pulses. It is applied through a D. C. amplier tube 69 to thegrid of the blocking oscillator tube 32, the grid circuit of tube 32 being. completed through the plate resistor 1| of the D. C. ampliiier. 'Ihus the grid bias on the blocking oscillator depends upon the voltage across condenser 68 and the oscillator is locked in with the synchronizing pulses as previously described.

`It may be noted that the synchronizing pulses are applied to terminal a with negative polarity, instead of with positive polarity as in Fig. 2, for the reason that the D. C. amplierin Fig. 4 reverses the control voltage polarity whereas the D. C. amplier of Fig. 2 does not; Therefore, the polarity is reverse at terminal ul in order to reverse the slope of the :front-of` the synchronizing pulse so that the oscillator control will bein the proper direction.

tion of the R-C combination 'l2-13 is to note that it prevents any sudden changes in the charge of the storage condenser 68. This reduces the tendency of the cricuit to oscillate since it reduces the A, C. gain in the loop circuit although it does not reduce the D. C. gain.

With respect to noise accompanying the synchronizing pulses, such noise is random so that the probabilities are that the frequency control circuit will not have successive incorrect signals applied thereto at vthe instant the circuit is keyed by an oscillator pulse. Since a single incorrect signal will be removed from the storage condenser 68 by the R-C combination 12-13, it follows that this R-C combination reduces the susceptibility of the circuit to noise. In this connection it may be noted that a Single synchronizing pulse by itself. has very little synchronizing eiect on the oscillator; a train of successive synchronizing pulses are necessary to produce substantial control of the oscillator.`

Figure 5 shows a simplied embodiment o f the invention whereinthe bridge circuit oi! four diodes are replaced by a'balanced bridge circuit containing only two diodes 16 and 11 which are in two armsof the bridge. The other bridge arms contain the upper and lower halves of a tertiary transformer winding 18 of the blocking oscillator The circuit of Fig. 4, as well as the other cirt make the circuit more stable and less susceptible to noise. Thus, in Fig. 4, a resistor 12 and a comparatively large capacity condenser 13 are connected across the condenser 68.

Perhaps the simplest way to consider the iiinc- 75 deiiection circuit of the type containing an ostransformer. By means of winding 'I3 the oscil- .lator pulses are applied across diodes 16 and 11, as `described withreference to diodes 38 and 40 in Fig.'2, to make them conducting during the pulse portion 58h.

The cathode o'f diode 16 and the anode of diode 11 are connected together through resistors 3l and 82, preferably of equal resistance, which are shunted by condensers 63 and 84, respectively, preferably of equal capacity. The R-C circuits 8|.-63 and 82-84 perform the same function as the R-C circuit 48-49 in Fig. 2, that is, a bias voltage builds up across them to maintain. the diodes in a nonconducting condition until they are keyed by a blocking oscillator pulse.

The synchronizing pulses are impressed be- -tween the junction point of resistors 8| and 62 and the center point of transformer coil 'I8 with the pulses positive at said junction point. n There is thus built `up acrossa storage condenser 86 a control voltage which changes with any change in phase relation of synchronizing and oscillator pulses just as described in connection with Fig. '2. vAs in Fig, 2, the control voltage from condenser 8611s applied to the oscillator I9 through a D. C. amplier 5I whereby the oscillator is locked in with the synchronizing pulses as previously described.

On the drawings various circuit constants have been indicated by way of example in ohms, megohms. microfarads and micromicrofarads.

I claim as my invention:

1. The method of synchronizing a cathode ray deilection circuit of the type containing an oscillator with synchronizing pulses each having a `sloping side which comprises the steps of obtaining from said oscillator pulses which are of short duration compared with the duration of said sloping side, producing periodically recurring pulses whichchange in eiective amplitude in response to a change in the phase relation of said two groups o f pulses,.integrating the pulses thus produced toprovide a frequency control voltage, and

controlling the frequency of said oscillator in accordance with said control voltage.

2.v The method of synchronizing a cathode ray cillator withsynchronizing pulseseach having a sloping side which comprises the steps of obtaining from said oscillator pulses which are of short i duration compared with the duration of said sloping side, combiningsaid synchronizing pulses andoscillator pulses to produce recurring pulses having a value which varies with any variation in the phase relation ofsaid two groups of pulses, integrating said pulses thus produced to provide a frequency control voltage, and controlling the frequency of said oscillator in accordance with said-control voltage whereby said oscillator is locked in with said synchronizing pulses.

3; The method of synchronizing a cathode ray deflection circuit of the type containing a blocking oscillator with synchronizing pulses each having a sloping side which comprises the steps of obtaining from said oscillator pulses which have a first occurring portion and a second occurring portion, said second portion being of short Vduration compared with the duration of said sloping side, combining said synchronizing pulses and said second occurring portions of the oscillator pulses to produce recurring pulses having a value which varies with any variation in the phase relation of said two groups of pulses', integrating said pulses thus produced to provide a frequency control voltage, and controlling the frequency of said oscillator in accordance with said control voltage whereby said oscillator is locked in with said synchronizing pulses.

4. A cathode ray deflection circuit -comprising an oscillator which is to be synchronized by synchronizing pulses having a sloping side, means for obtaining pulses from said oscillator which are of short duration compared with the duration of said sloping side, means for producing periodically recurring pulses which change in effective amplitude inresponse to a change in the phase relation of said two groups of pulses, means for integrating the pulses thus produced to provide a frequency control voltage, and means for .controlling the frequency of said oscillator in accordance with said control voltage.

5. A cathode ray deflection system comprising an oscillator, a balanced bridge circuit which may be keyed by signal applied across one diagonal to pass signal applied across the other diagonal, means for applying pulses obtained from said oscillator across said one diagonal, means for applying synchronizing pulses across said other diagonal, means for integrating the resulting output pulses of said bridge to obtain a frequency control voltage, and means for controlling the frequency of said oscillator in accordance with said control voltage.

6. A cathode ray deflection system comprising an oscillator, a balanced bridge circuit which may be keyed by signal across one diagonal to pass signal applied across the other diagonal, means for applying synchronizing pulses across one of said diagonals, each of said pulses having a sloping front, means for obtaining pulses of comparatively large amplitude from said oscillator and applying them across the other of said diagonals to key the-bridge circuit during the occurrance of said sloping front, means for integrating the resulting output pulses of said bridge to obtain a frequency control voltage, and means for controlling the frequency of said oscillator in accordance with said control voltage.

7. The method of operating a cathode ray deflection circuit comprising producing a sawtooth deflection wave having a return line portion, synchronizing said sawtooth wave with incoming synchronizing pulses, and making said return line portion begin at least as early as the start of a synchronizing pulse.

8. The method of synchronizing a cathode ray deflection circuit of the type containing an oscillator with synchronizing pulses which comprises the steps of obtaining from said oscillator frequency control pulses and sawtooth control pulses which occur earlier than said frequency control pulses, producing a sawtooth deflection wave having a return line portion which Starts at a time controlled by said sawtooth control pulses, producing periodically recurring pulses which change in effective amplitude in response to a change in the phase relation of said syn-- chronizing pulses and said frequency control pulses, integrating the pulses thus produced to provide a frequency control voltage, and controlling the frequency of said oscillator in accordance with said control voltage.

9. A cathode ray deflection circuit vcomprising an oscillator of the blocking type which is to be synchronized 1by synchronizing pulses, said oscillator producing pulses each having a first occurring portion and a second occurring portion, a frequency control circuit, means for Supplying said synchronizing pulses and said oscillator pulses to said control circuit, .means including said control circuit for producing periodically recurring pulses which change in effective amplitude in response to any change in the phase relation of said synchronizing pulses and said second occurring pulse portions, means for integrating the pulses thus produced to provide a frequency control voltage, and means for impressing said control voltage upon said oscillator for synchronizing it with said synchronizing pulses. i

l0. A cathode ray deflection system comprising a blocking oscillator which produces pulses each onal to -pass signal applied across the other diagonal, means lfor applying synchronizing pulses having a sloping front across one of said diagonals, means for applying said later occurring portions of the oscillator pulses across the other of said diagonals to key said bridge circuit while said sloping front is impressed thereon, means for integrating the resulting output pulses of said bridge to obtain a frequency control voltage, and means for controlling the frequency of said oscillator in accordance with said control voltage.

, 11.A A cathode ray deflection systemcomprising a blocking oscillator which produces pulses each having a first occurring portion and a later occurring portion, sawtooth generating means for producing a sawtooth deflection wave having a return line portion, means for causing said return line portion to start in response to and substantially simultaneously with the occurrence of said rst occurring portion of the oscillator pulses, a balanced bridge circuit which may be keyed by signal applied across one diagonal to pass signal applied across the other diagonal, means for applying synchronizing pulses having a sloping front across one of said diagonals,

means for applying said later occurring portions,

of the oscillator pulses across the other of said diagonale to key said bridge circuit while said sloping front is impressed thereon, means for integrating the resulting output pulses of said bridge t0 obtain a frequency control voltage, and

means for controlling the frequency of said oscillator in accordance with said control voltage.

12. A cathode ray deection circuit comprising an oscillator which is to be synchronized by synchronizing pulses, a frequency control circuit, means for supplying said synchronizing pulses and pulses from said oscillator to said control circuit, means including said control circuit for producing periodically recurring pulses which change in effective amplitude in response to any change in the phase relation of said two groups of pulses, a capacitor for integrating the pulses thus produced to provide a frequency control voltage, a resistor and acapacitor connected-,in series with each other and in shunt to said first capacitor, the capacity of the second capacitor being large compared with that of the first capacitor, and means for impressing said control voltage upon said oscillator for synchronizing it with said synchronizing pulses.

' 13, A cathode ray deflection circuit comprising an oscillator which is to be synchronized by synchronizing pulses, a frequency control circuit, means for supplying said synchronizing pulses and signal from said oscillator to said control circuit, means including said control circuit for producing periodically recurring pulses which change in effective amplitude in response to any change in the phase relation of said pulses and said signal, a capacitor for integrating the pulses thus produced to supply a frequency control voltage to a control channel, a resistor and a capacitor connected in series with each other and in shunt to said channel, the capacity of the second capacitor being large compared with that of the rst capacitor, and means for impressing the control voltage from said channel upon said oscillator for synchronizing it with said synchronizing pulses.

KARL R. WENDT.

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