SE453242B - DEVICE FOR AUTOMATIC PICTURE PREDICTION CONTROL COMPENSATED FOR VARIATION POINT DANCE DIFFERENCE VARIATIONS - Google Patents

Description

455 242 2 and thereby modifies the cathode bias voltage, thereby obtaining the desired black current level of the cathode as a result.

In an AKB device of the type described in the above-mentioned U.S. patent specification, control circuits respond to a periodically converged signal with a magnitude representative of the black current level of the cathode. The derived signal has a prescribed level that differs from zero when the black current level is correct and different levels (eg more or less positive) when the black current level is too high or too low. The derived signal is formed at a sensing point connected to control circuits which include locking and sampling networks to generate a picture tube bias correction signal in accordance with the magnitude of the closely derived signal.

For example, the derived signal may be sampled by a sampling amplifier which charges or discharges a storage capacitor in accordance with the level nos of the derived signal. The bias correction signal increases or decreases as needed to maintain a correct black current level.

It must be established that the control circuits to which the sensing point is connected can be adversely affected when the sensing point from which the black-current representative signal is derived has impedance variations which constitute a function of the picture tube drive bias level. Thus, a device for substantially canceling the effect of such impedance variations on the control circuits will be described below. The described device also has the advantage of increasing the immunity of a locking network cooperating with the control circuits against false signals, including locally generated disturbances which could otherwise distort or partially obscure the bias correction signal.

When the bias correction signal is derived or extracted from a storage capacitor, it takes the form of a bias correction voltage.

The bias correction voltage removed from the storage capacitor should remain unchanged when the level of the extracted signal in the form of a voltage pulse represents a correct black current level. This requires that the storage capacitor be neither charged nor discharged by output current from the sampling amplifier when the level of the voltage pulse represents a correct black current level. More specifically, in an AKB device of the type described in the said U.S. patent, this requires that the sampling amplifier not supply any current to the storage capacitor when the correct picture tube black level current is indicated by a representative voltage pulse having a predetermined magnitude other than zero. This result can be achieved by shifting the bias voltage of the sampling amplifier, such as by means of a preset manually adjustable potentiometer which is connected to a suitable bias voltage control point of the amplifier.

It can further be stated that such manual pre-made settings are not desirable in a signal control device which is otherwise automatic. Such manual adjustments are also time-consuming in an undesirable manner, in addition to which the required potentiometers entail an undesired cost increase in the device.

It should also be noted that the signal processing methods used in some AKB devices can give rise to a displacement error if the blocking voltages and signal amplifications of the individual picture tube electron guns are not identical, for example due to tolerances in the manufacture of the picture tubes. In such a case, the black level current established by the KB device may have a fault that can be compensated by means of preset manually adjustable potentiometers.

The described device advantageously facilitates the construction of AKB signal processing circuits which do not require manually adjustable control means in order to compensate for such displacement errors.

The present invention relates to a video signal processing plant in which a derived signal representative of the black current level guided by an image display device has a given amplitude which differs from zero when the black current level is correct. The derived representative signal is supplied via an input signal coupling path to a sampling amplifier which provides an output current for charging and discharging a charge storage device in accordance with the amplitude of the derived signal. In accordance with a principle of the present invention, an auxiliary signal is supplied to the input signal coupling path with such a magnitude and direction that the amplitude of the derived signal at the amplifier input is canceled when the amplitude of the derived signal is representative of a correct black current level. Thus, the current line of the sampling amplifier remains unchanged when the amplitude of the derived pulse corresponds to the correct black current level, and the voltage across the storage device remains unchanged.

According to a feature of the invention, the magnitude of the auxiliary signal is proportional to the magnitude of the blocking potential of the picture tube cathode generated during the AKB interval. According to another feature of the invention, the input to the sampling amplifier is locked at a reference voltage during a locking interval preceding the signal sampling interval. The black level representative derived signal is formed during the reading interval, so the reference voltage at which the amplifier input is locked during the reading interval is a function of the magnitude of the derived signal, and the auxiliary signal is formed during the subsequent sampling interval. The auxiliary signal has such a magnitude and direction that the input voltage of the amplifier is kept substantially unchanged when the magnitude of the derived signal corresponds to the correct black current level. According to a further feature of the invention, the amplifier input is locked at a reference voltage during the locking interval, and the derived Û signal and the auxiliary signal are both formed during the sampling interval.

According to the present invention, an automatic picture tube bias control device includes a capacitor for coupling a derived signal representative of the picture tube black current level and an auxiliary signal having a predetermined size and direction to the input of a sampling amplifier. The source of the derived signals has a variable output impedance that is proportional to the magnitude of the picture tube bias level. The derived representative signal is passed from the variable impedance output of the source of the derived signal to the capacitor via a switching impedance. The coupling impedance is large in relation to the variable output impedance in order to significantly reduce the impedance variations presented to the source of the auxiliary signal from the output of the source of the derived signal. In accordance with another feature of the invention, the switching capacitor is included in a locking network. The switching impedance further increases the immunity of the latch's response to false signals.

In the accompanying drawings, Fig. 1 shows a part of a color television receiver which includes an AKB device and a cooperating signal sampling network in which the principles of the present invention are applied, Fig. 2 illustrates signal waves related to the operation of the device shown in Fig. 1, Fig. 3 shows an alternative version of signal waves shown in Fig. 2, Fig. 4 shows circuit details in the sampling network according to Fig. 1 and Fig. 5 shows circuit details in a time signal generator which is connected to the device according to Fig. 1.

In Fig. 1, television signal processing circuits 10 supply separate luminance (Y-3 and chrominance (C)) components of a composite color television signal to a luminance / chrominance signal processing network 12. The processor 12 includes luminance and chrominance gain control circuits, as well as DC level setting. includes keyed black level latches), color demodulators for generating ry, gy and by color difference signals, and matrix amplifiers for combining the latter signals with processed luminance signals to provide low level color image representative signals r, g and b. These signals are amplified and otherwise processed by circuits in video output signal processing networks 14a, 14b and 14c, respectively, which supply high level amplified color picture signals R, G and B to their respective cathode intensity control electrodes 16a, 16b and 16c in a color picture tube 15. Networks l1a and lc also performs functions related to the AKB operation, as will be described below The lead tube 15 is of the self-converging in-line gun type with a commonly activated grid 18 cooperating with each of the electron guns including the cathode electrodes loa, 16b and 160.

Since the output signal processors or processors 11a, 14b and 14c are similar in this embodiment, the following discussion of the operation of the processor 14a also applies to the processors 14b and 14c. The processor 144a includes a picture tube driver comprising a common emitter input transistor 20, which transistor receives the video signal r from the processor 12 via an input resistor 21, as well as a common voltage output voltage transistor 22, which transistor together with the transistor 20 where a cascode video drive amplifier. The high level video signal R, which is suitable for driving the picture tube cathode 16a, is formed over a load resistor 2Ä in the collector output circuit of the transistor 22.

A working supply voltage for the amplifier 20, 22 is obtained from a source for a high direct voltage E + (eg + 250 volts). Negative DC feedback for the drive circuit 20, 22 is provided by a resistor 25. The signal gain of the cascode amplifier 20, 22 is determined primarily by the relationship between the value of the feedback resistor 25 and the value of the input resistor 21. The feedback network provides a suitable output. to stabilize the DC work level at the amplifier output. A sensing resistor 30 which is connected in direct current in series with and between the collector / emitter paths of the transistors 20, 22 serves to form a voltage at a node A with a relatively low voltage representing the level of picture tube cathode black current which is conducted during picture tube extinguishing intervals. The resistor 30 has a function in cooperation with the AKB device in the receiver, which function will now be described.

A time signal generator 40 containing logic control circuits responds to periodic horizontal synchronizing rate signals (H) and to periodic vertical synchronizing rate signals (V), both of which are taken from deflection circuits in the receiver, for generating time signals VB, VS, VC, VP and VG, which control the mode of the AKB function during periodic AKB intervals. Each AKB interval begins shortly after the end of the vertical sweep regression interval within the vertical blanking interval and encloses several horizontal line intervals also within the vertical blanking interval and during which video signal image information is missing. These time signals are illustrated by the waves in Fig. 2.

For the moment, reference is made to Fig. 2. The time signal Va, which is a video extinguishing signal, comprises a positive pulse which is generated shortly after the vertical sweep regression interval ceases at the time T1, as marked by the signal wave V. The extinguishing signal VB remains below the AKB interval duration and is fed to a slack control input terminal of the luminance / chrominance processor lä to cause the r, g and b outputs of the processor l2 to have a black picture representative DC reference V level corresponding to the absence of video signals. This can be done by reducing the signal gain of the processor 12 to virtually zero via the gain control circuits of the processor 12 in response to the signal VB and by modifying the DC level of the video signal processing path Via the DC level control circuits of the processor 12 to produce a black picture representative signal path. from the processor l2. The time signal VG, which is a positive grid drive pulse, encloses three horizontal line intervals within the vertical blanking interval. The time signal VC controls the operation of a latch circuit which is associated with the signal sampling function in the AKB device. The time signal VS, which is a sampling control signal, appears after the signal VC and has the task of determining the time of operation of a sampling and holding circuit which forms a direct current bias control signal for regulating the black current level of the picture tube cathode. The signal VS encloses a sampling interval, the beginning of which is somewhat delayed in relation to the end of the locking interval which is enclosed by the signal VC and whose end substantially coincides with the end of the AKB interval. A negatively directed auxiliary pulse VP, the function of which will be described in more detail below, coincides with the sampling interval. The signal time delays T, which are indicated in Fig. 2, are of the order of 200 ns.

To now return to Fig. 1, during the AKB interval, the positive pulse VG (eg of the order of +10 volts) is given a bias voltage in the forward direction of the picture tube grating 18, whereby the electron gun comprising the cathode 16a and the grating 18 is increased its power cord. At other times than during the AKB intervals, the signal VG forms the normal, less positive bias voltage for the grating 18. In response to the positive grating pulse VG, a positive current pulse with a similar phase occurs at the cathode 16a during the grating pulse interval. The amplitude of the cathode output current pulse thus formed is proportional to the level of the cathode black current line (usually a few microamperes).

The induced positive cathode output pulse occurs at the collector of transistor 22 and is connected to the base input of transistor 20 via resistor 25, thereby causing the current line of transistor 20 to increase proportionally while the cathode pulse is present. The increased current conducted by the transistor 20 causes a voltage to be generated across the sensing resistor 50. This voltage is in the form of a negative directional voltage change which occurs at the sensing node A and which is proportional in magnitude to the magnitude of the black current level representative cathode output pulse. the magnitude of the voltage change at node A is determined by the product between the value of the resistor 50 times the magnitude of the current flowing through the resistor 50.

The voltage change at node A is connected via a small resistor 51 to a node B at which a voltage change V1 is formed which substantially corresponds to the voltage change at node A. Node B is connected to a bias control voltage processing network 50. The network 50 includes an input connector. , an output lock and sampling operational amplifier 52 (e.g., a transconductance operational amplifier) having a cooperating feedback switch 54 operable depending on the lock time signal VC, and a charge storage capacitor 56 having an associated switch 55 dependent on the sampling switch VS . The voltage generated across capacitor 56 is used to supply a picture tube bias correction signal via the mains 58 and resistor mains 60, 62, 64 to the picture tube driver circuit via a bias control input at the base of the transistor 20. The network 58 includes signal conversion and buffer circuits for supplying the bias control voltage at a suitable level and with low impedance in accordance with the requirements of the transistor 20 for the bias control input signal.

The operation of the device according to Fig. 1 will now be described with particular reference to the waves 1 Fig. 2. The auxiliary signal VP is supplied to the circuit node B in Fig. 1 via a diode 35 and a voltage conversion impedance network comprising the resistors 32 and 34, which for example, can have the values 220 kohm and 270 kohm, respectively. The signal VP has a positive DC voltage level which amounts to about +8.0 volts at all times except during the AKB sampling interval so that the diode 35 is kept current-conducting so that a normal DC bias voltage can be formed at node B. When the positive DC voltage component of the signal VP If present, the connection point between the resistors 32 and 34 is locked at a voltage equal to the positive DC component of the signal VP minus the voltage drop across the diode 35.

The signal VP manifests itself as a negatively directed, less positive pulse component with constant amplitude during the AKB sampling interval. The diode 35 is made to become non-conductive in response to the negative pulse VP, whereby both resistors 32 and 34 are connected between the node B and ground. The resistor 31 causes an insignificant damping of the voltage change which occurs at the node A in relation to the corresponding voltage change (V1) which occurs at the node B since the value of the resistor 31 (of the order of 200 ohms) is small in relation to the values of resistors 32 and 34.

Before the locking interval but during the AKB interval, the pre-existing nominal direct voltage (VNOM) which occurs at node B charges the positive terminal on capacitor 51. During the locking interval when the grid drive pulse VG is formed, the voltage at the node A decreases in response to the pulse VG 1 to an extent representative of the black current level.

This causes the voltage at node B to decrease to a level that is substantially equal to VNOM - V1. Below the lock level, the time signal VC also causes the lock switch 54 to close (i.e., become current-conducting), whereby the inverting (-) signal input of the amplifier 52 is connected to the output of the amplifier, whereby the amplifier 52 will have the configuration of a follower amplifier with the amplifier 1 At this time, the storage capacitor 56 is disconnected from the amplifier 52 via the non-conduction switch 55. Consequently, a source of a constant reference DC voltage VREF (eg + 5 volts) applied to a non-inverting input (+) of the amplifier 52 will be disconnected. by means of the feedback action, the inverting signal input of the amplifier 52 is connected via the output of the amplifier 52 and the current 453 242 line switch 54.

During the locking interval, the voltage V5 across the capacitor 51 is a function of a reference connection voltage determined by the voltage VREF at the negative terminal of the capacitor 51 and a voltage at the positive terminal of the capacitor 51 corresponding to the difference between the previously described the existing nominal direct current level (VNOM) at node B and the voltage change V1 that occurs at node B during the locking interval. The voltage V3 across the capacitor 51 during the lock reference interval will thus be a function of the level of the black current representative voltage change V1 which may vary. The voltage V5 can be expressed as (VNOM - V1) - VREF.

During the immediately following sampling interval, the positive grid drive pulse VG is missing, which means that the voltage at node B increases positively to the originally prevailing nominal DC voltage level VNOM which occurred before the locking interval.

At the same time, the negative pulse VP appears, which gives the diode 55 bias voltage in the bow direction and which disturbs (ie briefly changes) the normal voltage conversion and switching action nos the resistors 32, 34 in such a way that the voltage at node B decreases to an extent V2 which is marked in Fig. 2. At the same time, the lock switch 54 is made non-conductive, and the sampling switch 55 is closed (becomes current) in response to the signal VQ, whereby the charge storage capacitor 56 is connected to the output of the amplifier 52.

Thus, during the sampling interval, the input voltage applied to the inverting signal input (-) of amplifier 52 will be equal to the difference between the voltage at node B and the voltage V5 across input capacitor 51. The input voltage applied to amplifier 52 is a function of magnitude. the voltage change V1, which can vary with changes in the black current level of the picture tube.

The voltage across the output storage capacitor 56 remains unchanged during the sampling interval when the magnitude of the voltage change V1 occurring during the locking interval becomes equal to the magnitude of the voltage change V2 occurring during the sampling interval, indicating a correct black current level of the picture tube. This is due to the fact that during the sampling interval the voltage change V1 at the node B increases in the positive direction (from the latch connection reference level) when the grid drive pulse is canceled, and the voltage change V2 gives rise to a simultaneous negative voltage disturbance at the node B.

When the picture tube bias voltage is correct, the positively directed voltage change V1 and the negative directional voltage change have equal magnitudes, whereby these voltage changes mutually cancel each other during the sampling interval so that the voltage at node B remains unchanged.

When the magnitude of the voltage change V1 is smaller than the magnitude of the voltage change V2, the amplifier 52 charges the storage capacitor 56 proportionally in such a direction that the cathode black current line is increased. Conversely, amplifier 52 discharges capacitor 50 proportionally so that the cathode black current conduction decreases when the magnitude of voltage change V1 is greater than the magnitude of voltage change V2.

More specifically, as shown by the waves in Fig. 2, the amplitude "A" of the voltage change V1 is assumed to be about 3 millivolts when the cathode black current level is correct, and it varies within a range of a few millivolts (¿A) as the cathode black current level increases and decreases in relation to the correct “level when the display parameters work parameters change.

Thus, the latch interval reference voltage V3 across capacitor 51 varies with changes in the magnitude of voltage V1 as the cathode black current level changes. The voltage change V2 at node B has an amplitude "A" of about 3 millivolts, which corresponds to the amplitude "A" associated with the voltage change V1 when the black current level is correct.

As indicated by the wave VCOR in Fig. 2, the voltage at the inverting input of the amplifier 52 remains unchanged during the sampling interval when the voltages V1 and V2 both have the amplitude "A". However, as indicated by the wave VH, the input voltage to the amplifier 52 increases to the extent ¿> when the voltage change V1 has the amplitude "A + .A", which corresponds to a high black current level. In this case, the amplifier 52 discharges the output storage capacitor 56 so that the bias control voltage applied to the base of the transistor 20 causes the collector voltage of the transistor 22 to increase, thereby decreasing the cathode black current toward the correct level.

Conversely, and indicated by the wave VL, the input voltage to the amplifier 52 decreases to an extent A during the sampling interval when the voltage change V1 has the amplitude "A - A", which corresponds to a low black current level. In this case, the amplifier 52 charges the output storage capacitor 56, which causes the collector voltage of the transistor 22 to decrease, thereby increasing the cathode black current toward the correct level. In both cases, several sampling intervals may be needed to achieve the correct black current level.

The voltage formed at node B during the AKB lock and sampling intervals is a function of the values of resistors B1, 32 and 54 and the value of an output impedance ZO which occurs at node A. When the signal VP expresses the positive DC voltage level (+ 8 volts) during the locking interval, the connection point between resistors 32 and 34 becomes voltage locked, and a current conducted by resistor 31 from node A to node B constitutes a function of the values of Zo, resistor 31 and resistor 34. When below the subsequent sampling interval the negative directional pulse component of the signal VD occurs, the diode 35 will become non-conductive and the connection point between the resistors 52 and 54 will become unlocked. At this time, another current of the resistor B1 is conducted from the node A to the node B as a function of the value nos the resistor 32 in addition to the values of Zo and the resistors 31, 34. The voltage change V2 obtained at the node B in response to the negative pulse pulse the true of the signal VP is proportional to the difference between these currents. The impedance 20 at node A may vary in an undesirable manner as a function of the cathode bias voltage level of the picture tube (ie, the barrier voltage level of the cathode) associated with the expected correct cathode black current level. The resistor 51 compensates for variations in the value of the impedance Zo and also serves to increase the immunity of the network 50 locking and sampling circuits to locally generated false signals such as horizontal rate disturbances.

Node A can be considered as a voltage source in series 13 with the above-mentioned impedance Zo. The value of the impedance ZO is a function of the value of the sensing resistor 30 divided by a control voltage factor which is a function of the operating point of the transistor 20. The operating point of the transistor 20 during AKB intervals is proportional to the blocking voltage of the cathode.

In practice, it has been shown that the impedance Zo can have minimum and maximum values of 30 ohms and 50 ohms, respectively, under correct black current conditions. The value of Zo at point A can thus vary by 67

Claims (15)

1. 453 242 22 PATENT CLAIM 1. Automatic bias control device in a video signal processing plant which includes an image reproducing device responsive to video signals supplied to an intensity control electrode in the plant, characterized by means (30) for generating a periodic signal representative of the black current level conducted by said intensity control electrode during video signal image blanking intervals, said derived signal having a size other than zero when said black current level is correct, information storage means (56), amplifying means (52) having a signal input and an output coupled to said storage means for modifying the information content of said storage means in accordance with the current conduction state of said amplifying means in response to input signals supplied, input signal coupling means (B1) for coupling said derived signal to said amplifying means, ) for providing a periodic idle signal (VP, V2) for said input signal switching means having a magnitude and direction to substantially cancel the response of said amplifier to the magnitude of said derived signal when the magnitude of said derived signal is representative of a correct black current level, and means (58) for applying a bias correction voltage obtained from said storage means to said image reproducing apparatus for maintaining a correct black current level. n Device according to claim 1, characterized in that impedance means (32, 54) are coupled to said input signal coupling means for establishing a bias voltage for said input signal coupling means 1 in the presence of said derived signal and that said auxiliary signal (VP, V2 ) is arranged to be coupled to said impedance means for modifying said established bias voltage in a direction to effect said cancellation of the amplifier response. 3. A device according to claim 2, characterized in that said auxiliary signal (VP, V2) cancels said derived signal in said input signal path to produce said canceled 453 242 23 amplifier response. Device according to claim 3, characterized in that said derived signal (V1) and said auxiliary signal (V2) comprise coincident pulses of mutually opposite polarity and substantially equal size when the derived signal is representative of a correct black current level. . Device according to claim 1, characterized in that said auxiliary signal (V2) has a magnitude proportional to a direct voltage component obtained in said intensity control electrode during said extinguishing interval. Device according to claim 1, characterized by locking means (51) connected to said input signal switching means at said amplifier input, switching means (54, 55) connected to said locking means and to said amplifier output, said means, means, means for bringing said switching means (92) operating during an initial locking interval to (l) lock said amplifier input at a reference voltage in response to a reference source coupled to said amplifier input during said locking interval and (2) disconnect said amplifier output from said storage means and bring said switching means to become operative during a subsequent sampling interval to (3) cancel the reading of said amplifier input and (A) connect said amplifier output to said storage means, and by generating said derived black current representative signal during said locking interval and coupling to said locking means , wherein said reference voltage, at which said amplifier input is locked, by said derived signal, and that said auxiliary signal (VP) is generated during said subsequent sampling interval. Device according to claim 6, characterized in that said image reproducing means is a picture tube (15) of an electron gun comprising a grid electrode (18) and a cooperating cathode intensity control electrode (16a), said automatic bias control device further including means (40) for biasing said picture tube electron gun during said locking interval to induce a cathode output signal with constitutes a further function of the magnitude which includes- -r- _ a. 453 242 24 a magnitude proportional to the cathode black current level, and that said derivative means (50) derives said periodic representative signal from said induced cathode output signal. Device according to claim 1, characterized by locking means (51) connected to said input signal switching means at said amplifier input, switching means (54, 55) connected to said amplifier output, to said locking means and to said storage means, means (92) for causing said switching means to operate during an initial locking interval to (1) lock said amplifier input at a reference voltage in response to a reference source connected to said amplifier input during said locking interval and to (2) disconnect said amplifier output from said storage means and causing said switching means to become operative during a subsequent sampling interval to (5) cancel the reading of said amplifier input and to (4) connect said amplifier output to said storage means, and said derived black current representative signal and said auxiliary signal are both formed. . Device according to claim 6 or 8, characterized in that said welding means comprises a capacitor (51) for coupling signals from said input coupling means to said amplifier input and that said auxiliary signal (VP, V2) has such a magnitude and direction that the voltage at said amplifier input remains substantially unchanged when the magnitude of the derived signal is representative of a correct black current level. Device according to claim 9, characterized in that said image reproducing means is a picture tube (15) including an electron gun comprising a grid electrode and a cooperating cathode intensity control electrode, said automatic bias control device further including means (40) for providing bias to said picture tube electron gun during said sampling interval to induce a cathode output signal having a magnitude proportional to the cathode black current level and that said derivation means (50) derives said periodic representative signal from said induced cathode output signal. V.L'.L. ";.;; ._._..._._..._._-.-..- v.. (453 242 25 Device according to any one of the preceding claims, characterized in that said amplifying means (52) comprises a differential input amplifier. Device according to claim 1, characterized in that an alternating current capacitor (51) is connected to the input of the amplifying means, that said input signal coupling means connects said derived signal to said capacitor to change its charge, and that said periodic auxiliary signal is coupled to said capacitor to change its charge, said auxiliary signal having such a magnitude and direction that the changed charge of said capacitor formed in response to said derived signal is substantially canceled when the magnitude of said derived signal is representative of a correct black current level, 13. A device according to claim 12, characterized in that said auxiliary signal has a magnitude as a function of a direct voltage component obtained in said intensity control electrode during said extinguishing interval. 1U. Device according to claim 1, characterized in that said means for deriving said periodic signal has a variable output impedance which is related to the bias voltage of said intensity control electrode, said input signal coupling means including an impedance (31) for coupling said derived representative signal from said output of said derivative means to said information storage means, said impedance being large relative to said variable output impedance to significantly reduce impedance variations presented to said means providing said periodic auxiliary signal from said output means. of said derivation means. Device according to claim 1U, characterized in that said derived signal is generated at a first circuit point (A) corresponding to said output of said derivation means, that said periodic auxiliary signal is connected to said capacitor at a second circuit point ( B), that said impedance (31) is connected from said first circuit point (A) to said second circuit point (B).