Video blanking circuit with controlled rate of unblanking

A circuit is disclosed for automatically blanking the kinescope of a color television receiver during channel changing intervals. The receiver also includes an automatic kinescope beam current limiter, plural kinescope driver stages, and a source of reference bias voltage to which video signals amplified by the kinescope driver stages are normally referenced, coupled in common to the plural kinescope driver stages. During channel changing intervals, the blanking circuit modifies the reference voltage with a given blanking response time so that the driver stages and kinescope are blanked. The blanking circuit exhibits an unblanking response time, slower than the blanking response time, for permitting the reference bias voltage to gradually return to a normal level after the channel change interval ends, at which time the driver stages and kinescope are unblanked and operate normally.

This invention concerns a circuit for blanking video signals processed by a 
television receiver when the receiver is being tuned from channel to 
channel, and for unblanking the video signals with a given unblanking 
response time after the receiver is tuned. In particular, the invention 
relates to such a circuit arranged to augment the control of kinescope 
beam current normally provided by an automatic beam current limiter 
circuit of the receiver. 
Television receivers often include apparatus for automatically limiting 
excessive beam currents conducted by an image reproducing kinescope of the 
receiver. The beam current limiter is typically arranged so as to sense 
when potentially damaging levels of kinescope beam current above a 
threshold level are present. A control voltage developed in response to 
the sensed current is utilized, for example, to control the television 
signal with a sense to limit the beam currents to a safe level. The beam 
current limiter typically does not react instantaneously to excessive beam 
current levels, due to time constant effects which cause the beam limiter 
operation to exhibit a slight delay. 
Many television receivers also include provision for blanking the kinescope 
display during intervals when the receiver is being tuned from channel to 
channel. Such channel change blanking is considered desirable since it 
serves to eliminate visual interference which would normally occur during 
tuning of the receiver. Such interference results from transient signals 
that are received as the receiver is tuned from one channel to the next, 
causing disturbing flashes, streaks, and other forms of transient 
interference to be displayed by the kinescope. 
The video signal level appearing after a given channel is tuned may be 
sufficient to cause the kinescope to conduct very high levels of beam 
current, several times greater than the threshold current level above 
which the automatic beam current limiter operates. The likelihood of such 
high level signal conditions occurring is increased by the often 
unpredictable behavior of the intermediate frequency (IF) signal 
processing stages of the receiver during channel change intervals. Under 
certain conditions the channel change operation can cause the peak 
detected white level output of the IF stages to be abnormally high and 
capable of causing abnormally high and unsafe levels of kinescope beam 
current, in response to white-going noise transients, for example when the 
response time of the automatic beam current limiter is insufficient to 
limit such abnormally high beam currents quickly, the kinescope and 
associated circuits would be subjected to potentially destructive 
electrical stresses. 
In many television receivers the high operating voltages for the kinescope 
are derived from a high voltage supply (e.g., a voltage tripler) which 
responds to horizontal flyback pulses generated locally by deflection 
circuits of the receiver during horizontal image retrace intervals. When 
such television receivers also employ a deflection voltage regulator 
including a switching circuit responsive to the flyback pulses, the 
operation of the deflection regulator, and thereby the operation of the 
deflection circuits, can be disturbed in the presence of excessively high, 
uncontrolled beam currents as derived from the high voltage supply. This 
disturbed operation can result in a badly distorted display including 
random streaks across the display, among other effects. 
In accordance with the present invention there is disclosed herein a 
channel change blanking network arranged to provide a measure of control 
over kinescope beam currents developed immediately after the channel 
change blanking interval ends. In addition, the blanking network is 
arranged so as to facilitate control over beam currents developed in 
response to both luminance and chrominance components of a video signal 
processed by a color television receiver. 
Channel change blanking apparatus according to the present invention is 
included in a color television receiver including a video signal 
processing channel for providing plural color image representative video 
signals, a kinescope for providing a picture display in response to the 
color signals, plural video output stages for respectively supplying the 
plural color signals to the kinescope, and a network for automatically 
limiting excessive beam currents conducted by the kinescope. The receiver 
also includes an input frequency selective tuning network, a switch 
network, and a source of reference bias potential coupled in common to the 
plural video output stages and to which video signals processed by the 
video output stages are normally referenced. The tuning network includes 
an adjustable channel selector operated during channel changing intervals 
for selectably tuning the receiver. The switch network is coupled to a 
source of potential, and exhibits a first switching state during the 
channel changing intervals and a second switching state at other times. In 
addition, a blanking network is coupled to the reference bias source and 
to the switch network. The blanking network exhibits a blanking response 
time for producing a modified reference bias potential during channel 
change intervals so that the video output stages and thereby the kinescope 
are blanked during channel change intervals. The blanking network also 
exhibits an unblanking response time slower than the blanking response 
time, for permitting the modified reference bias voltage to gradually 
return to normal after the channel change interval ends.

In the system of FIG. 1, broadcast composite color television signals are 
received by an antenna 10 and supplied to a radio frequency (RF) tuner 
stage 11. Tuner 11 includes a frequency selective channel selector 
mechanism, an automatic fine tuning (AFT) control circuit, and RF and 
mixer stages for providing an intermediate frequency (IF) output signal. 
Associated with the channel selector mechanism is a switch network coupled 
to an output terminal A of tuner 11 and operative when the receiver is 
being tuned from channel to channel, at which time the AFT network is 
disabled. 
The IF signals from tuner unit 11 are supplied to an IF signal processor 12 
including IF amplifier and video detector stages as well as automatic gain 
control (AGC) circuits. One signal output of IF unit 12 supplies a 
detected video signal to a frequency selection bandpass filter 13 (e.g., 
including a comb filter) for providing separated luminance (Y) and 
chrominance (C) components of the composite color television signal. 
Another detected video signal output of IF processor 12 is coupled to a 
sync separator 35 for deriving the horizontal line synchronizing (sync) 
component of the television signal. The derived sync component is supplied 
from an output of separator 35 to sync processing and deflection circuits 
38. Circuits 38 (e.g., including synchronized horizontal and vertical 
oscillators) provide horizontal and vertical deflection signals H and V 
for application to deflection control circuits of a color kinescope 28, 
and horizontal flyback pulses during horizontal line retrace intervals. 
Operatively associated with deflection circuits 38 is a horizontal 
deflection voltage regulator 42 including a switching network (e.g., an 
SCR switching network) timed to operate in response to horizontal flyback 
pulses received from deflection circuits 38. 
The separated chrominance component (C) from unit 13 is processed by a unit 
14 for providing r-y, g-y and b-y color difference signals. These signals 
are combined in a matrix 20 with processed luminance signals from a 
luminance processor 16 to provide low level, color image representative 
signals r, g and b. These color signals are amplified individually within 
a kinescope driver stage 25 to provide high level output color signals R, 
B and G, which are coupled to respective intensity control electrodes 
(e.g., cathodes) of kinescope 28. 
High operating voltage for kinescope 28 is provided from a high voltage 
supply 50 (e.g., a voltage tripler). One input of supply 50 is supplied 
with horizontal flyback pulses, and another input of supply 50 is coupled 
to a source of operating potential (B+) via a resistor 52. Current (I) 
supplied to supply 50 via resistor 52 is representative of the beam 
current demand of kinescope 28. 
Excessively high average beam currents above a threshold level are sensed 
by an average responding sensing unit 54 (e.g., including a filter 
capacitor) coupled to resistor 52. A signal developed by sensing unit 54 
in accordance with the level of excessive beam currents is applied to a 
beam current limiter control network 58, which develops a suitable output 
control voltage proportional to the level of excessive beam current. 
A keyed automatic brightness control network 60 develops an output signal 
suitable for controlling the brightness representative DC level of 
luminance signals processed by network 16 in response to a reference 
voltage derived from a viewer adjustable brightness control 61 (e.g., a 
potentiometer), the output control voltage from beam limiter control 
network 58, and the level of the b color signal from matrix 20 during the 
"back porch" interval of horizontal blanking intervals. For purposes of 
normal brightness control, unit 60 is keyed during the "back porch" 
interval of horizontal blanking intervals to compare the reference voltage 
from brightness network 61 with the blanking level then associated with 
the b color signal. The output signal from network 60 adjusts the DC level 
of the luminance signal in accordance with the setting of brightness 
control 61 via a closed control loop including luminance processor 16, 
matrix 20, and brightness control network 60. In the beam current limiting 
mode, control network 60 compares the blanking level of signal b with the 
control voltage from unit 58. The output signal then developed by network 
60 serves to modify the DC level of the luminance signal from processor 16 
in a direction to limit excessive kinescope beam current conduction via a 
second control loop. The arrangement including automatic brightness 
control 60 can be of the type disclosed in U.S. Pat. No. 4,209,808 and in 
U.S. Pat. No. 4,197,557 for example. The described automatic beam current 
limiter including control unit 58 can be of the type disclosed in a 
copending U.S. patent application Ser. No. 103,445 of L. A. Harwood, et 
al., titled "Automatic Kinescope Beam Current Limiter With Sequential 
Control Modes." 
The response time of the average beam current limiter is dictated by 
factors including the time constant associated with the circuits in the 
beam current limiter control loop. In this example, the automatic beam 
current limiter operation exhibits a delay (approximately 0.1 seconds) 
between the time excessive beam currents first appear, and the time when 
beam limiter operation is initiated. The average beam current limiter in 
this case typically becomes effective for steady state beam current 
limiting within about one-half second after excessive beam currents first 
appear. 
An auxiliary blanking control network 65 is coupled to kinescope driver 
stage 25 and to terminal A of tuner 11, and serves to blank the kinescope 
display when the receiver is being tuned from channel to channel. 
Additional details of blanking network 65 and driver 25 are shown in FIG. 
2. 
Referring to FIG. 2, kinescope driver stage 25 is shown as comprising red, 
green and blue signal amplifiers respectively including transistor pairs 
71 and 72, 73 and 74, and 75 and 76, each pair being arranged in cascode 
amplifier configuration. Low level r, g and b signals are applied 
individually to base inputs of transistors 71, 73 and 75 for developing 
high level R, G, B signals at collector outputs of transistors 72, 74 and 
76, respectively. 
Driver stage 25 also includes a bias network 80 associated in common with 
each of the red, green and blue signal amplifiers. Network 80 includes a 
PNP transistor 85 biased by means of a network including resistors 87 and 
88 to provide a bias reference voltage V.sub.R (approximately +3.2 volts) 
at the emitter output of transistor 85. Reference voltage V.sub.R is 
coupled in common to the emitter circuits of transistors 71, 73 and 75, 
and represents a reference potential to which the video signals amplified 
by the driver stage are referenced for the purpose of establishing a 
desired picture black level reference. Specifically, the value of voltage 
V.sub.R is such that substantially no signal currents flow in the color 
signal amplifiers, and the kinescope is biased at a threshold conduction 
point, when the input color signals exhibit a black level picture 
condition. This condition occurs, for example, when a viewer adjustable 
brightness control of the receiver (e.g., included in network 61 of FIG. 
1) is at mid-range setting. 
Blanking control network 65 (FIG. 1) is shown in FIG. 2 as comprising a 
capacitor 90, resistor 92 and diode 94 arranged in series between terminal 
A at the output of tuner 11 (FIG. 1) and a point of reference potential 
(ground). A voltage developed on capacitor 90 is applied to the base 
electrode of transistor 85. A switch 95 coupled to a source of DC 
potential (+27 v.) is located in tuner unit 11. 
Switch 95 is in an open or nonconductive position (as shown) when the 
receiver is tuned to a television channel. At such time a stable, 
quiescent voltage of approximately +2.5 volts appear on capacitor 90 and 
bias network 80 is permitted to develop reference voltage V.sub.R 
(approximately +3.2 volts). Switch 95 is rendered conductive (closed) 
during channel change blanking intervals when the receiver is being tuned 
from one channel to the next (i.e., between channels). In the case of a 
mechanical channel selector mechanism, for example, switch 95 opens and 
closes in response to the operation of a cam associated with the channel 
selector. 
During each channel change blanking interval, capacitor 90 charges rapidly 
to a voltage above the quiescent voltage, toward the +27 volt level of the 
supply connected to switch 95. The rate at which capacitor 90 charges 
(i.e., the charging time constant) is determined by the level of the +27 
volt supply, the (negligible) impedances of switch 95 when closed and 
terminal A, the impedance presented by diode 94 and resistor 92, and the 
value of capacitor 90. In this example the charging time constant of 
capacitor 90 is approximately 3.3 milliseconds. 
A normally nonconductive clamping diode 98 conducts to clamp the base 
voltage of transistor 85 to approximately +11.2 volts when the voltage on 
capacitor 90 reaches or attempts to exceed this level. The clamping action 
of diode 98 prevents the base voltage of transistor 85 and the voltage on 
capacitor 90 from exceeding +11.2 volts, at which time transistor 85 is 
rendered nonconductive (reverse biased). The emitter potential of 
transistor 85 then rises to a voltage V.sub.R ' which is more positive (on 
the order of +5 volts) than normal emitter reference voltage V.sub.R, and 
which is primarily determined by the impedance presented to the emitter of 
transistor 85 in conjunction with the level of the D.C. operating supply 
voltage (+10.7 volts) applied to the emitter circuit of transistor 85. The 
more positive reference voltage V.sub.R ' causes signal amplifier 
transistors 71, 73 and 75 to be cut-off or blanked, thereby also blanking 
the kinescope display. 
At the end of the channel change blanking interval, switch 95 is rendered 
nonconductive and capacitor 90 begins to discharge at a predetermined rate 
that is slower than the charging rate of capacitor 90 via resistor 92. The 
discharge time constant of capacitor 90 is on the order of fifty 
milliseconds, and is determined by the value of capacitor 90 and the value 
of the parallel combination of resistors 87 and 88. Diode 98 is rendered 
nonconductive when the voltage on capacitor 90 drops slightly below the 
+11.2 volt base clamping level. Transistor 85 begins to conduct when the 
voltage on capacitor 90 drops 1 V.sub.BE (the base-emitter junction offset 
voltage of transistor 85) below the voltage V.sub.R ' at the emitter of 
transistor 85. 
It is noted that the desired rapid rate of charging capacitor 90 to a 
desired level via switch 95 during channel change blanking intervals 
increases in accordance with the level of the charging voltage source 
coupled to the switch (+27 volts in this case). Thus the desired capacitor 
charging level can be reached sooner by increasing the magnitude of this 
voltage source. However, if capacitor 90 is permitted to charge to too 
high a voltage (e.g., approximately +27 volts in this case), the time 
required for the capacitor to discharge to a level low enough to permit 
transistor 85 to conduct after the channel change blanking interval ends 
may be excessively long. The clamping action of diode 98 assists to 
overcome this difficulty by limiting the voltage developed on capacitor 90 
to a maximum of +11.2 volts as discussed. 
The conduction of reference source transistor 85 increases gradually in 
accordance with the gradual, exponential discharging of capacitor 90 at 
the end of the channel change blanking interval, until the voltage on 
capacitor 90 reaches the stable quiescent level. This level is reached 
approximately 0.25 seconds after the channel changes blanking interval 
ends. At this time normal reference voltage V.sub.R again appears at the 
emitter of transistor 85. The described discharging action of capacitor 90 
serves to gradually (exponentially) increase the conduction of video 
output transistors 71-75 after channel change blanking ends, so that these 
transistors are biased for normal operation when the voltage on capacitor 
90 reaches the stable level. 
By gradually increasing the conduction of the video output transistors in 
this manner after channel change blanking ends, the kinescope beam current 
is permitted to increase gradually, rather than abruptly. The magnitude of 
the kinescope beam current during this time is a function of the 
(decreasing) level of reference voltage V.sub.R and the level of the 
signals then appearing at the inputs of video output transistors 71, 73 
and 75. When the level of these signals is associated with a signal 
condition capable of producing excessive beam currents, a point will be 
reached, some time after the end of the channel blanking interval, when 
the gradually decreasing reference voltage permits the beam current to 
reach the threshold level of the automatic beam current limiter operation. 
Automatic beam current limiting is initiated approximately 0.1 seconds 
after the threshold is reached, due to the initial beam limiter operating 
delay. From this time the rate of further increases of excessive beam 
current conduction above the threshold level is constrained by the initial 
(partial) limiting action of the automatic beam current limiter, until 
some later time when the beam limiter becomes fully effective for steady 
state beam current limiting (in this example approximately one-half second 
after the threshold current level is reached). 
The exponential discharge rate of capacitor 90 in this example is slow 
enough to constrain potentially damaging levels of excessive beam current 
until the automatic beam current limiter can become effective, but is fast 
enough so that a viewer is unlikely to notice a delay between the time 
that the channel change interval ends and the time that an image is 
displayed by the kinescope. More specifically, the fifty millisecond 
discharge time constant for capacitor 90 is chosen so that the voltage on 
capacitor 90 substantially reaches the normal quiescent voltage value 
approximately 0.25 seconds after the channel change blanking interval 
ends. Accordingly, normal reference voltage V.sub.R and associated 
kinescope beam current conduction are also developed approximately 0.25 
seconds after channel change blanking ends. This time interval is 
sufficient to permit the automatic beam current limiter, with an initial 
response time delay of 0.1 seconds in this example, to begin limiting 
excessive beam currents when present. 
The channel change unblanking delay provided by network 65 as discussed 
prevents unsafe, potentially damaging levels of beam current from 
appearing for a period of time before the automatic beam current limiter 
becomes effective. Such high levels of beam current can be caused by video 
signal information associated with the channel to which the receiver is 
tuned and which appears immediately after the receiver is tuned to such 
channel. The high levels of beam current also can be caused by the often 
unpredictable behavior of the tuner and IF signal processing networks of 
the receiver during channel change. In this case abnormally high white 
level signals including noise can be produced until the AFT and AGC 
circuits associated with the tuner and IF networks resume normal, 
stabilized operation. 
The unblanking delay provided by network 65 also assists to prevent the 
operation of the deflection circuits from being disrupted in the presence 
of abnormally high levels of beam current (i.e., several times greater 
than normally expected high beam current levels). With reference to FIG. 
1, it is noted that abnormally high levels of beam current tend to "load" 
the horizontal flyback output of deflection circuits 38, with an attendant 
reduction in the level of the flyback signal which is employed for timing 
the operation of the switching network in deflection regulator 42. This 
can lead to irregular operation of the regulator, causing large variations 
in the regulator output voltage. Such operation often causes a condition 
sometimes referred to as "squegging" wherein the image display is badly 
distorted and includes random streaks across the display, among other 
effects. 
The arrangement including auxiliary blanking control 65 and reference 
source 80 provides several significant advantages in addition to those 
mentioned previously. 
Since network 65 operates with respect to reference source 80 which 
supplies a signal reference bias in common to each of the R, G, B video 
driver stages, the unblanking delay provided by network 65 serves to 
control the R, G, B kinescope drive signals comprising luminance and color 
difference signals, both of which can contribute to the production of 
excessively high beam current levels. 
The blanking and unblanking control provided by network 65 is accomplished 
by means of DC rather than by AC signal control such that network 65 is 
essentially isolated from the video signal processing path. In addition, 
network 65 operates independent of the automatic brightness control loop. 
Therefore the use of a capacitance such as capacitor 90 for providing the 
blanking and unblanking delay control functions does not compromise the 
stability of the automatic brightness control loop. Also, the value of 
capacitor 90 can be tailored to suit the channel change blanking and 
unblanking timing requirements of a particular system without requiring a 
compensating adjustment of the signal processing parameters of the video 
signal processing circuits. 
The described arrangement is especially useful in a television receiver 
with limited access to luminance and chrominance signal control points, 
such as in a receiver where luminance and chrominance signal processing 
occur primarily in an integrated circuit with a limited number of external 
terminals available for control purposes.