Note: Descriptions are shown in the official language in which they were submitted.
lZ~4859
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"TELEVISION RECEIVER ALIGNMENT SYSTEM"
This invention concerns apparatus for setting
the bias of the G2 grid of a kinescope in a television
receiver.
In a color television receiver having a color
image reproducing kinescope with plural electron guns, the
black level bias voltage of the electron guns establishes
the peak beam current available fro~ the electron guns.
The black level voltage of each electron ~un is related to
the magnitude of the bias voltage applied to the kinescope
G2 grid, also referred to as the screen grid. A color
kinescope with an "in-line" electron gun structure has a
G2 grid electrode energized in common to all three
electron guns, while a kinescope with separate
"delta-type" electron guns has separately energized G2
grid electrodes for each gun. In either case, the G2 bias
voltage is often set at a value between 400 and 600 volts
so that a desired cathode-to-Gl grid voltage produces a
black level condition.
High brightne~s and high resolution in a
reproduced image require a high kinescope peak beam
current capability and small spot size. For each gun,
peak beam current capability increases with increasing
black level voltage, which is related to the G2 bias
voltage. The need for high brightness and high resolution
suggests ~hat the highest available G2 bias voltage should
be used, consistent with other requirements and
constraints of the receiver design.
Some television receivers also employ automatic
kinescope bias (AKB) control systems for maintaining a
desired black level kinescope cathode bias. Such systems
operate to maintain desired cathode-to-G1 grid bias for
each electron gun, and should be capable of operating over
a range of black level bias voltages at least as great as
the maximum difference in black level voltage between any
two kinescope electron guns, which can be on the order of
50 volts. To compensate for other system parameter
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tolerances as well, the operatin~ range of the AKB system
may be as great as 100 volts. The choice of an operating
point within that range is determined by the G2 grid bias
voltage. Consequently, the G2 bias voltage must be
manually adjusted on each receiver to insure that the
black level bias voltage of each kinescope electron gun is
within the operating range of the AKB system.
Furthermore, to obtain high brightness and resolution in a
displayed image, the G2 bias voltage should be adjusted so
as to make the black level bias voltage of the electron
gun with the highest (ie., most positive) black level
voltage nearly equal to the highest useable black level
bias voltage capable of being produced by the kinescope
driver stages.
When adjusting the G2 screen grid bias, it is
desirable to assure that adequate dynamic range in the
blacker-than-black direction exists for image blanking
reserve purposes, particularly in the presence of
variations of the operating supply voltage for the
kinescope drivex circuits. It is also herein recognized
that for establishing a desired cathode-to-Gl grid
voltage, circuit configurations which unduly increase the
cost or complexity of the system, or which compromise
other aspects of system performance, should be avoided.
Consistent with these objectives, in accordance
with the principles of the present invention there is
disclosed herein apparatus wherein the existing bias
voltages of both the kinescope Gl grid electrode and the
kinescope cathode electrode are modified to produce a
prescribed cathode-to-Gl voltage during alignment of the
receiver when the bias of the kinescope G2 grid electrode
is being adjusted.
The single FI~URE of the drawing depicts a
portion of a color television receiver including kinescope
bias adjusting apparatus according to the present
invention.
Television signal processing circuits 10 provide
separated luminance (Y) and chrominance (C) components of
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a composite color television signal to a
luminance-chrominance signal processor 12. Processor 12
includes luminance and chrominance gain control circuits,
... . .
DC level setting circuits, color demodulators for
S developing r-y, g-y and b-y color difference signals, and
matrix amplifiers for combining the latter æignals with
processed luminance signals to provide low level color
image representative output signals r, g and b. These
signals are respectively amplified by red, green and blue
video output kinescope driver amplifiers 14a, 14b and 14c
of similar configuration as that shown for red driver 14a.
Drivers 14a, 14b and 14c provide high level amplified
color image signals R, G and B to respective cathode
intensity control electrodes 16a, 16b and lSc of a color
k~nescope 15. In this instance kinescope 15 is of the
self-converging, "in-line" gun type with a commonly
energized Gl grid electrode associated with each of the
kinescope electron guns comprising cathode electrodes 16a,
16b and 16c, and a commonly energized G2 (screen) grid
electrode cou~led to a potentiometer 17 Providinq an
adjustable bias volta~e to the G2 grid.
Driver 14a includes an input common emitter
amplifier transistor 20 which receives input signal r via
a resistor 21, and a high voltage common base output
amplifier transistor 22 which forms a cascode video output
amplifier stage with input transistor 20. High level
video signal R suitable for driving kinescope cathode 16a
is developed across a load resistor 24 in the collector
output circuit of transistor 22. A high operating
potential for driver 20, 22 is provided by a source of
positive DC potential (+230 volts) coupled to the
collector circuit of transistor 22. Direct current
negative feedback is provided from the collector output of
transistor 22 to thP base input of transistor 20 by means
of a feedback resistor 25. The signal gain of cascode
amplifier 20, 22 is primarily determined by the ratio of
the value of feedback resistor 25 to the value of the
input impedance of driver 20, 22 comprising resistor 21.
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The feedback network provides a suitably low amplifier
output impedance, and assists to stablize the DC operating
level at the amplifier output.
Automatic kinescope bias (~KB) control networks
13a, 13b and 13c are respectively associated with each of
driver stages 14a, 14b and 14c. The automatic bias
control networks exhibit similar structure and operating
characteristics and serve to maintain a desired black
level bias for the respective cathodes of kinescope 15. A
sensing resistor 30 in series with driver transistors 20,
22 acts in conjunction with the AKB system by developing a
voltage at a sensing node A representative of the
kinescope cathode black current level conducted duxing
image blanking intervals.
A timing signal source 40 associated with the
AKB system responds to a horizontal line synchronizing
rate signal (~) and to a vertical field synchronizing rate
signal (V), both derived from deflection circuits of the
receiver, for generating periodic timing signals VB, VS
and VG which control the operation of the AKB function
during periodic AKB control intervals. Each AKB interval
begins shortly after the end of each video signal vertical
retrace interval within the vertical blanking interval,
and encompasses several horizontal line intervals also
within the vertical blanking interval and during which
video signal image information is absent,as shown, for
example, in U.S. patents 4,263,622 and 4,277,798, both of
Werner Hinn.
Timing signal VB is generated shortly after the
end of the vertical retrace interval, and exists for the
duration of the AKB interval. This signal is applied to
an input blanking control terminal of
luminance-chrominance processor 12 for causing the r, g
and b outputs of processor 12 to exhibit a DC reference
voltage corresponding to black video signal information.
This is accomplished by reducing the signal gain of
processor 12 to substantially zero via the gain control
circuits of processor 12 in response to signal VB, and by
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modifying the DC level control circuits of processor 12 to
produce a black representative reference voltage at the
outputs of processor 12.
Timing signal VS occurs during the AXB interval
S and enables sampling circuits within bias control networks
14a, 14b and 14c to operate for developing an output bias
control signal representative of the magnitude of the
kinescope black current.
Timing signal VG, a positive grid drive pulse
is developed during a prescribed portion of the AKB
interval (e.g., comprising two horizontal line intervals
within the vertical blanking interval~ and is amplified by
a network 42 before being coupled to the G1 grid of
kinescope 15 via a conductor 44.
During each AKB interval, positive pulse VG
forward biases grid Gl, thereby causing the electron gun
comprising cathode 16a and grid Gl to increase conduction.
In response to grid pulse VG, a similarly phased, positive
current pulse appears at cathode 16a during the grid pulse
interval. The amplitude of the cathode output current
pulse is proportional to the level of cathode black
current conduction (typically a few microamperes).
The induced positive cathode output pulse
appears at the collector of transistor 22. This pulse is
fed back to the base input of transistor 20 through
resistor 25, causing the current conduction of transistor
20 to increase proportionally while the cathode pulse is
present. The increased current conducted by transistor 20-
causes a voltage to be developed across sensing resistor
30. This voltage is in the form of a negative-going
voltage pulse which appears at sensing node A and which is
proportional in magnitude to the magnitude of the black
level representative cathode output pulse.
The recovered black current representative
voltage pulse is coupled from node A via an AC coupling
capacitor 34 to sampling and control signal processing
circuits in bias control network 13a. Keyed sample and
hold circutis within network 13a are enabled by sampling
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timing signal VS for developing a DC bias control voltage
proportional to the magnitude of the voltage pulse
developed at node A. The bias control voltage is stored
and is applied via a resistor 38 to a bias control input
at the base of transistor 20 for maintaining a desired
cathode bias voltage corresponding to a desired black
level cathode current. Illustratively, if the magnitude
of the induced cathode output pulse corresponds to a
condition of excessive black current, the bias control
voltage decreases to thereby increase the bias voltage of
cathode 16a at the collector of transistor 22. This
reduces the black current level to the correct level.
Networks 13a, 13b and 13c can employ signal
sample and hold networks of the type described in U. S.
Patent No. 4,331,981 and in U. S. Patent No. 4,331,982,
both of R. P. Parker, and can also employ sampling and
control voltage processing circuits of the type shown in
U. S. Patent 4,277,7g8 of Werner Hinn.
A double pole, double throw switch 50 for
facilitating receiver alignment in a service operating
mode of the receiver includes slideable contacts 52, 54
and terminals a-f. No external connections are made to
switch terminals "a" and "f". A +230 volt DC potential,
generated by a voltage source 48, is coupled via resistors
56 and 57 to switch terminals "b" and "c", respectively
and a resistor 58 is coupled from terminal "c" to ground.
A +11.2 volt DC potential, also generated by source 48, is
connected to switch terminal "d". The +230 volt DC
potential coupled to switch 50 also ser~es as the
operating supply voltage for red, green and blue driver
stages 14a, 14b and 14c.
In a "normal" position of switch 50 as shown,
red driver stage 14a is enabled to operate by means of the
base of output transistor 22 receiving a +11.2 volt bias
potential via switch terminal d, contact 54, terminal e,
conductors 60, 62 and input bias terminal "+". Green and
blue drivers 14b and 14c are enabled in similar fashion.
In addition, in this switch position automatic bias
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control networks 13a, 13b and 13c are enabled to operate
in response to the +11.2 volt potential being coupled to
operating supply volta~e input te~minals (+) of networks
13a, 13b and 13c via switch 50 and conductors 60, 64.
Normal bias for the G1 grid is provided by means of a
voltage divider comprising the +230 volt source and
resistors 57 and 58 as connected to G1 grid coupling path
44. The normal bias for the Gl grid is approximately +21
volts.
In a "service" position of switch 50 when it is
desired to adjust the bias of the kinescope G2 (screen)
grid via a potentiometer 17, switch contact 52 connects
terminals "b" and "c" and switch contact 54 connects
terminals "e" and "f". Thus the +11.2 volt source is
decoupled from the bias inputs of each driver stage,
whereby the upper rank video output transistor (eg.,
transistor 22) of each driver stage is rendered
non-conductive. The +11.2 volt source is also decoupled
from the operating supply voltage inputs of each of the
automatic bias control networks, thereby disabling the
normal operation of the bias control networks. Disabling
the normal operation of the bias control networks in the
service mode prevents the control networks from developing
bias control voltages which could lead to an excessive,
high current, bias correction in a white-going direction
the moment switch 50 is returned to the normal position.
In addition,resistor 56 is coupled across resistor 57 via
switch contact 52, thereby modifying the normal voltage
divider action of resistors 57 and 58 and changing the
bias of the Gl grid to approximately +40 volts from the
normally present +21 volts.
When video output transistor 22 is
non-conductive, the voltage developed across load resistor
~4, and thereby the bias voltage of kinescope cathode 16a,
is substantially equal to +214 volts as determined by the
voltage divider action of the circuit comprising the +230
volt supply potential, load resistor 24, feedback resistor
25 and the potential existing at the base electrode of
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transistor 20. The base of transistor 20 represents a
virtual ground point, ie., the Iquiescent base potential of
transistor 20 corresponds to a relatively small, fixed
potential equal to the sum of ground potential at the
emitter of transistor 20 plus the substantially constant
+0.7 volt base-emitter junction offset voltage of
transistor 20.
Thus it is seen that in the "service" position
of switch 50 a cathode-to-Gl grid voltage differential of
+174 volts, ie., 214 volts 40 volts, is developed. This
voltage was chosen for this system in view of the need to
maximize image brightness and resolution by biasing the
kinescope electron guns to the highest useable black level
voltage via adjustment of the G2 screen grid bias. In
this case it is desired for the electron gun which
exhibits the highest (most positive) black bias level to
produce a black image display when the associated driver
stage output is approximately 20 volts below (less
positive than) the black clipping level of the driver
stage, to provide a measure of blanking-reserve and to
accomodate the dynamic control range of the AKB system.
Thus the G2 screen grid bias is adjusted such that the
electron gun with the highest black level bias is at the
black threshold when the cathode-to-Gl grid voltage is
approximately +174 volts.
Screen grid bias control potentiometer 17 is
initially adjusted in a white-going direction until the
kinescope display screen "lights up" dimly, then
potentiometer 17 is adjusted in a black-going direction
until the display just extinguishes or is barely visible.
When the three electron guns exhibit different conduction
characteristics the display screen will "light up" with
one color corresponding to that associated with the
electron gun having the highest cutoff level. In such
case the electron gun with the highest black level point
is barely emissive. If two or more electron guns have the
same conduction characteristics, then two or more colors
will appear simultaneously.
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Switch 50 is returned to the "normal" position
after the G2 screen grid adjustment has been completed. At
this time the bias of the Gl grid is reduced to the normal
level of approximately +21 volts and the automatic bias
control networks are enabled. Since the G2 screen grid
wa~ adjusted to produce a black display for a
cathode-to-Gl grid voltage of +174 volts, and the Gl grid
voltage in the normal mode is +21 volts, it is seen that
in the normal mode a black level condition corresponds to
a cathode voltage of +195 volts as developed at the
collector output of transistor 22. It is necessary for
driver 20, 22 to exhibit a blanking reserve capability in
a blacker-than-black direction to assure that image
retrace scanning lines are not displayed by the kinescope.
Thus driver 20, 22 exhibits an additional dynamic range of
approximately 20 volts for producing a maximum
blacker-than-black collector potential of +214 volts when
transistor 22 is cut-off. As noted previously, this +214
volt collector potential is produced during the service
mode when the G2 screen grid is adjusted.
Obtaining the desired 174 volt cathode-to-Gl
grid voltage in the service mode is faciliated by
modifying both the bias of the kinescope cathode electrode
(by rendering transistor 22 non-conductive) and the bias
of the Gl grid electrode (via voltage divider resistors
56, 57, 58). In this regard it has been observed that in
some receivers it may be difficult or impossible to obtain
the desired cathode-to-G1 grid voltage in the service mode
by modifying(ie., reducing) the cathode bias alone, unless
more costly and complicated alternative procedures are
used, such as changing the values of the driver feedback
impedance or the driver load impedance via a switching
network, altering the base bias of output transistor 22 so
that transistor 22 conducts a desired, predictable
current, or reducing the value of the +230 volt supply,
for example. In addition to a~ding excessive cost and
complexity to the system, some of these alternatives may
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undesirably compromise system performance such as by
reducing the high ~requency response of ~he driver stages.
