Note: Descriptions are shown in the official language in which they were submitted.
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T I TLE OF THE I NVENT I ON
FOC~SSING ADJUSTMENT CHANGEOVER CIRCUIT FOR
PROJECTION TV RECEIVER
BACKGRO~ND OF THE INVENTION
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Field of Ap~lication
The present invention relates to a focus
adjustment changeover circuit for facilitating the
execution of focus adjustment and white balance
ad~ustment of a projection display television receiver.
BRIE~ DESCRIPTION OF THE DRAWINGS
Fig. 1 shows emission characteristics of green,
red and blue CRTs of a projection display television
receiver;
Fig. 2 is a circuit diagram of an example of a
prior art focus adjustment circuit for a projection
display television receiver;
Fig. 3 is a circuit diagram of a first embodiment
of a focus adjustment changeover circuit according to
the present invention; and,
Fig. 4 is a circuit diagram of a second
embodiment of a focus adjustment changeover circuit
according to the present invention.
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Prior Art Technology
With a generally utilized form of projection
display television receiver, three separate cathode ray
tubes ~hereinafter abbreviated to CRTs) are utilized
which have respective red, green and blue phosphors,
1~ for projecting respective red, green and blue
television pictures. These three CRT pictures are
projected onto a single display screen by projection
lenses. The red, green and blue television pictures
are thereby superimposed on the screen to form a
projected color television picture. Generally, ~he
emission characteristics of the phosphors of these
CRTs, i.e. the relationships between CRT drive voltage
and brightness of emitted light, are of the form
illustrated in Fig. 1. In Fig. 1, numeral 1 denotes
the emission characteristic for the green CRT and
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numeral 2 the emission characteristic for the red CRT,
when the respective electron beams of these CRTs are
precisely focused by the electron lenses of the CRTs,
e.g. to produce a minimum spot size by each electron
beam. Numeral 3 denotes the emission characteristic of
the blue CRT under such a condition of precise focus of
the electron beam, while numeral 4 denotes the
characteristic of the blue CRT under a condition in
which the electron beam of that tube is slightly
defocused by the electron lens of the tube. This
condition of slight defocusing will be referred to in
the following as the defocused state. As can be seen
for Fig. 1, the emission characteristic of each of the
red and green CRTs is substantially linear up to high
levels of drive voltage, i.e. up to high values of
electron beam current. However the emission
characteristic of the blue CRT exhibits a saturation
condition at high levels of beam current i.e. the
emission characteristic of the blue phosphor exhibits
saturation at lower levels of CRT drive voltage than do
the red and green phosphors. As a result, even if the
white balance of the projected television picture is
adjusted to be correct at some moderate level o
display brightness, the white color produced on the
display screen by combining light from the red, blue
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and green CRTs will have an insufficient level of blue
component and so will have an excessively low color
temperature, i.e. will not produce a pure white color,
at high levels of emitted light.
However as illustrated by curve ~, this problem of
saturation of the blue phosphor is alleviated to some
extent if the electron beam of the blue CRT is operated
in a slightly defocused condition, rather than being
precisely focused. For this reason it is usual to
operate the blue CRT in this slightly defocused
condition, in order to provide a more accurate white
balance at high levels of CRT drive voltage, and to
execute white balance adjustment of the television
receiver with the blue CRT in this defocused state.
This has however the disadvantage that problems
arise when focusing adjustment of the projection lens
of the blue CRT is executed. Specifically, this
projection lens adjustment (e.g. performed while
observing a crosshatch test pattern that is projected
on the display screen by the blue CRT) is made
difficult by the fact that the blue electron beam is
defocused, and also by the fact that a projected image
in blue light has inherently low visibility by
comparison with a green or a red image. For this
2S reason such a projection display television receiver is
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made operable in two type~ oE adiu~tment modes, i.e. a
mode in which electromagnetic focusing of the
respe~ive electron beams i~ established such that only
the blue CRT is set in the defocused state and the red
and green CRTS in the precisely ~ocused state, s~ that
white balance adjus-tment can be executed, and a mode in
which electromagnetic focusing is established such that
all of the CRTs are set in the precisely focused state r
so that projection lens focus adjustment carl be
executed.
An example of a prior art focus adjustment
changeover circuit will be described referring to Fig.
2, in which it is assumed that electromagnetic focusing
of each CRT is utilized. Numeral 5 denotes an input
terminal coupled to receive a video signal for
producing a normal television picturer while an input
terminal 6 is coupled to receive an adjustment signal
which is a crosshatch test pattern signal, to produce a
crosshatch test pattern picture that is utilized during
focus adjustment and white balance adjustment
operation. These signals from the input terminals 5
and 6 are selectively transferred to a video processing
circuit 7 by a switch 8~ and circuit 7 thereby
produces video projection signals that are applied to
drive red, green and blue CRTs (not shown in the
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drawings). Numerals 9, 10 and 11 denote respective
static focus coils for performing static electrical
focusing of the electron beams of the blue, red and
green CRTs. Numerals 12, 13 and 14 denote current
stabilizer circuits for supplying respective currents
to the static focus coils 9, 10 and 11 respectively for
static focus control. Each of the circuits 12, 13 and
14 is configured as shown for current stabilizer
circuit 12, i.e. including a pair of transistors 15 and
0 lf~ which are connected in common-emitter configuration,
and drive and feedback transistors 17 and 18.
An input terminal 19 is coupled to receive a
parabola waveform voltage signal during each vertical
scanning period, for vertical focus control.
This signal is amplified by a transistor 20, and is
then adjusted in amplitude by means of a potentiometer
21 and transferred through respective capacitors of a
capacitor group 22 to the bases of the respective
transistors 15 of the current stabilizer circuits 12,
13 and 14. In this way, electron beam focus deviations
between the center and the upper and lower regions of
the projected picture are corrected.
Numerals 23, 24 and 25 denote respective
potentiometers which are utilized for static focus
adjustment of the blue, green and red CRTs
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respectively. DC voltages derived from these
potentiometers, adjusted to provide appropriate levels
of electromagnetic focus currents, are applied through
respective ones of a set of resistors 26 to the bases
of the respective ones of transistors 15 of the current
stabilizer circuits 12, 13 and 14. Static focus
adjustment of the electromagnetic focus currents of the
focus coils 9, 10 and 11 of the blue, green and red
CRTs can thereby be mutually independently executed.
Focus adjustment of a projection display
television receiver by utilizing such a prior art
apparatus is executed in two stages. In a first stage,
the changeover switch 8 is set to supply the crosshatch
test pattern signal from the input terminal 6 to the
video processing circuit 7. A crosshatch test pattern
is thereby projected on the display screen of the
television receiver by the CRTs. In this condition,
the potentiometers 23, 24 and 25 are respectively
adjusted such as to vary the levels of DC voltage
applied to the bases of the transistors 15 of the
current stabilizer circuits 12, 13 and 14, to set the
levels of current flow in each of the electromagnetic
focus coils 9, 10 and 11 such as to precisely focus the
electron beams of each of the blue, green and red CRTs.
When this has been completed, optical focusing of the
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respective projection lenses of the blue, green and red
CRTs is executed to attain an optimum focus condition
oE each of the blue, green and red images projected on
the display screen.
~n the second stage, the changeover switch ~ is
set such as to supply the signal from input terminal 5
to the video processing circuit 7, for thereby
projecting a television picture. The focus adjustment
potentiometer 23 for the blue CRT is then adjusted such
as to slightly defocus the electron beam of the blue
CRT. White balance adjustment of the projection
display television receiver is then executed, with the
color temperature of the projected picture at high
levels of brightness being improved due to the slightly
defocused state of the electron beam of the blue CRT as
described hereinabove.
Each time that the installation conditions of a
projection display television receiver are altered,
(i.e. when the television receiver is moved to a
different location, or the distance to the display
screen is altered, etc) it is necessary to repeat the
two steps described above, in order to first adjust the
focus o~ the projection lenses and then re-adjust the
white balance. However with such a prior art focus
adjustment circuit, each time that these two steps must
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be repeated in this way, it is necessary to first
adjust the potentiometer 23 for changing the
electromagnetic focusing of the electron beam of the
blue CRT from the defocused state described above to
the precisely focused state, then to execute focusing
of the projection lenses, then to again adjust the
potentiometer 23 to return the potentiometer 23 to a
position which provides the defocused state of the blue
CRT. It is therefore necessary to frequently make
delicate adjustments to the electromagnetic focus
potentiometer 23. Furthermore since such adjustment of
the potentiometer 23 is executed manually, fixedly
p~edetermined adjustment statuses cannot be ensured, so
; that it is difficult to establish the defocused state
and the precisely focused state for the blue CRT each
time that such an adjustment procedure is performed.
SUMMARY OF THE INVENTION
It is an objective of the present invention to
provide a focus adjustment changeover circuit whereby
alteration of the setting of the static electromagnetic
focus state of the blue CRT of a projection display
television receiver, in order to execute projection
lens focus adjustment when the installation conditions
of the CRT are altered~ can be carried out in a simple
and easy manner.
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It is a further objective of the present invention
to provide a focus adjustment changeover circuit
whereby alteration of a projected picture used for
adjustment purposes can be executed simultaneously with
alteration of the static electromagnetic focus state of
the blue CRT of a projection display television
receiver, when projection lens focus adjustment is to
be executed.
To achieve the above objectives, a focus
adjustment changeover circuit according to the present
invention for a projection display television receiver
according to a first embodiment comprises:
externally operable mode changeover switch means;
and,
circuit means responsive to the changeover switch
means for selectively establishing as two mutually
separate modes of operation a first mode in which a
focus current level is set to provide precise
electromagnetic focusing of a cathode ray tube of the
2 projection display television receiver and a second
mode in which the focus current level is set to provide
an electromagnetically defocused condition of the
cathode ray tube;
and is characterized in that the first mode is
selected when projection lens focus adjustment of the
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projection display television receiver is to be
executed and the second mode is selected when white
balanee adjustment of the is to be executed.
According to a second embodiment, a focus
adjustment changeover cireuit according to the present
invention further comprises second changeover switch
means coupled to be actuated together with the mode
changeover switch means for selecting a test pattern
signal, genera~ed internally within the projection
display television receiver, to be supplied as a drive
signal to th-'. cathode ray tube when the first mode is
selected, and for selecting a color video signal to be
supplied as a drive signal to the cathode ray tube when
the second mode is selected~
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DESCRIPTION OF PREFERRED EMBODIMENTS
Fig. 3 is a circuit diagram of a first embodiment
of a focus adjustment changeover circuit according to
the present invention. In Fig. 3, components which are
iden~ical to components in the prior art example of
Fig. 2 are designated by corresponding reference
numerals, and Yurther description of these will be
omitted. Numerals 27 and 28 denote respective
electromagnetic focus adjustment potentiometers which
are used to ~et the levels of focus current flowing in
the focus coil 9 of the blue CRT to the precisely
focused state and ~he defocused state (described
hereinabove) respectively. A changeover switch 29
functions as a focus mode selection switch, for
selectively establishing two modes which will be
designated as modes A and B in the following, and which
are selec~ed when the input terminals A and input
terminals B of the mode changeover switch 29 shown in
Fig. 3 are selected, respectively. In this embodiment,
mode A is the precisely focused state, in which the
electron beams of all of the CR~s including the blue
CRT are optimally focused. This mode is used for
adjustment of the projection lens focus. Mode B is
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the condition in which the electron beam of the blue
CRT is set in the defocused state, and is used for
white balance adjustment. At the time of initial set-
up adjustment of the television receiver, the mode
changeover switch 29 is set for mode ~, so that a focus
adjustment voltage produced from the electromagnetic
focus adjustment potentiometer 27 is supplied to the
base of transistor 15 of the current stabilizer circuit
12 of the blue CRT. In addition, switch 8 is set to
supply a crosshatch test pattern signal to the video
processing circuit 7, as described above, such that
this pattern is projected on the display screen by the
blue CRT. The level of current which flows in the
blue static focus coil 9 is then adjusted by variation
of the potentiometer 27, to set the blue CRT in a
precise electromagnetic focus condition. In this
state, the projection lens of the blue CRT is rotated
until optimum optical focusing of that lens is
attained.
Next, switch 29 is set to select the B mode,
whereby a focus adjustment voltage from the
electromagnetic adjustment potentiometer 28 is supplied
to the base of transistor 15 of current stabilizer
circuit 12, and the switch 8 is changed over to supply
the color video signal from input terminal 5 to the
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video processing circuit 7. The level of current which
flows in the blue static focus coil 9 is then adjusted
by variation of the potentiometer 28 to set the
electron beam of the the blue CRT in the slightly
defocused state described hereinabove. White balance
adjustment is then carried out.
Subsequently, whenever it again becomes necessary
to execute adjustment of the projection lenses and
white balance adjustment, e.g. when the projection
display television receiver is moved to be installed at
a different location, then it is only necessary to
change from mode B to mode ~ by actuating the mode
changeover switch 29 (and also actuating switch 8), to
set the blue CRT in the precise electromagnetically
focused stater so that projection lens focus adjustment
can be executed. Thus, it is no longer necessary to
further adjust the focus current supplied to focus coil
9 of the blue CRT when such projection lens focusing is
to be performed. Similarly, after this projection
lens focus adjustment has been completed and the blue
CRT is to be returned to the defocused state for
execution of white balance adjustment, it is only
necessary to actuate switch 28 to restore mode B
operation (with switch 8 being changed over to select
input terminal 5), whereby the blue CRT is restored to
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the defocused state, so that white balance adjustment
can then be immediately performed without any
readjustment of the focus current of coil 9 of the blue
CRT. It can thus be understood that these
electromagnetic and optical focus adjustment operations
can be executed much more rapidly and conveniently than
has been possible in the prior art, while in addition
the two selectable levels of focus current for the blue
CRT can be initially fixed with a high degree of
accuracy, and thereafter left unchanged.
Fig~ 4 shows a second embodiment of a focus
adjustment changeover circuit according to the present
invention. ThiS is similar to the first embodiment
described above, but differs in that the mode
chanyeover switch 29 and the input signal changeover
switch 8 are mutually linked (i.e. ganged) such that
when mode A is selected by the mode changeover switch
29, the crosshatch pattern signal is selected by switch
8 to be supplied from input terminal 6 to the video
processing circuit 7, while when mode B is selected by
switch 29, the color video signal is selected by switch
8 to be supplied from the input terminal 5 to the video
processing circuit 7. otherwise, the operation of this
embodiment is identical to that of the second
embodiment described aboveO
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