Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE XNVENTION
Field of the Invention
The present inventiorl relates generally
to a video signal processing circuit, and is directed more
particularly to a video signal processing circuit for use
in a television receiver.
Description of the Prior Art
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In the art, when a video signal is applied
to the cathode of a cathode ray tube to drive the same,
it is selected such that, as shown in Fig. 1, the screen
of the cathode ray tube becomes darkest at a pedestal level
Vl of the video signal and becomes brightest at a white
level V2 of the video signal. When the cut off level
(black level3 of the cathode ray tube is selected equal
to the pedestal level of the video signal to completely
restore the D.C. component as set forth above, the fluc-
tuation of a set up leve.. between stations (channels3
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appears, as it is~ as the fluctuation o-f the black level
with the result that when a broadcast wave of a certain
station with a high set up level is received, the black
appears with somewhat greay. To avoid this defect,
in a prior art television receiver, the D.C.transferring
ratio is lowered to about 0.5 t:o 0.9 to reduce the fluc-
tuation of the black level due to the above fluctuation
of the set up level as to be relatively small. However,
the fluctuation of the black level can not be sufficiently
removed by the above manner.
OBJECT~ AND SU~ ~RY OF THE INVENTION
.
Accordingly, an object of the present
invention is to provide a novel video signal processing
circuit free from the defect inherent to the prior art.
Another object of the invention is to
provide a video signal processing circuit in which the
level nearest to the black side in the video period is
detected, taken as the black level and set as the cut-off
level of a cathode ray tube to avoid the fluctuation of
the black level due to the above fluctuation of the set
up level.
A further object of the invention is to
provide a video signal processing circuit in which .the
level nearest to the black side and with a predetermined
level in a video period`is detected, taken as the black
level-and set as the cut-off level of a cathode ray tube
to avoid the fluctuation of the black level due to the
above fluctuation of the set up level without losing the
naturality of a pickure screen.
Accordinq to an aspect of the present
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invention, a video signal processing circuit is provided
which comprises:
a) a wave-forming circuit for setting a level of an
input video signal to a first predetermined D.C. level,
which is higher than a pedestal level of said input
video signal, during at least a blanking period of
said input video signal and producing a wave-formed
signal;
b) a peak-hold circuit for holding a level of a black
-side peak of the wave-formed signal and producing a
peak-held signal; and
c) black level control means controlled by said peak
-held signal for controlling a D.C. level of said input
video signal so as to coincide the darkest level of
said input video signal with a second predetermined
D.C. level.
The other objects~ features and advantages
of the present invention will become apparent fromthe
following description taken in conjunction with the accom-
panying drawings through which the like references designate
the like elements and parts.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. l is a graph used to explain such a
case where a cathode ray tube is driven;
Fig. 2 is a block diagram showing an
example of the present invention;
Fig. 3 is a waveform diagram used to
explain the operation of the example shown in Fig. 2;
Fig~ 4 is a connection diagram showing
a practiacl example of the invention;
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Fig. 5 is a connection diagram showing
another example of a part of Fig. ~;
Fig. 6 is a block diagram showing another
example of the invention;
Fig. 7 is a waveform diagram used to
explain the example of the invention shown in Fig. 6;
Fig. 8 is a block diagram showing a further
example of the invention;
Fig. 9 is a waveform diagram used to
explain the operation of the example shown in Fig. 8;
Fig. 10 is a connection diagram showing
a practical example of that shown in Fig. 8; and
Fig. Il is a waveform diagram used to
explain the operation of the example shown in Fig. 10.
DESCRIPTION OF THE PREFERRED EMBODI~ENTS
The present invention will be hereinafter
described with reference to the attached drawings.
Turning to Fig. 2, the fundamental con-
structuion of the present invention will be now described.
In Fig. 2, 1 designates an input terminal
to which an input video signal So shown in Fig. 3A is fed
from a video detecting circuit (not shown in Fig. 2).
The input video signal So is then supplied to a blanking
circuit 2 where the video signal is blanlced over a period
somewhtat longer than the horizontal blanking period by
a blanking pulse Pl shown in Fig. 3B and fed thereto
through a terminal 3. Then, the blanking circuit 2
generates an output signal S2 which has, for example, a
voltage source potential or voltage, for example, +Vcc
corresponding to the blanking period of the input video
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signal as shown in Fig. 3C. The output signal S2 is
then fed -to a peak hold (bottom hold ) circuit 4 which
in turn produces a peak-hold output signal S3 which cor-
responds to the peak level at the most black-side as
shown in Fig. 3D. The input: video signal S0 Erom the
input terminal 1 and the peak-held signal S3 from the
peak hold circuit 4 are both supplied to a comparator
5. This comparator 5 delivers therefrom one o~ both
signals S0 and S3 which has a level nearer to the white
level than the other or output signal S4 shown in Fig. 3E,
so that this comparator 5 is an analog OR circuit.
The output signal S4 from the comparator 5 is supplied
to a clamp circuit 6 which is also supplied with a clamp
pulse P2, shown in Fig. 3F, through a terminal 7, so that
the clamp circuit 6 produces at its output terminal 8 a
video signal S5 whose blanking periods are made equal with
one another to a predetermined level as shown in Fig~ 3G.
Fig. 4 shows an example of the practical
circuit which will realize the fundamental construction
shown in Fig. 2. In Fig. 4, 9a and 9b designate a
pair of transistors whose emitters are connected together
to a constant current source consisting of a transistor
10, a diode 11 and a resistor 12. The terminal 3; con-
nected through the resistor 12 to the base of the transistor
10, is supplied with the blanking pulse Pl as set forth
above so that, during the period in which the transistor
10 is made OFF bv the blanking pulse Pl, the collector
of the transistor 9b becomes the power source voltage
~Vcc substantially. The collector output of this
transistor 9b is derived through a current mirror circuit
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eonsisting of a diode 13 and a PNP transistor 14,
A series connection of resistors 15 and
16 is inserted between the collector of the transistor 14
and the ground, and.the connection point between resistors
15 and 16 is eonneeted to the base o an NPN transistor
17 which has the emitter grounded and the eollector eon-
neeted to the base of the transistor 9b and also to the
eonnection point between a resistor 18 and a capacitor 19,
The series conneetion of the resistor 18 and eapacitor 19
. eonneeted between the voltage source terminal ~~ec and
the ground and the transistor 17 form the peak hold ei.r-
euit 4 shown in Fig. 2. When the voltage at the base
of the transistor 9a is higher than that at the base of
the transistor 9b, no eurrent flows through the diode 13
and the transistors 14 and 17 and the eapaeitor 19 is
charged up by the power souree voltage through the resijstor
-18. In this ease, the time eonstant is made very large.
When the terminal voltage acxoss the eapaeitor 19 (the
~oltage at the base of the transistor 9b) tends to beeome
higher than the base voltage of the transistor 9a, current
flows through the diode 13 and the transistors 1~ and 17 :.
and the eapacitor l9 is diseharged to lower its terminal
voltage. Thus, such a ~eedback is applied that the
base voltages of the transistors 9a and 9b become equal
each other, so that the level nearest to the black side
in the video period is held and the held-output signal S3
is applied to the base of a transistor 20a.
A transistor 20b is provided which has
the eolleetor and emitter respectively connected to those
of the transistor 20a. The connection point between
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the collectors of bo-th transistors 20a and 20b is conneeted
to the power source terminal +Vcc, and the connection point
between the emitters thereof is connected through a resistor
21 to the ground and also to an output terminal 5a. Both
transistors 20a and 20b form the comparator 5 shown in Fig.
2, and the input video signal S0 is applied to the base
of the transistor 20b. One of the signals S0 and S3 which
is higher than the other in level is derived as the output
signal S4 therefrom.
To the output terminal 5a of the comparator
5 connected is the clamp circuit 6 as described in connection
with Fig. 2. In practice, as the clamp circuit 6, a
well-known clamp cireuit sueh as a feedbaek elamp eireuit
or the like ean be used so that the elamp circuit 6 is
not shown in Fig. 4.
As shown in Fig. 5, such a comparator may
be used in which two diodes 22a and 22b are connected in
sueh a manner that their cathodes are connected together
to the ground through the resistor 21 and also to the
output terminal 5a.
As will be easily understood from the above
deseribed example of the invention, aecording to the present
invention, the level nearest to the black side in the
video period is deteeted, held, is taken as the blaek and
set as the eut-off level (blac]c level? of the cathode ray
tube automatically. so that the fluctuation of the black
level by the fluetuation of the set up level between the
stations as in the prior art can be effectively avoided
by the invention.
It may be considered that the video signaI S2,
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which is subjected to the blan]cing process, is used to
clamp the level nearest to the black side to a predetermind
level which corresponds to the cut-off level. However,
in this case the blanking periocl is selected wider than the
horizontal blanking period and the level of the former is
khe high level at the white side/ so that even if the blank-
ing process is performed at the stage prior to the cathode
ray tube, a pulsative signal in the high level at the white
side will remain. Such the defect is removed by the
present invention.
Fig. 6 shows another example of the invention.
In this example, an input video signal S0 shown in Fig. 7A
is supplied to a clamp circuit 23 and its pedestal level is
clamped therein to a reference voltage V0, which is the
voltage of a clamp voltage source 29, by a clamp pulse fed
to the clamp circuit 23 through a terminal 24. An output
signal Sl from the clamp circuit 23 is fed to a subtracter
25 and to a blanking circuit 2. This blanking circuit
2 is similar to that shown in Fig. 2 and produces an output
signal S2 shown in Fig. 7s which is fed to a peak hold cir-
cuit 4. An output signal S3 from the peak hold circuit
4 and the reference voltage V0 both shown in Fig. 7C are
fed to a comparator 26.
The comparator 26 compares both the signals
2~ and detects the level difference therebetween. The
output~from the comparator 26 is fed through a gate cir-
cuit 27 to the subtracter 25. The gate circuit 27 is
also supplied with a control pulse similar to the blanking
pulse Pl through a terminal 28 to be controlled, so that
the subtracter 25 is supplied with a signal S6 which is
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in a predetermined level in the period oE the control pulse
and in the level in response to the output from the com-
parator 26 in the video period other than the control pulse
period as shown in Fig. 7D. The output signal of the
gate circuit 27 is subtracted from the video signal Sl in
the subtracter 25 so that the subtracter 25 delivers to
the output terminal 8 an output signal S5 whose level
nearest to the black s,ide in the video period coincides
with the black level as shown in Fig. 7E.
The example of the invention shown in Fig.6
can avoid the fluctuation of the black level owing to the
fluctuation of the set up level similar to the first
example of the present invention.
In the above examples of the invention shown
in Figs. 2 and 6, an undesired phenomenon will appear in
accordance with the kind o~ the input video signals.
For example, in case of a video signal which is generally
bright and small in the contrast ratio, when this video
signal is processed by this invention, a part of the video
signal to be inherently bright becomes dark excessively
and hence a reproduced picture becomes unnatural.
A further example of the inventlon, which
can avoid the above undesired phenomenon, will be now
described with reference to Fig. 8. In brief, the
example of Fig. 8 is formed by such a manner that the
,clamp circuit 23 and the clamp voltage source 29 of the
example shown in Fig. 6 are added to the example shown
in Fig. 2, and further a threshold voltage source 30
and a diode 31 are added -thereto.
In the example of the invention shown in
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Fig. ~, the input video signal S0 from the video detecting
circuit i 9 fed through the input terminal 1 to the clamp
circuit 23 to which the clamp pulse is applied through the
terminal 24. Thus, the clamp circuit 23 produces a
video signal Sl whose pedestal level is clamped to a pre-
det~ermined level V0 as shown in Fig. 9~. This video signal
Sl is applied to the blanking circuit 2 and blanked over
a period somewhat wider than the horizontal blanking period
by a blanking pulse Pl shown in Fig. 9B and applied to be
blanking circuit 2 through a terminal 3. Then, the
blanking circuit 2 produces an output signal S2 whose part
corresponding to the blanked period is made at, for example,
the power source voltage as shown in Fig. 9C. This
output signal S2 is supplied to a peak hold circuit 4
which in turn produce a peak-held output signal S3 which
corresponds to the peak level nearest to the black side
in the video period as shown in Fig. 9D.
The output side of the peak hold circuit
~ is connected to the voltage point of (V0 -~ Vt) through
the diode 31 in the forward direction thereof, so that if
the forward voltage drop of the diode 31 is neglected,
when the held output S3 is lower than the voltage tVo + Vt)
in level, the diode 31 is cut off and the held output S3
corresponds to the video signal. While, when the held
output S3 is higher than (Vo -~ Vt) in level, the diode 31
is made conductive and the held output S3 becomes the level
of (V0 + Vt).
The video signal Sl from the clamp circuit
23 and the held output S3 from the peak hold circuit ~
are fed to a comparator ~. This comparator 5 produces
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one of both the signals S1 and S3 which is in level much
near to the white side than the other and hence delivers
an output signal S4 shown in Fig. 9E. The output signal
S4 from the comparator 5 is fed to a clamp circuit 6 to
which a clamp pulse P2 shown in Fig. 9F is fed through
a terminal 7. Thus, at an output terminal 8 led out
from the clamp circuit 6 delivered is a video signal S5
whose blanking periods are arranged equal to a predetermined
level as shown in Fig. 9G.
Fig. 10 shows an example of a practical
connection according to the present invention which will
realize the circuit construction shown in Fig. 8. In
Fig. 10, the parts and elements corresponding to those
of Figs. 4 and 6 are marked with the same references
and their description will be omitted.
Now, the difference between the examples
of Fig. 10 and those of Figs. 4 and 6 will be described.
In the example of the invention shown in Fig. 10, the
connection point between the resistor 18 and capacitor
19 is connected through a diode 31 to the emitter of a
transistor 2~t. This emitter of the transistor 24t
is grounded through a constant current resistor 25r whose
resistance value is selected large, and the collector
of the transistor 24t is connected to the power source
terminal of +Vcc The base of the transistor 24t is
connected through a resistor 26r to the clamp voltage source
29 and also through a resistor 27r to the power source
terminal of +Vcc. In this case, by selecting the
resistance values of the resistors 26r and 27r, the
-~oltage at the connection point therebetween i.e. the base
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voltage o~ the transis~or 24t can be made as (V0 -~ Vt),
and the base-emitter voltage drop o-f the transistor
24t has such a polarity to cancel the forward voltage
drop of the diode 31. Thus, the held output S3 is
prevented from exceeding over the voltage (Vo + Vt).
According to the example of the invention
shown in Fig. 8, when a video signal Sl, which corresponds
to a bright picture and whose level is higher than the
predetermined voltage (V0 -~ Vt) as shown in Fig. llA, is
fed to the circuit, the held output S3 becomes (V0 + Vt)
per se, and accordingly the output S4 from the comparator
5 becomes as shown in Fig. l]B by the solid line.
While, as in the examples of Figs. 2 and 6, if the level
nearest to the black side in the video period is merely
detected, held and made coincident with the cut off level
of the cathode ray tube, there may be such a fear that
the video signal corresponding to a bright picture and
small in contrast ratio is converted to a video signal
of a dark picture as shown in Fig. llB by the broken line.
According to the example of the invention
shown in Fig. 8, on the contrary, such the ear does
not occur because when the video signal has a level
higher than the predetermined level (V0 ~ Vt), no peak
hold operation is carried out.
The present invention can be applied not
only to a television receiver as in the above examples,
but also to such a case where the output from a tele-
vision camera is processed or such a process is carried
out that a level of the output signal from the television
camera, which is nearest to the black side and nearer
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than the predetermined level to the black level, is
detected, held and then made co:Lncident with the set up
level.
It will be apparen-t that many modifications
and variations could be effected by one skilled in the
art without departing from the spirits or scope oE the
novel concepts of the present invention so that the
spirits or scope of the invention should be determined
by the appended claims only.
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