Television – Image signal processing circuitry specific to television – Color balance or temperature
Reexamination Certificate
1998-04-30
2001-08-28
Hsia, Sherrie (Department: 2614)
Television
Image signal processing circuitry specific to television
Color balance or temperature
C348S687000, C348S689000, C348S706000, C348S707000
Reexamination Certificate
active
06281943
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to video display devices, and more particularly, to amplifiers and cut-off control circuits for adjusting the white balance of display devices.
2. Description of the Related Art
In a display device such as a monitor, a white balance adjustment makes a white object appear white regardless of the color temperature. In particular, in a cathode ray tube (CRT), a white balance adjustment adjusts gains and biases of signals applied to red, green, and blue (RGB) guns or cathodes in the CRT. The bias adjustment is often referred to as a cut-off adjustment. Cut-off control circuits can employ a DC-coupling or an AC-coupling when driving cathodes of a CRT. The coupling mode of the cut-off control circuit does not affect the gain adjustment since the gain controls an AC signal component that both AC and DC couplings transfer. However, the coupling mode does affect the bias adjustment, and cut-off control circuits typically require different integrated circuit chips according to the type coupling employed. A further concern is that brightness control depends on a combined RGB signal, but the cut-off control, which controls cathode biases, is carried out for each of the R, G, and B signals separately. In order to adjust the white balance, the brightness level is adjusted first, and then the cut-off control is performed.
FIG. 1A
is a circuit diagram of a conventional cut-off control circuit having a DC-coupling to the video portion of a monitor (e.g., a cathode in a CRT). The cut-off control circuit of
FIG. 1A
includes a video pre-amplifier
101
and a drive amplifier
102
for driving a CRT. For simplicity,
FIG. 1A
shows only one channel even though a color video system normally has three channels (R, G, and B). The cut-off control adjusts the respective CRT cathode bias voltages VA for the R, G, and B cathodes. In
FIG. 1A
, a video feedback voltage Vf provides a negative feedback to pre-amplifier
101
. Pre-amplifier
101
controls an output voltage Voutput so that video feedback voltage Vf is the same as or depends on a brightness control voltage Vbright. A current controller
103
controls video feedback voltage Vf and thus controls voltage Voutput and CRT cathode bias voltage VA. Equation 1 expresses CRT cathode bias voltage VA as a function of feedback voltage Vf, resistances R
1
and R
2
of respective resistors
104
and
105
, and a current Ic.
V
A
=
V
f
*
R1
+
R2
R2
+
Ic
*
R1
Equation
1
:
As can be seen in Equation 1, the CRT cathode bias voltage VA depends on the control current Ic, which a cut-off control signal Vcutoff can adjust.
FIG. 1B
is a circuit diagram of the conventional video pre-amplifier
101
shown in
FIG. 1A
, in which an external feedback brightness control method is used. Referring to
FIG. 1B
, the video pre-amplifier
101
includes an adder
110
a
, a drive amplifier
101
b
, a comparator
101
d
, and a switch
101
c.
FIG. 2A
is a circuit diagram of a conventional cut-off control circuit using an AC-coupling mode. The cut-off control circuit of
FIG. 2A
includes a video pre-amplifier
201
, a drive amplifier
202
, a coupling capacitor
203
, and a comparator
204
. This circuit uses a separate DC bias circuit that controls cathode bias voltage VA. Only an AC component of the output signal from drive amplifier
202
passes through a coupling capacitor
203
to voltage VA. Equation 2 expresses cathode bias voltage VA as a function of a reference voltage Vref, resistances R
3
, R
4
, and R
5
of respective resistors
213
,
214
, and
215
, and cutoff voltage Vcutoff.
V
A
=
Vref
*
R3
+
R4
+
R5
R3
*
R5
-
Vcutoff
*
R4
R5
Equation
2
:
As shown in the equation 2, cathode bias voltage VA depends on cut-off control voltage Vcutoff which is applied to a comparator
204
to adjusts a control current Ic.
FIG. 2B
is a circuit diagram of video preamplifier
201
shown in
FIG. 2A
, in which a built-in feedback brightness controlling method is used. The video pre-amplifier
201
of
FIG. 2B
includes an adder
201
a
, a drive amplifier
201
b
, a comparator
201
d
, and a switch
201
c.
As mentioned above, the video pre-amplifiers
101
and
201
of the conventional cut-off control circuits respectively using DC-coupling and AC-coupling differ from each other. Thus, conventional cut-off control circuits using different coupling modes require different integrated circuits and external parts. Further, the cut-off control circuits require a number of external parts, such as an operational amplifier and resistors, in addition to the video pre-amplifier. This makes the circuits more complex.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a video pre-amplifier that can be implemented on a single IC chip and can be used in cut-off control circuits using both AC-coupling mode and DC-coupling mode. Another object of the invention is to provide a cut-off control circuit that requires fewer external components other than a video pre-amplifier IC chip.
To achieve the above objects, a preamplifier according to an embodiment of the invention includes a switching unit for receiving control data, generating a bus control signal according to the control data, and outputting the bus control signal internally or externally. The switching unit provides a bus control signal internally, when the preamplifier operates in a cut-off control circuit that uses DC-coupling. The switching unit provides a bus control signal to an external bias circuit when the preamplifier operates in a cut-off control circuit that uses AC-coupling.
REFERENCES:
patent: 5304946 (1994-04-01), Sano
patent: 5452020 (1995-09-01), White
patent: 5726540 (1998-03-01), Klink
Heid David W.
Hsia Sherrie
Samsung Electronics Co,. Ltd.
Skjerven Morrill & MacPherson LLP
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