Brightness control apparatus

A brightness control circuit for a color television receiver including means for amplifying color representative signals (e.g., color difference signals) and means for amplifying luminance signals comprises a source of periodic keying signals occurring during blanking intervals of the luminance signals, and first and second keyed clamping networks. The first clamping network clamps the color representative signals to a first reference voltage in response to the keying signals, and the second clamping network clamps the blanking intervals of the luminance signals to a second reference voltage, representing a black tone of a reproduced image, in response to the keying signals. The first and second reference voltages are in predetermined relation and dependent upon the keying signals. An adjustable brightness control is coupled to the second clamping network for varying the second reference voltage level and therefore the black tone reference level and the brightness of a reproduced image.

This invention relates to brightness control apparatus for video signal 
processing systems and, more particularly, to such apparatus operatively 
associated with keyed clamping video signal processing circuits of a 
television receiver. 
Because of the nature of a composite television signal in which a reference 
black level occurs periodically, so-called keyed clamps are often employed 
in television receivers to conduct during intervals associated with the 
reference level and thereby change a coupling capacitor so as to restore 
or provide a reference DC component to a signal coupled by the capacitor. 
Such keyed clamping circuits are shown, for example, in my U.S. Pat. No. 
3,763,315 and in U.S. Pat. No. 3,927,255 granted to B. J. Yorkanis. A 
keyed clamping arrangement can also be employed in a kinescope driver 
stage for stabilizing the operating point and for establishing the 
blanking cut-off level of the driver stage, as described in U.S. Pat. No. 
3,970,895 granted to D. H. Willis and U.S. Pat. No. 3,959,811 granted to 
R. L. Shanley, II. 
The present invention relates to a brightness control arrangement suitable 
for use with video signal processing systems of the type described in the 
aforementioned U.S. patent of Willis. 
In the design of a brightness control circuit for a television receiver, it 
is desirable to provide an accurate and reproducible range of control. 
Where a number of circuit elements and voltage sources are direct current 
coupled to an image reproducing device, tolerances of the values of the 
circuit elements and supplies must be taken into account in determining 
the operating range of the brightness control. It is therefore customary 
for brightness controls to be coupled across a relatively large voltage 
supply but, in operation in particular receiver, only a small range of the 
control is used. The sensitivity of such controls is typically undesirably 
limited because of the small actual operating range and, at the same time, 
they are undesirably costly because of high breakdown voltage (insulation) 
requirements. 
The brightness control arrangement to be described herein desirably 
provides accurate and predictable operation such that the range of 
brightness control which can be provided by the viewer operated control is 
more readily determined for various operating conditions. In essence, the 
arrangement to be described exhibits relatively few circuit tolerances 
that require compensation, so that a reproducible range of brightness 
control is achieved. 
In accordance with the present invention, brightness control apparatus is 
provided for a color television system including means for amplifying 
color representative signals, means for amplifying luminance signals 
having periodic blanking intervals and image intervals containing 
brightness information disposed between adjacent blanking intervals, and a 
color image reproducing device. A keying circuit provides periodic keying 
signals during the blanking intervals. The color representative signals 
are coupled to the color signal amplifying means via a first network, and 
the luminance signals are coupled to the luminance signal amplifying means 
via a second network. A first clamping circuit is coupled to the keying 
circuit and to the first coupling network, and is responsive to the keying 
signals for clamping the color representative signals to a first reference 
voltage. A second clamping circuit is coupled to the keying circuit and to 
the second coupling network for clamping the blanking interval portions of 
the luminance signals to a second reference voltage representing a black 
tone of a reproduced image. The first and second reference voltages are in 
predetermined relation and dependent upon the keying signals. An image 
brightness control device is coupled to the second clamping circuit for 
varying the second reference voltage level and therefore the black tone 
reference level and brightness of a reproduced image.

In FIG. 1, a video processing unit 12 is shown for receiving radio 
frequency (RF) signals from an antenna 10 and for translating these 
signals through intermediate frequency (IF) amplifying and detecting 
stages (not shown) to provide a composite video signal. The composite 
video signal comprises chrominance, luminance and synchronizing 
components. 
A frequency selection unit 15 selectively couples the chrominance component 
to a chrominance channel 14, including a chrominance processing unit 16 
for processing the chrominance component to derive R-Y, B-Y and G-Y color 
difference signals. The color difference signals are coupled to respective 
inputs of kinescope driver stages 18a, 18b and 18c of a kinescope driver 
unit 20. Kinescope driver stages 18a, 18b and 18c are similar and each 
include an amplifier transistor 24a, 24b and 24c, and a keyed bias 
transistor 26a, 26b and 26c, respectively, as described in the 
aforementioned U.S. Pat. No. 3,970,895. The kinescope driver stages 
combine a luminance output signal, Y, of a luminance channel 100 with the 
R-Y, B-Y and G-Y color difference signals to form R, B and G color 
signals. The R, B and G color signals are applied to cathode electrodes of 
a kinescope 38. 
Video processing unit 12 is also coupled to a channel 74 for processing the 
synchronizing (sync) component of the video signal. A sync 60 derives 
periodic positive line sync pulses from the video signal. The derived sync 
pulses (FIG. 7) are in phase with the correspond to line sync of the video 
signal (FIG. 3) and are coupled to a horizontal deflection unit 62. 
Appropriate vertical sync pulses are also derived and are coupled to a 
vertical deflection unit 76. Periodic horizontal and vertical deflection 
signals are coupled from outputs of units 62 and 76 to appropriate 
deflection windings associated with kinescope 38. Horizontal deflection 
unit 62 also supplies negative-going periodic horizontal flyback pulses 
(FIG. 5) during the horizontal sync or retrace interval to a high voltage 
unit 78, and also provides high operating voltages for ultor and focus 
electrodes of kinescope 38. 
Horizontal deflection unit 62 further supplies horizontal flyback pulses to 
an input of a keying unit 130. Keying unit 130 generates periodic keying 
pulses (FIG. 6) during the horizontal retrace interval in response to and 
substantially coincident with the horizontal flyback pulses. The keying 
pulses control the operation of bias transistors 26a, 26b and 26c of 
kinescope driver stages 18a, 18b and 18c during the horizontal retrace 
interval as described in U.S. Pat. No. 3,970,895. 
A luminance processing unit 44 of luminance channel 100 amplifies and 
otherwise processes the luminance component to provide a "sync tips 
positive" luminance output signal (FIG. 3). The luminance component from 
unit 44 comprises periodic blanking pulses 306 and signal portions 308 
representing image information disposed between the blanking pulses. The 
blanking pulses are formed by a pedestal level 310 upon which are imposed 
sync pulses 312. Although the pedestal level 310 is generally considered 
to correspond to a blanking level of the kinescope, it is common to refer 
to this level as a black level, relating to a black tone of an image 
reproduced by the kinescope. 
The luminance component shown by FIG. 3 is coupled from luminance 
processing unit 44 via a coupling capacitor 104 to a keyed black level 
clamping unit 110. The clamped luminance signal is coupled via a resistor 
103 to a base electrode of a PNP luminance driver transistor 105. Periodic 
horizontal sync pulses (FIG. 7) from sync separator 60 and periodic keying 
pulses (FIG. 6) from gating unit 130 are combined to form a switching 
signal (FIG. 8) which controls the clamping (conduction) intervals of 
clamping unit 110. A clamped luminance component appearing at the junction 
of an output of clamping unit 110 and capacitor 104 is shown in FIG. 4. 
Horizontal deflection unit 62 and vertical deflection unit 76 also supply 
periodic horizontal and vertical blanking pulses to a blanking unit 160 
where they are amplitude limited and combined with the clamped luminance 
component to insure that kinescope 38 is substantially cut-off during the 
horizontal and vertical retrace intervals. The combined signal appears at 
the base electrode of luminance driver transistor 105. 
Additional control of clamping unit 110 is accomplished by an automatic 
brightness limiter unit 115 and by a brightness control unit 112. 
Brightness unit 112 includes a manually adjustable, viewer operated 
control to vary the conduction of clamp 110 and to thereby obtain a 
desired level of brightness of an image reproduced by kinescope 38, as 
will be discussed in connection with FIG. 2. Brightness limiter 115 
generates a voltage for controlling the conduction of clamping unit 110 to 
reduce the beam current of kinescope 38 when the current, as manifested by 
the current demand of high voltage unit 78, exceeds a predetermined 
maximum level. The operation of brightness limiter 115 is described in 
greater detail in my copending U.S. patent application Ser. No. 715,861, 
entitled, "Automatic Beam Current Limiter" and filed concurrently with the 
present application. 
Referring now to FIG. 2 together with FIG. 1, it is noted that reference 
terminals A-H of FIG. 2 correspond to reference terminals A-H of FIG. 1. 
The luminance signal from luminance processor 44 is coupled to an emitter 
follower buffer transistor 201 via a terminal A. A viewer operated control 
208 is operative to vary the amplitude of the luminance signal processed 
by transistor 201. An emitter output of transistor 201 is coupled to a 
bias resistor 202 and to a coupling capacitor 204 which is operatively 
associated with a PNP keyed level clamping transistor 210. 
The black level of the luminance signal coupled via capacitor 204 is 
clamped to a reference level, representing a black tone of an image, when 
transistor 210 is rendered conductive in response to periodic keying 
pulses applied to a base electrode of transistor 210. The conduction 
intervals of transistor 210 are controlled in a first instance by first 
keying pulses (FIG. 6) from a keying circuit 230. As is described in 
detail in connection with a similar keying circuit shown in allowed U.S. 
patent application Ser. No. 580,688, now U.S. Pat. No. 3,984,864 issued on 
Oct. 5, 1976, of D. H. Willis, keying circuit 230 also serves to couple a 
signal which may be called an "extra blanking signal" via a resistor 244, 
a diode 245 and a terminal F to cut-off kinescope 38 during horizontal 
trace portions of each vertical retrace interval. 
Keying circuit 230 comprises a PNP transistor 235 having an emitter output 
coupled to the emitters of keyed bias transistors 26a, 26b and 26c of 
kinescope driver 20 via a terminal G. The horizontal flyback voltage 
waveform generated by horizontal deflection unit 62 is coupled through a 
terminal E, a resistor 232 and a resistor 234 to a base of transistor 235. 
Negative amplitude excursions of the horizontal flyback pulse are limited 
by a diode 240 to prevent the development of excessive negative voltages 
at the junction of resistors 234 and 236. The amplitude limited flyback 
pulse is translated to a more positive DC level by a network including 
resistors 234, 236 and a resistor 242 coupled to a source of positive 
supply voltage (+22 v). 
The first keying pulses appear at the emitter output of transistor 235 and 
are coupled to kinescope driver stages 18a, 18b and 18c via terminal G. 
The first keying pulses are also coupled to the base of clamping 
transistor 210 via a DC voltage translating network including a source of 
positive voltage (22 volts) and resistors 248, 249 and 256. The keying 
voltage level of the first keying pulses corresponds to the minimum level 
of the keying pulse waveform, V.sub.K in FIG. 6. 
Second keying pulses (FIG. 7) for controlling the clamping intervals of 
transistor 210 are provided from sync separator 60 through a terminal B, a 
signal isolation diode 220 and an amplitude determining resistor 224. The 
first and second keying pulses are summed at the base of transistor 210 to 
form a combined keying signal (FIG. 8) having a keying voltage level 
corresponding to the minimum amplitude level of the combined keying signal 
waveform. It is noted that the second, positive sync, keying pulses serve 
to prevent transistor 210 from clamping to the level of the sync tip of 
the luminance signal, which may vary in amplitude and therefore adversely 
affect the clamping reference level provided by transistor 210. 
A clamped luminance signal (FIG. 4) appearing at a junction of capacitor 
204 and the emitter output of transistor 210 is coupled through resistor 
203 to a base input of a PNP luminance driver transistor 205, which 
provides an amplified clamped luminance signal through terminal F to 
amplifier transistors 24a, 24b and 24c of kinescope driver 20. Horizontal 
and vertical blanking pulses from deflection units 62 and 76 have time 
durations respectively corresponding to the horizontal and vertical 
retrace intervals. The horizontal blanking pulses are in time synchronism 
with the negative portion of the horizontal flyback pulse waveform. 
Horizontal blanking pulses coupled through terminal C, a resistor 264 and 
a signal isolation diode 262, and vertical blanking pulses coupled through 
a terminal D, a resistor 268 and a signal isolation diode 266, are 
amplitude limited by a clamping diode 270 and coupled to the base of 
transistor 205 by a resistor 271. 
A brightness control network comprising a variable resistor 212, a filter 
capacitor 213 and a resistor 214 serves to adjust the bias and therefore 
the level of conduction of keyed clamp transistor 210. Adjustment of 
resistor 212 varies the black level of the luminance signal and the 
brightness of a reproduced image. 
The operation of the circuit of FIG. 2 will now be considered together with 
kinescope driver stage 18c of of FIG. 1 as a representative one of the 
kinescope driver stages. The combination of complementary transistors 205 
and 24c serves to amplify and matrix the Y and R-Y signals to derive the R 
signal at a collector output of amplifier transistor 24c. As described in 
greater detail in U.S. Pat. No. 3,970,895 and the aforementioned U.S. 
patent application of Willis, amplifier transistor 24c and keyed bias 
transistor 26c are arranged in feedback relation, and the voltage 
developed at the emitter of transistor 24c is maintained substantially 
independent of the DC conditions of chrominance unit 16 and the 
base-emitter voltage variations of transistor 24c by a clamping network 
comprising a coupling capacitor 34c and keyed bias transistor 26c. The 
clamping network 34c, 26c also serves to establish the cut-off or blanking 
conduction level of amplifier transistor 24c and therefore that of 
kinescope driver stage 18c. Clamping action occurs when keyed transistor 
26c conducts in response to the keying voltage V.sub.K of the first keying 
pulse during the horizontal flyback blanking interval, when the keying 
voltage level V.sub.K appears at the emitter of transistor 26c. A 
reference voltage related to the keying voltage V.sub.K then appears at 
the junction of coupling capacitor 34c, a collector of transitor 26c, and 
a base input of amplifier transistor 24c. The reference voltage serves to 
establish a desired direct voltage component of the color difference 
signal amplified by transistor 24c. 
It is noted that the first keying pulse is employed both for keying 
clamping transistor 210 to establish the black or blanking level of the 
luminance signal, and for keying bias transistor 26c for establishing the 
blanking level of kinescope driver stage 18c. As discussed below, a 
predictable relationship exists between the voltage used for keying both 
clamping transistor 210 and bias transistor 26c during the blanking 
interval. 
During the blanking interval, the keying voltage level V.sub.K of the first 
keying pulses appears at the emitter of transistor 235 and at the emitter 
of keyed bias transistor 26c of kinescope driver stage 18c via terminal G. 
A voltage then appearing at the emitter of amplifier transistor 24c of 
stage 18c is substantially equal to the keying voltage V.sub.K plus the 
base-emitter voltage (0.6 volts) of transistor 26c. 
Also during the blanking interval, it is desired to render luminance driver 
transistor 205 non-conductive so that substantially no current flows in 
the base-emitter circuit of transistor 205 including resistor 203 and a 
variable bias control resistor 36c of stage 18c. The voltage (V.sub.K + 
0.6) appearing at the emitter of transistor 24c therefore corresponds to 
the voltage appearing at the emitter of luminance driver transistor 205. 
In order to render transistor 205 non-conductive at this time, the base 
voltage of transistor 205 should substantially equal or exceed the keying 
voltage level V.sub.K. 
Neglecting for the moment the blanking pulses coupled to the base of 
transistor 205 via resistor 271, the voltage then appearing at the emitter 
of clamp transistor 210 corresponds to the base voltage of transistor 205, 
that is greater than or equal to V.sub.K. Consequently, in order to render 
clamp transistor 210 conductive during blanking interval periods T.sub.1 
and T.sub.2, a keying voltage applied to the base of transistor 210 should 
be of a magnitude correspondingly equal to or greater than the emitter 
voltage of transistor 210, V.sub.K, less the base emitter voltage drop 
(0.6 volts) of transistor 210, or (V.sub.K - 0.6). 
As described above, the keying voltage (V.sub.K - 0.6) to be applied to the 
base of clamp transistor 210 is directly related to keying voltage V.sub.K 
applied to the individual kinescope driver stages. An accurate, 
predictable range of brightness control can be achieved if keyed clamp 
transistor 210 and the keyed bias transistors (e.g., 26c) of the kinescope 
driver stages are referenced to the same potential, such as ground, or to 
separate stable reference potentials. 
The relationship described above pertains to a keying voltage level for a 
nominal black level condition. The blanking pulses coupled via resistor 
241 insure that luminance driver transistor 205 is cut-off during blanking 
intervals for all settings of brightness control 212, and do not upset the 
premise upon which such relationship is based. The blanking pulses also 
serve to maintain a desired voltage across capacitor 204. In this regard 
it is noted that signals coupled to clamping network 204, 210 from buffer 
transistor 201 can cause a voltage to be developed across capacitor 204 
sufficient to cause transistor 210 to be cut-off during the blanking 
interval, by reverse biasing the emitter-base junction of transistor 210. 
The blanking pulses serve to prevent this condition by maintaining a 
differential voltage across capacitor 204 such that the emitter-base 
junction of clamp transistor 210 remains forward biased during the 
blanking interval. The values of resistors 203 and 271 are selected to 
provide a level of blanking current sufficient to recharge capacitor 204 
rapidly in the presence of rapid changes of the amplitude of signals 
coupled to capacitor 204 via transistor 201. 
The keying voltage level (V.sub.K - 0.6) for keying clamp transistor 210 is 
derived from the keying voltage level V.sub.K appearing at the emitter of 
transistor 235 by means of a voltage translating network including 
resistors 248, 249 and 256, and the associated +22 volt source. The 
combination of the +22 volt source and resistors 248, 249 serve as a 
voltage divider for translating the keying voltage level V.sub.K of the 
keying pulses appearing at the emitter of transistor 235 to a more 
positive level. The translated keying voltage level appears at the 
junction of resistors 248 and 249 and is reduced in magnitude by the 
voltage drop across resistor 256 to produce a keying voltage level greater 
than (V.sub.K - 0.6) at the base transistor 210. A keying voltage level 
greater than (V.sub.K - 0.6) is provided for reasons which will be 
explained subsequently. 
Brightness adjustment is provided by adjusting the position of the wiper 
arm of variable resistor 212. Such adjustment alters the base bias of 
clamp transistor 210, thereby causing the conduction, and hence the 
clamping voltage, of transistor 210 to change. A corresponding change in 
the black level of the luminance component results. 
The conduction of transistor 210 increases as the wiper arm of variable 
resistor 212 is adjusted from the extreme upper to the extreme lower 
position. When variable resistor 212 is set to the extreme lower position 
of zero ohms, transistor 210 exhibits maximum conduction such that the 
black reference level of the luminance component is clamped to a level 
(411 in FIG. 4) which corresponds to a condition of maximum desired image 
brightness. Conversely, when the wiper arm of variable resistor 212 is set 
at an extreme upper position, the conduction of transistor 210 decreases 
such that the black level is clamped to a level (414 in FIG. 4), which 
corresponds to a condition of reduced image brightness. In FIG. 4, level 
410 corresponds to a condition of average brightness between a range of 
brightness indicated by levels 411 and 414. 
The range of brightness control obtainable is related to the resistance 
values of variable resistor 212 and associated resistor 214, and to the 
values of resistors 248, 249 and 256. In this connection it is noted that 
in some cases it may be desirable to tailor the range of brightness 
control to provide a greater range of control in a "blacker-than-black" 
direction. This is accomplished in the present circuit by increasing the 
base voltage of clamp transistor 210 above the keying voltage level 
(V.sub.K - 0.6) which produces a nominal black level, by an amount V.sub.A 
in a positive direction. The additional positive voltage, V.sub.A, is 
provided by appropriately selecting the values of resistors 248, 249 and 
256 of the voltage translating network. Thus, when the resistors of the 
voltage translating network are selected to increase the keying voltage 
level applied to the base of transistor 210 by an amount V.sub.A above the 
keying voltage level (V.sub.K - 0.6) required for a nominal black level, a 
greater brightness control range results in the direction of 
"blacker-than-black" tones. 
It is noted that undesired variations in the level of the keying signals do 
not adversely affect the operation of the color signal amplifier 
transistors of the kinescope driver stages. Considering stage 18c, for 
example, such transistor 26c, and the level of the clamped luminance 
signal from transistor 205, to change in the same direction. The latter 
two voltage levels tend to alter the conduction of amplifier transistor 
24c in opposing directions, since they are applied to base and emitter 
electrodes of transistor 24c, respectively. Variations in the level of the 
keying signals are therefore nullified. 
Although the invention has been described in terms of a specific circuit 
embodiment, it should be appreciated that other circuit arrangements may 
be devised by those skilled in the art without departing from the scope of 
the invention.