Note: Descriptions are shown in the official language in which they were submitted.
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_ackground of the Invention
This invention relates to dynamic hue control networks.
More particularly, this invention relates to dynamic hue control
networks that are adapted to automatically vary hue dependent
upon the level of the luminance component of the composite
television signal.
By government regulation in North America a composite
colour television signal comprises a luminance component, a
chrominance component and a synchronising reference burst com-
ponent. The luminance component is amplitude modulated on a
~ carrier and contains information relating to the brightness of
the image that is being telecast. The chrominance information is
carried on a 3.58 MHz sub-carrier. This sub-carrier is amplitude
modulated in respect of the degree of colour saturation and phase
modulated in accordance with the tint of the colour. The colour
sync bursts are transmitted only during the blanking intervals
s for the horizontal scan lines and provide a means whereby the
reference signal can be reconstructed at the receiver.
The function of the sync bursts is to provlde reference
phase information. The locally generated reference signal must
be in phase with the transmitted sub-carrier if the true hue of
the telecast image is to be reproduced. Phase delay and distor- -
~; tion of colour signals during their transmission occur frequently
and, because of this, it is common practice to provide a customer
operated hue control that enables the viewer to ad~ust the phase -~
of the chrominance signal relative to the locally generated
reference signal, or vice versa, to obtain true colour reproduc-
tion.
The problem of hue errors is particularly acute at low
luminance levels. When the luminance level is low, even if the
¦ hue control is adjusted so that the high luminance flesh colours
are acceptably correct, it often will be found that the dark areas
~ are incorrect, being surrounded by green tinges.
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As the angle between the chroma demodulators departs
from quadrature, and specifically for angles greater than
quadrature, where the greens are siynificant]y reduced, the
larger demodulation angles produce considerably more acceptable
colour reproduction. This effect has been used in various
automatic tint control circuits. For example, reference may be
made to U.S. Patent 3,562,410, issued February 9, 1971. The
disadvantage of these automatic tint control circuits is that
colour reproduction is limited to the orange-cyan axis, which,
although producing acceptable flesh tones, severely limits the
reproduction of other colours, particularly greens which become
cyan, and purples which become red.
It has been shown by N.W. Parker in his paper entitled
"An Analysis of the Necessary Decoder Corrections for Colour
Receiver Operation with Non-Standard Receiver Primaries"(IEEE
Transactions-Broadcast and Television Receiyers - volume 12,
April 1966) that for the phosphors used in present day picture
tubes, a demodulation angle of 105 is about optimum. In practice,-
transmission standards and cable T.V. practices do not permit the
2~ use of angles much less than 120 if objectionable greenish areas
are to be avoided.
Summary of the Invention
In accordance with this invention there are provided
automatic hue control networks wherein the demodulation angles
are automatically increased at low luminance levels to reduce
the green tinge problem noted beforehand, while retaining a
greater range of correct colour reproduction for high luminance
levels.
In accordance with this invention there is provided,
in a colour television receiver comprising means responsive to
a composite colour television signal, which signal includes a
luminance component and a colour subcarrier component modulated -
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with chrominance information, for reconstituting a colour
image, which means include a demodulator coupled to a source
of chrominance signal and to a source of colour reference
signal for developing a colour control signal, means for
automatically increasing the phase angle between said reference
signal and said chrominance signal when said luminance signal
moves toward a signal representing black and for automatically
decreasing said phase angle when said luminance signal
moves toward a signal representing white, the last mentioned
means including a variable phase shift network for varying
said phase angle, means for deriving a signal that varies
with the level of said luminance signal, and means for apply-
ing the derived signal to said variable phase shift network
to automatically vary the phase shift imparted by said network
such that said phase angle increases and decreases as said
luminance signal moves towards signals representing black and
; white respectively.
Brief Description of the Drawings
, . -
Figure 1 is a block diagram of a colour television
.; 20 receiver showing one embodiment of the invention;
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.~j Figure 2 is a block diagram of a part of a colour
J television receiver illustrating a second embodiment of this
invention; and
Figure 3 is a circuit diagram showing the first
embodiment of the invention in greater detail.
Detailed Description of the Invention
Including the Preferred Embodiments
With the exception of circuitry which has been added
in accordance with this invention, the colour television
receiver shown in Figure 1 is essentially conventional in
nature, so only a brief description of how it is constituted
and its mode of operation is necessary.
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The transmitted signal is picked up by an antenna l~
and is processed in a tuner 1] that includes the usual R.F.
amplifying and heterodyning stages for developing an I.F. signal
; that is amplified by an I.F. amplifier 12. The amplified I.F.
signal is applied to a luminance and chrominance detector 13
wherein luminance information and chrominance information, the
latter being in the form of a composite signal, is derived. The
' luminance component is amplified by a luminance amplifier 14 and
is applied to the cathodes 15 of a three gun colour television
picture tube 16.
The I.F. signal is also applied to a sync and sound
detector 17 that derives both synchronising and sound components.
The sound components are applied to conventional audio circuits
. 18. In these circuits the sound components are demodulated and
amplified before being applied to a speaker l9. ~ :
~ The synchronising information in the form of horizontal
;¦ and vertical sync pulses is applied to horizontal and vertical
,~ deflection systems 20 and 21 which generate sawtooth scanning . .
signals to drive a deflection yoke 22 mounted on the neck of the
cathode ray tube 16.
The chrominance signal is processed in a chrominance
channel that includes a chrominance amplifier 23, a burst ampli-
fier 24, a reference oscillator 25, a phase shifting network 26
and chrominance demodulators 27. The chrominance channel develops
three colour difference signals generally referred to as B-Y,
R-Y and G-Y that are applied to the control grid electrodes 28
: of cathode ray tube 16.
The chrominance signal is amplified by chrominance
amplifier 23, the amplified signal being applied to two of the
input terminals of synchronous demodulator 27. ~.
~: Synchronising bursts of sub-carrier frequency borne
by the chrominance signal are derived by chrominance amplifier
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23 and are amplified by burst amplifier 24. Amplifier 24 is
gated by a signal from the deflection circuitry and applies its
output to reference oscillator 25 that includes suitable phase
comparison circuitry for generating a 3.58 MHz continuous wave
demodulation signal that is synchronised in phase and frequency
to the sync burst. The output of reference oscillator 25 is
applied to a phase shifter 26 which derives two phase dis-
placed reference signals that are applied to synchronous demodu-
lator 27.
In accordance with one embodiment of this invention,
a variable reactance network 29 is associated with phase shift
network 26, as will be more clear from a consideration of
Figure 3, to which reference is now made.
As seen in Figure 3, phase shift network 26 is con-
stituted by two channels 30 and 31. Channel 30 may be considered
to carry the R-Y reference signal, while channel 31 may be con-
sidered to carry the B-Y reference signal. Channel 31 includes
an inductance coil Ll, a resistor Rl and a capacitor Cl connected
as shown. These components constitute a phase shifting network
that typically may produce a 105 phase lag of the B-Y reference
signal.
The luminance component of the video signal is coupled
via a capacitor C2 to the base electrode of a transistor TRl The
base electrode of translstor TRl is clamped so that the black
level of the luminance signal is established at a specified
voltage. A portion of the luminance signal that is derived at
potentiometer Pl in the emitter circuit of transistor TRl is
applied via a low pass filter constituted by an inductance coil
L2 and a capacitor C3 to a varactor diode Dl. The filter removes
video signals above about 1 MHz, thus preventing any high frequency
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luminance or chrominance information in the luminance channel
from affecting the chrominance detector. These high frequency
signals are not required in any event since the highest
chrominance modulation frequency for equiband operation is
0.6 MHz.
The signal which is applied to varactor diode Dl
increases its capacitance for black signals. This increased
capacitance is in parallel with capacitor Cl and consequently
increases the delay or phase shift of channel 31, i.e. increases
the phase shift of the B-Y reference signal. This results in
a wider demodulation angle for the chrominance signals at
black levels. On the other hand, when the luminance signal
is at a high level for colours such as yellow, cyan and green,
the voltage across varactor diode Dl will increase, reducing
its capacitance and resulting in a narrower demodulation angle
with more accurate reproduction of these colours.
It should be noted that in the case where a phase
lead is applied to one demodulator and a phase lag is applied
to the other demodulator, the dynamic hue control network of
this invention can readily be rearranged to change the degree
of both lead and lag so as to change the demodulation angle.
In the embodiment of the invention shown in Figure
2, variable reactance network 29 of Figure l is removed and
becomes associated with a phase shift network 3? that is
included between chrominance amplifier 23 and one of its
inputs to synchronous demodulator 27. In this embodiment of
the invention the phase of the chrominance signal relative to
; the phase of the reference oscillation is varied, whereas in
the embodiment of the invention shown in Figures l and 3 the
phase of the reference oscillation in channel 31 is varied
relatlve to the phase of the chrominance signal.
While one form of variable reactance network using
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a varactor diode has been shown and described, it is to be
understood that any active device which changes its effective
reactance in response to changes in the luminance signal level
can be used. Thus, in the case where the phase shift network
is sensitive to changes in resistance to vary the degree of
phase shift, a voltage dependent resistor could be used.
It also should be understood that this invention is
not limited to a colour receiver in which R-Y, B-Y and G-Y
signals are derived from demodulator. It applies to colour
receivers generally including those using the R, G, B
chrominance signal system.
While preferred embodiments of this invention have
been disclosed herein, those skilled in the art will appreciate
that changes and modifications may be made therein without
- departing from the spirit and scope of this invention as
defined in the eppended ~laims.
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