Television – Image signal processing circuitry specific to television – Chrominance signal amplitude control
Reexamination Certificate
1999-06-30
2001-01-23
Hsia, Sherrie (Department: 2714)
Television
Image signal processing circuitry specific to television
Chrominance signal amplitude control
C348S624000, C348S663000, C348S679000
Reexamination Certificate
active
06177962
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a video signal processing apparatus, and in particular, to a chroma overload protection apparatus.
BACKGROUND OF THE INVENTION
Chroma overload circuits are commonly used to prevent oversaturated chroma which may result from “channel impairments” in a video system such as tuner tilt, multipath propagation effects, noise or the like.
A conventional form of chroma overload protection circuit operates by adjusting the gain of the chroma signal in response to an average chroma level during active video time intervals after automatic chroma control (ACC) processing.
FIG. 1
illustrates an example of such a system. The conventional system of
FIG. 1
comprises series connected ACC unit
10
and chroma overload unit
20
(each outlined in phantom) followed by chroma signal separator circuit
30
which separates the chroma signal into its component vector components, U and V for further processing (e.g., demodulation to baseband and matrixing to provide baseband color difference output signals R-Y and B-Y).
ACC unit
10
comprises controllable gain amplifier
12
to which a chroma input signal is applied and a feedback path from the output of amplifier
12
to control input
14
thereof. The feedback path comprises a series connection of burst gate
16
and automatic chroma control circuit (ACC)
18
. Chroma overload circuit
20
is also feedback controlled and comprises controllable gain amplifier
22
which is coupled at the input thereof to receive the ACC controlled chroma from unit
10
and is provided with feedback to control input
24
thereof via a series connection of gate
26
and average detector
28
. Chroma signal separation into its component vectors is performed after chroma ACC and chroma overload protection by chroma signal separator circuit
30
connected to the output of overload circuit
20
to provide separated chroma vector component output signals U and V.
In the operation of ACC unit
10
, burst gate
16
passes the color burst component of the chroma signal to ACC unit
18
which compares the burst amplitude with a reference level and provides a control signal to control input
14
of amplifier
12
to adjust the chroma signal amplitude to a predetermined level. In this way, the amplitude of the overall chroma signal C is stabilized at a predictable value based on the burst amplitude relative to the reference or desired level. If the burst amplitude increases, the gain of amplifier
12
decreases to thereby stabilize the average chroma signal level. Some smoothing may be included in ACC unit
18
to prevent noise which may be present on the gated burst signal from disturbing the regulated chroma output signal level.
In the operation of chroma overload circuit
20
, gate
26
is open during the active video trace interval and closed otherwise to thereby pass only chroma to average detector
28
. Recall that ACC detector
18
serves to regulate the chroma level based on the gated burst amplitude. Overload detector
28
serves to provide chroma overload limiting based on the average chroma level (rather than the burst level). For this purpose, the time constant of average detector
28
as well as the gain characteristic of gain controlled amplifier
22
determine the behavior of the overload circuit. In some systems of this type, the attack and decay time constants of detector
28
may be different, and detector
28
may be implemented as a so-called “leaky peak detector” (e.g., a capacitor with a fast charging circuit and a parallel connected “leak” resistor to provide a relatively slow discharge time constant).
SUMMARY OF THE INVENTION
It is herein recognized that a problem exists with conventional chroma overload circuits and that problem relates to the relatively long time constants involved. The response time constant is usually on the order of a video field (e.g., 17 milli-Seconds). This long time constant may have a tendency to reduce visible artifacts associated with modulating the chroma gain. On the other hand, it also may allow instantaneous levels of the chroma signal to exceed desired levels resulting in undesirable oversaturation of the chroma of displayed images.
A further aspect of the problem, as herein recognized, is that in an analog system, “head room” is provided to accommodate this instantaneous chroma signal condition which may cause oversaturation. By contrast, in a digital system, for example, one supporting a CCIR 601/656 interface standard, there may be little “head room” provided for chroma in the system standard. By way of example, in a digital system, chroma levels exceeding the 8-bit (or sometime 10-bit) range for the chroma components (e.g., U and V vector components) may be “hard limited” (e.g., by truncation). Since U and V may be limited separately after demodulation, chroma overload may produce an objectionable tint shift in displayed images.
The problem of chroma overload is further illustrated in the phasor (vector) diagrams of
FIGS. 2A and 2B
. In the diagram of
FIG. 2A
there is shown an over-saturated chroma vector C which exceeds a desired saturation limit represented by grid
200
, and its constituent component vectors, U and V.
FIG. 2B
shows the effect of limiting the constituent vectors. In this example, vector U is limited to a magnitude of U-LIM (“LIM” is the limiting value), vector V is not limited. The resultant vector C-LIM has a different angle, and therefore a different tint, than vector C. This can result in a particularly objectionable type of picture artifact to the eye. Imagine, for example, an over-saturated blue object becoming red at the areas of highest saturation.
In accordance with the principles of the present invention, the solution to the problem of chroma tint shifts upon overload of one or more vector components is provided by controlling chroma saturation on a pixel-by-pixel basis. Advantageously, this approach prevents tint shifts on over-saturated pixels. Moreover, this solution also eliminates (not just reduces) visible artifacts associated with larger time-constant methods of modulating the chroma gain.
Chroma overload protection apparatus in accordance with the invention comprises a source for providing a chrominance signal having first and second vector components; a separator, coupled to said source, for separating said chrominance signal into the first vector component and the second vector component; and an overload compensator, coupled to the separator, for adjusting the magnitude of the first and second vector components as a given function of a detected saturation level of the chrominance signal on a pixel by pixel basis.
In a desirable application of the principles of the invention, the compensator comprises a saturation calculator responsive to the first and second vector components for providing a saturation indicating signal converter responsive to the saturation indicating signal for providing a gain control signal; and gain control means, responsive to the gain control signal, for concurrently adjusting the gain of the first vector component in a first signal path and of the second vector component in a second signal path.
A method, in accordance with the invention, for providing chroma overload protection, comprises the steps of: providing a chrominance signal having first and second vector components; separating the chrominance signal into the first vector component and the second vector component; and adjusting the magnitude of the first and second vector components as a given function of a detected saturation level of the chrominance signal on a pixel by pixel basis.
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patent: 4962417 (1990
Keen Ronald Thomas
Rumreich Mark Francis
Zukas Mark Robert
Hsia Sherrie
Kiel Paul P.
Thomson Licensing S.A.
Tripoli Joseph S.
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