Comb filtered signal separation

Details Comb filtered signal separation Comb filtered signal separation

1999-02-04

2001-01-16

Hsia, Sherrie (Department: 2714)

Television

Image signal processing circuitry specific to television

Chrominance-luminance signal separation

C348S665000, C348S668000

Reexamination Certificate

active

06175389

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an improved comb filter system and method, and in particular an improved system and method for separation of composite signals.
2. Background
Due to limitations on available bandwidth and the increased demand to transmit additional information on existing bandwidth, it is often necessary to multiplex or combine two or more information signals into a single composite signal.
A color television signal is an example of a composite signal. A color television signal comprises a luminance (brightness) component and a chrominance (color) component. These components are often represented as Y and C components wherein Y represents the luminance component and C represents the chrominance component.
Originally, broadcast television in the United States began with black and white broadcast and therefore lacked the chrominance component, C, of modern television's composite signal. Television standards and technology required that the black and white television signal, that is, the luminance component (Y), reside within 6 megahertz (MHz) of bandwidth space.
Eventually, however, technology advanced to provide color television. To allow black and white televisions to receive the new color signal broadcast, the color signal standard located the color information within the same 6 MHz of bandwidth space allotted to each channel of the black and white signal. Under this standard, the color information overlaps with the luminance information.
FIG. 1
illustrates a composite television signal on a coordinate system in which the horizontal axis
100
represents frequency and the vertical axis
102
represents amplitude. Signal line
104
represents the luminance information (Y) while line
106
represents the chrominance information (represented as I and Q) of the composite signal. As shown, the frequencies of these signals
104
,
106
overlap. In an NTSC (National Television Standards Committee) system, the luminance information occupies the range DC to 5.5 MHz of bandwidth while the chrominance signal is bandlimited to the range approximately 0.6 to 1.3 MHz and is modulated onto a carrier at 3.58 MHz. The audio portion of the signal is at 4.5 MHz. While these two data signals conveniently fit within the 6 MHz of bandwidth space they are allotted, obvious decoding challenges are presented in order to separate the luminance information from the chrominance information.
The first decoding scheme adopted to separate the overlapping luminance (Y) and chrominance (C) signals comprises simple notch filtering in combination with band pass filtering.
FIG. 2
illustrates a block diagram of a basic notch filter
152
and band pass filter
154
. An incoming composite signal on line
150
is presented to both of the notch filter
152
and the band pass filter
154
.
FIG. 3
illustrates the frequency response of a notch filter
152
and a band pass filter
154
. The output of the notch filter generally comprises the luminance portion
174
of the composite signal while the output of the band pass filter generally comprises the chrominance portion
176
of the composite signal.
In particular, for NTSC video, the notch filter removes a portion of the composite signal centered at 3.58 MHz, but allows the remainder
174
to pass. While the notch filter
152
allows the majority of the luminance information
174
to pass, it undesirably removes components of the luminance signal having frequencies within the range of the notch filtered frequencies
177
. The notch filtered frequencies that are removed range from 2.5 to 4.5 MHz. Stated another way, the notch filter allows the frequency band below 2.5 MHz and the frequency band above 4.5 MHz to pass.
The band pass filter
154
configured to operate in accord with the NTSC standard video allows a 2 MHz portion of the composite signal centered at 3.58 MHz to pass while removing portions outside of this band. This portion of the composite signal contains all the chrominance information. Undesirably, however, the output of the band pass filter also contains luminance components having frequencies within the band pass filter's frequency band.
Notch and band pass filtering suffers from numerous drawbacks as can easily be understood with reference of FIG.
3
. In particular, the band pass filtered chrominance portion of the composite signal also contains luminance information, i.e., band pass filtering does not remove all luminance information from the chrominance signal. The unwanted luminance information in the chrominance signal introduces artifacts into the video image. This is most noticeable in pictures that contain closely spaced black and white lines, such as when the video display is of person is wearing a herringbone jacket.
Likewise, notch filtering the composite signal to remove the chrominance information from the composite signal to obtain the luminance information removes valuable portions of the luminance signal. A loss of luminance information is especially critical due to the human eye's sensitivity to brightness and contrast variations in a projected image.
Therefore, a need exists for a method and apparatus for video separation that is more robust than prior systems, requires less memory, and more completely separates the components of the composite signal.
SUMMARY OF THE INVENTION
In accordance with the purpose of the invention as broadly described herein, there is provided a method and apparatus for separating overlapping components in a composite signal, such as for example, separating the chrominance and luminance components in a quadrature amplitude modulated (QAM) signal. A novel technique is employed in which the composite signal is notch filtered to create a luminance signal containing all but a portion of the luminance components. The composite signal is also band passed filtered to create a chrominance signal containing all the chrominance components and the luminance components missing from the luminance signal.
Next, the chrominance signal is demodulated from the subcarrier frequency. Thereafter, the demodulated chrominance signal is comb filtered to separate the luminance components from the chrominance components. The chrominance signal is thus isolated.
Next, the isolated chrominance signal is subtracted from the original chrominance signal that contains the luminance components to yield a signal comprising only luminance components.
This luminance components signal is next remodulated and added back to the original luminance signal that is missing these luminance components. The entire luminance signal is thus created.
In an alternative embodiment, the sampling rate, i.e. format, of the demodulated chrominance signal is modified prior to comb filtering to reduce memory and processing requirements. This significantly reduces memory requirements for each line store in the comb filter. After comb filtering, the original sampling rate is restored to facilitate the combination of the luminance components with the notch filtered luminance signal.
In alternative embodiments, the number of taps or line delays in the comb filter is varied depending on one or more design parameters.
In an alternative embodiment, the signal separation system adopts an adaptive signal weighting scheme to dynamically adjust the portion of the final signal that is derived from a particular filtering scheme. In one configuration, a line difference detector forms a luminance coefficient and a chrominance coefficient depending on the amount of change occurring between video scan lines. The value of the luminance coefficient controls the ratio of notch filter separated luminance signal to comb filter separated luminance signal in the final output of the luminance signal. The value of the chrominance coefficient controls the ratio of band pass filter separated chrominance signal to comb filter separated chrominance signal in the final output of the chrominance signal.
Thus, the adaptive system dynamically selects which type of signal separation method is used a

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