In an NTSC analog television signal, the signal in a picture period (video signal except for blanking-period) consists of a mixture of a luminance signal (Y) that is amplitude modulated and a color signal modulated by a color subcarrier signal. In addition, the NTSC analog television signal itself includes the vertical and horizontal sync component signals and the so-called color burst signal that is used by a television receiver as a reference to demodulate color. The signal of this type is a composite signal, which is usually called a color composite video signal. To display the color composite video signal, the television receiver side separates the luminance signal and the color signal by decoding the color composite video signal (compound signal), and further decodes the color signal into the RGB color signals.
The so-called NTSC analog television signal, which is also called a composite video signal, includes the picture period expressed by the following equation.
                              Comp          ⁡                      (            t            )                          =                ⁢                              Y            ⁡                          (              t              )                                +                      C            ⁡                          (              t              )                                                              =                ⁢                              Y            ⁡                          (              t              )                                +                                    U              ⁡                              (                t                )                                      ⁢                          sin              ⁡                              (                                  ω                  ⁢                                                                          ⁢                  t                                )                                              +                                    V              ⁡                              (                t                )                                      ⁢                          cos              ⁡                              (                                  ω                  ⁢                                                                          ⁢                  t                                )                                                        where Comp(t) is a television signal during the picture period, Y is the luminance signal, C is a chrominance signal, U and V are color signals (U=R−Y and V=B−Y, where R=red signal and B=blue signal), ω is an angular frequency of the color subcarrier signal and t is time.
The Y/C separation of the NTSC analog television signal refers to restoring Y(t) and C(t) independently from the Comp(t) signal. Since the frequency bands of Y(t) and C(t) overlap on each other, complete separation is usually difficult. However, a variety of Y/C separating methods have been proposed and performed conventionally which utilize the color subcarrier signal frequency, or the relationship between the color subcarrier frequency and the horizontal or vertical scanning frequency of the television signal.
A Y/C one-dimensional separating method utilizes the output of a bandpass filter, having a central frequency that equals the color subcarrier frequency of the color signal, as the color component signal, and its remainder as the luminance component signal. Another method, a two-dimensional Y/C separation (comb filter) method, utilizes the fact that the relationship between the color subcarrier frequency and the horizontal scanning frequency in the television signal, that is, the phase difference of the color subcarrier frequency between adjacent scanning lines, is 180 degrees. Assuming that the images of the adjacent scanning lines have a correlation, the luminance signal is computed from the sum of the adjacent scanning lines, and the color signal from the difference between the adjacent scanning lines. This two-dimensional Y/C separating method utilizes as the data of the adjacent scanning lines: (1) the data of the upper or lower scanning line of a target scanning line; or (2) the data of both the upper and lower scanning lines of the target scanning line. Alternatively, there is a method that utilizes, instead of the scanning line data in the same field, the data of the upper and lower scanning lines in adjacent fields or the data of the corresponding scanning line in the adjacent frame. This method, utilizing the data of the inter-field or inter-frame scanning line(s), is referred to as three-dimensional Y/C separation. It is used in conjunction with motion detection in the time axis direction. An adaptive Y/C separating method of switching between the three-, two- and one-dimensional Y/C separating methods is usually used: When no motion is detected, three dimensional Y/C separation is used; when the motion is detected, the two-dimensional Y/C separating method, specifically one of the variety of the two-dimensional Y/C separating methods is used in accordance with the correlation between the scanning lines; and when no correlation is present between the scanning lines, the one-dimensional Y/C separation is used.
The above-mentioned various types of adaptive Y/C separation are selected and used in accordance with the demand for the reduction in the amount of the luminance component signal appearing in the separated color signal, or the reduction in the amount of the color component signal appearing in the separated luminance signal. Generally, the reduction in the amount of the color component signal appearing in the separated luminance signal is referred to as dot crawl elimination, and the reduction in the amount of the luminance component signal appearing in the separated color signal is referred to as a cross-color interference elimination. Furthermore, as for the Y/C separation, a variety of methods are proposed and performed such as those implemented without degrading the total image quality, or those with high cost performance which meet the purpose of equipment adopting the methods. However, they have various problems to be solved at present.
The foregoing two-dimensional Y/C separating methods were carried out using a glass delay line or an analog method utilizing a CCD previously, and now use digital processing.
However, the mainstream of the Y/C separating methods in the conventional digital processing assumes a digital signal that is sampled by a sampling clock signal with a frequency that is an integer multiple of the color subcarrier frequency, such as a frequency 2 fsc or 4 fsc, twice or four times the color subcarrier frequency fsc. This is due to the fact that the Y/C separating methods process the digital signal instead of processing the analog video signal by an analog circuit, and output an analog signal, that is, a signal with analog timing, at the final stage.
Recently, a novel technique has been proposed. Although the conventional techniques digitize the analog signal for carrying out the digital processing, that is, digitize only the level of the analog signal with maintaining the time axis of the analog signal, the new technique digitizes not only the level, but also the time axis, thereby carrying out complete digitization to carry out transmission or recording.
As for the NTSC color television signal, the color subcarrier frequency fsc and the horizontal sync frequency fh have a relationship of fsc=(455/2)fh, and the horizontal sync frequency fh and the vertical sync frequency fv have a relationship of fh=(525/2)fv.
As for the formats of the digital video signal, the International Telecommunication Union (ITU) brings forth recommendation ITU-R BT.656 (formerly known as CCIR 656). The recommendation provides the standard of interfaces for digital component video signals in the 525-line and 625-line television systems operating according to the 4:2:2 level of ITU-R BT.601. The ITU-R BT.601 defines the standard of the studio encoding parameters of a digital television for the standard aspect ratio of 4:3 or for the wide-screen aspect ratio of 16:9.
According to the foregoing ITU recommendation, it is necessary to generate a clock signal (abbreviated to fc from now on) with a frequency 27 MHz, that is, 1716 times the horizontal sync frequency of the NTSC standard television signal with the 525-lines. The frequency is specified considering the interconversion between the PAL and NTSC television signals, and differs from an integer multiple of the color subcarrier frequency of the NTSC television signal. The ratio between fsc and fc is given by (455/2)/1716=455/(2×1716)=(13×7×5)/(13×11×3×2×2×2)=(7×5)/(11×3×2×2×2)=35/264.
The sampling frequencies in the standard “4:2:2 digital component television signal” in the foregoing ITU-R BT.601 are 13.5 MHz for the Y signal and 6.75 MHz for the Cb and Cr signals. Thus, comparing 4 fsc with 13.5 MHz, the clock interval of the former corresponds to phase rotation of 90 degrees of the color subcarrier signal, and that of the latter corresponds to the phase rotation of 95.4545 degrees. As for 2 fsc and 6.75 MHz, they correspond to 180 degrees and 190.91 degrees, respectively.
Consequently, a Y/C separating method is required which can operate at a clock frequency different from an integer multiple of the color subcarrier frequency such as the clock interval of 90 degrees or 180 degrees.
In addition, although the relationship is maintained between the color subcarrier frequency and the horizontal sync frequency in the analog NTSC television signal, the phase correlation between the two signals is usually indefinite. When the sampling is carried out by the clock signal synchronizing with the color subcarrier frequency, the uncertainty of the phase correlation can bring about discontinuity (or mismatch) between the phase of the horizontal sync signal, which is sampled and finally output after undergoing digital processing, and the phase of a signal bearing the phase of the sampled image itself. In addition, the problem brings about an increase in the horizontal blanking of the output signal, that is, a reduction in the horizontal width of the picture period. To solve the problem, new standard RS-170A is proposed which is an improved version of conventional RS-170, the standard of the NTSC television signal. The new standard limits the phase difference between the horizontal sync signal and the color subcarrier signal to within certain bounds with reference to the color subcarrier frequency.
In digital equipment adapted to the NTSC television signal based on the RS-170A standard, the shift in the horizontal direction of an image on a display screen (the horizontal shift is produced because of the above-mentioned discontinuity) does not occur in the television signal after the digital processing. However, comparing the input source signal with the signal passing through the digital processing makes it clear that the shift (horizontal shift) occurs, whose maximum value is the above-mentioned “specified certain boundaries”.
To solve the above-mentioned horizontal shift, a technique is implemented, which generates the clock signal in synchronism with the horizontal sync signal of the input signal. In this case, a method called digital PLL has been conventionally used. It generates the color subcarrier signal composed of 8-bit quantized bits using the clock signal, compares the phase of the generated color subcarrier signal with that of the color burst signal in the input signal, and controls the phase of the generated color subcarrier signal such that the phase difference is maintained at a fixed value.
As for a signal source involving the fluctuations in the time axis such as a reproduced signal of an analog VCR or analog VTR, a broadcasting station or the like eliminates the fluctuations in the time axis of the input source signal (particularly the luminance signal) with an expensive, high performance apparatus called a time-base corrector. Generating the clock signal (sampling clock signal) in synchronism with the horizontal sync signal by inputting the signal source involving the fluctuations in the time axis without using the time-base corrector will presents the following problem. When the NTSC television signal digitized by the clock signal is subjected to the above-mentioned conventional Y/C separating method, the dot crawl on the time axis, the so-called cross-color effects, takes place because of the fluctuations in the clock period. A similar problem becomes conspicuous when the relationship between the color subcarrier frequency and the horizontal sync frequency of the input source signal does not maintain the relationship of the above-mentioned NTSC standard as a result of the frequency variations rather than the jitter in the horizontal sync signal of the input source signal.
Therefore, there must be two different methods to carry out the digital processing including the Y/C separation processing: one method is for a NTSC television signal source such as a terrestrial broadcasting source, satellite broadcasting source, and cable television capable of neglecting the time-base fluctuations (jitter); and the other method is for a general NTSC analog television signal involving the time-base fluctuations (jitter).
The present invention is implemented to solve the foregoing problems. Therefore an object of the present invention is to provide a Y/C separator and Y/C separating method of the NTSC television signal capable of reducing the amount of the luminance component signal appearing in the separated color signal, or the amount of the color component signal appearing in the separated luminance signal in carrying out the digital processing of the NTSC analog television signal, even if the sampling frequency is not an integer multiple of the color subcarrier frequency.
Another object of the present invention is to provide a Y/C separator and Y/C separating method of the NTSC television signal capable of reducing the amount of the luminance component signal appearing in the separated color signal, or the amount of the color component signal appearing in the separated luminance signal, even when the clock signal is synchronized with the horizontal sync signal.
Still another object of the present invention is to provide a Y/C separator and Y/C separating method of the NTSC television signal capable of reducing the amount of the luminance component signal appearing in the Y/C-separated color signal, or the amount of the color component signal appearing in the Y/C-separated luminance signal, even in such a case where a signal source such as a reproduced signal of an analog VCR or analog VTR is input which involves the frequency variations in the horizontal sync signal included in the signal source and involves the fluctuations in the time axis, and the quantization is carried out by using the clock signal synchronizing with such a horizontal sync signal, when carrying out the digital processing of the NTSC analog television signal.