TELEVISION SIGNAL FRAME NUMBER CONVERTER

Abstract: In a television signal frame number converter for converting an input television signal having a first frame number per second into an output television signal having a second frame number per second in accordance with an interpolation coefficient, a processing unit (11) processes two successive frames on the input television signal by a frame interpolation process to produce a processed signal. A memory unit (12) memorizes the processed signal in synchronism with a write-in address signal (141) given by a first frame synchronizing signal (13) of the input television signal and is read out by a read-out address signal (171) given by a second frame synchronizing signal (16) of a reference television signal having the second frame number per second. The converter determines (15) the interpolation coefficient in accordance with a time interval between a write-in start timing of the memory unit and a read-out start timing of the memory unit.

Description: Background of the Invention: This invention relates to a frame rate converter for a television signal. Such a television signal frame rate converter is particularly useful in a television standard converter. The NTSC system, the PAL system, and the SECAM system are well-known as television standards widely adopted in the world. When it comes to convert television standards, from the NTSC system to the PAL system for example, it may be necessary to convert a signal having 30 frames per second to a signal having 25 frames per second. Generally, a television signal frame rate converter has a frame interpolation process, carried out by using two successive frames of one television standard signal, and converting them into another standard signal in accordance with an interpolation coefficient. The interpolation coefficient is closely related to the timing of two standard signals at the interpolation time, and varies with lapse of time. However, the conventional method for determining the interpolation coefficient is a stereotyped one. The interpolation coefficient has been determined by the nominal timing ratio of the two television standards concerned. Taking the NTSC system and the PAL system for instance, the PAL system's 5 frames can be processed from the NTSC system's 6 frames, using constant coefficients of 1.0, 0.8, 0.6, 0.4, and 0.2 for each, if the NTSC system and the PAL system have exactly 30 and 25 frames per second, respectively. One known standards converter is described in European Patent Application EP-A-0 136 770. In reality, the frame rate of each television standard signal deviates from its nominal value; furthermore, the NTSC's real frame rates are not equal to 30 but 59.940052/2. The difference between the nominal and real coefficient values causes degradation of the picture quality of the converted signal. In order to obtain a converted signal having a good picture quality, we have appreciated that it is desirable to determine the interpolation coefficient in real time. Summary of the Invention: The invention in its various aspects is defined in the independent claims below, to which reference should now be made. By using the invention it is possible to determine optimum interpolation coefficients for any given situation in a television frame standards converter. Brief Description of the Drawings: Figure 1 is a block diagram of a television signal frame rate converter according to a first embodiment of this invention; Figure 2 is a view for use in describing an operation of the television signal frame rate converter illustrated in Figure 1; Figure 3 is a block diagram of a television signal standards converter to which this invention may be applied; and Figure 4 is a block diagram of a television signal frame rate converter according to a second embodiment of this invention. Description of the Preferred Embodiments: Referring to Figure 1, description will be made as regards a television signal frame rate converter according to a first embodiment of this invention. By a frame interpolation process, the television signal frame rate converter converts an input television signal S₁ having a first number of frames per second into an output television signal S₂ having a second number of frames per second. The first number of frames is not smaller than the second number of frames. In other words, the input television signal S₁ has a first frame frequency while the output television signal S₂ has a second frame frequency. The television signal frame rate converter comprises a processing unit 11 and a memory unit 12. A first synchronizing signal separator 13 is supplied with the input television signal S₁. The first synchronizing signal separator 13 separates a first vertical synchronizing signal from the input television signal S₁ and delivers the first vertical synchronizing signal to a write-in address generating unit 14. The write-in address generating unit 14 comprises a write-in address generator 141 and a vertical write-in address generator 142. Supplied with the first vertical synchronizing signal, the write-in address generator 141 generates a write-in address signal S₃ representing a write-in address for the memory unit 12. The write-in address signal S₃ has the first frame frequency. In response to the write-in address signal S₃, the vertical write-in address generator 142 generates a vertical write-in address signal S₄ corresponding to horizontal scanning lines of a television image. The vertical write-in address signal S₄ has a constant cycle and may therefore be called a first timing signal. The vertical write-in address signal S₄ is supplied to a coefficient generator 15. As will later be described, the coefficient generator 15 is for generating interpolation coefficients in the form of first and second coefficient signals S₅ and S₆. The first coefficient signal S₅ represents a first coefficient k which has a value in the range 0 to 1. The second coefficient signal S₆ represents a second coefficient (1 - k). In a manner known in the art, a reference television signal S₇ of the second number of frames per second is supplied to a second synchronizing signal separator 16. The synchronizing signal separator 16 separates a second vertical synchronizing signal from the reference television signal S₇ and delivers the second vertical synchronizing signal to a read-out address generating unit 17. The read-out address generating unit 17 comprises a read-out address generator 171 and a pulse signal generator 172. Supplied with the second vertical synchronizing signal, the read-out address generator 171 generates a read-out address signal S₈ for the memory unit 12. The read-out address signal S₈ has the second frame frequency. In response to the read-out address signal S₈, the pulse signal generator 172 generates a start pulse signal S₉ at the head of the read-out address signal S₈ and delivers the start pulse signal S₉ to the coefficient generator 15. The start pulse signal S₉ indicates a start time of the read-out address signal S₈ and may be called a second timing signal. The coefficient generator 15 calculates the first and the second coefficients k and (1 - k) in accordance with the vertical write-in address generated when the start pulse signal S₉ is generated. Referring to Fig. 2, description will be directed to principles of the frame interpolation process and a method of determining the first and the second coefficients k and (1 - k). The frame interpolation process is carried out in connection with each field of the input television signal by a method, such as a weighted mean method known in the art. In Fig. 2, zeroth to fourth input fields of the input television signal are depicted at wf₀, wf₁, wf₂, wf₃, and wf₄ which are arranged at a first time interval T₁. Zeroth to third output fields of the output television signal are depicted at rf₀, rf₁, rf₂, and rf₃ which are arranged at a second time interval T₂. The first time interval T₁ is not longer than the second time interval T₂. The first time interval T₁ is equal to a write-in cycle time of the memory unit 12 (Fig. 1) for each field and is defined by a write-in start time and a write-in end time of a write-in cycle. The write-in start and the write-in end times are depicted by t₁ and t₂, respectively. On the other hand, the second time interval T₂ is equal to a read-out cycle time of the memory unit 12. In the example being illustrated, the zeroth output field rf₀ is derived from the zeroth and the first input fields wf₀ and wf₁ by a frame interpolation process and is read out of the memory unit 12 at a read-out start time depicted by t₃. The read-out start timing may be called a frame interpolation timing. The zeroth and the first input fields wf₀ and wf₁ are weighted by first and second weighting coefficients, respectively. The first and the second weighting coefficients are determined by a third time interval T₃ between the write-in start time t₁ and the read-out start time t₃. Although the third time interval T₃ varies with lapse of time, it can be represented by the write-in address or the vertical write-in address described before. Thus, the first and the second weighting coefficients are determined in real time and serve as the first and the second coefficients k and (1 - k), respectively. Following the principles described above, the remaining first to third output fields rf₁ to rf₃ are derived from the first and the second input fields wf₁ and wf₂, the second and the third input fields wf₂ and wf₃, and the third and the fourth input fields wf₃ and wf₄, respectively. The above-described frame interpolation process may be called a linear frame interpolation process. Referring back to Fig. 1, an analog-to-digital (A/D) converter 18 is supplied with the input television signal S₁ and converts the input television signal S₁ into a digital television signal. The digital television signal is supplied to a delay circuit 19 and to the processing unit 11. The delay circuit 19 gives the digital television signal a delay equal to one frame of the digital television signal to deliver a delayed digital television signal to the processing unit 11. By the frame interpolation process, the processing unit 11 processes the digital television signal and the delayed digital television signal in accordance with the first and the second coefficient signals S₅ and S₆ to produce a processed signal. In other words, the processing unit 11 processes two successive frames of the digital television signal at a time. For this purpose, the processing unit 11 comprises first and second multipliers 21 and 22 and an adder 23. The first multiplier 21 multiplies the digital television signal by the first coefficient k and delivers a first multiplied signal representing a first multiplied value to the adder 23. The second multiplier 22 multiplies the delayed digital television signal by the second coefficient (1 - k) and delivers a second multiplied signal representing a second multiplied value to the adder 23. Thus, the digital television signal and the delayed digital television signal are weighted by the first and the second coefficients k and (1 - k), respectively. Supplied with the first and the second multiplied signals, the adder 23 calculates a mean value between the first and the second multiplied values and produces a weighted mean signal as the processed signal. The processed signal is supplied to the memory unit 12. The memory unit 12 memorizes the processed signal as a stored television signal in synchronism with the write-in address signal S₃. Although the processed signal, namely, the stored television signal has an unnecessary frame for the output television signal, the unnecessary frame is skipped in read-out operation of the memory unit 12 because the memory unit 12 is read out at the second frame frequency which is not higher than the first frame frequency of the write-in address signal. Read from the memory unit 12, the read-out television signal is supplied to a digital-to-analog (D/A) converter 24. The digital-to-analog converter 24 converts the read-out television signal into an analog signal and produces the analog signal as the output television signal S₂ having the second number of frames per second. Referring afresh to Fig. 3, description will be directed to a television signal standards converter to which this invention is applied. The television signal standards converter is operable to convert an input television signal S₁₀ of the NTSC system into an output television signal S₂₀ of the PAL system. Accordingly, the input television signal S₁₀ has 30 frames (namely, 60 fields) per second while the output television signal has 25 frames (namely, 50 fields) per second. The illustrated converter comprises a frame rate converter 30 according to a preferred embodiment of this invention and the other portions which may be structured in a known manner. In this connection, the portions except the frame rate converter 30 will be simply described in comparison with the frame rate converter 30. The input television signal S₁₀ is converted by an analog-to-digital (A/D) converter 31 into a digital television signal. The digital television signal is supplied to a detector 32 and a Y/C signal separator 33 of a motion adaptive type. Responsive to the digital television signal, the detector 32 detects moving parts and edge portions appearing in the television signal and produces a motion detection signal and an edge detection signal on detection of a moving part and an edge portion, respectively. The motion and the edge detection signals are supplied to the Y/C signal separator 33 together with the digital television signal so as to generate a motion-adapted Y/C signal from the digital television signal in a known manner. More specifically, the Y/C signal separator 32 comprises a temporal filter and a spatial filter which are adaptively switched in response to the motion and the edge detection signals. At any rate, the Y/C signal separator 32 carries out Y/C signal separation in a still part specified by no detection of the motion detection signal by utilizing correlation between two successive frames of the digital television signal. When a moving part is specified by detection of the motion detection signal with no edge detection signal, the Y/C signal separator 32 carries out the Y/C signal separation by utilizing the correlation in the vertical direction of the television image, namely, the correlation between two horizontal scanning lines which are adjacent to each other. On the other hand, if the edge detection signal is present, the Y/C signal separator 32 carries out the Y/C signal separation by utilizing the correlation in the horizontal direction of the television image. Thus, the Y/C signal separator 32 produces a separated luminance (Y) signal and a separated chrominance (C) signal, which may appear in an interlaced manner and which may be called interlaced Y and C signals. The interlaced Y and C signals are sent to a non-interlace converter 34 of a motion adaptive type. The non-interlace converter 34 of motion adaptive type is also supplied with the motion detection signal from the detector 32. As known in the art, the non-interlace converter 34 of motion adaptive type comprises a plurality of interpolation filters and selects one of the interpolation filters in response to the motion detection signal. In the still part of the television image, the non-interlace converter 34 interpolates the horizontal scanning line by using information from two successive fields. In the moving part of the image, the non-interlace converter 34 interpolates the horizontal scanning line by using the information from one field. Thus, the interlaced Y and C signals are converted by the non-interlace converter 34 into non-interlaced signals as a converted signal S₁₁. The converted signal S₁₁ is supplied to the frame rate converter 30. In the manner described in conjunction with Fig. 2, the frame rate converter 30 converts the converted signal S₁₁ having 30 frames per second into a frame converted signal S₁₂ having 25 frames per second. Each frame of the converted signal S₁₁ is formed by 525 horizontal scanning lines and should be converted into a frame formed by 625 horizontal scanning lines, as well known in the art. To this end, the frame converted signal S₁₂ is supplied to a scanning line number converter 35. The scanning line number converter 35 converts the frame converted signal S₁₂ having 525 horizontal scanning lines per frame into a line converted signal having 625 horizontal scanning lines per frame. Then, the scanning line number converter 35 interlaces the line converted signal in a known manner and produces a line interlaced signal. The line interlaced signal is sent to an encoder 36. The encoder 36 encodes the line interlaced signal into an encoded signal of the PAL system and delivers the encoded signal to a digital-to-analog (D/A) converter 37. The digital-to-analog converter 37 converts the encoded signal into an analog signal. The analog signal is produced as the output television signal S₂₀. Referring to Fig. 4, the description will be made as regards the frame rate converter 30 illustrated in Fig. 3 in detail. The frame rate converter 30 comprises a delay circuit 41, a write-in address generator 42, an extractor 42', a read-out address generator 43, a coefficient generator 44, a processing unit 45, and a memory unit 46. The coefficient generator 44 and the processing unit 45 may be similar in structure to those illustrated in Fig. 1 and will therefore not be described in detail. In addition to the linear frame interpolation process, the frame rate converter 30 carries out a motion compensating frame interpolation process for compensating for the moving parts of the television image. For this purpose, the frame rate converter 30 further comprises a motion vector detector 47, a multiplying unit 50 comprising first and second multipliers 51 and 52, and a compensating unit 60 comprising first and second motion compensating circuits 61 and 62. The frame rate converter 30 still further comprises a selecting unit 70 comprising first and second selecting circuits 71 and 72. Each of the first and second selecting circuits 71 and 72 is simultaneously switched to carry out a selected one of the linear frame interpolation process and the motion compensated frame interpolation process. Each of the processes is selected in a manner to be later described. As described in conjunction with Fig. 1, the write-in address generator 42 produces a write-in address signal S₄₁ in response to the first synchronizing signal derived from the input television signal. In response to the second synchronizing signal derived from the reference television signal of the PAL system, the read-out address generator 43 produces a read-out address signal S₄₃. Supplied with the write-in address signal S₄₁ and the read-out address signal S₄₃, the extractor 42' extracts the write-in address signal S₄₁ at a start time of the read-out address signal S₄₃ and produce an extracted write-in address signal. Responsive to the extracted write-in address signal, the coefficient generator 44 calculates first and second coefficients k and (1 - k) in accordance with the extracted write-in address signal and produces first and second coefficient signals S₄₅ and S₄₆ representing the first and the second coefficients k and (1 - k), respectively. The motion vector detector 50 is operable to detect a motion vector for the moving part of the television image in a manner known in the art. In response to the converted signal S₁₁ produced from the non-interlace converter 34 (Fig. 3), the motion vector detector 47 produces first and second motion vector signals and a control signal S₄₇ on detection of the motion vector. Supplied with the converted signal S₁₁, the delay circuit 41 produces a delayed signal having a delay equal to one frame of the converted signal S₁₁. In the absence of the control signal S₄₇, the first selecting circuit 71 selects the converted signal S₁₁ and supplies the processing unit 45 with the converted signal S₁₁. Similarly, the second selecting circuit 72 selects the delayed signal produced from the delay circuit 41 and supplies the processing unit 45 with the delayed signal in the absence of the control signal S₄₇. This means that the frame rate converter 30 carries out the linear frame interpolation process described in conjunction with Fig. 1. The linear frame interpolation process will therefore not be described any longer. Next, description will be made as regards the motion compensated frame interpolation process. The first and the second motion vector signals produced from the motion vector detector 47 are weighted by the first and the second multipliers 51 and 52 in accordance with the first and the second coefficient signals S₄₅ and S₄₆, respectively. Supplied with the first motion vector signal and the first coefficient signal S₄₅, the first multiplier 51 multiplies the first motion vector signal by the first coefficient k and delivers a first weighted motion vector signal to the first motion compensating circuit 61. In response to the second motion vector signal and the second coefficient signal S₄₆, the second multiplier 52 multiplies the second motion vector signal by the second coefficient (1 - k) and delivers a second weighted motion vector signal to the second motion compensating circuit 62. Supplied with the first weighted motion vector signal and the converted signal S₁₁, the first motion compensating circuit 61 carries out a compensating operation for the moving part in accordance with the first weighted motion vector signal and produces a first compensated signal. Similarly, supplied with the delayed signal and the second weighted motion vector signal, the second motion compensating circuit 62 carries out a compensating operation in accordance with the second weighted motion vector signal and produces a second compensated signal. When the motion vector detector 47 produces the control signal S₄₇, the first and the second selecting circuits 71 and 72 select the first and the second compensated signals, respectively, and send the first and the second compensated signals to the processing unit 45. The processing unit 45 comprises first and second multipliers 451 and 452 and an adder 453. Responsive to the first and the second compensated signals and the first and the second coefficient signals S₄₅ and S₄₆, the processing unit 45 carries out a motion compensating frame interpolation process and produces a processed signal. The processed signal is supplied to the memory unit 46. In synchronism with the write-in address signal S₄₁, the memory unit 46 memorizes the processed signal as a stored signal. As described in conjunction with Fig. 1, the memory unit 46 is read out by the read-out address signal S₄₃. Read from the memory unit 46, a read-out television signal is produced as the frame converted signal S₁₂ having 25 frames per second. The television signal frame rate converter according to this invention is applicable to a frame rate conversion between the NTSC system and the SECAM system and between the SECAM system and the PAL system.

Claims: A television signal frame rate converter for converting an input television signal having a first number of frames per second into an output television signal having a second number of frames per second, said converter comprising coefficient generating means (15) for generating first and second coefficient signals representing first and second interpolation coefficients, respectively, processing means (11) for processing two successive frames of said input television signal by a frame interpolation process in accordance with said coefficient signals to produce a processed signal, and memory means (12) for memorizing said processed signal in synchronism with a write-in address signal derived from a first synchronizing signal of said input television signal, said memory means being accessed by a read-out address signal derived from a second synchronizing signal of a reference television signal having said second number of frames per second, characterized by: first signal generating means (142) for generating a first timing signal defined by said write-in address signal; and second signal generating means (172) for generating a second timing signal representing a start time of said read-out address signal in accordance with said read-out address signal; and in which said coefficient generating means is connected to said first and said second signal generating means and generates said first and said second coefficient signals in response to the first timing signal generated when said second timing signal is generated. A television signal frame rate converter for converting an input television signal having a first number of frames per second into an output television signal having a second number of frames per second, said converter comprising coefficient generating means (44) for generating first and second coefficient signals representing first and second interpolation coefficients, respectively, processing means (45) for processing two successive frames of said input television signal by a frame interpolation process in accordance with said coefficient signals to produce a processed signal, and memory means (46) for memorizing said processed signal in synchronism with a write-in address signal derived from a first synchronizing signal of said input television signal, said memory means being accessed by a read-out address signal derived from a second synchronizing signal of a reference television signal having a said second number of frames per second, characterized by: means (42') responsive to said write-in address signal and said read-out address signal for extracting said write-in address signal at a start time of said read-out address signal to produce an extracted write-in address signal; and in which said coefficient generating means is connected to said means and generates said first and said second coefficient signals in response to said extracted write-in address signal. A television signal frame rate converter as claimed in claim 2, wherein said converter further comprises: detecting means (47) responsive to said input television signal for detecting a motion vector by utilizing correlation between said two successive frames to produce a motion vector signal representing said motion vector on detection of said motion vector, said detecting means further producing a control signal together with said motion vector signal; weighting means (50) responsive to said coefficient signal and said motion vector signal for weighting said motion vector signal by said coefficient signal to produce a weighted motion vector signal; compensating means (60) for compensating said two successive frames in accordance with said weighted motion vector signal to produce two successive compensated frames; and selecting means (70) responsive to said two successive frames, said two successive compensated frames, and said control signal, for selecting one of said two successive frames and said two successive compensated frames to deliver a selected one of said two successive frames and said two successive compensated frames to said processing means, said selecting means selecting said two successive compensated frames in the presence of said control signal and selecting said two successive frames in the absence of said control signal.

Inventors: ONOSATO, MASASHI, SHIKINA, CHOKEI, TANAKA, YUTAKA, TSUJI, MASAKAZU, TSUTSUI, TAKEO, YOROZU, MASATOSHO
Applicants: NEC CORPORATION, NIPPON HOSO KYOKAI
Application year: 1988
Main classification: H04N7
IPC codes: H04N7