Horizontal synchronizing signal separation circuit

Abstract: In a horizontal synchronizing signal separation circuit, from a composite synchronizing signal which includes a horizontal synchronizing signal and a vertical synchronizing signal, a detection signal indicative of detecting a rising edge of said composite synchronizing signal is generated. A gate is provided through which the detection signal is passed in accordance with a control signal. The control signal is a pulse signal having a predetermined pulse width, and generated by a circuit to which the output of the gate is input. The output of the gate is also supplied to another circuit which generates a separated horizontal synchronizing signal.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a horizontal synchronizing signal separation circuit, and more particularly to a horizontal synchronizing signal separation circuit for extracting the timing of a horizontal synchronizing signal from a composite synchronizing signal which includes a horizontal synchronizing signal and a vertical synchronizing signal. Hereinafter, the term "synchronizing signal" is abbreviated as "sync signal". 2. Description of the Prior Art: In a television system such as the NTSC (National Television System Committee) system or the PAL (Phase Alternation Line) system, only a composite sync signal in which a vertical sync signal and horizontal sync signal are combined is supplied as a sync signal to a television apparatus. In a matrix type liquid crystal display ( LCD ) apparatus widely used in recent years, a clock signal for sampling image signals in the LCD apparatus is generated. Since this clock signal must be precisely synchronous with the horizontal sync signal when the display is performed based on the image information in the above-mentioned television system, the clock signal is generated using a PLL (Phase-Locked Loop) circuit 100 as shown in Figure 9. The PLL circuit 100 has a loop which comprises a voltage-controlled oscillator ( VCO ) 101 , a frequency divider 102 , a phase comparator 103 and a low-pass filter ( LPF ) 104 . It is desirable to supply the horizontal sync signal as the sync signal Sync which is the input signal of the PLL circuit 100 . In the prior art, however, the composite sync signal is supplied as it is. Figures 10A to 10C show composite sync signals used in the NTSC system. Figure 10A shows a composite sync signal in a transition period from an even field to an odd field. Figure 10B shows a portion of a composite sync signal in one field. In Figure 10C, a composite sync signal in a transition period from an odd field to an even field. As shown in Figures 10A and 10C, in addition to the horizontal sync signal 21 , a vertical sync signal and an equalizing pulse 22 exist in the composite sync signal in the transition period from one field to the next field. The equalizing pulse 22 is inserted to equalize the waveforms of the composite sync signal in the portion of the vertical sync signal and in the peripheral portion thereof, during the transition period from the even field to the odd field and during the transition period from the odd field to the even field. In the peripheral portion of the vertical sync signal, the width of the horizontal sync signal 21 and that of the equalizing pulse 22 are half as compared with that of the normal horizontal sync signal 21 . In the prior art, since such a composite sync signal is input to the PLL circuit 100 (Figure 9) as it is, the phase relationship in the PLL circuit 100 is disturbed due to the presence of the vertical sync signal and equalizing pulse in the composite sync signal shown in Figures 10A and 10C. This disturbance causes the oscillation frequency of the VCO 101 to fluctrate. If this fluctuation in the oscillation frequency of the VCO 101 is not absorbed even when entering a display period during which image information for the display area is supplied to the LCD apparatus, a problem arises in that the resulting image is distorted. In order to avoid the image distortion from occurring, it is necessary to absorb the fluctuation in the oscillation frequency of the VCO 101 during a period prior to the above display period (i.e., a vertical retrace line interval). This is a major hindrance in the simplification of the design of the PLL circuit for a matrix type display apparatus such as an LCD apparatus. Further, in some prerecorded video tapes which are commercially available, an AGC (Auto Gain Control) signal for the luminance signal is purposely inserted in the composite video signal in order to destabilize the playback of video tapes produced by copying the prerecorded video tapes. In the playback of these video tapes, the AGC signal cannot be completely removed by a low-pass filter in the extraction of the composite sync signal from the composite video signal, and therefore, a pulse (which may be a spurious sync signal such as that shown in Figure 11) is mixed in immediately after the vertical sync signal in the extracted composite sync signal. When a composite sync signal in which the spurious sync signal exists immediately before the display period as shown in Figure 11 is input to the PLL circuit 100, it is practically impossible to stabilize the PLL circuit 100 which is disturbed by the spurious sync signal, before the display period. In order to solve this problem in the prior art, such measures as narrowing the area in which the image is actually displayed on the display screen are employed, but it is difficult to completely hide the image distortion appearing in the upper portion of the display screen, and satisfactory display can not be obtained in many cases. SUMMARY OF THE INVENTION This invention, as defined by claim 1, provides a horizontal synchronizing signal separation circuit for producing a derived horizontal sync signal from a composite sync signal which includes vertical and horizontal sync signals, comprising means for generating a detection pulse in response to a signal edge in said composite sync signal, a gate which is coupled to receive said detection pulse and which is controlled by a control signal to selectively output said detection pulse, and control signal generation means for generating said control signal, characterized in that said control signal generation means comprises a monostable multivibrator which receives the detection pulses passed by said gate and generates a pulse signal containing a pulse of predetermined width in response to each received detection pulse wherein said predetermined pulse width is greater than one half of one horizontal scanning period, and an inverter which inverts said pulse signal to form said control signal, and by pulse signal generation means which produces said derived horizontal sync signal by generating pulses in response to the detection pulses output by said gate. The preamble of claim 1 relates to the disclosure of GB-A-2 151 422. The dependent 2 to 5 are directed to preferred features of the invention. According to the invention, a horizontal synchronizing signal separation circuit can be provided which correctly extracts the timing of a horizontal sync signal from a composite sync signal. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram showing a horizontal sync signal separation circuit in a first embodiment of the invention. Figures 2 to 4 are timing charts illustrating the operation of the first embodiment. Figure 5 is a circuit diagram showing a second embodiment of the invention. Figure 6 is a circuit diagram showing a third embodiment of the invention. Figure 7 is a timing chart illustrating the operation of the third embodiment of the invention. Figure 8 is a timing chart showing a horizontal sync signal separated from a composite sync signal according to the invention. Figure 9 is a block diagram showing a PLL circuit for generating a sampling clock signal used in a prior art LCD apparatus. Figures 10A to 10C show composite sync signals used in the NTSC system. Figure 11 shows a composite sync signal obtained from a video tape which was subjected to a duplication prevention process. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 is a block diagram of a first embodiment of the invention. A composite sync signal C sync is input to a rising edge detection circuit 1 . When detecting the rise of the composite sync signal C sync , the rising edg. detection circuit 1 generates a pulse signal HED which is then supplied to one input terminal of an AND gate 2 . A control signal TPF is input to the other input terminal of the AND gate 2 , and the pulse signal HED passes through the AND gate 2 when the control signal TPF is HIGH . The pulse signal HED that has passed through the AND gate 2 is input to two monostable multivibrators 3 and 5 . The monostable multivibrator 3 is triggered by the pulse signal HED that has passed through the AND gate 2 and generates a pulse PULSE1 with a predetermined pulse width. The pulse PULSE1 output from the monostable multivibrator 3 is logically inverted by an inverter 4 , and the pulse output from the inverter 4 is supplied to the AND gate 2 as the control signal TPF . The monostable multivibrator 5 is triggered by the pulse signal HED that has passed through the AND gate 2 and generates a pulse with a predetermined pulse width. The pulse output from the monostable multivibrator 5 becomes the separated horizontal sync signal H syn . The time of rise of the separated horizontal sync signal H syn is essentially the same as the time when the pulse signal HED passes through the AND gate 2 . The separated horizontal sync signal H syn becomes an input signal for the PLL circuit 100 shown in Figure 9 . Next, the operation of this embodiment is described. The timing chart of Figure 2 shows signals in various parts of the horizontal sync signal separation circuit in Figure 1, when the separated horizontal sync signal H syn is synchronous with a horizontal sync signal 21 in the composite sync signal C sync . The rising edge detection circuit 1 outputs the pulse signal HED at the rise of the composite sync signal C sync . The pulse PULSE1 is output from the monostable multivibrator 3 in response to the pulse signal HED that has passed through the AND gate 2 . The pulse width of the pulse PULSE1 is set to be longer than one half (H/2) of the horizontal scanning period H and shorter than the horizontal scanning period H (H/2 < PULSE1 < H). The pulse PULSE1 is inverted by the inverter 4 and supplied to the AND gate 2 as the control signal TPF . The fall of the control signal TPF is slightly delayed behind the generation of the pulse signal HED due to the delay caused by the monostable multivibrator 3 and the inverter 4 . Therefore, the control signal TPF is already HIGH when the pulse signal HED is input to the AND gate 2 , thus allowing the pulse signal HED to pass through the AND gate 2 . The separated horizontal sync signal H syn is output from the monostable multivibrator 5 in response to the pulse signal HED that has passed through the AND gate 2 . The width of the signal H syn is set to be approximately equal to that of the horizontal sync signal 21 . Since the pulse width of the pulse PUSLE1 is set as described above, the control signal TPF is LOW when the pulse signal HED corresponding to the equalizing pulse 22 is generated. As a result, this pulse signal HED will not pass through the AND gate 2 . Therefore, the signal H syn is not generated at the timing of the equalizing pulse 22 , and the synchronizing condition between the separated horizontal sync signal H syn and the horizontal sync signal 21 is not disturbed by the equalizing pulse 22 . The process in which the horizontal sync signal separation circuit in Figure 1 is synchronized with the horizontal sync signal 21 in the composite sync signal C sync is described by referring to Figure 3. Figure 3 shows the composite sync signal C sync which is in a transition period from an even field to an odd field. The output of the monostable multivibrator 3 is LOW in its initial state. As shown in Figure 3, the pulse signal HED generated at timing T1 of the equalization pulse 22 in the vertical sync signal passes through the AND gate 2 , so that the pulse PULSE1 is generated. The control signal TPF becomes LOW at this time, and therefore, the pulse signal HED generated at timing T2 of the next rising edge of the composite sync signal C sync (i.e., at the timing of the horizontal sync signal 21 ) cannot pass through the AND gate 2 . Thereafter, the control signal TPF becomes HIGH as the pulse PULSE1 returns to LOW, and the pulse signal HED generated at timing T3 which is the timing of the equalization pulse 22 passes through the AND gate 2 . The pulse PULSE1 is made HIGH again by this pulse signal HED . The same operation is repeated. Of the pulse signals HED corresponding to the timings of the rise of the composite sync signal C sync , those at timings T5 , T7 , T9 and T11 pass through the AND gate 2 , but those at timings T4 , T6 , T8 , T10 and T12 do not pass through the AND gate 2 . In the example of Figure 3, the timings T5 , T7 , T9 and T11 are the timings of the equalization pulse 22, and the timings T4 , T6 , T8 , T10 and T12 are the timings of the horizontal sync signal 21. Therefore, up to the timing T12 , the horizontal sync signal separation circuit in Figure 1 is not synchronous with the horizontal sync signal 21 . However, since the equalization pulse 22 does not exist after the timing T12 , the control signal TPF keeps HIGH until the timing T13 . Therefore, the pulse signal HED corresponding to the horizontal sync signal 21 at the timing T13 passes through the AND gate 2 , and the pulse PULSE1 is generated at that timing. Thereafter, the pulse PULSE1 is generated at the timing of the horizontal sync signal 21 . That is, after the timing T13 , the horizontal sync signal separation circuit is synchronous with the horizontal sync signal 21 , and therefore, the separated horizontal sync signal H syn is also synchronous with the horizontal sync signal 21 . As can be seen from Figure 3 and the above description, regardless of the timing by which the pulse PULSE1 is initially generated, the separated horizontal sync signal H syn is synchronous with the horizontal sync signal 21 not later than the point of time when the period during which there is no equalization pulse 22 of the composite sync signal C sync (i.e., the display period) starts. The operation of this embodiment is further described with reference to Figure 4. Figure 4 shows signals in various parts of the horizontal sync signal separation circuit in Figure 1, in a transition period from an even field to an odd field. In Figure 4, the separated horizontal sync signal H syn is in synchronous with the horizontal sync signal 21 in the even field. Therefore, in the even field, the pulse signal HED corresponding to the equalization pulse 22 does not pass through the AND gate 2 because the control signal TPF is LOW. The pulse signal HED corresponding to the horizontal sync signal 21 passes through the AND gate 2 . The pulse signal HED that has passed through the AND gate 2 maintains the synchronizing condition between the horizontal sync signal separation circuit, and triggers the monostable multivibrator 5 to generate the separated horizontal sync signal H syn . As shown in Figure 4, the synchronizing condition between the separated horizontal sync signal H syn and the horizontal sync signal 21 is maintained even in the odd field. In this manner, according to this embodiment, the vertical sync signal and equalization pulse 22 can be removed from the composite sync signal C sync , and the separated horizontal sync signal H syn synchronous with the horizontal sync signal 21 can be extracted alone. Figure 5 shows a second embodiment of the invention. In Figure 5, components similar to those of the horizontal sync signal separation circuit in Figure 1 are designated by the same reference numerals as those used in Figure 1. In this embodiment, the rising edge detection circuit 1 uses a CR circuit comprising a resistor 151 and a capacitor element 152 in order to detect the rising edge of the composite sync signal C sync . In addition to the CR circuit, the rising edge detection circuit 1 also comprises a buffer 153 , an inverter 154 and an AND gate 155 . The pulse width of the pulse PULSE1 output from the monostable multivibrator 3 is determined by the value C₁·R₁ obtained from the resistance R₁ of a resistor 351 and the capacitance C₁ of a capacitor 352 connected to the monostable multivibrator 3 . Further, the pulse width of the separated horizontal sync signal H syn is determined by the value C₂·R₂ obtained from the resistance R₂ of a resistor 551 and the capacitance C₂ of a capacitor 552 connected to the monostable multivibrator 5 . Figure 6 shows a third embodiment of the invention. In Figure 6, components similar to those of the horizontal sync signal separation circuit in Figure 1 are designated by the same reference numerals as those in Figure 1. In this embodiment, at the output side of the monostable multivibrator 5 there is provided an AND gate 6 to which the pulse PULSE2 output from the monostable multivibrator 5 and the composite sync signal C sync are input. The output of the AND gate 6 is the separated horizontal sync signal H syn . The pulse width of the pulse PULSE2 is made slightly larger than that of the horizontal sync signal 21 contained in the composite sync signal C sync by appropriately selecting the resistance R₃ of a resistor 561 and the capacitance C₃ of a capacitor 562 connected to the monostable multivibrator 5 . Therefore, only the horizontal sync signal 21 contained in the composite sync signal C sync passes through the AND gate 6 in the period during which the pulse PULSE2 is output from the monostable multivibrator 5 . As shown in Figure 7, the pulse width of the horizontal sync signal 21 contained in the composite sync signal C sync is accurately reflected in the separated horizontal sync signal H syn in this embodiment, and a horizontal sync signal that is more faithful than in the circuit in Figure 5 can be extracted. Figure 8 shows a composite sync signal C sync into which a spurious sync signal similar to that in Figure 11 is mixed, and the separated horizontal sync signal H syn obtained from this composite sync signal C sync by the horizontal sync signal separation circuit in Figure 5 . As can be seen from Figure 8, the timing of the horizontal sync signal can be completely extracted according to the invention, even if the composite sync signal C sync is deformed by a spurious sync signal. According to the invention, a horizontal sync signal separation circuit that can extract the timing of a horizontal sync signal from a composite sync signal is provided. The timing of the horizontal sync signal can be extracted in the same way, even if the composite sync signal contains an AGC signal for the luminance signal. The horizontal sync signal separated from the composite sync signal by the horizontal sync signal separation circuit of the invention at the timing of the horizontal sync signal may be used as the input of a PLL circuit. Therefore, a sampling clock signal can be obtained that is much more stable than in the case where the unchanged composite sync signal is used as the input to the PLL circuit, whereby a stable good display can be attained in a matrix type display apparatus such as an LCD apparatus. A particularly good effect can be attained in the special effect playback of a video tape such as in fast forwarding and in the playback of a video tape which was subjected to a duplication prevention process. Furthermore, the output of the horizontal sync signal separation circuit can be used as the input of the PLL circuit for generating the sampling clock signal in a matrix type display apparatus, whereby the design of such PLL circuits can be simplified vis-à-vis that in the prior art.

Claims: A horizontal synchronizing signal separation circuit for producing a derived horizontal sync signal ( H syn ) from a composite sync signal ( C syn ) which includes vertical ( 21,22 ) and horizontal ( 21 ) sync signals, comprising means ( 1 ) for generating a detection pulse ( HED ) in response to a signal edge in said composite sync signal, a gate ( 2 ) which is coupled to receive said detection pulse and which is controlled by a control signal ( TPF ) to selectively output said detection pulse, and control signal generation means ( 3,4 ) for generating said control signal ( TPF ), characterized in that said control signal generation means comprises a monostable multivibrator ( 3 ) which receives the detection pulses ( HED ) passed by said gate ( 2 ) and generates a pulse signal ( PULSE1 ) containing a pulse of predetermined width in response to each received detection pulse wherein said predetermined pulse width is greater than one half of one horizontal scanning period, and an inverter ( 4 ) which inverts said pulse signal to form said control signal ( TPF ), and by pulse signal generation means ( 5 ) which produces said derived horizontal sync signal ( H syn ) by generating pulses in response to the detection pulses output by said gate ( 2 ). A horizontal synchronizing signal separation circuit according to claim 1, wherein said pulse signal generation means comprises a monostable multivibrator ( 5 ). A horizontal synchronizing signal separation circuit according to claim 2, further comprising a resistor ( 551 ) and a capacitor ( 552 ) connected to said monostable multivibrator ( 5 ), and wherein said derived horizontal sync signal ( H syn ) has a pulse width determined by said resistor and said capacitor. A horizontal synchronizing signal separation circuit according to claim 2, wherein said pulse signal generation means further comprises an AND gate ( 6 ) having a first input which receives the pulse output ( PULSE2 ) of said monostable multivibrator ( 5 ) and a second input which receives said composite sync signal ( C sync ), the output of the AND gate ( 6 ) comprising said derived horizontal sync signal ( H syn ). A horizontal synchronizing signal separation circuit according to claim 4, further comprising a resistor ( 561 ) and a capacitor ( 562 ) connected to said monostable multivibrator ( 5 ), and wherein the pulse ( PULSE2 ) output by said monostable multivibrator ( 5 ) has a width determined by said resistor and said capacitor which is slightly larger than that of the horizontal sync signal ( 21 ) contained in said composite sync signal ( C sync ).

Inventors: OKADA HISAO, TANAKA KUNIAKI, UEHIRA SHIGEYUKI, OKADA, HISAO, TANAKA, KUNIAKI, UEHIRA, SHIGEYUKI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1991
Main classification: H04N5
IPC codes: H04N5