Signal detecting apparatus

Abstract: A detecting apparatus (1) for a composite signal containing a video signal and an audio signal. A filter (15) for attenuating the video and audio signals is provided in a PLL circuit loop disposed in the detecting apparatus, thus video or audio signal component generated in the output of the PLL circuit (9) can be reduced and a beat component is not generated in the output of the detecting circuit.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a signal detecting apparatus, and is applicable to an intermediate frequency detecting circuit for detecting, for example, a video intermediate frequency signal containing a video and an audio component, used in a color television receiver and so on. DESCRIPTION OF THE RELATED ART A conventional color television receiver receives a vestigial side-band (VSB) broadcasting signal by a tuner and demodulates a video intermediate frequency signal (VIF signal) containing a sound carrier (f A ) and a chrominance subcarrier (f SC ). Thus, a beat signal of, for example, 920 [KHz] is generated proportional to the magnitude of the sound carrier and the chrominance subcarrier. When the beat signal changes the envelope of the carrier chrominance signal, non-linear distortion is generated in the luminance signal and the chrominance signal, which appears as stripes on the screen due to beat interference. To reduce the beat interference, it is sufficient to make the sound carrier and/or chrominance subcarrier small in comparison to the picture carrier in the VIF signal, and then the signal is input to the video intermediate frequency detecting circuit for detection. Therefore, the color television receiver supplies the VIF signal to a surface acoustic wave filter (hereinafter referred to as a SAW filter) for waveform shaping so that the sound carrier and the chrominance subcarrier are significantly attenuated as compared with that in monochrome television. In the NTSC system and the PAL system, upper and lower side bands of the carrier chrominance signal need to be symmetrical when demodulating the chrominance signal, to eliminate crosstalk between the two chrominance signals which are obtained by quadrature two-phase modulation of the chrominance subcarrier. Thus, the SAW filter having characteristics in which the amplitude of the chrominance subcarrier frequency is at -3 [dB] to -6 [dB] from the level is selected, so that the setting range for setting the magnitude of the sound carrier and the chrominance subcarrier is limited. Recently, the television is increased in its size so that high picture quality is required even for ground wave broadcasting. Particularly in the tuner systems, the video characteristics depend on the video intermediate frequency detecting circuit. There exists a video intermediate frequency detecting circuit with an inter-carrier system simply arranged by using a PLL circuit. This type of video intermediate frequency detecting circuit provides the VIF signal, for example, to two multipliers and to a PLL circuit respectively, and provides the output from the PLL circuit to the two multipliers to detect an image. This makes it possible to obtain the video signal from one of the multipliers and an audio signal by demodulating a signal at a difference frequency between the above signal and the signal output by the other multiplier. However, since the margin of the setting range is narrow when the attenuation level of the sound carrier and the chrominance subcarrier is set in the SAW filter as described above, the sound carrier and the chrominance subcarrier remain in the VIF signal which has passed through the SAW filter. In addition, since it is necessary to extract the sound signal while assuring sufficient S/N ratio, the sound carrier is never sufficiently attenuated. Thus, a voltage control oscillator (hereinafter referred to as a "VCO") which forms part of the PLL circuit in the aforementioned video intermediate frequency detecting circuit operates based on the VIF signal in which the sound carrier and the chrominance subcarrier remain, and outputs to the multiplier a switching carrier containing the sound carrier and the chrominance subcarrier. Accordingly, there occurs the problem that the multiplier to which the switching carrier is given produces a detected video signal which contains a beat component having a frequency which is the difference between the sound carrier and the chrominance subcarrier, and the image quality is thus deteriorated. To avoid such a problem, it could be contemplated to reduce the components of the sound carrier and the chrominance subcarrier by increasing carrier purity of the VCO. However, such a method narrows the operating range of the PLL circuit, and causes the disadvantage that it is difficult to compatibly set both the stable operation of the VCO and reduction of the components of the sound carrier and the chrominance subcarrier. Instead of the inter-carrier system, it could also be contemplated to use respective SAW filters exclusively for picture and sound, and to sufficiently reduce the sound carrier in the VIF signal with the picture SAW filter. With this method, it is possible to reduce the beat interference in the picture while maintaining S/N ratio of the audio output signal. However, this method has the disadvantage that the requirement for exclusive SAW filters makes the structure complicated. Also, in this method, since the wiring pattern for high frequency signals becomes complicated in mounting on a board, there arise problems that signals are easily coupled, and that a board is increased in its area when the circuit is implemented in an integrated circuit. Incidentally, the sound carrier in the VIF signal of television broadcasting system M is at 4.5 [MHz], and the sound carriers in the VIF signals of television broadcasting system B and system G are at 5.5 [MHz], respectively. In addition, the chrominance subcarrier of the NTSC system is at 3.58 [MHz] and the chrominance subcarrier of the PAL system is at 4.43 [MHz], respectively. Document US 4 811 096 discloses a video detector employing a PLL system. In this document, a PLL video detector comprises a phase detector circuit receiving a video intermediate frequency signal and an oscillation output of a VCO for phase-detecting the same. The phase of the oscillation output of the VCO is controlled by a signal obtained by smoothing a detection output of the phase detector circuit by a low-pass filter. In addition, a trap circuit having a center frequency of fH is provided between the phase detector and the VCO, so that an unnecessary component having a frequency of fH is removed from an output signal of the low-pass filter. Consequently, the VCO stably oscillates, so that occurrence of a buzz sound in a sound output and degradation of the picture quality can be prevented. OBJECT AND SUMMARY OF THE INVENTION In view of the foregoing, an object of the present invention is to provide a signal detecting apparatus which can reduce the beat components resulting from the sound carrier and the chrominance subcarrier remaining in a video signal detected using a PLL circuit with a simple arrangement. The foregoing object and other objects of the invention have been achieved by the provision of a signal detecting apparatus according to claims 1, 14 or 15. In this apparatus,a filter for attenuating the second signal superposed on the input signal (e.g., a sound carrier component or a chrominance subcarrier component), for example, a sound carrier filter or a chrominance subcarrier filter, is provided in a PLL loop of the PLL circuit. The second signal in the PLL loop is attenuated with the filter allocated in the PLL loop (e.g., the sound carrier filter or the chrominance subcarrier filter) to reduce the sound carrier component to be output from the PLL circuit, so that the beat component resulting from the second signal (the sound carrier and the chrominance subcarrier) which remains in the first signal detected by using the PLL circuit can be easily reduced with a simple arrangement. The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawing. BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing, Fig. 1, is a connection diagram showing a video signal receiving apparatus according to an embodiment of the signal detecting apparatus of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the present invention will now be described with reference to Fig. 1. 1 generally indicates a video signal receiving apparatus, in which a color television broadcasting signal S1 input from an antenna 2 is selected by a tuner 3 and then converted into a VIF signal S2. The VIF signal S2 passes through a SAW filter 4, and is input into a video detecting integrated circuit 5 as a VIF signal S3 in which a sound carrier and a chrominance subcarrier are attenuated to predetermined values. The VIF signal S3 input to the video detecting integrated circuit 5 is amplified and converted into a VIF signal S4 by a video intermediate frequency amplifier 6, and is input to a video detector 7, a sound detector 8, and a PLL circuit 9. The video detector 7 consists of a multiplier, to which an intermediate frequency carrier signal with phase 0°, i.e., a switching carrier S5 is input from the PLL circuit 9 to synchronously detect the video component of the VIF signal S4. This causes the video detector 7 to give a demodulated signal S6 to a video signal amplifier 10 so as to make a video signal S7 output. On the other hand, the sound detector 8 consists of a multiplier, to which a switching carrier S8 with phase 90° is input from the PLL circuit 9 for causing beat down of the VIF signal S4. This causes the sound detector 8 to give a signal S9 to a sound FM detector 11 so as to make the sound signal S10 output. The PLL circuit 9 inputs the VIF signal S4 and the switching carrier S8 output from a VCO 12 into a phase comparator 13 consisting of a multiplier for phase comparison. The phase comparator 13 gives a comparison output signal S11 to a PLL filter 14, and inputs a phase error signal S12 to a trap filter 15. The trap filter 15 gives to the VCO 12 a phase error signal S13 in which the sound carrier and the chrominance subcarrier are attenuated, and controls the oscillation frequencies and the phases of the switching carriers S5 and S8 output from the VCO 12. The characteristics of the trap frequencies of the sound carrier and the chrominance subcarrier to be attenuated by the trap filter 15 are adjusted by an adjuster (not shown) above the video detecting integrated circuit 5. Incidentally, the trap filter 15 may well have uneven characteristics due to production error; this is required to be adjusted. In this case, as an adjuster of the trap filter 15, a self-adjusting system is used which is disposed in the video detecting integrated circuit 5 for adjusting characteristics of other filters. Thus, there is no need to provide an adjusting system exclusively used for the trap filter 15. In the above arrangement, the PLL circuit 9 applies a feedback loop to an oscillation output of the VCO 12 so as to synchronize with the phase of a video intermediate carrier component of the VIF signal S4. The VIF signal S4 contains the sound carrier and the chrominance subcarrier which are also contained in the comparison output signal S11. When the comparison output signal S11 passes through the trap filter 15, the phase error signal S13 is output in which the sound carrier and the chrominance subcarrier are sufficiently attenuated. Thus, the control of the VCO 12 by the phase error signal S13 causes significant attenuation of the sound carrier and the chrominance subcarrier generated in the switching carriers S5 and S8 as compared with that in the prior art. Accordingly, if a video component is detected by multiplying the switching carrier S5 with the VIF signal S4, no beat component determined by the frequency of f A - f SC (beats of 920 [KHz] in the system M and the NTSC system in Japan and North America) would be generated even if the sound carrier and the chrominance subcarrier components exist in the VIF signal S4. With the above arrangement, the sound carrier and the chrominance subcarrier in the PLL loop are attenuated by the trap filter 15 disposed in a stage before the VCO 12 in the PLL loop, in order to reduce the sound carrier and the chrominance subcarrier components generated in the switching carrier S5 in the PLL circuit 9, so that the beat component of 920 [KHz] resulting from the sound carrier and the chrominance subcarrier, both of which remain in the demodulated signal S6 detected by using the PLL circuit 9, can be easily reduced. In addition, since the beat component of 920 [KHz] which is difficult to eliminate after detection can be reduced in the video detecting integrated circuit 5, there is no need to add external parts to the video detecting integrated circuit 5. Moreover, since the phase error signal S12 passes through the trap filter 15, the sound carrier and the chrominance subcarrier can be easily attenuated with simpler construction as compared with the case where the high frequency signal passes through, which requires a complicated wiring arrangement; thus the characteristics of the video signal S7 can be improved as described above. In the embodiment described above, the trap filter 15 is disposed in a stage before the VCO 12. However, the present invention is not limited only to this, instead the filter position for attenuating the sound carrier and the chrominance subcarrier can be at an arbitrary location as long as it is in the PLL loop. Also in this case, the aforementioned effect can be attained. Incidentally, the beat component of 920 [KHz] between the sound carrier and the chrominance subcarrier can also be reduced even by inserting a filter for attenuating the sound carrier and the chrominance subcarrier between the VCO 12 and the video detector 7. However, in this case, its characteristics should be established quite carefully because the switching carrier S5 is at a high frequency range. Also, in the embodiment described above, the sound carrier and the chrominance subcarrier are simultaneously attenuated by the trap filter 15. However, the present invention is not limited just to this, but can be applied to a case where only the sound carrier is sufficiently attenuated by a filter. Furthermore, as the trap filter 15 in the aforementioned embodiment, both an active filter and a passive filter can be used. In this case, the filter and the other circuits may have either a bipolar transistor circuit construction or a field-effect transistor circuit construction. As described above, according to the present invention, the first signal in the PLL loop is attenuated by the filter disposed in the PLL loop to reduce the first signal component output from the PLL circuit, so that the signal detecting apparatus can be realized which can easily reduce the beat component remaining in the second signal detected by using the PLL circuit. While there has been described in connection with the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made. It is intended, therefore, to cover in the appended claims all such changes and modifications as fall within the true scope of the invention.

Claims: A signal detecting apparatus (1) having a PLL circuit (9) for receiving an input signal containing a first signal and a second signal, and a detecting circuit (7) for detecting said first signal from said input signal based on an output from said PLL circuit (9), the signal detecting apparatus further comprising a filter (15), for attenuating said second signal component contained in said input signal, in a PLL loop of said PLL circuit (9), characterised in that it further comprises a second detecting circuit (8) for detecting said second signal from said input signal based on a second output from said PLL circuit (9). The signal detecting apparatus according to claim 1, wherein a filter (15) for attenuating a third signal component contained in said first signal is provided in said PLL loop of said PLL circuit (9). The signal detecting apparatus according to claim 1 or 2, wherein said filter (15) for attenuating said second signal component is disposed in a stage before an oscillator (12) of said PLL circuit (9). The signal detecting apparatus according to claim 2, wherein said filter (15) for attenuating said third signal component is disposed in a stage before the oscillator (12) of said PLL circuit (9). The signal detecting apparatus according to claim 2 or 4, wherein a common filter (15) for attenuating said second and third signal components is disposed in a stage before the oscillator (12) of said PLL circuit (9). The signal detecting apparatus according to any previous claim, wherein: said first signal is a composite signal; and said second signal component is a chrominance subcarrier component. The signal detecting apparatus according to any of claims 1 to 5, wherein: the input signal is a VIF signal; and the second signal component is a sound carrier component. The signal detecting apparatus according to claim 2 or 4, wherein the third signal component is a chrominance subcarrier component. The signal detecting apparatus according to any of claims 1 to 5 implemented as an integrated circuit, wherein: the input signal is a VIF signal; the first signal is a video signal; the second signal component is a sound carrier component and the filter for attenuating said second signal component is a bipolar active filter. The signal detecting apparatus according to claim 2 or 4, wherein the third signal component is a chrominance subcarrier component and the filter (15) for attenuating said third signal component is a bipolar active filter. The signal detecting apparatus as in any of claims 1 to 10, said first and second output signals from said PLL circuit having different phases. The signal detecting apparatus as in claim 11, a phase difference between said first and second output signals being 90°. The signal detecting apparatus according to any of claims 1 to 12, further comprising a SAW filter (4) receiving said input signal. A signal detecting apparatus, comprising: an input terminal for receiving an input signal containing a video signal and an audio signal; an oscillator (12) for receiving a control signal and generating first and second output signals according to said control signal, a first calculating circuit (13) for receiving said input signal and the first output signal from said oscillator (12) and outputting a calculated signal; a filter (14) for attenuating high frequency of the output signal from said first calculating circuit (13); a trap filter (15) in which a trap is disposed within the frequency band range of said input signal, for receiving the output signal from said filter (14); a second calculating circuit (7) for receiving and performing a calculation on said second output signal output from said oscillator (12) and said input signal when an output signal from said trap filter (15) is input to said oscillator (12) as said control signal; and a third calculating circuit (8) for inputting and performing a calculation on said input signal and the first output signal of said oscillator (12). A signal detecting apparatus, comprising: an input terminal for being supplied an input signal containing a video signal and an audio signal; an oscillator (12) for being supplied a control signal and generating first and second output signals according to said control signal, a first calculating circuit (13) for receiving said input signal and the first output signal from said oscillator (12) and outputting a calculated signal; a filter (14) for producing an output signal obtained by attenuating high frequency of the output signal from said first calculating circuit (13), said output signal of said filter (14) being used as control signal for the oscillator (12) ; a filter (15) in which a trap is disposed within the frequency band range of said input signal, for receiving said second output signal of said oscillator (12); a second calculating circuit (7) for receiving and performing a calculation on the output signal from said trap filter (15) and said input signal; and a third calculating circuit (8) for receiving and performing a calculation on said input signal and the first output signal of said oscillator (12). The signal detecting apparatus according to claim 14 or 15, wherein the trap frequency of said trap filter (15) is set to a chrominance subcarrier frequency of said video signal. The signal detecting apparatus according to claim 14 or 15, wherein the trap frequency of said trap filter (15) is set to a sound carrier frequency. The signal detecting apparatus according to claim 14 or 15, wherein the trap frequency of said trap filter (15) is set to a chrominance subcarrier frequency of said video signal and a sound carrier frequency. The signal detecting apparatus according to any of claims 14 to 18, wherein said first and second output signals of said oscillator (12) are made different from each other in their phases. The signal detecting apparatus as in claim 19, a phase difference between said first and second output signals being 90°. The signal detecting apparatus according to any of claims 14 to 20, wherein said first calculating circuit (13) is used as a phase detecting circuit. The signal detecting apparatus according to any of claims 14 to 21, wherein: said second calculating circuit (7) is used as a video signal synchronous detecting circuit; and said third calculating circuit (8) is used as a sound detecting circuit. The signal detecting apparatus according to any of claims 14 to 22 implemented as transistor integrated circuit, wherein said filter (14) and said trap filter (15) are active filters. The signal detecting apparatus according to claim 23, wherein the trap filter has self-adjusting operation for adjusting filter characteristics of said filter. The signal detecting apparatus as in any of claims 14 to 24, further comprising a SAW filter (4) for receiving said input signal.

Inventors: NATSUME YOSHITAKA, NATSUME, YOSHITAKA, NATSUME, YOSHITAKA, C/O SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1995
Main classification: H04N5
IPC codes: H04N5, H03D1