JPH0362075B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video synchronous detection device suitable for use in a television receiver, and relates to a video carrier signal extracted from the previous stage of a video intermediate frequency amplification circuit and an output signal of the video intermediate frequency amplification stage. By converting the frequency and suppressing local jitter from tuners, etc., and detecting the signal using a synchronous detection circuit, accurate and reliable phase detection is performed, and the operation of the video intermediate frequency filter is utilized in an optimal state. The purpose of the present invention is to provide a video synchronous detection device that can perform the following functions. In television receivers currently on the market, a tuner converts the frequency of a television signal received by an antenna into a video intermediate frequency. This video intermediate frequency signal is passed through a video intermediate frequency filter, amplified by a video intermediate frequency amplification circuit, and then supplied to a video detection circuit. This video detection circuit demodulates a video signal from a video intermediate frequency signal, and this video detection circuit employs two types of methods. One of these methods performs envelope detection using a diode detection method, and the other method is a synchronous detection method using a double-balanced configuration or the like. By the way, television radio waves have residual sideband characteristics in which most of the lower sideband is suppressed in order to improve frequency utilization, and on the receiver side, Nyquist characteristics are used to obtain flat frequency characteristics during demodulation. I'm leaning on it. This Nyquist characteristic is obtained by a video intermediate frequency filter, and the frequency characteristics and envelope delay characteristics are determined by the performance of this filter.
By the way, on May 1, 1971, Kenrokukan Publishing Co., Ltd. published "Broadcasting Technology May 1947 Vol. 25 No.
In the paper titled ``Performance of video demodulator for measurement'' described on pages 68 to 78 of ``5'', the envelope of the modulated wave with residual sideband characteristics is no longer similar to the modulated signal, For this reason, it is stated that orthogonal distortion cannot be avoided in the envelope detection method. Then, the carrier is attenuated to 1/2 by a video intermediate frequency filter and envelope detection is performed, which increases the apparent degree of modulation and generates large orthogonal distortion, and this carrier receives phase modulation as well as amplitude modulation. It is also stated that this causes deterioration of the differential phase in the envelope detection output. Since the details are described in the aforementioned paper, a detailed explanation will be omitted here, but it can be seen that there are various problems inherent in the envelope detection method. In order to completely eliminate this orthogonal distortion, it is said that there is no suitable method other than employing a synchronous detection method. This synchronous detection method is the latter of the two methods described above. This synchronous detection method creates a carrier wave for demodulation that has the same frequency and phase as the carrier wave of the modulated wave, and extracts the signal wave from the multiplicative product with the modulated wave through a low-pass filter, but there are problems with this synchronous detection method. The point is that it is a method of obtaining synchronous carrier waves, and it is argued that it has drawbacks such as a complicated circuit. In other words, the synchronous detection method also has various problems and cannot be called the best method. However, this synchronous detection method is extremely advantageous in integrating circuits, and has been adopted as a pseudo-synchronous detection circuit in recent integrated circuits for video intermediate frequency amplification and video detection.
This pseudo-synchronous detection circuit has generally obtained good results in current receivers, but in receivers for audio multiplex broadcasting, where the number of full-scale broadcasting channels is expected to increase further in the future, video buzz due to differential phase characteristics may occur. Noise is generated and further improvements need to be made. In addition to audio multiplex broadcasting, which requires high quality and high performance, we have also made further improvements to TV receivers that require similar performance, such as teletext broadcasting and anti-ghosting. Performance is required. In particular, with the currently used quasi-synchronous detection method, in addition to the above-mentioned problem, the output waveform of the detection circuit may fold back during overmodulation, and therefore, in a receiver with ghost countermeasures, this folding phenomenon may occur. In this case, not only the ghost cannot be removed, but also a ghost component is superimposed on the image, which in turn deteriorates the quality of the image receiving screen. As a countermeasure to this problem, a method of performing synchronous detection using a phase-controlled circuit type oscillator has been developed. For example, in the ``Television Society Journal 1978, February issue'' published by the Television Society on February 1, 1978,
It is described in detail in the paper titled ``Video Demodulator Using Follow-up Phase Synchronous Detection Method'' published on pages 126 to 132. According to this paper, in television broadcasting equipment, the carrier wave is attached to the amplitude modulation of the video signal.
It may undergo phase modulation called AM-PM conversion, and when receiving TV broadcast waves that have undergone such AM-PM conversion, the carrier wave of the synchronization signal part of the received signal is extracted and this is generated as the reference phase. It is said that in synchronous detection using a synchronous carrier wave, the distortion of the differential phase characteristic is greater than that in envelope detection. In order to further improve this synchronous detection method, we developed a tracking phase synchronous detection method in which the phase of the synchronous carrier wave follows the phase modulation of the received signal.
A method is described in which the distortion of the differential phase characteristic due to PM conversion is the same as that of envelope detection, and which eliminates the orthogonal distortion and nonlinear type that are disadvantages of envelope detection. Although these systems have good characteristics as a single detector, there are inherent problems that need to be solved when viewed as a whole system. In other words, as mentioned above, this method would be extremely effective when applied to television receivers that are currently widely used, but it is difficult to reproduce images with higher quality and fidelity. Sometimes it is necessary to further eliminate the distortion of the differential phase characteristics caused by this AM-PM conversion. Therefore, although the method described in this paper has great advantages compared to conventional methods, it is necessary to further improve the differential phase characteristic distortion by AM-PM conversion in the transmitter. However, they do not necessarily have sufficient performance to fully satisfy the demands of television receivers with higher quality differential gain, differential phase characteristics, and waveform distortion, and further improvements are required. The present invention focuses on these points and provides an improved video synchronous detection device, in which the video intermediate frequency signal from the tuner is split into two, and one of the video intermediate frequency signals is sent to the video intermediate frequency amplification stage. The other branched video intermediate frequency signal is supplied to the second mixing circuit via the carrier amplifier circuit, and the second mixing circuit amplifies the oscillation signal from the oscillation circuit. After frequency conversion, the signal component corresponding to the carrier signal is extracted by passing through a narrow band band pass filter and limiter circuit, and this carrier signal is added to the first mixing circuit and frequency converted with the video intermediate frequency signal. This conversion output is added to a synchronous detection circuit, and the oscillation signal of the oscillation circuit is synchronously detected with a phase-shifted signal, thereby suppressing local oscillation jitter that occurs in tuners, broadcasting stations, etc. The object of the present invention is to obtain a video synchronous detection device that is faithful to a transmission signal on the transmitting side and that has improved various defects by performing synchronous detection. Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 shows the basic configuration of a video coherent detection device according to the present invention. An antenna 1 receives television broadcast waves having vestigial sideband characteristics from the transmitting side. It is supplied to tuner 2. In this tuner 2, a desired channel is selected by a frequency synthesizer type channel selection device 3, and the high frequency television signal is converted into a video intermediate frequency signal, and the signal shown in FIG. 2a is output. This video intermediate frequency signal is branched into two, and one of the video intermediate frequency signals is supplied to a video intermediate frequency filter 5 constituting a video intermediate frequency amplification stage 4 . This filter 5 is configured to have a Nyquist characteristic and obtains the necessary frequency characteristics. The output of this filter 5 is supplied to the video intermediate frequency amplification and automatic gain control circuit 6 constituting the video intermediate frequency amplification stage 4, where it is amplified as shown in FIG. 2b, and then supplied to the first mixing circuit 7. be done. On the other hand, the other side video intermediate frequency signal of the two-branched output of the tuner 2 is transmitted to a carrier amplification and automatic gain control circuit 8.
The output is amplified and controlled to produce the output shown in FIG. 2c, which is supplied to the second mixing circuit 9. This mixing circuit 9 is supplied with a reference oscillation signal from an oscillation circuit 10, frequency-converted by the mixing circuit 9, and then supplied to a narrowband bandpass filter 11. Only the video carrier signal component (actually, as described later, the oscillation signal of the oscillation circuit and jitter component are also included) is extracted. Figure 2 d shows the waveform as a horizontal component, and Figure 2 e shows the waveform as a vertical component.Although most of the horizontal signal has been removed, the vertical signal component still remains as shown in Figure 2 e. are doing. This video carrier signal component is further supplied to a limiter circuit 12, where the signal is clipped at a predetermined level, and the output shown in FIG. 2f from which the vertical signal component is removed is obtained. This narrowband bandpass filter 11 and limiter circuit 12
The video carrier signal passed through the video intermediate frequency amplification stage 4 is added to the first mixing circuit 7 and mixed with the signal passed through the video intermediate frequency amplification stage 4 . The output of this mixing circuit 7 is applied to a synchronous detection circuit 13. On the other hand, the phase of the oscillation signal of the oscillation circuit 10 is set by the phase shift circuit 14 to become a waveform signal shown in FIG. The signal is detected and the video signal is reproduced on the output side. The carrier output from the limiter circuit 12 also has substantially the same signal waveform as shown in FIG. 2g. With the above configuration , the video carrier signal of the video intermediate frequency amplification stage 4 is now f _p , the oscillation frequency of the oscillation circuit 10 is f ₁₀ , the frequency jitter component in the tuner 2 is Δf, and the video signal is fv. Then, the output of the tuner 2 becomes f _p +fv+Δf, and the output of the video intermediate frequency amplification stage 4 also becomes f _p +fv+Δf. On the other hand, if we consider the case where the second mixing circuit 9 is of the down-conversion type, f _p +fv + △f and f ₁₀ are supplied to this mixing circuit 9, so the output of the mixing circuit 9 is f _p A signal of +fv+Δf−f ₁₀ is obtained.
Then, since the video signal fv is removed by the narrowband bandpass filter 11, the limiter circuit 12
The signal supplied to the first mixing circuit 7 through f _p
+△f−f ₁₀ . Therefore, this mixing circuit 7 receives f _p +fv+△f from one side and f _p +△ from the other side.
Since f−f ₁₀ is added, if this mixing circuit 7 is configured in a down-conversion method, the output of the mixing circuit 7 is f _p +fv+△f−(f _p +△f−
f ₁₀ ), which results in a signal of fv+f ₁₀ . In other words, it can be considered that the video carrier signal among the signals applied to the synchronous detection circuit 13 has been replaced with the oscillation signal of the oscillation circuit 10. Furthermore, it can be seen that the local jitter component of the tuner 2 is also removed at the time of input to the synchronous detection circuit 13. Therefore, by synchronously detecting the output of the first mixing circuit 7 and the oscillation signal of the oscillation circuit 10 that has passed through the phase shift circuit 14 in the synchronous detection circuit 13, it operates with overmodulation, and the differential gain and differential phase are It can be used as a good synchronous detection device. Although the second mixing circuit 9 is a lower conversion method, it is also an upper conversion (up-conversion) method.
The same method can be used. The narrowband bandpass filter 11 constituting the video synchronous detection apparatus constructed as described above will be explained in more detail.It is constructed as shown in FIG. 3. That is, in FIG. 3, the output signal from the mixing circuit 9 is supplied to a terminal 21, and further supplied to the base of an amplifying transistor 25 via a series circuit of capacitors 22 and 23 and a resistor 24. The connection point between the capacitors 22 and 23 is grounded through a resistor 26, and the connection point between the capacitor 23 and the resistor 24 is connected to the midpoint of voltage dividing resistors 27 and 28 for bias. Note that the resistor 24 is a resistor for preventing oscillation. The emitter of the transistor 25 is grounded through an emitter resistor 29, and the collector is connected to a power supply line through a parallel circuit of a matching resistor 30 and a coil 31. Therefore, after the video intermediate frequency signal is amplified by this transistor 25, it is taken out from the collector and supplied to a crystal filter circuit 33 via a coupling capacitor 32. A matching capacitor 34 is further interposed between the input side of the crystal filter circuit 33 and ground. A configuration example of this filter circuit 33 will be described later. This filter circuit 33 removes only the video signal from the video intermediate frequency signal, and supplies this output via a coupling capacitor 35 and an oscillation prevention resistor 36 to the base of a transistor 37 constituting a drive stage. The base of this transistor 37 is also biased by voltage dividing resistors 38 and 39, and the output terminal of the filter circuit 33 is grounded through a parallel circuit of a capacitor 40 and a resistor 41. The emitter of the transistor 37 is grounded via a resistor 42, and the collector is connected to a power supply line. The output of this transistor 37 is taken out from the emitter and supplied to the base of the next stage transistor 43. The emitter of this transistor 43 is similarly grounded via a resistor 44, and the collector is connected via a resistor 45 to a power supply line. The signal supplied to this transistor 43 is taken out from the collector and supplied to the base of the final stage transistor 46. The collector of this transistor 46 is directly connected to the power supply line, and the emitter is grounded via an emitter resistor 47, and only the signal component from which the video signal is removed is transferred to the resistor 48.
It can be obtained at terminal 50 via a series circuit of capacitor 49. The power supply line is connected from a terminal 51 connected to a predetermined voltage source to each transistor 37, 43 of the drive stage via series resistors 52, 53.
46, and is configured to supply a predetermined bias to the transistor 25 from the midpoint of the resistors 52 and 53 via the resistor 54, and smoothing capacitors 55, 56, 57, 58 as necessary. ,5
9 is interposed between the ground and ground. A specific example of the crystal filter circuit 33 is shown in FIG. 4a. This filter circuit 33 has two crystals 71 and 72, and capacitors 73 and 7 are connected in parallel to the crystals 71 and 72, respectively.
4 are connected, the output sides of this parallel circuit are connected in common, and the input sides are connected to both ends of the secondary winding 76 of the transformer 75, respectively. And this transformer 7
Input terminals 78 and 79 are arranged at both ends of the primary winding 77 of 5, a tap is provided at the middle point of the secondary winding 76 to serve as one of the output terminals 80, and the other output terminal is connected to the parallel circuit in common. A terminal is led out from this point and configured as the other output terminal 81. This filter circuit 33 is generally called a Yarman type, and its equivalent circuit is formed as shown in FIG. 4b. This filter circuit 33 makes it possible to obtain passband characteristics as shown in FIG. As for the characteristics of this filter circuit 33 itself, when the frequency of the video carrier signal fp is fo, the filter characteristics are set so that this fo becomes the center frequency, and at a position -3 dB from its peak, its bandwidth ±
It is set to stay within 7.5KHz. Therefore, the filter circuit 33 has fp+fv+△f as described above.
A signal of -f ₁₀ is supplied, but due to this filter characteristic, a signal of fp + △f - f - f ₁₀ with fv removed is obtained at the output side of the filter circuit 33, and this output has a frequency spectrum that does not affect the video signal. This results in a signal that approximates the carrier on the transmitting side. However, even if such a high-performance filter circuit 33 is employed, the low frequency spectrum of the vertical synchronization portion cannot be separated. Therefore, this narrowband bandpass filter 1
A limiter circuit 12 is further provided on the output side of the filter 1, which is a narrowband bandpass filter 1 that varies depending on the low frequency spectrum of the vertical synchronization part and the degree of video modulation.
1. Prevents output signal level fluctuation. An example of a specific circuit configuration of this limiter circuit 12 is shown in FIG. The output of the aforementioned narrowband bandpass filter 11 is supplied to a terminal 91, and this input signal is supplied to the base of a transistor 94 via capacitors 92 and 93. A connection point between the capacitors 92 and 93 is grounded through a parallel circuit of a resistor 95 and a coil 96, and bias resistors 97 and 98 are connected to the base of the transistor 94. The emitter of this transistor 94 is grounded via a resistor 99, the collector is connected to one end of the resistor 97 via a resistor 100 and a coil 101, and this connection point is grounded via a capacitor 102 and connected via a resistor 103. Connected to power line.
The signal amplified by this transistor 94 is taken out from the collector and sent to the next stage transistor 10.
4 base. The emitter of the transistor 104 is grounded through a parallel circuit of a resistor 105 and a series circuit of a resistor 106 and a capacitor 107. Collector is resistor 108 and coil 109
connected to the power supply line through a parallel circuit. The carrier signal amplified by this transistor 104 is supplied to a primary winding 112 of a transformer 111 via a coupling capacitor 110. The secondary winding of this transformer 111 is divided into two parts, and one end of the secondary winding 113 is connected to a first diode 11.
The anode side of the diode 4 and the cathode side of the second diode 115 are connected to each other. Further, the anode side of a third diode 117 and the cathode side of a fourth diode 118 are connected to one end of the other secondary winding 116, respectively, and the other ends of the secondary windings 113 and 116 are collectively connected to a resistor 119. It is connected to a power supply line via a capacitor 120 and grounded via a capacitor 120. The cathode side of the diode 114 is connected to the anode side of the fourth diode 118 and the two 1st side of the output transformer 121.
It is connected to one end of one winding 122 of the next winding. Similarly, the cathode of the third diode 117 and the anode of the second diode 115 are connected and also connected to one end of the other primary winding 123. The other ends of both primary windings 122 and 123 are collectively grounded via a resistor 124, and one end of the secondary winding 125 is grounded and the other end is connected to the base of a transistor 127 via a capacitor 126. do. A resistor 1 is connected in parallel to the secondary winding 125.
28 is connected to the base of the transistor 127, and bias resistors 129 and 130 are connected to the base of the transistor 127. One end of this resistor 129 is connected to the power supply line via a resistor 131 and to the collector of a transistor 127 via a resistor 132, and the emitter of the transistor 127 is grounded via a resistor 133. The output of the transistor 127 is taken out from the collector and supplied to the base of the transistor 135 via the oscillation prevention resistor 134. A parallel circuit of a coil 136 and a resistor 137 is connected in parallel to the resistor 132, and is grounded via a capacitor 138. transistor 135
The emitter is grounded through a resistor 139 and connected to an output terminal 142 through a resistor 140 and a capacitor 141, and its collector is directly connected to a power supply line. Therefore transistor 1
The output of 35 is taken out from the emitter and transmitted to output terminal 142. The power line is connected to terminals 143 and 144 that are connected to a voltage source, and the terminal 143 is connected to two terminals of the transformer 111 via a resistor 145.
Voltage is supplied to each diode 114, 115, 117, 118 connected to the next winding 113, 116, and the transistor 9 is supplied via the coil 146.
4 and 104, respectively. And smoothing capacitors 147 and 14 are connected to the power line.
8 and 149 are respectively interposed between the ground and ground. On the other hand, a resistor 150 is connected to another terminal 144 to supply voltage to transistors 127 and 135, and smoothing capacitors 151 and 152 are interposed between the terminal and ground. According to the limiter circuit 12 configured in this way, the carrier signal from the narrowband bandpass filter 11 in the previous stage is introduced from the terminal 91, and after being amplified by the transistors 94 and 104,
It is clipped at a predetermined level by a bridge-type limiter section consisting of diodes 114, 115, 117, and 118, and its output is connected to transistors 127 and 1.
35 to obtain the carrier signal limited to the output terminal 142, and this limiter circuit 12
A carrier signal is supplied to the mixing circuit 7 while suppressing level fluctuations and the like. By configuring the video synchronous detection device as described above , the output signal of the video intermediate frequency amplification stage 4 is supplied to the first mixing circuit 7, and the video intermediate frequency signal branched before the video intermediate frequency filter 5 is supplied. After converting the frequency of the signal, a carrier signal is supplied through a narrowband bandpass filter 11 and a limiter circuit 12. In other words, the first mixing circuit 7
After the carrier signal component is replaced with the oscillation frequency signal of the oscillation circuit 10, it is supplied to the synchronous detection circuit 13, and the oscillation signal from the same oscillation circuit 10 is supplied to the synchronous detection circuit 13 as a carrier signal to perform the detection operation. Since the local jitter caused by the tuner 2 and the broadcasting station is completely removed, excellent DG/DP characteristics can be obtained. Next, another embodiment will be described with reference to FIG. In FIG. 7, the same parts as those shown in FIG. 1 are given the same numbers, and detailed explanation thereof will be omitted. The embodiment shown in FIG. 7 differs from the previous embodiment in that an automatic phase control device 161 is provided on the output side of the phase shift circuit 14.
This automatic phase control device 1 6 1 is a phase difference detection circuit 1
62 and a phase control circuit 163, and the output of the phase shift circuit 14 is supplied to the phase control circuit 163. The output of this phase control circuit 163 is supplied to the synchronous detection circuit 13 and also to the phase difference detection circuit 162. This phase difference detection circuit 16
2 is also supplied with the output of the first mixing circuit 7, and the phase difference detection circuit 162 detects the carrier signal of the output of the first mixing circuit 7, that is, _f10 , and the carrier signal that has passed through the phase control circuit 163. Compare the phase of the signal, that is, the oscillation signal of the oscillation circuit 10, and if there is a phase difference between the two signals so as to match the phase of _f10 , generate a control signal in the phase difference detection circuit 162,
This control signal controls the phase control circuit 163 to change the phase of the signal passing through the phase shift circuit 14.
_f10 , and by matching the phases of both signals and adding them to the synchronous detection circuit 13, a reliable demodulated output can be obtained. With the apparatus configured in this way, it is possible to obtain a more accurate demodulated signal than in the embodiments described above. FIG. 8 shows still another embodiment, in which the antenna 1 receives television broadcast waves having vestigial sideband characteristics from the transmitting side, and the television broadcast waves are supplied to the tuner 2. This Chuyuna 2
Then, the desired channel is selected by the channel selection device 3, and the high frequency television signal is converted into a video intermediate frequency signal and output. This video intermediate frequency signal is supplied to a video intermediate frequency filter 5 constituting a video intermediate frequency amplification stage 4 . This filter 5 is configured to have a Nyquist characteristic and obtains the necessary frequency characteristics. The output of this filter 5 is a video intermediate frequency amplification and automatic gain control (hereinafter simply abbreviated as AGC) that constitutes the video intermediate frequency amplification stage 4 .
After being supplied to the circuit 6 and amplified, it is supplied to the mixing circuit 7, and after frequency conversion, it is supplied to the detection means 15 . The detection means 15 is composed of an envelope detection circuit 16 and a synchronous detection circuit 13, to which the outputs of the mixing circuit 7 are applied in parallel. On the other hand, the output of tuner 2 is carrier amplified and
The signal is also supplied to a narrowband bandpass filter 11 via the AGC circuit 8 and the mixing circuit 9, and this filter 11 extracts only the video carrier signal component. This video carrier signal component is further supplied to a limiter circuit 12, where the signal is clipped at a predetermined level. The video carrier signal that has passed through the narrowband bandpass filter 11 and the limiter circuit 12 is applied to the mixing circuit 7 described above. On the other hand, the envelope detection circuit 16 is constituted by, for example, a diode detector, and directly demodulates the video signal from the video intermediate frequency signal. That is, the detection means 15 generates the outputs of different detection circuits 13 and 16, respectively, and supplies these to the changeover switch 17. A changeover switch control circuit 18 is provided on the output side of the limiter circuit 12, and a control signal is generated by detecting the output level of the limiter circuit 12, etc., and is supplied to the changeover switch 17 to control either of the detection circuits 13, Select 16 outputs. Since the output of the phase shift circuit 14 is applied to the synchronous detection circuit 13, the synchronous detection circuit 13 performs phase detection with the supplied frequency-converted video intermediate frequency signal. In other words, in a normal and correctly tuned television signal reception state, both the envelope detection circuit 16 and the synchronous detection circuit 13
Detection operations are performed at the detection circuits 3 and 16, respectively, and demodulated video signals are obtained at the output terminals of the detection circuits 13 and 16, respectively, and both video signals are supplied to the changeover switch 17. The output of the limiter circuit 12 is supplied to a changeover switch control circuit 18, and the changeover switch control circuit 18 detects whether or not there is an output from the limiter circuit 12 (whether there is a carrier signal), for example, by a level detection circuit. etc., and generate a control signal. This control signal is supplied to the changeover switch 17, but when there is a carrier signal, the output of the synchronous detection circuit 13 is output to the changeover switch 17 output side, and when there is no carrier signal, the output of the envelope detection circuit 16 is output. The changeover switch 17 is controlled so that the signal is output to the changeover switch 17 output side. Therefore, in the state described above, since a carrier signal is generated in the limiter circuit 12, a control signal is sent from the changeover switch control circuit 18 to the changeover switch 17 so that the output of the synchronous detection circuit 13 is output to the changeover switch 17 output side. and selects the output of the detection stage 15 . By the way, television receivers are sometimes used not only for normal television signal reproduction, but also for display purposes. This type of usage is used, for example, as a display device such as a magnetic recording/reproducing device, a video game, or a video disk.In these devices, signals converted to television signals are generally connected to the antenna terminal board of a television receiver. The tuner 2 is supplied with signals from these devices 19 instead of the signals from the antenna 1. . Generally, the signal from the device 19 to be reproduced is converted into a video intermediate frequency signal using an empty channel portion of the tuner 2. Even if the carrier signal component in the high-frequency signal converted into the television signal supplied from the reproduced device 19 to the television receiver is accurately set to the channel frequency of the tuner 2, the accuracy is poor, so it cannot be set correctly. Frequency errors often occur when the signals are not in tune.
In this case, the output from the tuner 2 is supplied to the narrowband bandpass filter 11 via the carrier amplification and AGC circuit 8. However, since the frequency characteristic of the narrow band band pass filter 11 is narrow band, the carrier signal component cannot be detected by the narrow band band pass filter 11 due to the above-mentioned error.
No carrier signal component is output to the output. Therefore, no carrier signal component is generated on the output side of the limiter circuit 12, and therefore the changeover switch control circuit 1
Since the carrier signal is not supplied to the selector switch 17, the selector switch control circuit 18 detects that there is no carrier signal and sends the selector switch 17 to the synchronous detection circuit 13.
A switching control signal is supplied to control the switching of the switching switch 17 so that the output of the envelope detection circuit 16 is obtained from the output side of the switching switch 17. In this way, when there is no complete carrier signal, the synchronous detection circuit 13 is incomplete, so in this case, the envelope detection circuit 16 output is switched to be used as a demodulation output, Since the detection circuit 16 operates even if the carrier signal frequency is slightly different, there is no problem that a detection output cannot be obtained. By configuring the video detection device as described above, in the case of a general accurate television signal, the demodulated output of the synchronous detection circuit 13 is selected and extracted to obtain a demodulated video signal that is more similar to the video signal on the transmitting side. Even during overmodulation, the phase of the carrier signal does not change and good linearity can be obtained. When the carrier signal is fluctuating for the signal from the reproduced device 19, a demodulated video signal can be obtained by selecting the demodulated output of the envelope detection circuit 16 as the output of the detection stage 15 .
There are no disadvantages such as failure to obtain a detection output due to carrier signal fluctuations. Of course, if the carrier signal from the reproduced device 19 is accurate and has no errors, it will be extracted by the narrowband bandpass filter 11 in the same way as a regular television signal. Even if there is a signal, a control signal is generated from the changeover switch control circuit 18 so that the detected output of the synchronous detection circuit 13 is selected, and the changeover switch 17 is automatically switched so that the output of the synchronous detection circuit 13 is obtained. It is convenient to use because it is automatically controlled according to the usage pattern. In the above explanation, we have explained the case where the selector switch control circuit 18 is operated using the output of the limiter circuit 12, but it is also possible to use the output of the AGC circuit 8, and the point is whether there is a correct carrier signal or not. Any method can be used as long as it can detect the location and method. Further, the present invention is not limited to the above-mentioned embodiment, but can take other configurations, and this embodiment will be described with reference to FIG. 9. In this explanation, the same parts as in each of the embodiments described above are given the same numbers, and detailed explanation thereof will be omitted. This embodiment differs from the previous embodiment in that the configuration of the detection stage and the signal to be switched by the changeover switch are different. That is, the output of the video intermediate frequency amplification stage 4 is sent to the mixing circuit 7.
The signal is supplied to a single synchronous detection circuit 13 via the synchronous detection circuit 13, and is also supplied to a bandpass filter 20 at the same time. The output of the bandpass filter 20 is supplied as one input of the changeover switch 17, and the output from the phase shift circuit 14 is supplied as the other input of the changeover switch 17. The changeover switch 17 is controlled by a control signal from a changeover switch control circuit 18. With this configuration, a single synchronous detection circuit 13 is used, and by switching and selecting the carrier signal applied to this detection circuit 13, the detection operation mode of the detection circuit 13 is changed. It is something. Now, if we consider the case where the correct television signal is being received, a carrier signal appears at the output end of the limiter circuit 12, and this carrier signal is transmitted to the selector switch 17.
is supplied to. and band pass filter 2
0 is configured as a filter having a characteristic wider than that of the narrowband bandpass filter 11, and for example, a filter having a characteristic of approximately ±2 MHz is used. Therefore, even in this receiving state, the bandpass filter 2
A carrier signal component is obtained at the output terminal of 0, and this carrier signal is also transferred to the changeover switch 17.
is supplied to. However, since the changeover switch control circuit 18 detects the presence of a carrier signal at the output of the limiter circuit 12, for example, even if there is an output to the bandpass filter 20, the carrier signal at the output of the phase shift circuit 14 is automatically changed. Switching control is performed so that the signal is supplied to the synchronous detection circuit 13, so the detection operation is performed using the carrier signal of the narrowband bandpass filter 11 system, and an accurate demodulated video signal is obtained at the output of the detection circuit 13. . When a signal is received from the reproduced device 19 in which the carrier signal has fluctuated, the changeover switch control circuit 18 is The changeover switch 17 is controlled to be switched from the phase shift circuit 14 side to the bandpass filter 20 side. For this purpose, a carrier signal component is extracted from the video intermediate frequency signal that has passed through the video intermediate frequency amplification stage 4 using the bandpass filter 20, and this carrier signal component is supplied to the synchronous detection circuit 13 via the changeover switch 17. . The synchronous detection circuit 13 performs an analogous synchronous detection operation based on the carrier signal component that has passed through the bandpass filter 20, and obtains a demodulated output on the output side of the synchronous detection circuit 13. In this embodiment, the detection operation of the detection stage 15 is selectively switched using a changeover switch 17, and unlike the previous embodiment, there is no need to provide multiple detection circuits of different types, and a synchronous detection circuit is installed at the end. It is only necessary to switch and use the carrier signal added to 13. Moreover, FIG. 10 shows still another embodiment, and FIG.
By changing the arrangement of the changeover switch 17 and its control circuit system in the device shown in the figure, the output of the changeover switch 17 is not supplied to the synchronous detection circuit 13, but is added to the mixing circuit 7, and the oscillation from the oscillation circuit 10 is supplied to the synchronous detection circuit 13. The output is phase-adjusted through a phase shift circuit 14 and then supplied to a synchronous detection circuit 13 as a carrier signal. That is, the same parts as in FIG. 9 are given the same numbers and detailed explanations are omitted, but the output of the mixing circuit 9 is divided into two, one of which is sent to the narrowband bandpass filter 11, and the other to the bandpass filter 20. supply each. The output of this narrowband bandpass filter 11 is transferred to the selector switch 17.
and a changeover switch control circuit 18, and a bandpass filter 2 is connected to the changeover switch 17.
An output of 0 is added. The changeover switch control circuit 18 controls the changeover switch 17 according to the output of the narrowband bandpass filter 11 so that either the output of the narrowband bandpass filter 11 or the bandpass filter 20 is outputted to the output side of the changeover switch 17. do. The output of the changeover switch 17 is supplied to the mixing circuit 7 via the limiter circuit 12, mixed with the signal from the video intermediate frequency amplification stage 4, and supplied to the synchronous detection circuit 13. This synchronous detection circuit 13
The oscillation output from the oscillation circuit 10 is supplied as a carrier signal through the phase shift circuit 14, and a detection operation is performed. With this configuration, both the narrowband bandpass filter 11 and the bandpass filter 20 input the output of the mixing circuit 9 to extract the carrier signal component, and the switching and control system of the carrier signal is all done in the mixing circuit. The output of the video intermediate frequency amplification stage 4 is not particularly used. For this reason, even if a wider bandpass filter 20 is used than the narrowband bandpass filter 11, it will not be affected by the Nyquist characteristics of the filter 5 constituting the video intermediate frequency amplification stage 4, and will not be affected by the Nyquist characteristics of the filter 5 constituting the video intermediate frequency amplification stage 4. Therefore, the bandpass filter 20 can be set to have characteristics unrelated to the Nyquist characteristics, and is not subject to any restrictions on the filter characteristics. Since the output of the changeover switch 17 is supplied to the limiter circuit 12, the limiter operation is performed by the limiter circuit 12.The limiter circuit provided on the output side of the bandpass filter 20 is omitted, and the limiter circuit for the output of the bandpass filter 20 is used. Since the limiter circuit 12 can be used as a circuit, the limiter circuit 12 can be shared (unified) and the circuit configuration can be simplified. In the above embodiments, the changeover switch 17 is automatically switched and operated depending on the presence or absence of a carrier signal present in the narrowband bandpass filter 11 system. The changeover switch 17 is automatically changed according to the state of the received signal, and the best output of the detection stage 15 can be selected and set, which is very convenient, but inconvenient cases may occur only in special cases. This is a problem that occurs when the reproduced device 19 is used. For example, the carrier signal of the reproduced device 19 may alternate in a short period of time between being correctly tuned to the carrier frequency of the set channel and being incorrectly tuned. This is the case when fluctuations such as those that occur in When such a fluctuating state occurs, a correct carrier signal is generated at a certain moment, and a carrier signal is also generated at the output of the limiter circuit 12. Therefore, the changeover switch 17 is also switched to the synchronous detection circuit 13 side or the phase shift circuit 14 side. However, at the next instant, the carrier signal fluctuates and deviates, so that no carrier signal component is generated at the output of the limiter circuit 12. Therefore, selector switch 1
7 is switched to the envelope detection circuit 16 side or the bandpass filter 20 side. Then, at the next moment, when the carrier signal becomes correct again, the operation is switched to the above-mentioned first operation again. If the carrier signal starts to fluctuate in this way, starting from the correct carrier signal, the changeover switch 17 will continue to perform the switching operation, and in the worst case, normal playback may occur due to screen jitter, screen shake, or screen flickering. There may also be cases where the screen cannot be obtained. If such a possibility exists, the automatic detection type changeover switch control circuit 18 is removed and a display means is connected in place of this changeover switch control circuit 18. In addition, by changing the configuration of the changeover switch 17 from an automatic changeover type to a manual changeover type, such problems can be eliminated. That is, an LED or a lamp is used as a display means, and the light is turned on or off according to the output of the limiter circuit 12, and whether or not it is a correct carrier signal is judged according to the display state of this display means, and the switch is manually switched based on this result. By switching the switch 17 so as to obtain a predetermined detection output, it is possible to prevent the screen from being adversely affected by fluctuations in the carrier signal. In response to such carrier signal fluctuations, the changeover switch 17 is switched to the envelope detection circuit 1.
It is used by switching to the 6 side or the band pass filter 20 side. Furthermore, by further developing this concept and adding display means to the circuit configuration of the above embodiment, the changeover switch 17 can be connected either automatically or manually. Only in cases where this is done for a short period of time, check the lighting status of the display means and manually switch the changeover switch 17 to prevent any adverse effects from occurring at the detection stage 1.
5 outputs may be selected. Of course, this display means may be omitted and the playback screen may be flashed to indicate that this abnormal switching operation is occurring, the screen may be fixed to a specific color so that the user does not notice it, or the screen may directly change. In such a situation, there is no problem in constructing the system so that it can act as a substitute for the display means without providing any special display means. It should be noted that the present invention can be modified in various ways other than those described above. For example, the insertion positions of the first mixing circuit and the video intermediate frequency amplification stage may be changed, and the first mixing circuit and the video intermediate frequency amplification stage may be inserted in order from the output side of the tuner. It can also be configured as a connection between intermediate frequency amplification stage and synchronous detection circuit.
Alternatively, the carrier amplification and AGC circuit can be omitted if the signal gain is sufficient, and when using this carrier amplification and AGC circuit, as a general rule, it has the same characteristics as the video intermediate frequency amplification circuit and the AGC circuit, or a similar one. By adopting products with specific characteristics, the characteristics can be unified. Furthermore, it is possible to omit the phase shift circuit, and in particular, when an automatic phase control device is inserted, further simplification is possible. Various other modifications and applications are possible, but they fall within the scope of the present invention as long as they do not depart from the gist of the present invention. As explained above, according to the present invention, the detection carrier signal supplied to the detection circuit is outputted from the tuner first so that it is not affected by the Nyquist characteristic of the filter constituting the video intermediate frequency amplification circuit. After the frequency is converted by this mixing circuit, the video carrier signal (oscillation signal of the oscillation circuit) is supplied to a narrowband bandpass filter.
It is not affected by the Nyquist characteristics of the video intermediate frequency amplification circuit. A comparison of differential gain (DG) and differential phase (DP) between the synchronous detection device of the present invention and a conventional analogous synchronous detection device is as follows. [Table] From the results of the differential gain (DG) of the measured values shown in this table, it can be seen that the measured value is smaller with the synchronous detection device of the present invention, and the hue change on the receiving side is smaller than the hue on the transmitting side. . Therefore, the video signal demodulated in the synchronous detection circuit is approximated by the video signal on the transmitting side, and therefore the amplitude modulation-phase modulation conversion on the transmitting side is absorbed and the video signal is approximated by the video signal on the transmitting side. At the same time, the problem of orthogonal distortion is also solved, and even during overmodulation, the phase of the carrier signal does not change and good linearity can be obtained. Furthermore, since the first and second mixing circuits are used, even if the local oscillation frequency changes by △f in the tuner, this frequency change △f no longer exists in the signal supplied to the synchronous detection circuit. It is possible to perform stable circuit operation even in the face of fluctuations. If a crystal oscillation type circuit using a highly stable crystal is used for the oscillation circuit, frequency fluctuations will be almost eliminated in the entire system. Furthermore, when used in combination with a frequency synthesizer type tuning system, the jitter in the frequency synthesizer is dependent on power supply ripple, and this power supply ripple can be removed with a stabilizing circuit, etc., thus preventing the occurrence of jitter. It can be effectively used in combination with the video synchronous detection device of the present invention. In the commonly used voltage synthesizer and potentiometer channel selection systems, the local oscillation frequency changes due to drift, making it impossible to extract a carrier signal.Also, the power source depends on the degree of video modulation based on changes in electric field strength. Even if there is frequency jitter due to fluctuation, it may not work, but by adopting the frequency synthesizer method, the frequency can be controlled in a separate loop and stabilized, which is particularly suitable when applied to the present invention. It is something to do. Furthermore, if a carrier amplification and AGC circuit are added, the limiter circuit can be operated reliably by the tuner's output terminal level fluctuation due to strong and weak electric fields, and if an automatic phase control device is installed, a synchronous detection circuit can be performed. The detection operation can be performed more accurately. Furthermore, compared to the phase control type synchronous detection method, there is no error in oscillation frequency and phase, and a reliable and stable detection operation can be performed. Furthermore, for input signals from devices to be played back such as magnetic recording/playback devices, video games, and video disk players, the detection output of the detection stage is selectively changed according to the signal state of the device to be played back. Therefore, even if the detection stage is operated with high accuracy during normal television broadcast reception, the present system will not become inapplicable, so it can be used over a wide range of areas. In addition, when a carrier amplification and AGC circuit is attached, reliable limiter operation can be performed even in strong and weak electric fields, and the carrier amplification and AGC circuit can be made in the same form as the circuit used in the video intermediate frequency amplification stage. Since it is possible to relatively prevent phase fluctuations, accurate and stable detection operation can be performed in a wide range of electric field conditions. Furthermore, if a frequency synthesizer-type channel selection device is used in the tuner's tuning system, it will be hardly affected by local jitter, making it possible to achieve even better results and receive normal television signals. This video detection device has various excellent advantages, such as being able to be applied even when the carrier signal is deviated due to the state of the electric field strength or an extreme drop in the local oscillation frequency of the tuner. This is something that can be provided.

[Brief explanation of drawings]

FIG. 1 is a circuit configuration diagram showing a video synchronous detection device according to the present invention, FIG. 2 is a waveform diagram showing operating waveforms of each part of the device shown in FIG. 1, and FIG. 3 is a video synchronous detection device according to the present invention. 4 is a circuit diagram showing a crystal filter circuit configuring the narrowband bandpass filter shown in FIG. 3, and FIG. 5 is a circuit diagram showing the characteristics of the narrowband bandpass filter. FIG. 6 is a circuit diagram showing a limiter circuit constituting the video synchronous detection device according to the present invention, and FIG. 7 is a circuit configuration diagram showing another embodiment of the video synchronous detection device according to the invention. , 8th
FIGS. 9 and 10 are circuit configuration diagrams showing still other embodiments. 2... Tuner, 3... Tuning device, 4... Video intermediate frequency amplification stage, 7, 9... Mixing circuit, 8... Carrier amplification and AGC circuit, 10... Oscillation circuit,
11... Narrowband band pass filter, 12... Limiter circuit, 13... Synchronous detection device, 14... Phase shift circuit, 16... Envelope detection circuit,
17... Changeover switch, 18... Changeover switch control circuit, 33... Crystal filter circuit, 1 6 1...
Automatic phase control circuit.

Claims (1)

[Claims] 1. A video intermediate frequency amplification stage that branches the video intermediate frequency signal frequency-converted by the tuner into two and amplifies one of the signals, and a first mixing circuit provided after the video intermediate frequency amplification stage. , a carrier amplifier circuit that amplifies the other branched signal from the tuner, and a second mixing circuit to which the output of the amplifier circuit and the oscillation signal from the oscillation circuit are supplied.
A narrowband bandpass filter whose bandwidth is set below the horizontal fundamental frequency of the video signal that extracts a signal component corresponding to the video carrier signal from the video intermediate frequency signal frequency-converted by the mixing circuit; a limiter circuit that clips the output at a predetermined level and supplies it to the first mixing circuit; a phase shift circuit that shifts the phase of the oscillation signal of the oscillation circuit; and an output of the phase shift circuit and the output of the first mixing circuit. 1. A video synchronous detection device comprising: a synchronous detection circuit that performs synchronous detection using the synchronized detection circuit.