Video signal detection circuit and video signal processing device

A video signal detection circuit of the present invention is provided with a trapping circuit which traps frequencies corresponding to color sub-carrier wave signal components and which is disposed between a phase comparing circuit and a voltage controlled oscillation circuit in a PLL circuit performing synchronous detection.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a video signal detection circuit and a 
video signal processing device, and, more specifically, relates to a video 
signal detection circuit for PLL synchronous detection which suppresses 
leakage onto a picture plane of so called C/S beat signals representing 
beat components of color sub-carrier waves and sound carrier waves in an 
intermediate frequency amplifier circuit (IF circuit) of a video signal 
processing device, such as a TV receiver or a VTR. 
2. Background Art 
In a video signal processing device such as a TV receiver or a VTR, 
intermediate frequency signals (IF signals) which are transmitted from a 
tuner in response to channel selection are normally received by a video 
signal detection circuit by a SAW filter (a comb filter). In this device, 
there are two methods for separating video signals from the sound signals. 
In one method, respective SAW filters for the video relating signals and 
for the sound relating signals are provided and these signals are 
separated at the IF stage, and in the other method the video relating 
signals and the sound relating signals are received by a common single SAW 
filter and the sound signal components are separated by a band pass filter 
(BPF) and transmitted to a sound detection circuit after video signal 
detection. 
On the other hand, at the detection output of the video signal detection 
circuit other than luminance signals and chroma signals such as sound 
carrier wave signals of 4.5 MHz and C/S beat signals representing the beat 
of the sound signals and the chroma signals as explained above, for 
example a beat component normally of 920 kHz in an NTSC system, are 
generated. Since reproduced picture images are likely to suffer from video 
disturbance by the C/S beat signals, the sound relating signals are 
normally attenuated by about several tens of dB with respect to the video 
relating signals so as to suppress the C/S beats. In particular, since the 
frequency of the voice sound carrier waves of 4.5 MHz is high, these 
components are removed after the video signal detection with the provision 
of a trapping circuit. 
In the former method in which the two separate SAW filters are provided, 
since the sound relating signals and the video relating signals are 
separated from each other, an advantage that the influence of the C/S beat 
signals is suppressed accordingly is obtained; however, the number of 
circuits which are to be formed into ICs is increased correspondingly to 
thereby increase the cost thereof. On the other hand, in the latter method 
in which the single common SAW filter is provided, the circuit scale 
thereof is reduced to thereby realize a system of a low cost, however, 
since sound signals flow through the video signal detection circuit, C/S 
beat signals are generated and the generated C/S beat signals appear on 
the picture plane which poses a problem of reducing the picture quality. 
In particular, in a case of a video signal detection circuit for a PLL 
synchronous system, since phase detection outputs are returned to a PLL, 
the circuit is more likely affected by the C/S beat signals in comparison 
with other detection systems. 
SUMMARY OF THE INVENTION 
An object of the present invention is to resolve the above problems in the 
conventional art and to provide a video signal detection circuit which 
suppresses the generation of C/S beat signals even if video relating 
signals and sound relating signals are signal processed via a common SAW 
filter. 
Another object of the present invention is to provide a video signal 
processing device which suppresses an adverse influence by C/S beat 
signals to the picture quality of reproduced picture images. 
A video signal detection circuit and a video signal processing device 
according to the present invention which achieve the above objects are 
characterized in that the video signal detection circuit which receives 
outputs from a video intermediate frequency amplifier circuit and performs 
detection of video signals therein comprises a synchronous detection 
circuit which receives the outputs and performs synchronous detection of 
the video signals; a PLL circuit including a phase comparing circuit, a 
voltage controlled oscillation circuit, and a smoothing circuit, in which 
the phase comparing circuit compares the outputs from the video 
intermediate frequency amplifier circuit with the outputs of the voltage 
controlled oscillation circuit, a signal corresponding to the comparison 
result is applied to the voltage controlled oscillation circuit via the 
smoothing circuit and the output of the voltage controlled oscillation 
circuit is transmitted to the synchronous detection circuit as a reference 
signal for the synchronous detection; and a trapping circuit which is 
disposed in a signaling passage from the phase comparing circuit to the 
voltage controlled oscillation circuit and traps signals having 
frequencies corresponding to color sub-carrier wave signal components 
contained in the video signals. 
With the provision of the trapping circuit which traps the signals having 
frequencies corresponding to the color sub-carrier wave signal components 
at a position between the phase comparing circuit and the voltage 
controlled oscillation circuit in the PLL circuit which performs the 
synchronous detection, the level of the color sub-carrier wave signals in 
the synchronous detection reference signal for the synchronous detection 
is lowered. 
Thereby, the level of the C/S beat signals in the synchronous detected 
video outputs is lowered and the leakage of the C/S beat signals onto the 
picture plane is suppressed. Moreover, in this instance it suffices to 
provide a single trapping circuit which traps only the signals having 
frequencies corresponding to the color sub-carrier wave signal components, 
and the trapping frequency can be increased so that the circuit can be 
realized only by adding a simple circuit. Further, a common single SAW 
filter can be applied for the sound relating signals and the video 
relating signals.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, numeral 10 is a video signal detection circuit. Radio wave 
receiving signals (radio frequency signals) received at an antenna 1 are 
amplified in high frequency by a tuner 2 and then are frequency mixed with 
signals from a built-in local oscillation circuit therein, whereby the 
radio wave receiving signals are frequency converted into intermediate 
frequency signals signals of a selected channel (in an NTSC system the 
video frequency is at 58.75 MHz and the sound frequency is at 54.25 MHz). 
The outputs from the tuner 2 are subjected to a filtering process in a SAW 
filter 3 and thereafter transmitted to a video intermediate frequency 
amplifier circuit (VIFAMP) 4. 
The VIFAMP 4 amplifies in high frequency the selected intermediate 
frequency band and transmits the same as the output to the video signal 
detection circuit 10. The video signal detection circuit 10 receives the 
amplified signals and transmits the same to a synchronous detection 
circuit 11 and a phase comparing circuit (PC) 12. The synchronous 
detection circuit 11 transmits the detection outputs to a video signal 
amplifier circuit 5 as well as to a sound detection circuit 7 via a BPF 6 
which is positioned at a post stage for the amplifier circuit 5. 
The synchronous detection circuit 11 is a so-called PLL type synchronous 
detection circuit and receives signals of reference frequency locked via 
the PLL from a voltage controlled oscillator (VCO) 13 as the reference 
signal for the synchronous detection. By way of example, the synchronous 
detection circuit multiplies the received reference signal with the output 
from the VIFAMP 4 via a multiplying circuit therein and outputs the 
resultant multiplied signal as a video detection signal, such as to the 
video signal amplifier circuit 5 via a low pass filter (LPF). 
The VCO 13 further transmits the output thereof to the PC 12. The PC 12 
performs phase comparison between the output of the VCO 13 and the output 
of the VIFAMP 4 and transmits the resultant compared signal to a smoothing 
circuit 14. 
The smoothing circuit 14 is a kind of low pass filter and the output 
voltage thereof is applied to the VCO 13 so that the center oscillation 
frequency is controlled to assume the above indicated frequency of 58.75 
MHz. At the output thereof other than the smoothed voltage, the color 
sub-carrier wave component of 55.17 MHz and the sound carrier wave 
component of 54.25 MHz corresponding to the video carrier wave component 
of 58.75 MHz, and color sub-carrier wave component of 3.58 MHz and sound 
carrier wave component of 4.5 MHz corresponding to the detuning frequency 
components of their carrier wave components, respectively appear. Further 
higher high frequency components are sufficiently attenuated such as by a 
parasitic capacitor Cs. Further, other than the above, through the cross 
modulation of the color sub-carrier wave component of 3.58 MHz and the 
sound carrier wave component of 4.5 MHz, C/S beat components are further 
generated. Namely, at the output terminal 12a of the PC 12 in addition to 
the generated control voltage, the signal component of 3.58 MHz, the 
signal component of 4.5 MHz and the signal component of 920 kHz are 
superposed with comparatively high output levels. 
Therefore, through attenuation of either the color sub-carrier wave 
component of 3.58 MHz or the voice sound carrier wave component of 4.5 MHz 
the C/S beat components can be reduced, however such reduction is limited. 
In the present invention, regardless of the above attenuation of either 
the color sub-carrier wave component of 3.58 MHz or the sound carrier wave 
component of 4.5 MHz, the frequency components corresponding to the color 
sub-carrier wave components are, in particular, attenuated. Thereby, the 
C/S beat components are reduced. 
The smoothing circuit 14 normally includes a series circuit of a capacitor 
C and a resistor R connected successively between the output terminal 12a 
of the PC 12 and the ground GND serving as a PLL filter, and a low pass 
filter constituted by the parasitic capacitor Cs connected between the 
output terminal 12a and the ground GND. A capacitor having a capacitance 
of about 0.1 .mu.F.about.1 .mu.F is used for the capacitor C and a 
resistor having a resistance of about 150.OMEGA. is used for the resistor 
R. As a result, the time constant determined by the capacitor C and the 
resistor R is small. If a large time constant is used therefor, a problem 
is posed that the response characteristic and the pulling characteristic 
of this type of PLL circuit are varied, which limits the increase of the 
time constant. However, because of the small time constant determined by 
the capacitor C and the resistor R, the attenuation of the signal 
components such as the color sub-carrier wave of 3.58 MHz and the sound 
carrier wave of 4.5 MHz is difficult. 
Accordingly, in the present embodiment a color sub-carrier wave trap 15 
constituted by a capacitor Ct and a coil Lt connected successively in 
series between the junction point N of the capacitor C and the resistor R 
and the ground GND is provided. With the trap 15 the level of the color 
sub-carrier wave signal component which is superposed over the control 
voltage is reduced. As a result, the level of the color sub-carrier wave 
signal component among the outputs of the VCO 13 is reduced. 
Thereby, the video detection is performed with a synchronous detection 
reference signal in which the color sub-carrier wave component in the 
oscillation output of the VCO 13 is attenuated. Thus, the level of the C/S 
beat signals in the detected video signals is reduced without reducing the 
sound signal components. Further, in this instance it is possible to trap 
the sound carrier wave component together with the color sub-carrier wave 
component and to further reduce the level of the C/S beat signals if a 
trapping circuit is increased. However, if a signal trapping circuit 
exclusive only for the color sub-carrier wave component is provided as 
explained previously, the circuit structure is simplified correspondingly 
and a sufficient advantage is obtained by trapping only the color 
sub-carrier wave component having a higher frequency. Further, reduction 
of the sound quality is prevented. 
FIG. 2 is a trapping circuit applicable for both an NTSC system and a 
system, in which a color sub-carrier wave trap 17 is provided which has a 
reduced Q as illustrated in FIG. 3 with respect to that of the color 
sub-carrier wave trap 15 (a trap filter) as illustrated in FIG. 1. Such Q 
reduction can be normally realized by shifting the center oscillation 
frequency through simple selection of the values of the capacitor Ct and 
the coil Lt. As illustrated in FIG. 3, the position of the center 
oscillation frequency (the trough in the filter characteristic curve) is 
set at the intermediate position of 3.58 MHz and 4.43 MHz which 
respectively correspond to the color sub-carrier wave components for the 
NTSC system and the system. With this measure, the levels of the 
frequencies of 3.58 MHz and 4.43 MHz are equated which respectively 
correspond to the color-carrier wave components for the NTSC system and 
the system. In other words, the position of the center oscillation 
frequency (the trough of the filter characteristic curve) is set in such a 
manner that the attenuation levels of these two frequencies assume 
substantially the same level. As a result, both color sub-carrier wave 
components can be properly attenuated, for example, to about a few dB to 
10 dB. 
The color sub-carrier wave trap 17 having such a trap filter characteristic 
is inserted between the voltage control terminal of the VCO 13 and the 
output 12a of the PC 12. The abscissa in FIG. 3 represents frequency 
(tuning frequency). 
With the provision of the trapping circuit having the above explained 
characteristic, a video signal detection circuit which is applicable both 
to the color sub-carrier wave frequency of 3.5 MHz for the NTSC system and 
to the color sub-carrier wave frequency of 4.43 MHz for the system is 
realized. 
Further, a single filter can be used in common for other TV broadcasting 
standards or broadcasting modes, for example, a SECAM system, having the 
color sub-carrier wave frequency close to the above frequencies.