JPH0683460B2 - Video camera signal processing circuit - Google Patents

Description

The present invention relates to a signal processing circuit for a color video camera or the like,
In particular, the present invention relates to a circuit for effectively limiting the amplitude of an excessive carrier color signal level of a composite video signal obtained at an encoder output without causing phase distortion and luminance distortion.

[Conventional technology]

As an example of a signal processing circuit for a conventional color video camera, Nishikawa et al .: “Color Video Camera IC”, National Technical Report Vol. 28, No. 2, April 1982, pages 183-192, or Hitachi Consumer IC Data Book : ICs for VTRs "ICs for video cameras" Some are described on pages 93-128, February 1984.

[Problems to be Solved by the Invention] The above conventional signal processing circuit effectively enables an excessive carrier color signal level exceeding the maximum white level of the composite video signal obtained at the encoder circuit output, that is, the color video camera circuit output. The limiting signal processing circuit is not particularly considered, and has the following problems. As we all know,
In television broadcasting, a carrier wave for broadcasting by a composite video signal is amplitude-modulated, but when the brightness of the subject is high and the signal is large, the amplitude of the carrier wave is decreased, which is a so-called negative modulation method. Therefore, from the viewpoint of the linearity of the modulation characteristics, it is specified that the carrier wave remains up to 12.5% of the maximum value (synchronization part) for the brightness signal of the brightest white subject. When the carrier color signal is combined with the luminance signal, if the amplitude range becomes too large, overmodulation occurs and it becomes impossible to provide a sufficiently satisfactory transmission characteristic. Therefore, as shown in the luminance order color bar signal waveform example shown in FIG. 3, the carrier color signal level is regulated not to exceed 33% of the luminance signal 100% level. This carrier color signal level increases as the color saturation increases, and the carrier color signal increases as the luminance increases even for colors of the same chromaticity. Due to the above, the color television signal of FIG. 3 is represented by the following equation.

E _Y : Luminance signal E _R- E _Y : Red difference signal E _B- E _Y : Blue difference signal _s : Subcarrier frequency As shown in this equation, in the camera circuit, the color difference signal E _R-
E _Y, E _B -E _Y are each The level diagram is set to limit the amplitude of the signal and perform balanced modulation. On the other hand, in recent years, as seen in the widespread use of home color video cameras or home VTR integrated color video cameras, not only recording broadcast signals but also recording and playing back on VTR is also a big issue. I have been turned to. When the composite video signal is recorded / reproduced on the home VTR in this way, even if the carrier color signal is limited, for example, a highly saturated yellow or red flower,
Beat interference may appear in a reproduced image such as a blue sky. This beat interference is caused by low-frequency conversion of the carrier color signal, FM modulation of the luminance signal, and recording / reproduction of the luminance signal.
It is caused by cross-modulation between chromaticity signals, and occurs in eyes with higher saturation and higher brightness, which is visually unpleasant interference. Conventionally, the above-mentioned inconvenience occurs when recording / reproducing on a VTR. As a countermeasure against this, for example, the third
Predetermined level (V _c ) shown in the composite video signal waveform in the figure
It is considered to clip and record the level exceeding 0, and it is often adopted. Although beat interference is reduced by this, only one side of the carrier color signal is clipped, and as a result, a DC component is generated and luminance reproduction is deteriorated, or a hue change is caused in the carrier color signal and color reproduction is deteriorated. Magenta below the clip level (V _c ) shown in Fig. 3,
Inconvenience occurs, for example, red and parts cannot obtain any effect. It is also possible to limit the amplitude of a color signal having high saturation, that is, a high carrier color signal level, to a predetermined level at the stage of the carrier color signal before mixing the luminance signal and the carrier color signal. It is difficult to clip the positive and negative sides in a balanced manner due to imbalance in frequency characteristics, clipping characteristics, etc. As a result, the phase characteristics of the carrier color signal change and color reproduction deteriorates.

The present invention improves the above-mentioned conventional inconvenience, that is, effectively limits the amplitude of the carrier color signal without deteriorating the luminance reproduction, the hue reproduction, and the like, which occurs when the composite video signal is input to the VTR and recorded / reproduced. The purpose is to reduce the occurrence of interference.

[Means for solving problems]

The purpose is to obtain a red color difference (RY) generated in the camera circuit,
Blue-difference (BY) baseband (eg band: 500K
H _z ) Chromatic difference signal, a circuit that can clip at any desired level in each path is installed, and the signal passing through the circuit is balanced-modulated and added by subcarriers whose phases are different by 90 °. .

[Action]

The clipping circuit can perform signal processing in the low frequency region. Therefore, the amplitude value of the baseband color difference signal is well limited at the desired level. As a result, after the respective color difference signals are balanced-modulated by the subcarriers, the vector synthetic wave generated by addition, that is, the excessive amplitude of the carrier color signal is effectively and at a desired level, the phase distortion, or It is possible to limit the brightness without affecting the reproduction of brightness.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings. An embodiment shown in FIG. 1 shows an example of a circuit configuration centering on an encoder circuit for obtaining a composite video signal. The luminance signal and the two color difference signals generated by the in-camera signal generation circuit are input to terminals 1, 2, and 3, respectively. The terminal 7 is a clamp pulse applying terminal and the terminal 8 is a blanking pulse applying terminal. The luminance signal input to the terminal 1 is clamped by the clamp circuit 4, signal-processed by the signal processing circuit 9 such as white clipping, black clipping, blanking processing, and then input to the mixing circuit 10. On the other hand, since the color difference signals R-Y and B-Y are limited at the above levels, Input to terminals 2 and 3 at the level ratio of. After the RY color difference signal input to the terminal 2 is clamped by the clamp circuit 5, the amplitude limiting circuit 11 limits the upper and lower amplitude values of the color difference signal exceeding a predetermined level. The color difference signal B-
Y is mixed with the burst flag pulse located in the blanking period and input to the terminal 3, similarly clamped by the clamp circuit 6 and amplitude-limited by the amplitude limiting circuit 12. The color-difference signal (RY) whose amplitude is limited and the color-difference signal (BY) whose amplitude is limited and to which a burst flag pulse is added are different in phase from each other by 90 ° which are input to terminals 17 and 18, respectively. subcarrier (NTSC: = 3.58MH _z) was equilibrated modulated by the balanced modulator circuits 19 and 20, thereafter, to generate the added carrier chrominance signal of the burst signal by vector synthesis by summing circuit 21. The carrier chrominance signal after band-limited by the band-pass filter 22 (e.g. 3.58MH _z ± 0.5MH _z), are input to the mixing circuit 10 is input the luminance signal input from the other terminal, and the terminal 23 A composite video signal having a limited amplitude of the excessive carrier color signal is obtained at the terminal 24 by being combined with the synchronizing signal. The terminals 13 and 14 are amplitude limiting voltage application terminals, and are supplied from the amplitude limiting voltage generating circuit 15. The limit level can be changed and set by the DC control voltage applied to the terminal 16. The amplitude limit level may be set to an appropriate value from the viewpoint of occurrence of beat interference.

According to this embodiment, the baseband color difference signals, that is, in a low frequency region (dc ~500KH _z) can amplitude limitation process,
Almost unaffected by the frequency characteristics of the circuit,
Amplitude limiting processing can be performed well. Therefore, it is possible to effectively limit the excessive amplitude level of the carrier color signal after the balanced modulation.

FIG. 2 shows an amplitude limiting circuit 11,1 which is a main part of the embodiment shown in FIG.
2 shows a specific circuit example of 2 and the amplitude limiting voltage generating circuit 15. The amplitude limiting circuit 12 has a circuit configuration exactly the same as that of the amplitude limiting circuit 11 in FIG. As the signals input to the amplitude circuits 11 and 12, the color difference signal waveform diagram in the case of the luminance forward color bar signal shown in FIG. 3 is shown.
V _Ref is a clamp potential, and the numerical value corresponding to each color is a value for obtaining the composite video signal of FIG. 3, that is, a value corresponding to level 1 of the luminance signal (black to white) of the luminance order color bar signal. Is. In the transistors Q1 and Q2, the lower clipping circuit detects a signal having a voltage equal to or higher than the voltage applied to the base of Q2, that is, the terminal 141 at the common emitter and inputs the signal to the base of the transistor Q3. Next, in the transistors Q3 and Q4, a signal below the voltage applied to the base of Q4, that is, the terminal 142 is detected in the common emitter in the upper clipping circuit. Therefore, if desired potentials V _cu1 and V _cd1 in the figure are applied to the bases of the transistors Q2 and Q4, the amplitude value above a predetermined value can be detected. On the other hand, the amplitude limiting voltage generating circuit is a current mirror circuit composed of resistors R ₁ , R ₂ , R ₇ , R ₈ and transistors Q5, Q6, Q7, Q8, Q9, resistors R ₃ , R ₄ , R ₅ , R ₆ , and It is composed of a reference voltage source (V _Ref ). If an appropriate control voltage is applied to terminal 16, ignoring the _hFE difference of the transistors, the same current i will flow in the collectors of transistors Q5, Q6, Q8, Q9 (however, R ₁ = R ₂ = R ₇ = and R _8).
Therefore the potential of the intersection of the resistor _{R 3, R 4 (V Ref} + iR4) is the intersection of the R _5, R ₆ potential of generating (V _Ref -iR5). On the other hand, the collector of the transistor Q5 has (V _Ref + i
The potential of (R ₃ + R ₄ ) is (V _Ref −i (R ₅
+ Potential of R ₆₎₎ is generated. Amplitude control circuit for the former potential
With the limiting voltages V _cu2 and V _cd2 of 12, the latter is _used as the amplitude limiting circuit 11
If the limiting voltages V _cu1 and V _cd2 are given, the desired amplitude limiting effect can be obtained.

〔The invention's effect〕

As described above, according to the present invention, when the composite video signal generated by the camera circuit is input to the VTR and recorded / reproduced, a carrier color signal which is excessively large enough to cause beat interference due to intermodulation between luminance and chromaticity. Only the amplitude value can be limited without causing any adverse effects such as phase distortion and luminance distortion, and a good reproduced image with reduced beat interference can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG.
FIG. 1 is a circuit diagram showing a specific example of the amplitude limiting circuit and the amplitude limiting voltage generating circuit which are the main parts of FIG. 1, and FIG. 3 is a waveform diagram showing a composite video signal of luminance order color bar. 1 …… Luminance signal input terminal 2 …… RY color difference signal input terminal 3 …… BY color difference signal input terminal 11,12 …… Amplitude limiting circuit 16 …… Amplitude limiting voltage generating circuit 19,20 …… Balanced modulation Circuit 21 …… Adding circuit 22 …… Bandpass filter 10 …… Mixing circuit 23 …… Sync signal input terminal 24 …… Composite video signal output terminal

Claims (2)

[Claims] 1. A first and a second amplitude limiting circuit for limiting the amplitude to a desired level are provided in respective signal transmission paths of two different color difference signals, and the respective color difference signals whose amplitude is limited, The first and second balanced modulation circuits that balance-modulate subcarrier signals having mutually different phases, and an adder circuit that combines the two balanced-modulated signals to obtain a carrier color signal are provided. A signal processing circuit for a video camera, which obtains a carrier color signal. 2. A composite video signal comprising a mixing circuit for synthesizing the carrier color signal and the luminance signal, wherein a composite video signal in which the carrier color signal of which excessive amplitude is limited is superimposed is obtained at the output of the mixing circuit. A signal processing circuit for a video camera according to claim 1.