JPH0352273B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video signal processing circuit.
It is most suitable for use as a pre-emphasis and white/dark clip circuit in the recording system of a video recording and reproducing apparatus such as, or as a de-emphasis and expand circuit in the reproducing system.

Generally, in a VTR, the high frequency range of the video signal is emphasized, then FM modulated and recorded, and during playback, the high frequency range of the FM demodulated signal is attenuated to reduce the noise inherent in the recording medium (magnetic tape). when recording
To prevent FM modulators from overmodulating, a white/white signal that clips sharp overshoots and undershoots of the emphasized signal is usually
A dark clip circuit is used.

Figure 1 is a block circuit diagram showing the main parts of the recording and playback system of such a VTR.The input video signal is fed to a pre-emphasis circuit 1 to emphasize the high frequencies, and then to a clip circuit 2 for white/dark processing.・
Clipping is performed, modulated by an FM modulator 3, and then recorded on a magnetic tape 5 via a magnetic head 4. On the playback side, the video signal demodulated by the FM demodulator 6 is expanded by the expander circuit 7, and the high frequencies are attenuated by the de-emphasis circuit 8.

The recording-side clip circuit generally uses the conduction impedance of a diode, but it requires a large number of elements and has factors that make it unsuitable for IC implementation, such as characteristic fluctuations and variations due to temperature.

In order to improve this problem, a method is known in which a limiter is used to perform both pre-emphasis and clipping operations. In this method, the IC
However, when changing the clip level for adjustment, the output DC level (video signal pedestal level or sync level) changes at the same time, and the FM carrier frequency changes. Ta.

Furthermore, as shown in FIG. 1, a pre-emphasis circuit 1, a de-emphasis circuit 8, a clip circuit 2, and an expand circuit 7 are generally provided separately for the recording system and the reproduction system, respectively, so the circuit configuration is complicated. In addition, it is difficult to completely provide the inverse transfer function of the recording system in the reproduction system, and there are problems such as the transmission frequency characteristics not being flat.

In view of the above-mentioned problems, the present invention has a simple structure, can be easily integrated into an IC, can share the circuit between the recording system and the playback system, and can handle the DC level of the video signal processed even when the clip level is changed. The present invention provides a video signal processing circuit (emphasis/de-emphasis and clip/expand) in which the signal does not change.

Embodiments of the present invention will be described below with reference to the drawings.

Figure 2 shows the recording/recording of a VTR showing an embodiment of the present invention.
FIG. 3 is a block circuit diagram showing the main parts of the reproduction system. During recording, the input video signal is applied to the high-pass filter 11 through the REC contact of the record/re-select switch 10, and the high frequency is separated and passed to the current limiter 1.
given to 2. In the current limiter 12, the high frequency component is linearly amplified to a predetermined emphasis gain. Further, this limiter 12 operates as a limiter at white/dark peak parts such as overshoot of the high-frequency differential signal, and the input waveform is clipped at a set clip level or higher.

The clip level is set by adjusting the limiting current of the current limiter. Since changing the limiting current also changes the output DC level, a DC level control circuit 13 is provided.
The DC level of the output of the clip circuit is always controlled to be constant even if the clip level is changed.

The output of current limiter 12 is summed with the input video signal in adder 4 to form a pre-emphasized and white/dark clipped video signal. This video signal is FM modulated as described above and then recorded on the magnetic tape 5.

At the time of reproduction, the green/re-switching switch 10 is connected to the PB side, and the signal passing through the high-pass filter 11 and current limiter 12 is combined with the reproduction signal in a subtracter 15 by negative feedback. As a result, a circuit having characteristics complementary to those during recording is constructed, and expand and de-emphasis operations are performed.

As shown in the modified example of FIG. 3, a completely equivalent circuit can be realized as a feedback circuit during recording and a feedback circuit during reproduction.

According to the configuration of FIG. 2 or 3, recording/
Since the circuit is used for both playback and emphasis/clip, the number of elements is extremely small. The clip method also does not use the conduction impedance of a diode as in the past, so there is no need for a large-capacity coupling capacitor as an external component, making it extremely suitable for IC implementation. Furthermore, since the recording/reproducing characteristics are complementary, waveform reproducibility is also good.

Furthermore, the most important feature is that even if the limiting current (limiter level) is changed while using the current limiter 12 that operates with direct current,
Since the output signal is configured to have a constant DC level, the carrier center of the FM modulator 3 in the subsequent stage does not change, and the dynamic range of the subsequent circuit is not affected in any way. Furthermore, by using an asymmetric limiter (with an offset) as the limiter, it is also possible to change the balance of the clip level between the white side and the dark side.

Next, FIG. 4 is a specific circuit diagram of the main parts (high-pass filter 11, current limiter 12, and adder 14) of FIG. 2. Regarding recording and playback,
Only the configurations of the adder 14 and the subtracter 15 are different, and other similar circuits are used, so only the recording system will be described.

In FIG. 4, a differential amplifier 17 consisting of transistors Q ₂ and Q ₃ and a differential amplifier 18 consisting of transistors Q ₆ and Q ₇ constitute a high-pass filter 11, a current limiter 12, and a DC level control circuit 13. Note that the differential amplifier 17 is the differential amplifier 18.
connected in series on one wing of the transistor Q ₄ ,
_Q5 is inserted into the other wing of the differential amplifier 18 for balance.

Input video signal is buffered transistor _Q1
The signal is applied to the positive-phase input of the differential amplifier 17, and is also applied to the negative-phase input through a low-pass filter consisting of a resistor _R2 and a capacitor C. Therefore, the differential amplifier 17 operates as a high pass filter 11. The high-frequency component is amplified by the differential amplifier 17 and output to the buffer transistor _Q8 .

The current flowing through the current source _I2 of the differential amplifier 18 is the current _Ic flowing through the differential amplifier 17 and the transistors _Q4 and _Q5.
The current flowing through the current I ₂ −I _c is shunted. Therefore, the dynamic range of the signal amplified by transistors Q ₂ and Q ₃ is determined by I _c × R ₄ (load resistance of Q ₃ ), and high amplitude signals are clipped by this, resulting in the so-called current limiter. operates as

By adjusting the variable voltage source E ₃ connected to the base of one transistor Q ₇ of the differential amplifier 18, I _c changes, thereby changing the dynamic range, that is, the clip level. . The DC level of the limiter output (connection point of resistor R ₄ and transistor Q ₃ ) is always V _cc −
It is fixed at R ₄ ×I ₂ /2 and is not affected by the adjustment of E ₃ . In other words, since transistors Q ₆ and Q ₇ are differentially coupled, for example, when I _c increases, I ₂ − _{I c} decreases by the same amount, so the DC component flowing through resistor R ₄ does not change. .

Further, in the differential amplifier 17, transistors Q ₂ ,
When the resistance of the emitter region of Q ₃ is r _e , r _e ≪
If R ₅ and R ₆ are set, the limiter gain G ₁ is
It is determined by G ₁ R ₄ /R ₅ + R ₆ and does not change by adjusting E ₃ . Therefore, it operates as a clip circuit whose output DC level and gain are always constant.

The clipped output is the transistor _Q8 ~
It is added to the input video signal in an adder 15 composed of Q ₁₀ , resistors R ₉ to R ₁₁ , and current sources I ₃ to _{I 5} . Therefore, the characteristics of the pre-emphasis circuit _are as follows:
When the low-pass characteristic of capacitor _C1 is defined as LPF,
1+G ₁ (1-LPF), and the transfer function is 1+G ₁ SC ₁ R ₂ /1+SC ₁ R ₂ = (1+G ₁ )SC ₁ R ₂ +1/1
+SC ₁ R ₂ . That is, T ₁ = CR ₁ , T ₂ = (1+
G ₁ )・C ₁ R ₂ emphasis circuit.

Next, FIG. 5 shows a modification of the high-pass and current limiter circuit section of FIG. 4. In the emphasis circuit shown in Fig. 4, if you want to set the gain _G1 large, you need to reduce the resistors _R5 and _R6 , but in order to change the clip level, it is necessary to reduce the _{resistors R5} and _R6 . When I _c changes, the gain G ₁ changes, and the emphasis time constant also changes. Therefore, in the modification shown in FIG. 5, the differential amplifier 17 consisting of Q ₂ and Q ₃ is current-driven by the outer differential pair Q ₁₁ and Q ₁₂ . This is the configuration of a gain control amplifier, and the gain is determined by the ratio of the operating currents I ₂ and I ₁ of the differential amplifier 17 and the differential pair Q ₁₁ and Q ₁₂ , G ₂ R ₄ /R ₅ +R ₆ ×I ₂ /I ₁ .

Therefore, if you want to increase the gain, I ₁ and I ₂
It is sufficient to set the ratio to a large value. Since the resistors R ₅ and R ₆ can be large, when the operating currents of transistors Q ₂ , Q ₃ , Q ₈ , and Q ₉ are changed to change the clip level, r _e which is a function of the collector current Even if the value changes, the gain does not change significantly.

The clip level is determined by the dynamic range I ₂ ×R ₄ , and by adjusting E ₃ I ₂ , that is, the clip level, can be changed. However, E ₃
Assume that when changing , I ₁ and I ₂ move at a constant ratio. The output DC level is determined by V _cc −I ₂ +I ₃ /2×R ₄ and remains constant regardless of the adjustment of E ₃ . Note that the output is taken out via the buffer transistor _Q8 and added to the input signal in the adder 15 as in FIG.

FIG. 6 shows yet another modification of the high-pass and current limiter circuit section of FIG. In this modification, DC level control is used for the buffer transistor _Q8 in the output stage of the current limiter. Fourth
By changing E ₃ as shown in the figure, the current flowing through R ₄ changes, and the clip level changes.
If the emitter side resistance R ₁₂ of Q ₈ is set to R ₁₂ = R ₄ /2, the DC level of the limiter output is V _cc −I _c /2×R ₄ −V _BE −R ₄ /2 (I ₂ − Since I _c ) = V _cc −V _BE −R ₄ /2I ₂ , it is constant regardless of the adjustment of E ₃ .

As described above, since the limiter 12 and the DC level control circuit 13 of the embodiments shown in FIGS. 4 to 6 are configured by differential coupling, they can easily be integrated into an IC.

As described above, the present invention configures an emphasis circuit with a high-pass section, a current limiter, and an addition section,
Even if the limit level of the current limiter is changed, the DC level of the limit output does not change, so there is little effect on the subsequent circuit. For example, when an FM modulator is connected to the subsequent circuit, its carrier center does not change, and the dynamic range or modulation depth does not change, making it possible to perform high-quality signal processing. .

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram showing the main parts of the recording/playback system of a conventional VTR, FIG. 2 is a block circuit diagram showing the main parts of the recording/playback system of a VTR showing an embodiment of the present invention, and FIG. is a block circuit diagram showing a modification of FIG. 2, FIG. 4 is a specific circuit diagram of the main part of FIG. 2, and FIGS. 5 and 6 each show a modification of the main part of FIG. 4. It is a circuit diagram. In addition, in the symbols used in the drawings, 11
...high pass filter, 12...current limiter, 13...DC level control circuit, 14...adder, 17, 18...differential amplifier.

Claims (1)

[Claims] 1. A high-frequency component is extracted from the input video signal, and this high-frequency component is supplied to a current limiter configured with a differential amplifier, and the current limiter is adjusted by adjusting the operating current source of the differential amplifier. The limiter level of the limiter is made variable, and the DC level at the output end of the differential amplifier is controlled in accordance with the adjustment of the operating current source, and the video signal from the output end is controlled as described above. A video signal processing circuit that applies weight to the input video signal.