JPH03290854A - Video signal regenerating circuit - Google Patents

Abstract

PURPOSE:To attain auto tracking with high accuracy by providing a non-linear output circuit for outputting a DC voltage signal in accordance with envelope strength with higher sensitivity in an area in which an output value from a detecting circuit is at a low level than in a high level area. CONSTITUTION:This circuit is provided with a non-linear output circuit 18h for outputting a DC voltage signal in accordance with envelope strength with higher sensitivity in an area in which an output value from a detecting circuit 18e is in a low level than in a high level area. In such a state, in the case envelope strength of an input video signal is in a low level, a current flows to the circuit 18h and the DC voltage signal is outputted in accordance with video envelope strength with sensitivity prescribed by its resistance value, and in the case of a high level area in which video envelope strength exceeds a prescribed value, a current flows not only to the circuit 18h but also to a circuit 18i, and a DC voltage signal is outputted with sensitivity prescribed by a combined resistance of these two pieces of circuit 18h, 18i. In such a way, even in the case the envelope strength of a video signal is at a low level, auto tracking can be executed with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a video signal reproducing circuit, and particularly to a video signal reproducing circuit equipped with a video envelope detection circuit for auto-tracking a video tape recorder (VTR).

[Prior Art] Conventionally, an auto-tracking technique for accurately tracing a video track recorded on a videotape in a VTR with a video head is well known.

When performing auto-tracking, first record a signal indicating the rotational position of the video head on a control track formed in a predetermined area of the video tape when recording information, and then record the signal recorded on this control track during playback. The signal is read by the control head and the video head is positioned on the video track based on this signal.

After the video head is roughly positioned on the video track in this way, the video head plays back the video signal recorded on the video track, and the envelope of this playback signal is detected to maximize the playback output. Control the video head to the center position of the video track.

In this way, by holding the video head at the optimal position on the video track using the signal from the control track and the envelope detection signal, it is possible to reliably reproduce the video signal.

FIG. 3 shows a schematic block diagram of a conventional video signal reproducing circuit. As described above, the video head is positioned over the video track based on the signal read from the control track of the video tape by the control head (not shown), and the video signal recorded on the video track is reproduced.

The reproduced video signal is first transmitted to an IC head amplifier 10 installed near the video head (not shown).
preamplified by The amplified video signal is separated into a luminance signal and a color signal (Y-C separation), and further FM demodulated to reproduce the video signal.

On the other hand, the video signal preamplified by the head amplifier 10 is also input to a video envelope detection circuit 12, where the envelope is detected, and a DC voltage signal corresponding to the signal envelope strength is output.

This DC voltage signal is input to the control microcomputer 14, which controls the servo system 16 of the video head to perform auto-tracking so that the output of the video signal is maximized, and the position of the video head is controlled. It will be done.

FIG. 4 shows the configuration of a conventional video envelope detection circuit 12 used for envelope detection.

The video signal read out from the video track by the video head is preamplified by the head amplifier 10, and the transistor 1 in the video envelope detection circuit 12
Enter into the base of 2a.

On the other hand, a power supply is connected to the collector of this transistor 12a, and a constant current circuit for lowering the output impedance is connected to the emitter.

Furthermore, a bandpass filter 12b that passes only a predetermined frequency band (3.5 MHz to 6 MHz) is connected to the emitter of the transistor 12g in parallel with the constant current circuit. And this band pass filter 1
The output signal from 2b is input to the amplifier 12c for 2-hour normal mode (SP) and the amplifier 12d for 6-hour long mode (EP), and the signal is amplified by either amplifier depending on the mode during playback. is output. The amplified video signal is input to the detection circuit 12e, where frequency detection is performed.

The detected video signal is input to the non-inverting input terminal of the DC amplifier 12f, and only the positive voltage is taken out and a DC voltage signal is output via the transistor 12g.

Here, a circuit in which a resistor, a diode, and a DC power supply are connected in series to the output of the DC amplifier 12f (indicated by the dotted chain line in the figure) is connected in parallel to the transistor 12g, and this circuit controls the video signal. The correlation between the envelope strength and the DC voltage signal output from the emitter of the transistor 12g is defined.

[Problems to be Solved by the Invention] However, the conventional video signal reproducing circuit described above has several problems.

FIG. 2(B) shows the output characteristics of the video envelope detection circuit in this conventional video signal reproducing circuit. In the figure, the horizontal axis represents the envelope strength (mV, .) of the manually input video signal, while the vertical axis represents the output DC voltage (■).

Normally, the output from the video envelope detection circuit 12 is input to the servo system 1 for auto-tracking.
The dynamic range of the microcomputer 14 that controls 6 is ~5
Therefore, the resistance value of the circuit connected to the output of the DC amplifier 12f is adjusted so that the maximum value of the input video signal envelope strength becomes the upper limit of this dynamic range, and the output DC voltage The profile exhibits a linear characteristic that increases almost linearly with respect to the video envelope intensity, as shown in FIG. 2(B).

However, if the DC voltage output has a linear relationship with the video envelope strength, the detection sensitivity will be relatively low in the region where the video envelope strength is low compared to the region where the video envelope strength is high. When the servo system 16 is operated using a DC voltage signal, there is a problem in that the servo is not sufficiently applied in a region where the video envelope strength is at a low level, making auto-tracking difficult.

The present invention has been made in view of the above-mentioned conventional problems, and an object thereof is to provide a video signal reproducing circuit that can perform auto-tracking with high precision even when the envelope strength of the video signal is at a low level.

[Means for Solving the Problems] In order to achieve the above object, the video envelope detection circuit in the video signal reproducing circuit of the present invention includes a detection circuit that detects a high-frequency video signal and a detection circuit that outputs a low output value from the detection circuit. The level region is characterized by including a nonlinear output circuit that outputs a DC voltage signal in accordance with the envelope strength with higher sensitivity than in the high level region.

[Function] The video signal reproducing circuit of the present invention has such a configuration, and the video envelope detection circuit nonlinearly outputs a DC voltage signal according to the envelope strength. That is,
A DC voltage signal is output with high sensitivity when the envelope strength is low, and with low sensitivity when the envelope strength is high.

Then, the dynamic range of the microcomputer that controls auto-tracking can be effectively used, and tracking can be performed with high precision.

[Embodiments] Hereinafter, preferred embodiments of the video signal reproducing circuit according to the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of the configuration of the video envelope detection circuit 18 in this embodiment.

The video signal preamplified by head amplifier 10 is input to the base of transistor 18a.

On the other hand, a power supply is connected to the collector of this transistor 18a, and a constant current circuit for lowering the output impedance is connected to the emitter.

Further, a bandpass filter 18b is connected to the emitter, and a video signal in a predetermined frequency band including a luminance signal and a color signal is extracted. Then, the output signal from the bandpass filter 18b is manually inputted to the amplifier 18c for the 2-hour normal mode (S P) and the amplifier 18d for the 6-hour long mode (E P), as in the conventional case, and is output according to the mode during playback. is amplified by one of the amplifiers and output. Then, the amplified video signal is subjected to double-wave detection by the detection circuit 18e.

As a detection method, either peak detection for detecting the peak of the video signal envelope or average value detection for detecting the average value of the envelope intensity may be used, but in this embodiment, peak detection is used.

The video signal, which has been peak-detected by the detection circuit 18e and converted into a DC signal, is input to the non-inverting input terminal of the DC amplifier 18f, and only the positive voltage is taken out. This DC
Circuits 18h and 18i in which a resistor, a diode, and a voltage source are connected in series are connected to the output terminal of the amplifier 18f in parallel with the transistor 18g, and a nonlinear output circuit is configured by these DCC amplifier 18f circuit 18hs 18L) transistor 18g. There is.

The video envelope detection circuit 18 of this embodiment has such a configuration and outputs a DC voltage signal according to the envelope strength of the input video signal, but its output characteristics are nonlinear with respect to the envelope strength. There is. That is, the circuit 18 consisting of a resistor, a diode, and a voltage source is connected to the output terminals α and β in the figure of the DC amplifier 18f as described above.
h, 181 are connected, and when the output potential of the DC amplifier 18f exceeds the power supply voltage of the circuit 18h at point α, a current flows through the circuit 18h, and this current causes the base of the transistor 18g to change the resistance value of the circuit 18h. A voltage proportional to is applied and a DC voltage signal is output. On the other hand, the power supply voltage of the circuit 18i is set higher than the power supply voltage of the circuit 18h, and therefore the threshold voltage at which current starts flowing through the circuit 18i is higher than the threshold voltage of the circuit 18h.

Therefore, as shown in FIG. 2(A), when the envelope intensity of the human-powered video signal is in the low level region (region a in the figure), a current flows through the circuit 18h, and the slope is defined by the resistance value of the circuit 18h. That is, a DC voltage signal is output according to the video envelope strength at the sensitivity, and when the video envelope strength is in a high level region (region marked in the figure) above a predetermined value, a current flows in the circuit 18i as well as the circuit 18h. A DC voltage signal is output with a sensitivity defined by the combined resistance of these two circuits 18h and 18i.

As described above, the circuits 18h and 18i are connected in parallel, so their combined resistance is lower than the resistance value of the circuit 18h.

Therefore, as shown in Figure 2 (A), a nonlinear characteristic is exhibited in which the DC voltage signal is output with high sensitivity in the low-level region of the video envelope intensity, and with low sensitivity in the high-level region compared to the low-level region. .

In this way, in a region where the video envelope strength is at a low level, the output sensitivity of the DC voltage signal is set higher than in a region at a high level, so that the DC signal output from the video envelope detection circuit 18 is inputted to perform auto-tracking. When performing this, it is possible to efficiently use the dynamic range of the microcomputer 14 to output a sufficient DC voltage signal even when the video envelope strength is at a low level, and to perform auto-tracking with high precision.

In this embodiment, the nonlinear characteristics were realized using the circuits 18h and 181, but the nonlinear characteristics could also be realized by further connecting a third or fourth circuit consisting of a resistor, a diode, and a voltage source in parallel. Good too.

Furthermore, in this embodiment, the bandpass filter 18b
Although a video signal including a luminance signal and a chrominance signal is extracted using a bypass filter that transmits only the luminance signal, it is also possible to perform envelope detection of the luminance signal.

[Effects of the Invention] As described above, the video signal reproducing circuit according to the present invention has the advantage that auto-tracking can be performed with high precision even when the envelope strength of the video signal is at a low level.

[Brief explanation of drawings]

FIG. 1 is a block diagram of the configuration of a video envelope detection circuit in an embodiment of the video signal reproducing circuit according to the present invention, FIG. 2 is a graph diagram showing the relationship between envelope strength and DC voltage signal output, and 3rd rI! J is a block diagram of the configuration of a conventional video signal reproduction circuit, and FIG. 4 is a block diagram of the configuration of a video envelope detection circuit in the conventional video signal reproduction circuit. 0 2 4 6 8 Head amplifier video envelope detection circuit Microcomputer servo system video envelope detection circuit

Claims (1)

[Scope of Claims] A video signal reproducing circuit comprising a video envelope detection circuit that detects a high frequency video signal output from a video head and outputs a DC voltage signal according to the envelope strength, the video envelope detection circuit comprising: , a detection circuit that detects a high-frequency video signal, and a nonlinear output circuit that outputs a DC voltage signal in accordance with envelope strength with the output value from the detection circuit having higher sensitivity in a low level region than in a high level region. Features video signal regeneration circuit.