JPH04104584A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To obtain a high-frequency signal in which there is no break of a chrominance signal or no irregularity of the hue by switching the gain of the automatic gain control circuit at the time of normal reproduction and specific reproduction and reducing the gain at the time of specific reproduction lower than that of the normal reproduction. CONSTITUTION:The automatmic gain control circuit is provided with a reference voltage source 11 equipped with first reference voltage, and a reference voltage source 12 equipped with second reference voltage lower than this, and either of these are supplied to a changeover circuit 13. The changeover circuit 13 switches the output of the first and second reference voltage sources by a control signal supplied to a control input terminal 14 to be supplied to a comparison circuit 8 as the reference voltage. In short, the reference voltage source of the comparison circuit at the time of normal reproduction and specific reproduction to be supplied to the comparison means 8, and the comparison means 8 compares this reference voltage with the detection signal of the level detection means so as to be supplied as the control signal of a gain control means 5. At the time of specific reproduction, the reference voltage lower than that of the normal reproduction is selected. Thus, even at the time of the mountain part of the amplitude of the high-frequency signal, the irregularity of the hue by the saturation of the chrominance signal in a chrominance signal processing part can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an automatic gain control circuit used in a magnetic recording/reproducing device such as a video tape recorder.

[Conventional technology]

Video tape recorder for home use (hereinafter referred to as VTR)
When reproducing an FM-modulated video signal, the high-frequency signal obtained from a pair of magnetic heads does not have a constant amplitude due to variations in the heads between channels, variations in amplifier gain, and the like. Therefore, an automatic gain control circuit is provided to control this to a constant value and output the video signal continuously. FIG. 2 is a block diagram showing an example of a conventional automatic gain control circuit. In this figure, input terminals 1.2 are alternately supplied with signals read from a rotary head attached to a cylinder (not shown), and the outputs thereof are supplied to a pair of input ends of a switching circuit 3. The switching circuit 3 selects the output of either the A head or the B head based on a head switching signal applied to an input terminal 4, and its output is applied to a gain control circuit 5. The gain control circuit 5 amplifies the signal by changing its gain based on the control signal, and its output is given to the output terminal 6 and the level detection circuit 7. The level detection circuit 7 detects this output level,
Its output is given to a comparator circuit 8. A constant reference voltage source 9 is connected to the comparator circuit 8, and by comparing this level with the output of the level detection circuit 7, a control signal is obtained and fed back to the gain control circuit 5. In this way, the signals obtained from the pair of heads are controlled so that they always remain constant and form a bell.

(Problem to be Solved by the Invention) However, in such a conventional automatic gain control circuit, the output of the automatic gain control circuit is kept constant during normal playback, but during special playback such as high-speed playback, slow play, and still playback, the output of the automatic gain control circuit is kept constant. Since one head traces multiple tracks,
The high-frequency signal output from the head amplifier has a wavy enhelobed waveform with peaks and valleys. Therefore, the amplitude is larger in the peak part than in normal reproduction, and the color signal may be saturated at the peak part of the amplitude in the subsequent luminance signal and color signal reproduction processing section, resulting in a disadvantage that the hue may be disturbed.

The present invention has been made in view of these conventional problems, and by switching the gain of the automatic gain control circuit during normal playback and special playback, it is possible to prevent color signals from being saturated even during special playback. A technical problem is to obtain a high frequency signal that does not cause the color signal to become blurred or the hue to be disturbed.

[Means to solve the problem]

The present invention is an automatic gain control circuit used in a magnetic recording/reproducing device, which comprises: a first selection means for selecting video signals read out from a plurality of video heads in synchronization with a head switching signal; gain control means for varying the output gain of the gain control means, level detection means for detecting the output level of the gain control means, a plurality of reference voltage sources having different voltages, and a reference voltage different during normal playback and special playback. A device characterized by comprising: a second selection means for selecting a voltage source; and a comparison means that compares the output of the level detection means and the output of the second selection means and provides it as a control signal to a gain control circuit. It is.

[Function] According to the present invention having such features, the reference voltage source of the comparison circuit is switched and applied to the comparison means during normal playback and special playback. The comparison means compares this reference voltage with the detection signal of the level detection means and provides it as a control signal to the gain control means. By selecting a lower reference voltage during special playback than during normal playback,
Even at peaks in the amplitude of the high-frequency signal, the color signal will not be saturated in the color signal processing section and the hue will not be disturbed.

〔Example〕

Next, an embodiment of the automatic gain control circuit of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of this embodiment, and the same parts as those of the conventional example described above are given the same reference numerals and detailed explanations will be omitted. In this embodiment as well, a high frequency signal obtained by amplifying the signal from the pair of rotary heads from the rotary cylinder is applied to the input terminal 1.2. These signals are switched by a switching circuit 3, which is a first selection means, by a head switching signal applied to an input terminal 4.
The signal is applied to the gain control circuit 5. The gain control circuit 5 changes its gain in response to a control signal and provides an output to the output terminal 6 and the level detection circuit 7, as in the conventional example described above. The present embodiment has a reference voltage source 11 having a first reference voltage and a reference voltage source 12 having a lower second reference voltage, both of which are supplied to a switching circuit 13. The switching circuit 13 is a second selection means that switches between the outputs of the first and second reference voltage sources in response to a control signal applied to the control input terminal 14 and supplies the outputs of the first and second reference voltage sources to the comparator circuit 8 as a reference voltage. The comparison circuit 8 compares the level detection signal from the level detection circuit 7 based on the reference voltage, and provides a control signal to the automatic gain control circuit, as in the conventional example described above.

Next, the operation of this embodiment will be explained. First, input terminal 1
．． When the hexagonal amplifier signal from the A head and the B head is input to the input terminal 2, the output thereof is switched by a head switching signal input to the input terminal 4, and is applied to the gain control circuit 5. During normal playback, the switch 13 is activated by a control signal applied to the second control input terminal 14.
is switched to select the first reference voltage source 11;
The signal is applied to the comparator circuit 8. In this case, as in the conventional example described above, the first reference voltage and the level detection signal are compared and given to the gain control circuit 5 as a control signal to keep the output constant.

Now, during special playback such as slow still playback, the second reference voltage source 12 having a voltage lower than the first reference voltage is selected by the switching circuit 13 and applied to the comparison circuit 8. The second reference voltage and the detected level are then compared and provided to the gain control circuit 5 as a control signal. Therefore, the gain during special playback is smaller than the gain during normal playback. Therefore, even the peaks of the undulating high-frequency signal are saturated beyond the dynamic range in the color signal reproduction processing section at the subsequent stage, and the color signal is not blurred or the hue is disturbed.

[Effects of the Invention] As described above in detail, according to the present invention, the gain of the automatic gain control circuit is switched during normal playback and special playback, so that the gain during special playback is lower than during normal playback.

Therefore, during special reproduction, the output level is increased beyond the dynamic range of the color signal in the color signal processing section, and it is possible to obtain a high frequency signal without distorting the color signal or disturbing the hue.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an automatic gain control circuit according to an embodiment of the present invention, and FIG. 2 is a block diagram showing an example of a conventional automatic gain control circuit. 3, 1: 3------- switching circuit, 5--
Gain control circuit, 7--- Level detection circuit, 8---
--1 comparison circuit, 11--1 first reference voltage source, 12--2nd reference voltage source. Figure 1 Figure 2 3.13-- Switching concave camellia

Claims (1)

[Claims] (1) An automatic gain control circuit used in a magnetic recording and reproducing device, comprising: a first selection means for selecting video signals read out from a plurality of video heads in synchronization with a head switching signal; and the first selection. gain control means for varying the gain of the output of the means; level detection means for detecting the output level of the gain control means; a plurality of reference voltage sources having different voltages; and a plurality of reference voltage sources having different voltages during normal playback and special playback. a second selection means for selecting a reference voltage source; and a comparison means for comparing the output of the level detection means and the output of the second selection means and providing it as a control signal to the gain control circuit. An automatic gain control circuit featuring: