JPS58170177A - Automatic gain controller - Google Patents

Abstract

PURPOSE:To assure the stable characteristics of an automatic gain controller even to the shrinkage of a noise and a synchronizing signal, by limiting the width of a horizontal synchronizing signal within a prescribed period from the front edge of said signal in case the width of the signal is extraordinarily increased so that the rear edge of the reference pulse lags by a prescribed time from the rear edge of the horizontal synchronizing signal in a normal state. CONSTITUTION:The composite video signals of the Fig. A are supplied, and the signals of the Fig. B are obtained at an output of a synchronizing signal separating circuit 5. In this case, signals (c) and (d) are generated by the malfunction of the circuit 5 which are due to a noise pulse (a) and the shrinkage (b) of a synchronizing signal. When such signals (c) and (d) are applied to a pulse delaying circuit 9, a pulse (c) of a narrow width is eliminated and the signals of the Fig. C are obtained. When the signals of the Fig. B are applied to a monostable multivibrator 10, an output of the Fig. D is obtained regardless of the pulse width. Therefore signals of the Figures C and D are applied to a multiplier 11, and the signals of the Fig. E are obtained at the output of the multiplier 11. Then the signal of the Fig. F emerge to the output of a mixing circuit 4, i.e., the input of a peak value detecting circuit 7. Thus the normal signal level V0 is detected to keep the stable gain of a gain control circuit 2.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic gain control device for video signals used in magnetic recording and reproducing devices, television devices, and the like.

Generally, an automatic gain control device for a video signal inserts a pulse of a constant (7) level value into the blanking portion of the video signal extracted from the composite video signal, and then adjusts the magnitude of this pulse to the magnitude of the horizontal synchronizing signal. The gain of the composite video signal is controlled by detecting the sum.The pulses inserted into the blanking portion are generated based on the horizontal synchronization signal extracted from the composite video signal by the synchronization separation circuit.

Therefore, if the video signal contains noise (noise) or if only the synchronization signal part becomes lower than the normal level for a certain period of time, the synchronization separation circuit will malfunction.
If an extra pulse other than the horizontal synchronization signal appears in the output, or the pulse width becomes abnormally long, normal level detection is not performed, and as a result, the gain of the composite video signal is abnormally suppressed. There were various inconveniences.

FIG. 1 shows a block diagram of a conventional automatic profit system (wealth) system. In the figure, an input signal such as a video signal is applied to a terminal 1, passes through a gain control circuit 2, is taken out from an output terminal 3, and is also applied to a mixing circuit 4 and a sync signal separation circuit 5. The output of 6 is applied to the delayed pulse generation circuit 6, which generates a pulse of a predetermined level value with a predetermined phase delay n time from the horizontal synchronizing signal n1, and is applied to the mixing circuit 4. In the mixing circuit 4, after the horizontal synchronizing signal part of the video signal is clamped, blanking circuit 1
The pulse of the predetermined level value mentioned above is mixed with the output portion of the signal, and this output is applied to the peak value detection circuit 7. Peak value detection circuit 7
detects the peak value of the applied signal, and supplies, for example, a DC voltage to the gain control circuit 2 via the field circuit 8 in accordance with the magnitude of the peak value.

In the apparatus configured as described above, a composite video signal as shown in FIG. 2, for example, is supplied to the input terminal 1 f'1. In this case, a signal as shown in FIG. 2B is obtained at the output of the synchronizing signal separation circuit 6.
Ru. C and d shown in Figure 2B are pulses generated as a result of malfunction of the synchronization signal separation circuit 6 due to the noise pulse a and the compression of the synchronization signal b contained in the composite video signal shown in Figure 2B. be.

When such a pulse signal C9d is applied to the delayed pulse generation circuit 6, if the delayed pulse generation circuits integrate the output of the synchronizing signal separation circuit 6 and then perform waveform shaping, the output In this case, a narrow pulse like the noise pulse C can be removed, but the wide pulse d generated by the contraction b of the synchronization signal remains as it is. Appears in the output as a pulse. Since the mixing circuit 4 mixes the signals having the waveforms shown in FIG. 2 and C, the output thereof is as shown in the second diagram.

If the delay pulse generation circuit e detects the rising edge of the output of the synchronizing signal separation circuit 6 and delays it for a certain period of time to operate the monostable multivibrator, then a signal as shown in FIG. 2 R is obtained as its output. Although the phase is different from the wide pulse d generated by the contraction of the synchronization signal b, a normal width pulse can be generated within the range of the blanking part, but a narrow noise pulse A pulse with a constant width also appears in the output for C. As a result, the output of the mixing circuit 4 becomes as shown in FIG. 2F.

In the second diagram, when the signal shown at F is applied to the peak value detection circuit 7, the signal level vl is higher than the normal signal level vo.

v2 was detected and the gain of gain control circuit 2 was abnormally suppressed. In general, the hold circuit 8 connected between the peak value detection circuit 7 and the gain control circuit 2 has a discharging time constant set to be much smaller than the charging time constant, so once v1 or v2 is detected, the It takes a long time to return to the above state, and during this time the video signal level output to the terminal 3 continues to be lower than the normal level value.

The object of the present invention is to eliminate such drawbacks and provide an automatic gain control device that can obtain stable GC characteristics even against noise, synchronization signal compression, and the like.

An embodiment of the present invention will be described below with reference to FIG. 3 and FIG. 4. FIG. 3 is a block diagram of an automatic gain control device which is an embodiment of the present invention.

In FIG. 3, an input signal such as a video signal is applied to a terminal 1, passes through a gain control circuit 2, is output from an output terminal 3, and is also applied to a mixing circuit 4 and a synchronizing signal separation circuit 6. . The output of the synchronization signal separation circuit 6 is sent to the pulse delay circuit 9.
is added to the monostable multivibrator 1o. The pulse delay circuit 9 integrates the input pulse to form a trapezoidal wave or a triangular wave, and then shapes the waveform to obtain a delayed pulse.The pulse delay circuit 9 sets the delay time to be shorter than the width of the bank pouch in the blanking portion. The monostable multivibrator 10 is triggered at the leading edge of the horizontal synchronizing signal, and the pulse width is set to be at least 2μ longer than the width of the horizontal synchronizing signal and shorter than the blanking part. Stable multi piperator 1
The output of 0 is applied to the multiplier 11, and the output of the multiplier 11, that is, the reference pulse, is mixed at a predetermined level with the blanking portion of the video signal clamped by the horizontal synchronizing signal portion in the mixing circuit 4. Ru. The output of the mixing circuit 4 is sent to the peak value detection circuit 7
In addition to n1, a DC voltage fC is supplied to the gain control circuit 2 via the hold circuit 8 according to the magnitude of the peak value n1.

Next, consider a case in which a composite video signal as shown in FIG. 4B is supplied to the input terminal 1 of this apparatus, and a signal as shown in FIG. Figure 4B
C and d shown in FIG. 4 are generated as a result of the synchronization signal separation circuit 6 malfunctioning due to noise pulses a and compression of the synchronization signal b included in the composite video signal shown in FIG. When such a signal is applied to the pulse delay circuit 9, nJ
When C, a narrow pulse like the noise pulse C is removed and an output signal as shown in FIG. 4C is obtained.Also, a signal as shown in FIG.
In addition to 2" I-ta, an output signal as shown in Figure 4 is obtained regardless of the width of the pulse. Therefore, Figure 4 C2D
Since a signal such as shown in FIG. 4 is applied as an input to the multiplier 11, a signal as shown in FIG. Since a signal such as F appears and a normal signal level vo is detected, the gain of the gain control circuit 2 is kept stable. - In this way, in this embodiment, the trailing edge of the pulse signal input to the mixing circuit 4 (FIG. 4E) is the output of the pulse delay circuit 9 (FIG. 4C), that is, the synchronizing signal separation circuit 6 during normal operation.
The output of the monostable multivibrator 10 is determined by a signal whose trailing edge (FIG. 4B) is delayed by a predetermined time. (7) Since the trailing edge of the output (FIG. 4D), that is, the leading edge of the output (FIG. 4B) of the synchronizing signal separation circuit 5, is determined by a signal delayed by a predetermined time, the pulse delay circuit 9
Normal operation is possible even if both the delay time and the output pulse width of the monostable multivibrator 10 have relatively large variations.

In this example, monostable multivibrator 1
The width of the reference pulse is limited by 0 and the multiplier 11, but instead of this, a pulse is generated at a predetermined delay time from the leading edge of the horizontal synchronizing signal, and the pulse delay circuit is activated at the leading edge of the pulse. Even if the output of 9 is reset, the same effect cannot be obtained.

In the above example, the input of the synchronizing signal separation circuit 6 is supplied from the output terminal 3, but the same effect can be obtained even if the input is supplied from the input terminal 1.

As described above, according to the present invention, stable gain control operation is possible even when the synchronization signal separation circuit malfunctions due to noise pulses, contraction of the synchronization signal, or the like.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional automatic gain control system, FIG. 2 is a waveform diagram of the same device, and FIG. 3 is a block diagram of an automatic gain control device that is an embodiment of the present invention. Figure 4 is a waveform diagram of the same device. 1...terminal, 2...gain control circuit, 3
...Output terminal, 4...Mixing circuit, 6.
... Synchronization signal separation circuit, 6 ... Delay pulse generation circuit, 7 ... Peak value detection circuit, 8 ...
...Hold circuit, 9...Pulse delay circuit,
10... Monostable multivibrator, 11...
...Multiplier. Name of agent: Patent attorney Toshio Nakao and 1 other person
Ill @3wA II 4 Figure Director General of the Patent Office l Case Display Patent Application No. 62500 of 1982 Applicant (L+・1 10 Oaza Kadoma, Kadoma City, Osaka Prefecture)
06 Address Name 41H (582) Matsushita Electric Industrial Co., Ltd. Representative Toshihiko Yamashita 4 Agent 571 Address: 1 1006 Kadoma, Kadoma City, Osaka Prefecture
3 consecutive active duty (Mushibite 437-1 t21 holding branch)! Subject to 6 corrections

Claims (1)

[Claims] a gain control circuit that controls the gain of the composite video signal; a synchronization separation circuit that extracts a horizontal synchronization signal from the composite video signal; and a delay circuit that generates a signal delayed by a predetermined time from the leading edge of the horizontal synchronization signal; a delay pulse generation circuit that generates a signal in which the trailing edge of the horizontal synchronization signal is delayed by a predetermined time; a pulse generation circuit that generates a reference pulse using the output of the delay circuit and the output of the delay pulse generation circuit; a mixing circuit that mixes the reference pulse at a certain level into a part of the output signal of the gain control circuit so that the polarity is opposite to that of the horizontal synchronization signal in the output signal; a peak value detection circuit for controlling the gain of the gain side circuit according to the sum of the magnitude of the larger one of the output signals and the magnitude of the horizontal synchronization signal; The trailing edge of the reference pulse is a predetermined delay time from the trailing edge of the horizontal synchronizing signal during normal conditions, and a predetermined delay time from the leading edge of the horizontal synchronizing signal when the width of the horizontal synchronizing signal becomes abnormally wide. An automatic gain control device characterized in that the automatic gain control device is configured to prevent an abnormally large signal from being input to the peak value detection circuit.