JPH0312832B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides
This concerns the AGC circuit.

(Conventional configuration and its problems) Figure 1 shows a conventional VIF/AGC circuit and its peripheral block diagram. The IF signal from the tuner is input from 1 and is amplified by IF amplifier 2. The gain of this amplifier can be controlled by applying a voltage from the terminal 24. The signal 3 amplified to a constant output is sent to the detector 4
The signal is detected and output to 25 as a video signal. It is further amplified by amplifier 5, and its output is applied to the next differential amplifier as _VB . transistor 6,
One end of the differential amplifier constituted by 11 is fixed at the voltage V _C , and the other end detects the synchronization signal tip of the signal V _B. That is, when the detection output 25 becomes 2V _pp , the synchronous signal tip voltage of the amplified signal V _B is V _C
It's in line with that. Here, when the tip of the synchronizing signal falls below V _C , a current flows to the collector of the transistor 11 and discharges the charge from the capacitor 18 . Also, since the potential other than the V _B synchronization signal section is higher than V _C , current flows through transistor 6 and due to the potential drop across resistor 7 and diode 8, resistor 9 and transistor 10 act as a current mirror, and the current flows to capacitor 18 and charges it. be done. Figure 2 shows this change in detail, showing the terminal voltage of the capacitor.
V _A rises and falls due to charging and discharging, as shown in Figure 2b. In particular, in the equivalent pulse period and the vertical synchronization period of the transmitted video signal shown in FIG. That is, the signal period has a long interval between synchronizing signals, and charging and discharging are designed based on this period and remain constant, but when the equivalent pulse starts, the interval between synchronizing signals becomes narrower and the charging period becomes shorter. Then capacitor 1
8 voltage V _A gradually decreases as shown in Figure 2b.
Furthermore, during the vertical synchronization pulse period, the charging/discharging period is reversed and V _B particularly drops. However, it does not go down gradually, but becomes constant in the middle. It transfers this V _B voltage to terminal 24 through transistor 23, resistor 22, diode 21, and Zener diode 20.
This is to guide the signal from the signal to the IF amplifier for gain control. 1, as the IF input becomes smaller, the detection output 25 also becomes smaller, and the tip of the synchronization signal applied to the transistor 6 also becomes higher than V _C . Then, the capacitor 18 is no longer discharged and the potential of the voltage V _A becomes high, and this voltage V _A amplifies the gain of the IF amplifier and the detection output 25 of the detector 4 also increases and remains constant. Conversely, if the IF input is large, the AGC voltage will decrease and the IF input will decrease.
Lower the amplifier gain. The AGC circuit operates in this manner to keep the detection output 25 constant. but,
As mentioned above, in this method, the AGC voltage drops in a part of the waveform (before and after the vertical synchronization pulse), so if you control the IF amplifier with this voltage, the detection output 2
5, as shown in FIG. 2c (illustrated with negative polarity for ease of understanding), the vertical synchronization period is reduced and faithful waveform reproduction is not possible. As a result, vertical synchronization becomes unstable, ghost detection using the first step pulse of the vertical synchronization pulse, and changes in the IF amplifier due to AGC voltage fluctuations result in phase changes in the SIF output, resulting in audio S/S/ N deteriorates. One way to improve this problem is to make the resistor 19 in FIG. 1 smaller, but the detection output 25 at the time of a weak electric field deteriorates.

(Objective of the Invention) The present invention provides an AGC circuit that eliminates the above-mentioned drawbacks, maintains a faithful detected waveform, and increases weak electric field sensitivity.

(Structure of the Invention) The present invention is a peak value type AGC circuit in a VIF block, which compares the leading edge of a synchronization signal of a television signal obtained by video detection of a VIF signal with a first reference voltage. a first comparator; a charging/discharging circuit in which a first detection output current proportional to the potential difference between the tip of the synchronizing signal from the first comparator and a first reference voltage is charged or discharged; and the charging/discharging circuit. a second comparator that compares the charging potential of the charging and discharging circuit with a second reference voltage, and a second detection output that is proportional to the potential difference between the charging potential of the charging and discharging circuit and the second reference voltage from the second comparator. and current adding means for adding a current to the first detection output current that is charged to the charging/discharging circuit, and the gain of the IF amplifier of the VIF block is controlled by the charging voltage of the charging/discharging circuit. It is.

(Description of Embodiment) An embodiment of the present invention will be described below with reference to FIG. 3.

An IF signal input 1 from the tuner passes through an IF amplifier 2 and a detector 4 and appears as a video detection output 25. Further, it is amplified by an amplifier 5 and inputted to the base of the transistor 6 side of an operational amplifier consisting of transistors 6 and 11 forming a first comparator.
The tip of the synchronization signal of the video signal V _B is the transistor 11
When the voltage applied to the base of the transistor 6 falls below V _C (when the detected output 25 exceeds a predetermined amplitude, for example, 2V _pp ), the transistor 11 becomes conductive, and the transistor 6
is cut off, and no current flows through diode 8, resistors 7 and 9, and transistor 10. When transistor 11 becomes conductive, the current flows through capacitor 18.
This occurs as a discharge current from the capacitor 18, and the terminal voltage V _A of the capacitor 18 drops. However, the width of the horizontal synchronization signal is only 4 μsec, and the discharge immediately stops. Next, the video signal V _B enters the video period, but its voltage is
Since it is higher than V _C , transistor 6 conducts and current i ₆ flows, and current flows through resistors 7 and 9 (current i ₁₀ flowing through resistor 9 (resistance value R ₂ ) is equal to resistor 7 (resistance value R ₁ ). i ₁₀ ≒ i ₆ ×R ₁ / _R2 ), which is determined by the resistance ratio of i 10 ≒ i 6 ×R 1 /R2), and a charging current flows into the capacitor 18 through the transistor 10, and the voltage V _A increases. However, part of the charging current is also shunted to the charging merging resistor 19. This video period is long and the charging voltage gradually increases. When the next horizontal synchronization signal comes, the discharge occurs again, and the V _A voltage remains constant, but the above charging and discharging periods are different during the equivalent pulse period and the vertical synchronization pulse period, and the V _A voltage drops, creating a step and causing harmful effects. bring. Here, as the charging shunt resistor 19 of the capacitor 18 is made smaller, the charging shunt increases, so the charging current to the capacitor 18 decreases. Then, the V _A level during the horizontal synchronization period decreases, and the difference between it and the vertical synchronization period disappears, and the initial adverse effects are reduced. However, the input of radio waves decreased,
When the AGC voltage V _C rises and the gain of IF amplifier 2 is set close to full, noise increases in the detection output 25.
The horizontal synchronizing signal pulse width appears to be wide due to noise before and after the horizontal synchronizing signal section. Then, the discharge current increases because the charging and discharging to the capacitor 18 has been balanced and the IF gain has been controlled as described above. That is, the AGC voltage V _A decreases and the gain of the IF amplifier 2 does not reach its maximum.
The present invention uses the second comparators of the transistors 30 and 32 to detect that the voltage V _A rises during this weak electric field and that the voltage V _A generates the voltage V _D determined by the intersection of the resistors 17 and 28. When the current is made conductive and the transistor 29 is also made conductive, the resistor 27 (resistance value R ₃ ) is connected in parallel with the resistor 9 and the charging current i ₁₀ ≒i ₆ ×R ₁ /R ₂ ×R ₃ /R ₂ +R ₃ and increases.

Since the discharge current remains unchanged, the AGC voltage V _A rises, enough to bring the IF amplifier to its maximum gain.

(Effects of the Invention) As described above, the present invention faithfully reproduces a video signal, reduces the instability of vertical synchronization, and improves the fidelity of ghost detection using the first step pulse of the vertical synchronization pulse and the AGC voltage. This prevents changes in the IF amplifier due to fluctuations from causing phase changes in the SIF output and deterioration of audio S/N. Since the second detection output current is added to the first detection output current that is charged to the charge/discharge circuit for AGC control, the AGC voltage can be well averaged.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a conventional AGC device.
Figure 2 is a waveform diagram for explaining the operation of Figure 1;
The figure is a circuit diagram of an AGC device in an embodiment of the present invention. 2...IF amplifier, 4...Detector, 5...Amplifier, 6, 10, 11, 29, 30, 32...Transistor, 7, 9, 17, 27, 28...Resistor,
18...Capacitor, 19...Charging shunt resistor.

Claims (1)

[Claims] 1. A peak value type AGC circuit in the VIF block, which comprises a first circuit that compares the leading edge of the synchronization signal of the television signal obtained by video detection of the VIF signal with a first reference voltage. a charging/discharging circuit in which a first detection output current proportional to the potential difference between the tip of the synchronizing signal from the first comparator and a first reference voltage is charged or discharged; and charging/discharging circuit. a second comparator that compares the potential with a second reference voltage; and a second detection output current proportional to the potential difference between the charging potential of the charging/discharging circuit and the second reference voltage from the second comparator. , current adding means for adding to the first detection output current charged to the charging/discharging circuit, the IF of the VIF block is controlled by the charging voltage of the charging/discharging circuit.
AGC circuit configured to control amplifier gain.