JPH0430706Y2 - - Google Patents

Description

[Detailed explanation of the invention] (a) Industrial application field The present invention relates to an ALC circuit in a high-fidelity video tape recorder (HiFi VTR), etc.

(b) Conventional technology Conventionally, tape recorders and high-speed VTRs
In this case, two types of settings are used for setting the recording level of the audio input signal: automatic and manual. For example, β-method Hi-Fi VTR (β-HiFi)
In this case, the automatic adjustment is made so that the average level becomes β level (-8 dB) in automatic mode, and the volume needs to be manually adjusted in manual mode so that the input level does not exceed 0 dB at its peak. For this reason, when the adjustment is off to a high level or when a signal with an excessive level is input, the maximum allowable input level of β-HiFi +
The 7dB point may be exceeded, in which case more than 100% modulation will occur and the sound will be extremely distorted. Therefore, it was devised to operate the ALC circuit as a limiter during manual operation to suppress the input signal to a certain level and prevent signal distortion at excessive levels.
An example of this is disclosed in Japanese Utility Model Publication No. 56-23792.

A circuit according to the known technique is shown in FIG.
When automatic, the output of amplifier 1 is extracted as an ALC signal, rectified and smoothed, and then sent to ALC transistor 2.
The impedance between the collector and emitter of the ALC transistor 2 is successively changed according to the level of the output signal, and the output signal is automatically adjusted. During manual operation, the input level of the amplifier 1 is adjusted by a variable resistor 3,
By inserting a resistor 5 between the output terminal of the rectifying and smoothing circuit 4 and the ground, the electric charge of the capacitor 6 in the rectifying and smoothing circuit 4 is discharged through the resistor 5, so the value of the resistor 5 can be set appropriately. If you select this, you can obtain a recovery time suitable for limiter operation.
Furthermore, the resistor 5 can also be used to set the starting point of the limiter operation. That is, it can be used as a limiter.

However, in the known technique, as described above, both the setting of the start point of the limiter operation and the setting of the recovery time are greatly influenced by the resistor 5, and therefore the value of the resistor 5 must be adjusted to achieve the desired setting for one of them. If one is selected, the other cannot be set as desired, that is, it is impossible to independently set both the start point of limiter operation and the recovery time to free values.

(c) Purpose of the invention This invention was created in view of the above points,
The purpose of this study is to propose an ALC circuit that can set both the start point and recovery time of limiter operation to desired values, and that operates as a limiter during manual operation.

(d) Structure of the invention The invention comprises a first changeover switch that changes the level adjustment of the audio input signal between automatic and manual, an amplifier that amplifies the input signal, and an input switch of the amplifier when the first changeover switch is set to the manual side. It consists of a variable resistor that adjusts the level, a voltage divider circuit that extracts a part of the output of the amplifier as a control signal and divides the control signal, and a diode and a capacitor. In an ALC circuit consisting of a rectifier and smoothing circuit that performs rectification and smoothing, and a transistor whose collector-emitter path is connected between the input side of the amplifier and ground, and the output of the rectifier and smoothing circuit is applied to the base, the circuit responds to the changeover switch. A first resistor is inserted between the output end of the voltage dividing circuit and ground when the voltage dividing circuit is operated manually, and a switch means is provided for removing a part of the capacitor of the rectifying and smoothing circuit, so that the voltage dividing circuit operates as a limiter during the manual operation. This is an ALC circuit.

(e) Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram of an ALC circuit according to the present invention, and FIG. 3 is a characteristic diagram of the ALC circuit. The input terminal 7 is connected to the input end of the amplifier 1 via a fourth resistor 8, and furthermore, an input signal transmission line 9 subsequent to the fourth resistor 8 is connected to the collector of the transistor 2, and at the same time, one terminal c is opened and the movable contact b
A variable resistor 3 whose terminal d is connected to a fixed terminal e of a grounded first changeover switch 10 is connected to the input signal transmission line 9. Note that the fixed terminal f of the first changeover switch 10 is open. That is, only when the first changeover switch 10 is closed on the fixed terminal e side, the variable resistor 3 is inserted between the input signal transmission line 9 and the ground. The output terminal of the amplifier 1 is connected to the output terminal 11 of the entire ALC circuit, and a second resistor 12 and a third resistor 13 are inserted in series between the output terminal 11 and the ground, and the second resistor 12 and the third resistor 13 are connected in series. By taking out the output from a point g between 12 and 13, the output of the amplifier 1 is taken out as a control signal and the second and third resistors 12,
13 will result in partial pressure. Furthermore, the first resistor 14
is connected to be inserted in parallel with the third resistor 13 by closing the movable contact piece of the grounded second changeover switch (switch means) 15 of the fixed terminal h to the terminal i side. Said first, second and third resistors 1
The output of the voltage divider circuit 16 consisting of 4, 12, 13
The divided control signal is applied to the base of the transistor 2 whose emitter is grounded through the rectifier and smoothing circuit 4 consisting of 9 and the fifth resistor 20, and the control signal of the transistor 2 is controlled by the voltage-divided control signal. has been done. However, the configuration of the rectifying and smoothing circuit 4 is as follows:
The diode 17 is inserted in series between the output terminal of the voltage dividing circuit 16 and the base of the transistor 2, and the first capacitor 18 is inserted between the diode 17 and the subsequent control signal transmission line 21 and ground, Further, the second capacitor 19 is inserted in parallel with the first capacitor 18 only when the movable contact piece of the second changeover switch 15 is closed toward the fixed terminal j. Note that the second changeover switch 15 is different from the first changeover switch 1.
Switches in response to 0. That is, when the ALC is on, by closing the movable contact piece of the first changeover switch 10 toward the fixed terminal f, the movable contact piece of the second changeover switch 15 automatically closes toward the fixed terminal j side.
When the ALC is off, the first changeover switch 10
By setting the terminal to the fixed terminal e side, the second changeover switch 15 is automatically switched to the fixed terminal i side.

Next, circuit operation will be explained. When the recording level is automatically set, that is, when the ALC is on, the first selector switch 10 is switched to the fixed terminal f side, and the second selector switch 15 is switched to the fixed terminal j side, as described above. Therefore, a part of the output of the amplifier 1 taken out to the control signal transmission line 21 as a control signal is divided by the second and third resistors 12 and 13, and then passed through the rectifying and smoothing circuit 16. After being rectified and smoothed, it is applied to the base of the transistor 2. That is, the transistor 2 is controlled by the control signal, the impedance between the collector and emitter changes successively, and the amplifier 1 is controlled by the voltage division between the fourth resistor 8 and the impedance.
controls the input level of the signal and adjusts the output signal level. In other words, when the output of the amplifier 1 becomes high level, the voltage level applied to the base of the transistor 2 also becomes high, and the impedance between the collector and emitter becomes small, and the amplifier 1
The input level of is suppressed and the output signal level is adjusted. For example, as shown in FIG. 3b, in a β-type high-fidelity VTR (β-HiFi), the average level is adjusted to −8 dB (β level). However, in this case, the recovery time is determined by the sum of the capacitances of the first capacitor 18 and the second capacitor 19 and the time constant by the fifth resistor 20. When setting the recording level manually, as described above, the first changeover switch 10 is switched to the fixed terminal e side, and the second changeover switch 15 is switched to the fixed terminal i side. Therefore, the output signal level can be adjusted by manually operating the variable resistor 3 and adjusting the resistance between a and b. Also, a first resistor is connected in parallel with the third resistor 13.
Since the resistor 14 is inserted, the voltage division ratio of the control signal taken out from the output terminal of the amplifier 1 is different from when the ALC is on, and the ALC operation start point as a limiter is set at a higher level (for example, β-
In HiFi, it can be set to +7 dB), and since the second capacitor 19 is removed, the time constant becomes smaller and the recovery time becomes shorter. Therefore, as shown in FIG. 3c, it is possible to perform an ALC operation, that is, a limiter operation, which has a short recovery time and operates only when the output signal to which an excessive input signal is input reaches an excessive level. Note that a in FIG. 3 shows the case where no limiter operation is performed.

(F) Effects of the invention As mentioned above, according to the invention, when the recording level is set automatically, it works as an ALC circuit, and when it is set manually, it works as a limiter circuit that suppresses an excessive output signal level to a constant level. Moreover, the first
By appropriately selecting the resistance value of the resistor, it is possible to set the desired limiter operation starting point.
By appropriately selecting the capacitance of the capacitor, it is possible to easily set the desired recovery time, which is extremely effective as it provides a degree of freedom in design.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of the present invention, FIG. 2 is a circuit diagram of a conventional example, and FIG. 3 is a characteristic diagram of the circuit of FIG. Explanation of main figure numbers, 1... Amplifier, 2... Transistor, 3... Variable resistor, 4... Rectifying and smoothing circuit, 10... First changeover switch, 14... First resistor, 15... Switch Means, 16... Voltage dividing circuit,
17...diode, 18...first capacitor, 19...second capacitor.

Claims (1)

[Scope of utility model registration claim] Automatic level adjustment of audio input signal (ALC on)
Alternatively, a first changeover switch that switches to manual mode (ALC off); an amplifier that amplifies the input signal; a variable resistor that adjusts the input level of the amplifier when the first changeover switch turns off the ALC; a voltage divider circuit that extracts a part of the control signal as a control signal and divides the voltage of the control signal; a rectifier and smoothing circuit that includes a diode and a first capacitor and rectifies and smoothes the control signal voltage divided by the voltage divider circuit; In an ALC circuit composed of a transistor in which a collector-emitter path is connected between the input side and ground, and the output of the rectifying and smoothing circuit is applied to the base via a time constant resistor, in response to the first changeover switch, A switch means is provided for inserting a first resistor between the output end of the voltage dividing circuit and the ground when the ALC is turned off, and a second capacitor inserted between the output end of the rectifying and smoothing circuit and the ground when the ALC is turned on. An ALC circuit characterized in that it operates as a limiter by switching the ALC operation start point and ALC time constant.