JPH0325049B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to an automatic level adjustment circuit that automatically adjusts the input level during recording, for example, with a tape recorder. This relates to improvements that can prevent this.

[Technical background of the invention and its problems] Generally, for example, in a tape recorder that uses batteries as a power source, an automatic level adjustment circuit (hereinafter referred to as ALC circuit) that automatically adjusts the input level during recording is used.
As shown in FIG.
The signals are supplied to and amplified, respectively, and converted into DC voltage signals by rectifier circuits 15 and 16 comprising coupling capacitors C1 and C2, distortion reduction resistors R1 and R2, and voltage doubler rectification diodes D1 and D2, respectively. And level detection diodes D3, D
4. After setting the attack time and recovery time of the DC voltage signal using the charging time constant and discharging time constant of the capacitor C3 in the time constant circuit 17 consisting of the capacitor C3 and the discharging resistor R3, This level adjuster 1
The level adjustment amounts of Nos. 1 and 12 are variably controlled.

In other words, if the above ALC circuit receives a reference signal of +10 [dB] relative to the input reference voltage level Vref at time t1, for example, its output will reach the saturation level as shown in Figure 4. However, the level is gradually lowered and limited to a stable level. Usually, the time T from when the reference signal is input to the stable level of +3 [dB] is called attack time. This attack time is determined by a time constant set by the level detection diodes D3, D4 and capacitor C3.

FIG. 5 shows the input/output characteristics of the level adjusters 11 and 12 when the ALC circuit is operated and when it is not operated. That is, the above ALC
When the circuit is operated, as shown in A in the figure, when the input level exceeds the reference voltage level Vref, the output level is limited so that it does not exceed the normal operation saturation level V0, but if the ALC circuit is not operated, B
As shown in the figure, the input resistance becomes low, and the output level exceeds the normal operation saturation level V0 at the input level V1, resulting in the output signal clipping.
In particular, in a tape recorder that uses a battery as a power source, the power supply voltage level +B is low, so the input resistance level V1 is also extremely low.

Incidentally, tape recorders are increasingly being equipped with noise reduction (hereinafter referred to as NR) systems such as the Dolby (trademark) system for the purpose of expanding the dynamic range during recording and reducing tape hiss noise. . However, when the Dolby NR system and the ALC circuit are used together, it is required to set the attack time to about 5 [msec] under the input conditions explained in FIG. 4. However, this only applies when the input signal is a radio reception signal or a line input signal from the AUX terminal, and does not apply to the microphone input. In other words, in microphone input mode, if the attack time is fast, when shock noise is input due to hitting the microphone, blowing into the microphone, etc., the ALC will be applied too deeply and the recording level will become low, so the attack time should be slowed down. There is a need. However, the ALC circuit shown in Figure 3 uses a diode as the level detection element, so the attack time is approximately
100 [msec], which cannot be used in conjunction with the above-mentioned Dolby NR system.

In order to improve this, as shown in Figure 6,
It has been proposed to use transistors Q4 and Q5 in place of the diodes D3 and D4, thereby reducing the operating resistance and increasing the attack time. However, such an ALC circuit
If there is pulse noise etc. in the input signal, capacitor C3 will be overcharged and the recovery time of the ALC circuit will become longer, causing the problem that it will take a long time to return to the original operating state after the noise disappears. There is.

[Purpose of the Invention] This invention was made to improve the above-mentioned problems.In the line input mode, it prevents overcharging of the capacitor for setting the time constant, and in the microphone input mode, it slows down the attack time of the recording signal. do,
An object of the present invention is to provide an automatic level adjustment circuit for an audio signal recording device that can prevent malfunctions such as excessive application of ALC due to noise in each mode.

[Summary of the Invention] That is, the present invention provides an automatic level adjustment circuit for an audio signal recording device that selects either an audio signal from a line input signal or a microphone input signal, automatically adjusts the level, and records it on a recording medium. a signal conversion circuit that converts the input signal selected by the audio signal selection operation into a DC voltage signal; and a signal conversion circuit that inputs the DC voltage signal converted by this circuit and outputs a current according to the voltage level. The output current of the level detection transistor is charged and discharged with a time constant determined by the values of both elements, and the attack time and recovery time of the DC voltage signal are a time constant circuit that sets a time constant circuit; a level adjustment means that adjusts the level of the input signal selected in the selection operation according to the charging/discharging output of this circuit;
a changeover switch that connects a current source to the level detection transistor when a line input signal is selected and disconnects the current source and operates the level detection transistor as a diode when the microphone input signal is selected; The present invention is characterized by comprising overcharge prevention means for restricting the driving voltage level of the transistor to prevent overcharging of the capacitor when the current source is connected to the level detection transistor.

[Embodiment of the Invention] Hereinafter, an embodiment of the present invention will be described in detail with reference to FIG. However, in FIG. 1, the same parts as in FIGS. 3 and 6 are denoted by the same reference numerals, and only the different parts will be described here.

Figure 1 shows the configuration when the ALC circuit according to the present invention is applied to a cassette tape recorder with a radio equipped with a Dolby NR system.
In the figure, reference numeral 19 denotes a first changeover switch which is interlocked with a function switch (not shown) for changing over the input modes of AUX, radio, and microphone. This first changeover switch 19 has a movable terminal a of +B.
Connected to the power supply, when this movable terminal a is connected to fixed terminal b, it is in AUX input mode, when connected to fixed terminal c, it is in radio input mode, and fixed terminal d
When connected to the microphone input mode.
The fixed terminals b and c are both connected to the respective collectors of the transistors Q4 and Q5 via a voltage limiting resistor R5, and the fixed terminal d is connected to the terminal b of the second changeover switch 20. This second changeover switch 20 is a recording (REC) switch (not shown).
This works in conjunction with the mode changeover switch for changing the mode and playback (PLAY) mode.
-b is set to REC mode when connected,
When terminals b and c are connected, PLAY mode is set. Then, this second changeover switch 2
0's terminal a is open, and the terminal c is connected to the switching transistor Q via the bias resistor R6.
Connected to the base of 6. This switching transistor Q6 has a collector connected to the time constant circuit 17.
When the second selector switch 20 is connected to the PLAY mode side, the output of the time constant circuit 17 is short-circuited to stop the operation of the ALC circuit.

The operation of the above configuration will be explained below.

In other words, the attack time is required to be fast in the ALX input mode and the radio input mode, and in the microphone input mode, as mentioned above, if the attack time is fast, it is difficult to hit the microphone or breathe into the microphone. When shock noise from spraying etc. is input, the ALC is applied too deeply and the recording level becomes low.
You need to slow down your attack time. Therefore,
In the above ALC circuit, the function switch is
When switching to AUX input mode or radio input mode, that is, when switching movable terminal a of first changeover switch 19 to fixed terminal b or c,
The +B power supply voltage is applied to the collectors of the level detection transistors Q4 and Q5 via the resistor R5, thereby reducing the operating resistance of the transistors Q4 and Q5 and increasing the attack time. At this time, since the collector potentials of the transistors Q4 and Q5 are limited by the resistor R5, the capacitor C3 will not be overcharged.

Furthermore, when the function switch is switched to the microphone input mode, that is, when the movable terminal a of the first changeover switch 19 is connected to the fixed terminal d, the +B voltage is not applied to the transistors Q4 and Q5. Conventional diodes D3, D4
This is equivalent to connecting the . Therefore, the attack time in the microphone input mode can be set late. Note that the second changeover switch 20
When set to the REC mode, the switching transistor Q6 is in an off state and the ALC circuit is in an operating state. Furthermore, when the second changeover switch 20 is set to the PLAY mode, the switching transistor Q6 is turned on.
The above ALC circuit is in a non-operational state. In other words, this means that the tape recording signal is all adjusted to the optimum level, so the tape playback signal
This is because there is no need to apply ALC.

Therefore, the ALC circuit configured as described above can speed up the attack time only in line input mode and slow it down in microphone input mode, and even if the attack time is fast, the collector potential of transistors Q4 and Q5 remains The resistance R
5 prevents overcharging of capacitor C3, thereby preventing noise-induced
This makes it possible to prevent malfunctions such as excessive ALC application.

FIG. 2 shows another embodiment of the ALC circuit according to the present invention, in which the common collectors of the transistors Q4 and Q5 are grounded via a Zener diode D5. In other words, when this ALC circuit is operating,
Transistors Q2 and Q3 are in the on state, and the base potential of transistor Q1 is approximately 1.2 to 1.4
It is about [V]. From this, capacitor C
The maximum charging voltage level of No. 3 may be set to approximately this value. Therefore, by setting the collector potential of the transistors Q4 and Q5 to be equal to the base potential of the transistor Q1 using the Zener diode D5, overcharging of the capacitor C3 can be completely prevented.

[Effects of the Invention] As described above, according to the present invention, overcharging of the time constant setting capacitor is prevented in the line input mode, and the attack time of the recording signal is slowed down in the microphone input mode, thereby reducing noise caused by noise in each mode.
It is possible to provide an automatic level adjustment circuit for an audio signal recording device that can prevent malfunctions such as excessive ALC.

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram showing one embodiment of the automatic level adjustment circuit of an audio signal recording device according to the present invention, FIG. 2 is a block circuit diagram showing another embodiment of the invention, and FIG. is a block circuit configuration diagram showing the configuration of a conventional automatic level adjustment circuit, FIGS. 4 and 5 are characteristic diagrams for explaining the characteristics of the above automatic level adjustment circuit, and FIG. 6 is a block circuit diagram showing the configuration of a conventional automatic level adjustment circuit. FIG. 3 is a block circuit configuration diagram showing an improved version. 11, 12... Level adjuster, 13, 14... Amplifier, 15, 16... Rectifier circuit, 17... Time constant circuit,
18... Control signal generation circuit, 19... First changeover switch, 20... Second changeover switch, R5... Voltage limiting resistor, D5... Zener diode.

Claims (1)

[Scope of Claims] 1. An automatic level adjustment circuit of an audio signal recording device that selects either one of a line input signal and a microphone input signal, automatically adjusts the level, and records the audio signal on a recording medium, comprising: A signal conversion circuit that converts the input signal selected by the audio signal selection operation into a DC voltage signal, and a level detection circuit that inputs the DC voltage signal converted by this circuit and outputs a current according to the voltage level. a transistor for level detection, a capacitor, and a resistor for discharging, the output current of the level detection transistor is charged and discharged with a time constant determined by the values of both elements, and the attack time and recovery time of the DC voltage signal are set. a time constant circuit; a level adjustment means for adjusting the level of the input signal selected in the selection operation according to the charging/discharging output of this circuit; a changeover switch that connects a current source to the detection transistor and disconnects the current source when the microphone input signal is selected to operate the level detection transistor as a diode; and the current source is connected to the level detection transistor. An automatic level adjustment circuit for an audio signal recording device, comprising: overcharge prevention means for restricting the driving voltage level of the transistor to prevent overcharging of the capacitor when the transistor is overcharged.