JPS647531B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an improvement in an automatic volume control circuit for telephones and the like.

[Technical background of the invention and its problems]

2. Description of the Related Art Conventionally, an automatic voice volume adjustment circuit employed in, for example, a button telephone has been constructed as shown in FIG. In the figure, 1 is a subscriber line terminal, 2 is a power switch (for example, a central line button switch) that opens and closes the connection with the subscriber line, and 3 is a line transformer.
4 is a variable attenuation circuit that performs attenuation control of the signal obtained from the line transformer 3; 5 is an amplifier circuit; 6 is a speaker; 7 is a power amplifier circuit for driving the speaker 6; 8 is a rectifier for the output of the amplifier circuit 5. The rectifier circuit 9 is a holding circuit that smoothes and holds the output from the rectifier circuit 8. Moreover, these variable attenuation circuits 4,
The amplifier circuit 5, the rectifier circuit 8, and the holding circuit 9 constitute a volume control circuit.

The automatic listening volume adjustment circuit configured as described above operates as follows. First, the power switch 2 is closed, and the reception signal a input from the subscriber line terminal 1 is transferred to the line transformer 3 and the variable attenuation circuit 4.
The signal is input to the amplification circuit 5 via the amplification circuit 5, where it is amplified. The amplified output b of the amplifier circuit 5 is rectified by the rectifier circuit 8 and then held by the holding circuit 9, and the attenuated signal c
The signal is supplied to the variable attenuation circuit 4 as a signal. Since the attenuation amount of the variable attenuation circuit 4 changes in response to this attenuation signal c (DC voltage), the output level of the amplifier circuit 5 remains constant even if the input level input via the subscriber line terminal 1 changes. is maintained.

Therefore, even if the input level input via the subscriber line terminal 1 fluctuates, the input level of the power amplifier circuit 7 is kept constant by the variable attenuation circuit 4, the amplifier circuit 5, the rectifier circuit 8, and the holding circuit 9. Therefore, the volume of the received sound emitted from the speaker 6 is always kept constant.

However, in such a conventional automatic receiving volume adjustment circuit, the rectified signal input to the holding circuit 9 is input to the variable attenuation circuit 4 with a delay time when the power switch 2 starts to close.
This causes a delay in the automatic volume adjustment operation by the variable attenuation circuit 4 and the like, and as shown in FIG. 2, the level due to the attack response of the output b of the amplifier circuit 5 increases significantly when the signal is applied or increased. For this reason, for example, when you operate the central office line button switch on a button telephone to make a call, the dial tone from the exchange (after the line is connected) becomes abnormally loud at the beginning, resulting in poor hearing and problems in use. occurred. In order to prevent this problem, if the delay time of the holding circuit 9 is simply shortened, the feedback loop made up of the variable attenuation circuit 4, the amplifier circuit 5, the rectifier circuit 8, and the holding circuit 9 becomes unstable, causing vibrations, level fluctuations, etc. This caused problems such as oscillation in some cases.

[Purpose of the invention]

The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide an automatic listening volume adjustment circuit that can prevent excessive noise immediately after a line is connected with a simple configuration and is advantageous in use.

[Summary of the invention]

Therefore, the present invention generates a differential signal in conjunction with a first switch connected to a wired communication line and controls connection to the wired line, and supplies this differential signal as an attenuation signal to the variable attenuation circuit. Achieved the purpose.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 3 is a diagram showing an embodiment of the present invention applied to an amplification circuit for a key telephone set, and the same main parts as in FIG. 1 are given the same reference numerals and their explanation will be omitted. In the same figure, 10 is a DC power source, 11 is a switch that operates in conjunction with the electric power switch 2, which is opened when the electric power switch 2 is opened and closed when the electric power switch 2 is closed, and 12 is a DC voltage that is applied when the switch 11 is closed. A diode 13 is a differentiation circuit that differentiates the signal d and outputs a differential signal e. Further, the DC power supply 10, the switch 11, the differentiating circuit 12, and the diode 13 constitute a click prevention circuit.

Next, the operation of this embodiment will be explained with reference to the signal waveforms of each part shown in FIG. First, for example, when the central line button switch of a key telephone is operated, the power switch 2 is closed, and in conjunction with this, the switch 11 is also closed. Thereafter, the dial tone input from the exchange through the subscriber line terminal 1 passes through the line transformer 3, the variable attenuation circuit 4, and the amplifier circuit 5, and is amplified by the power amplifier circuit 7 to drive the speaker 6. At this time, the switch 11 is also closed in conjunction with the electric power switch 2, so the DC voltage signal d is supplied to the differentiating circuit 12, where it is
It is differentiated with a time constant of S, and is introduced into the control input terminal of the variable attenuation circuit 4 via the diode 13 as the attenuation signal c. In the variable attenuation circuit 4, the amount of attenuation (passage loss) is determined based on this attenuation signal c (DC voltage).
changes, so the amount of attenuation (loss) immediately after closing the switch 11 becomes extremely large and gradually decreases. Also, in the volume control circuit composed of the rectifier circuit 8, the holding circuit 9, etc., the gain is at the minimum in the initial state, so the volume set by the amplifier circuit 5 and the power amplifier circuit 7 is emitted from the speaker 6. .

Therefore, in the conventional circuit shown in Fig. 1, the attenuation amount (passage loss) is lower than the steady level for about several hundred milliseconds from this initial state to the steady state where the amount of attenuation (passage loss) is set based on the attenuation signal c. The amplified output b of the amplifier circuit 5 was obtained at a high level. but,
In the circuit according to this embodiment, the differential signal e is supplied to the control input terminal of the variable attenuation circuit 4 during this period of approximately several hundred milliseconds due to the delay of the holding circuit 9, so that the amplified output b of the amplifier circuit 5 is as described above. It will not become louder, and the speaker 6 will not emit excessive sound. Furthermore, the clicking noise caused by the closure of a DC loop with an exchange when a wired communication line is connected can also be suppressed by this temporary change in attenuation.

It should be noted that during normal operation, the diode 13 is reverse biased and a direct current potential is applied to it, so that it is turned off, and the electrical connection between the volume control circuit and the click prevention circuit is cut off. Therefore, during steady state, there is no influence between the volume control circuit and the click prevention circuit.

By the way, the present invention is not limited to the above embodiment, and the switches 2 and 11 may be a central line switching relay of an automatic dialing device. Further, these switches 2 and 11 may be electronic switches other than mechanical interlocking switches.

[Effects of the Invention] As described above, according to the present invention, it is possible to prevent excessive sound caused by the attack delay time of the volume control circuit, and the call characteristics are improved. Furthermore, the clicking noise that occurs immediately after a wired communication line is connected can also be reduced, further improving call characteristics. Furthermore, since the click prevention means is not affected by electrical influences from external circuits due to the diode means, the click prevention means can be quickly restored without being influenced by the outside.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing a conventional automatic listening volume adjustment circuit, Fig. 2 is a diagram showing signal waveforms of each part thereof, Fig. 3 is a block diagram showing an embodiment of the present invention, and Fig. 4 is a diagram showing each part thereof. FIG. 2... Electric power switch, 4... Variable attenuation circuit,
5...Amplification circuit, 8...Rectifier circuit, 9...Holding circuit, 10...DC power supply, 11...Switch, 12
...Differential circuit, 13...Diode, c...Attenuation signal, e...Differential signal.

Claims (1)

[Claims] 1. A first switch connected to a wired line and controlling the connection to the wired line, a variable attenuation means for controlling attenuation of the signal transmitted via the first switch, and a variable attenuation means based on the output of the variable attenuation means. automatic volume adjustment means comprising attenuation amount setting means for forming an attenuation signal for setting an attenuation amount and outputting this attenuation signal as a control input for the variable attenuation means; and an automatic volume adjustment means connected to a DC power source and interlocked with the first switch. A second switch and a DC voltage applied when the second switch is closed are differentiated to form a differential signal, and for a predetermined period of time, this differential signal is made into a damping signal and output as a control input of the variable damping means. What is claimed is: 1. An automatic reception volume adjustment circuit comprising: a click prevention means comprising a differentiating means that performs a differentiating operation; and a diode means for cutting off electrical influence from the automatic volume adjustment means to the click prevention means.