JPS59147561A - Loudspeaker telephone set circuit - Google Patents

Abstract

PURPOSE:To prevent the distortion in a transmission output even if an input to a microphone is execessive and to eliminate the difficulty of interruption of other party by providing an automatic loss control circuit. CONSTITUTION:When an input of the microphone 1 is excessive and an output level of a line transmission amplifier 4 exceeds a prescribed value depending on the setting position of a variable resistor 16, an FET13 is conductive and a resistor 18 is connected in parallel with a resistor 14. Thus, the negative feedback resistance value of a microphone amplifier 12 takes a new value decided by the resistors 14, 18 so as to reduce the gain of the amplifier 12. Then, the output level of the amplifier 4 is reduced and the generation of the distortion of the amplifier 4 is prevented, the transmission input level to a comparator 11 is limited, the interruption of the receiving is made easy so as to smooth the switching of transmission/receiving.

Description

[Detailed Description of the Invention] Technical Field of the Invention The present invention relates to a loudspeaker telephone circuit that switches between transmitting and receiving calls using a voice switch, and particularly relates to a loudspeaker telephone circuit that switches between transmitting and receiving calls using a voice switch. It concerns no public address telephone circuit.

Prior Art and Problems (1) A loudspeaker telephone uses a microphone for transmitting calls and a speaker for receiving calls, and allows calls to be made without having to pick up a handset. In a public address telephone, a microphone and a speaker are installed in relatively close proximity to transmit and receive calls, so a voice switch is used to switch between a transmitting state and a receiving state to prevent howling and echo.

FIG. 1 shows the configuration of a conventional loudspeaker telephone circuit. In the figure, ■ is a microphone, and the audio electrical signal generated from the microphone 1 by the transmitted voice is amplified to a certain degree via the microphone amplifier 2, and then is amplified by the variable loss circuit 3 according to the transmitting or receiving state. It is transmitted to line 6 via hybrid 5, subject to a variable amount of loss.

On the other hand, the other party's audio electrical signal sent via the line 6 is applied to the receiver amplifier 7 via the hybrid 5, where it is amplified to a certain degree, and then variable loss circuit 8 changes the signal depending on the transmitting or receiving state. The signal is then amplified to a certain extent by the speaker amplifier 9, and is emitted as sound through the speaker 102).

11 is a comparator which compares the output level of the microphone amplifier 2 and the output level of the receiving amplifier 7, and when the output level of the microphone amplifier 2 is large, reduces the loss of the variable loss circuit 3 and also reduces the loss of the variable loss circuit 3. Increase the loss of 8. As a result, a transmission signal is sent to the other party via the line 6 (transmission state). On the other hand, when the output level of the receiving amplifier 7 is high, the loss of the variable loss circuit 8 is decreased and the loss of the variable loss circuit 3 is increased. As a result, the reception signal sent from the line 6 is emitted as sound from the speaker 10 (reception state).

In this way, by changing the loss of the variable loss circuit 3.8 based on the comparison result of the comparator 11, transmission through the microphone 1 and reception through the speaker 10 are performed alternately. The comparator 11° variable loss circuits 3 and 8 constitute a voice switch.

However, in the conventional loudspeaker telephone circuit, the microphone amplifier 21 line transmission (3) amplifier 4 in the transmission system has a transmission distance of 50 cm from the microphone 1.
It is designed to be highly sensitive so that a sufficient transmission level can be obtained on the line 6, even if the transmission level is low. Therefore, if you are too close to the standard transmission distance or transmit at an excessive volume, the line transmission amplifier 4 easily saturates, causing distortion in the transmitting speech output, causing a reduction in speech quality during transmitting.

Furthermore, when using a level comparison type voice switch as shown in Figure 1, if the human power level on the transmitting side is large, the level detection output of the microphone amplifier 2 will be excessive, even if the other party is not able to speak. There was a problem in that it was easy to interrupt.

OBJECT OF THE INVENTION The present invention attempts to solve the problems of the prior art, and its purpose is to provide a loudspeaker telephone circuit that uses a voice switch to switch losses between a transmitting circuit and a receiving circuit. The object of the present invention is to provide a circuit type which does not distort the transmitting output even if the input to the microbond is excessive, and which does not pose a risk of making it difficult for the other party to interrupt.

Embodiment of the Invention FIG. 2 shows the structure of an embodiment of the loudspeaker telephone circuit of the invention. In this figure, the same parts as in Figure 1 are indicated by the same numbers, and their operations are also the same as in Figure 1.
It is the same as in the figure.

Reference numeral 12 denotes a microphone amplifier, which provides negative feedback from the output side to the input side via the resistor 14 when the FET 13 is non-conducting (the negative bias power supply V is applied to the gate of the FET 13 through the diode 17). It operates with a constant gain. 1
A rectifier circuit 5 rectifies the output of the line sending amplifier 4 and generates a positive voltage DC output corresponding to the sending level on the line 6. A variable resistor 16 divides the DC output from the rectifier circuit 14 into an arbitrary ratio to generate an output. A diode 17 becomes non-conductive when the output of the variable resistor 16 exceeds a certain value. At this time, FET 1
The gate voltage of FET 3 becomes the same potential as the source via the resistor 19, thereby making the FET 13 conductive.

(5) When the input to the microphone 1 becomes excessive and the output level of the line sending amplifier 4 exceeds a certain value determined by the setting position of the variable resistor 16, PE713 becomes conductive and the resistor 1 becomes a resistance. 14 in parallel. Therefore, the negative feedback resistance value for the microphone amplifier 12 takes on a new value determined by the parallel resistor 14.18, and the gain of the microphone amplifier 12 decreases. As a result, the output level of the line sending amplifier 4 is reduced, preventing the generation of distortion in the line sending amplifier 4, and limiting the level of human power for sending speech to the comparator 11.
It is less likely that the difference with the comparison input level on the receiver side will become extremely large. Therefore, it becomes easy to interrupt the receiving call, and switching between sending and receiving calls becomes smooth.

As described in detail, according to the loudspeaker telephone circuit of the present invention, the transmitting amplification of the loudspeaker telephone circuit uses a voice switch to control the loss between the transmitting circuit and the receiving circuit according to the magnitude of the transmitting and receiving level. In the circuit, means for rectifying the output of the amplifier that generates the line sending output to generate a DC signal corresponding to the line sending level, and means for reducing the gain of the preceding stage audio amplifier in accordance with the DC output. An automatic loss control (ALC) circuit consisting of
Even when the input to the microphone is excessive, the transmitted speech output is not distorted, and the other party's interruption is not made difficult.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the structure of a conventional loudspeaker telephone circuit, and FIG. 2 is a block diagram showing the structure of an embodiment of the loudspeaker telephone circuit of the present invention. 1-Microphone, 2-Microphone amplifier, 3-Variable loss circuit, 4-Line transmission amplifier, 5-Hybrid, 6
- Line, 7- Receiving amplifier, 8- Variable loss circuit, 9- Speaker amplifier, 10- Speaker, 11- Comparator, 12-
Microphone amplifier, 13-FET, 14-resistance, 1
- rectifier circuit, 16 - variable resistor, 17 - diode,
18-Resistance patent applicant Fujitsu Ltd. agent Patent attorney Gobe Tamamushi (3 others) (7)

Claims (1)

[Claims] Rectifying the output of the amplifier that generates the line sending output in the transmitting amplifier circuit of the loudspeaker telephone circuit, which uses a voice switch to control the attenuation of the transmitting circuit and the receiving circuit according to the respective magnitudes of the transmitting and receiving speech levels. 1. A loudspeaker telephone circuit comprising: means for generating a DC signal according to a circuit output level; and means for reducing the gain of a preceding stage audio amplifier according to the DC output.