JPS60223260A - Loudspeaking telephone circuit - Google Patents

Abstract

PURPOSE:To prevent assuredly a speech head cut phenomenon by adding a simple circuit by setting the band characteristic of a band stop filter at a telephone circuit band or wider than the band. CONSTITUTION:A band stop filter 17 is put into a transmitting system circuit 3, and the transmission signal fed from a microphone 4 is supplied from the output of a transmission amplifier 5 via the filter 17 as a control input signal of a comparator 15. The frequency band of the filter 17 is set approximately equal to a telephone circuit band for example. Therefore no signal can pass through the filter 17 even in case the received sounds delivered through a speaker 13 are put into the microphone 4. Thus the comparator 15 is never controlled.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a loudspeaker telephone for communicating using a speaker and a microphone.

In a loudspeaker telephone, a voice signal spoken by a subscriber is amplified by an amplifier in a transmission system circuit, then supplied to a telephone line via a hybrid circuit, and transmitted to the other party.

After the width is increased, the speaker is driven.

In this case, in order to prevent howling between the receiving circuit and the transmitting circuit and malfunctions described later,
The signal from one circuit increases the loss of the other circuit.

Therefore, if a sufficient audio signal is not input to the transmission system circuit through the microphone when transmitting a voice, the voice transmission mode cannot be set. Therefore, the problem of cutting off at the beginning of the story occurred.

The present invention aims to reliably prevent this interruption at the beginning of a conversation by adding a simple circuit.

Next, a conventional loudspeaker telephone circuit will be explained with reference to the block diagram of FIG. 1 is a connection terminal with a telephone line, 2 is a receiving circuit, and 3 is a transmitting circuit.

The transmission signal input to the microphone 4 is increased by the transmission signal []]
5, bandpass filter 6, iJ variable loss circuit 7, addition 1]
The signal is supplied to the telephone line via the converter 8 and the hybrid circuit 9, which is a 2-wire/4-wire converter, and is transmitted to the other party. Here, the bandpass filter 6 is used to limit the transmitting signal to a specified frequency band when sending the signal to the telephone line, and for example, the passband is 3001-Iz to 3.4K1-
It has been selected as 1z.

On the other hand, the reception signal from the other party is received by the high print circuit 9,
Receiver increase + lj device 10, variable loss circuit 11 and amplifier 1
The sound is emitted from the speaker 13 via 2. While the reception signal is arriving, the reception signal is applied to one input circuit of the comparison circuit 15 via the variable loss circuit 14, so that the comparison circuit 15 controls the variable loss circuits 7 and 16, respectively. Let the loss of be a dog.

Therefore, the transmission system circuit 3 becomes inoperative, and the speaker 13
This prevents howling and the like due to the sound emitted from the system.

In FIG. 1, the variable loss circuits 14 and I6 are provided on the input circuit side of the comparator circuit 150, respectively, because the above-mentioned non-operating state of the transmitting circuit 3 (or non-operating state of the receiving circuit 2) is provided. This is to ensure that the signals from the inactive side are applied to the input circuit of the comparator circuit 150, thereby eliminating a malfunction in which the inactive state is reversed.

In the dual conventional example, in order to switch from the receiving mode to the transmitting mode, a transmitting signal of a predetermined level must be applied to the other input circuit of the comparator circuit 15 via the variable loss circuit 16. , This causes the aforementioned problem of cutting off at the beginning of the story.

FIG. 2 is a block diagram showing an embodiment of the present invention, and parts that are the same as those in FIG. The newly added circuit according to the present invention is basically a band cut filter 17, and by inserting a band pass filter 18 as necessary, more stable performance can be ensured.

Next, the operation shown in FIG. 2 will be explained. In the receiving circuit 2 and the transmitting circuit 2, the operation of the block numbered +1, which is the same as in FIG. 1, is the same as described above. However, the receiver circuit 2
A band-pass filter 18 is inserted in the amplifier 18, but its frequency band characteristics are selected to be equal to the telephone line band, so that even if a signal outside the above-mentioned band is output from the receiver amplifier 10, the amplifier 12 so as not to be input.

This makes it possible to limit the frequency range of the received sound emitted from the speaker 3. However, the bandpass filter 18 is not essential since the frequency range of the transmitted signal is usually limited in each telephone set.

Now, similar to FIG. 1, the comparison circuit 15 and the variable loss circuits 7 and 16 prevent the noise caused by the reception sound emitted from the speaker 3 being applied to the microphone 10 and the phon 4. In the present invention, as described above, the band elimination filter 7 is inserted in the transmission system circuit 3, and the transmission signal from the microphone 4 is compared with the output of the transmission amplifier 5 via the band elimination filter 7. Circuit 150 is configured to be applied as a control input signal.

The frequency band of the band-rejection filter 170 is selected to be equal to, for example, the telephone line band.

Therefore, even if the received sound emitted from the speaker 3b enters the microphone 4, the signal cannot pass through the band rejection filter 17, so the comparison circuit 15 is not controlled. However, if the subscriber inputs voice to the microphone 4 for sending a call, the voice has frequency components in a wider range than the telephone line band, so the voice signal passes through the band elimination filter. , is applied to the other input circuit of the comparator circuit 15, and is compared with the level of the control input signal applied from the receiving system circuit 2 side to one input circuit of the comparator circuit 15, and based on the result, the transmitting mode is selected. It becomes possible to convert to In this case, the control input signal applied to the comparator circuit 15 from the transmitter system circuit 3 is applied directly through the band-rejection filter 17 as described above, in addition to passing through the variable loss circuit 16. There are advantages such as being able to lower the level of the transmitting signal at the time of transition than in the past, that is, improving the problem of interruptions at the beginning of a conversation, and making it possible to interrupt the transmitting voice even during reception. Furthermore, as can be seen from the above explanation, the frequency characteristics of the above band-rejection filter are as follows:
It is only necessary to remove the input signal that comes from the speaker 13 and pass the audio input signal.
Set it to at least the telephone line bandwidth.

[Brief explanation of the drawing]

FIG. 1 is a block diagram for explaining a conventional loudspeaker telephone, and FIG. 2 is a block diagram of a loudspeaker telephone illustrating an embodiment of the present invention. 4. Microphone, 9. Hybrid circuit 13.
...Speaker 15... Comparison circuit 17 Band-rejection filter Patent applicant Taiko Electric Seisakusho Co., Ltd. President and Chief Executive Officer Takehiko Takehiko

Claims (1)

[Claims] In a public address telephone that uses a speaker and a microphone to make calls, there is a variable loss circuit inserted in each of the transmitting and receiving systems, a comparison circuit that compares the signal levels of each of the transmitting and receiving systems and controls the variable loss circuit, and The system includes a band-removal filter that removes a predetermined band from the transmission signal input to the variable loss circuit inserted in the system and adds it to the comparison circuit, and the band-removal filter is configured to control the band characteristics of the band-removal filter. A loudspeaker telephone circuit characterized by being set to a telephone line band or higher.