JPS6038962A - Conference telephone system - Google Patents

Abstract

PURPOSE:To offer a conference telephone system including a howling prevention circuit without deterioration of talking quality such as interruption of voice by the application of an echo canceller used for satellite communication. CONSTITUTION:A signal from a 2-wire telephone line 10 is converted into a 4- wire signal by a 2-wire/4-wire converter 1. After the converted reception signal is converted into a digital signal by an A/D converter 2, the converted result is fed as a transmission signal of the 1st echo canceller 3. The transmission output of the 1st echo canceller 3 is given as the reception input to the 2nd echo canceller 4 and also restored into an analog signal by a D/A converting circuit 5 again and electroacoustic-transduced by a speaker 6. It is possible to suppress the sound coupling between microphones and the sneaking to the transmission incoming reception side at the 2/4-wire converter 1 by 20-30 dB respectively, and since no howling is produced even when the sound volume of the speaker and the sensitivity of the microphones are increased without using a voice switch, the conference telephone with natural sound volume is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a so-called conference telephone device that allows a plurality of participants to talk to each other simultaneously at a telephone terminal using a general telephone line.

Conventionally, in this type of conference call, a so-called loudspeaker telephone equipped with a microphone and a speaker has been used in place of the telephone end set of an ordinary telephone. Since this loudspeaker telephone is connected to a 2-wire subscriber line, a 2-wire to 4-wire conversion circuit is installed,
A microphone and speaker are connected to the wire. This loudspeaker telephone has problems such as howling due to acoustic coupling from the speaker to the microphone and electrical echo paths due to mismatching in the 4-wire to 2-wire conversion circuit.

Conventionally, in this type of loudspeaker telephone, in order to prevent howling, a loss circuit is installed in the sending and receiving paths, and the sending and receiving states are determined by detecting the respective speech levels. A point switch method is used in which the loss circuit is driven to suppress the gain of the four-wire loop to a value below which howling does not occur. However, with this voice switch method, as the volume of the speaker is increased, a greater loss must be inserted on the transmitting side.
This method has the drawback that a slight but unavoidable deterioration in sound quality such as cutting off of the beginning of a conversation when interrupting a transmission and fluctuations in the level of the received voice is inevitable, resulting in the loss of a natural telephone conference.

SUMMARY OF THE INVENTION In view of the above-mentioned drawbacks, an object of the present invention is to provide an echo canceller (CCITT) used for satellite communications.

Yellow Book Vol l11. H+ec,
G165) to provide a conference telephone device including a howling prevention circuit that does not cause deterioration in call quality such as voice disconnection.

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

FIG. 5 is a block diagram illustrating one embodiment of a conference telephone device according to the present invention. In the figure, a signal from a two-wire telephone line 10 is converted into a signal for a four-wire line by a two-wire/four-wire converter I. The converted reception signal is converted into a digital signal by an analog/digital (A/l) converter 2, and then sent to a first echo canceller (E/C) 3.
is supplied as a transmitting signal. The transmitting output of the first echo canceller 3 is given as the receiving power of the second echo canceller 4, and is also a digital/analog (D/A
) The signal is converted back into an analog signal by a conversion circuit 5 and then converted into an acoustic signal by a loudspeaker 6.

On the other hand, the transmission signal from the microphone 7 is sent to the A/D converter 8.
After being converted into a digital signal, it becomes the transmitting power of the second echo canceller 4. The transmitting output of the second echo canceller is given as the receiver's power to the first echo canceller 3, and is again converted into an analog signal by the D/A converter 9, and is then converted into the transmitter's power of the 2-wire and 4-wire converter l. Become.

The first echo canceller 3 is inserted at the 2-wire/4-wire conversion point for the purpose of canceling echo signals that circulate from the receiving side to the receiving side. Considering the echo path delay on the second line side, the echo cancellation delay capacity of the first echo canceller 3 is generally about 30 m5, which is sufficient. On the other hand, the second echo canceller is inserted to cancel the signal that goes around to the transmitting side due to the acoustic coupling between the microphone and the speaker. This acoustic coupling involves not only direct coupling from the speaker to the microphone, but also complex reverberation signals from the speaker that are reflected from the walls, ceiling, equipment, etc. of the room where the microphone and speaker are placed, and are coupled with the microphone, resulting in a relatively long delay. Create a time echo signal. Therefore, the second echo canceller generally requires an echo cancellation delay capacity of about 20 ()-500 mS. These first and second echo cancellers can be easily constructed using LSI or lugal processors due to recent improvements in LSI technology and digital signal processing technology.

According to the configuration described above, in the conference telephone device,
It is possible to suppress the acoustic coupling between the speaker and the microphone and the transmission upstream from the 2-wire 4-wire converter to the receiving side by about 20 to 30 dB.

Without using a voice switch, there is no feedback even when the speaker volume or microphone sensitivity is increased, making it possible to have a natural telephone conference.

The configuration of the present invention described in the above explanation is only the basic configuration, and even if an amplifier or other control function is added to either the transmitter or the receiver to improve the function, the basic configuration of the present invention will not change. It is clear that it is included in the configuration.

[Brief explanation of drawings]

The figure is a block diagram showing one embodiment of the conference telephone device of the present invention. 1... 2-wire 4-wire converter, 2.8... Old analog/digital conversion circuit, 3... First echo canceller, 4... Second echo canceller,
5, 9... Digital/analog conversion circuit, 6
...Speaker, 8...Microphone.

Claims (1)

[Claims] In a conference telephone device connected to a two-wire telephone line and comprising a two-to-four-wire conversion circuit, a microphone, and a speaker, the microphone and the speaker are connected to a transmission path of the four-wire line by acoustic coupling. A first echo canceller for canceling the signal and a second echo canceller for canceling the signal that the transmitting signal from the microphone enters the receiving channel due to mismatching of the two-wire and four-wire converter are provided. A conference telephone device characterized by: