JPS6041908B2 - public address telephone equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a loudspeaker telephone device that improves speech quality by changing the amount of loss in a bypass circuit inserted in a transmitting system and a receiving system according to the state of loop gain.

Figure 1 shows the configuration of a loudspeaker telephone, where 1 is a microphone, 2 is an amplifier, 3 and 6 are variable loss circuits, 4 is a speaker, 5 is a power amplifier, 7 is an audio switch control circuit, and 8
is a hybrid transformer, A is a volume variable circuit, B is an AG
It is a C circuit.

Now, in order to prevent howling caused by the closed loop formed from the speaker 4 via the microphone 1 and the hybrid 8, variable loss sections 3 and 6 are inserted.

As the conventionally known variable loss sections 3 and 6, there is a circuit as shown in FIG. 9 is a semiconductor element or analog switch operated by an audio switch control signal; 10; 1;
1 is resisted.

In this circuit, the resistors 10 and 11 are set so that the amount of loss (hereinafter referred to as basic loss amount) is a limit that does not cause howling when the analog switch 9 is in the off state.

However, although it is possible to improve the basic loss amount by adjusting the volume of the volume variable circuit A and reducing the gain of the M℃ circuit B, etc., this circuit has a fixed amount of loss, so no improvement in call quality can be expected. . The present invention solves the above-mentioned drawbacks and aims to improve call quality by connecting a bypass circuit whose gain operates reciprocally depending on the state of the loop gain in parallel with the variable loss circuit. .

This will be explained below along with examples. FIG. 3 shows an embodiment of the present invention, in which the variable loss circuits 3 and 6 in FIG.
It is installed at a position corresponding to

12 is a variable loss circuit, 13 is a loss circuit, 14 is a volume variable reciprocal circuit, 15 is an AGC reciprocal circuit, 16 is an addition circuit, 1
7 is an audio switch control signal.

Here, the reciprocal circuits 14 and 15 are those whose gain increases or decreases in opposition to a circuit having a factor that changes the loop gain, such as a volume variable circuit or an AGC circuit.

The level at the output of the bypass circuit composed of blocks 13, 14, and 15 is in a communication state, that is, in Fig. 3, variable loss circuit 1.
2 is set to a value obtained by subtracting the change width of the bypass circuit from this level so that the change width of the bypass circuit does not exceed the level when the switch 2 is in a conductive state (because the change width of the gain of the bypass circuit is larger than the change width of the gain of the variable loss circuit). Here, the loss amount of the variable loss circuit 12 in FIG. 3 is L.

If the amount of loss in the B bypass circuit is LBdB, the amount of loss Lx seen at the output of the adder circuit 16 can be expressed as follows.

Here, when LO<LB (when the gain of the bypass circuit does not change), equation (1) becomes as follows.

The LO is set to be the basic loss amount of Lx at this time. Next, when LO>LB (when the gain of the bypass circuit is maximum)
Similarly to the above, both Lx and LB become LB. As mentioned above, when the loss amount of the bypass circuit is larger than the loss amount of the variable loss circuit, it depends on the loss amount of the variable loss circuit, that is, it becomes the basic loss amount, and the loss amount of the bypass circuit is greater than the loss amount of the variable loss circuit. If it is small, it depends on the amount of loss in the bypass circuit.

If a call is being made and the gain of the bypass circuit is large, the output level of the adder circuit may be higher than the rated value. Therefore, the loss circuit 13 is operated by the inverted signal 18 of the audio switch control signal 17 in FIG. 3 to provide a loss.

If this loss is increased, the information at the start-up time will become smaller if the variable loss circuit method is a soft switch.
It is desirable to set this loss to one track DB. Here, the operation of the reciprocal circuit in the bypass circuit will be explained.

First, an example of the configuration of a volume variable circuit corresponding to FIG. 1A and its reciprocal circuit (corresponding to FIG. 3 14) is shown in FIGS. 4A and 4B, respectively. 19 is an operational amplifier.

Assuming that the gain in the circuit shown in FIG. 4a is G1 and the gain in the circuit shown in FIG. 4b is G2, the following equation is obtained. -Here, if RO=RJ, R=R'' (using two variable resistors), the gain G in the closed loop will be, and will be a constant value regardless of changes in the variable resistors.

Next, the AGC circuit and its reciprocal circuit (
Fig. 5A and Fig. 5A, respectively, show an example of the configuration of
Shown in b.

21 is a rectifier circuit, 20 and 23 are semiconductor elements whose resistance changes depending on the output voltage of the rectifier circuit 21, and 22 and 24 are operational amplifiers.

Here, the gain of the circuit shown in FIG. 4a is G3, and the gain of the circuit shown in FIG. 4B is G4.

Here, when R=R″Rx=Rx″, the gain G in the closed loop becomes, and the gain becomes a constant value.

As described above, even if the volume, AGC, etc. change, the loop gain does not change.

As explained above, according to the present invention, the amount of loss is set to the optimum amount of loss determined from the loop gain, so it is highly effective in improving call quality.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a loudspeaker telephone, FIG. 2 is a wiring diagram of a conventional variable loss section, FIG. 3 is a block diagram of main parts of a loudspeaker telephone according to an embodiment of the present invention, and FIGS. 4A and b 5 is a circuit diagram of a portion that performs a reciprocal operation of variable volume, and FIG. 5 is a circuit diagram of a portion that performs a reciprocal operation of AGC. 12... Variable loss circuit, 13... Loss circuit, 14,
15... Reciprocal circuit.

Claims (1)

[Claims] 1. A variable loss circuit for preventing howling in the transmitting system and the receiving system, an audio switch that compares the transmitting level and the receiving level and controls the variable loss circuit, and a volume adjustment A.
In a loudspeaker telephone device where the range of change in loop gain due to GC etc. is larger than the limit loss that does not cause howling,
A loudspeaker telephone device characterized in that the variable loss circuit and a bypass circuit whose gains operate reciprocally depending on the state of the loop gain are connected in parallel via an adder circuit. 2. The loudspeaker telephone device according to claim 1, further comprising means for causing a loss by activating a loss circuit connected in series with a bypass circuit of a receiving system or a transmitting system by a transmitting or receiving control signal of an audio switch.