JPS6331142B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a voice switch circuit in a public address telephone.

There are known loudspeaker telephones that use a speaker instead of a receiver and a microphone instead of a transmitter in a normal telephone.Such loudspeaker telephones are equipped with an audio switch circuit as a means to prevent howling. It is provided. FIG. 1 is a block diagram including the audio switch circuit of a conventional loudspeaker telephone, in which 1 is a microphone, 2, 4, 6, 10,
13 and 16 are amplifiers, 3 is a transmission variable loss circuit, 5
is an auxiliary transmission variable loss circuit, 7 and 9 are rectifier circuits, and 8 is a rectifier circuit.
1 is a comparator, 11 is a side sound protection circuit such as a hybrid coil, 12 is a speaker, 14 is a receiving variable loss circuit, and 15 is an auxiliary receiving variable loss circuit.

A transmitting circuit is constituted by a microphone 1, an amplifier 2, a transmitting variable loss circuit 3, and an amplifier 4, and a speaker 12, an amplifier 13, and a receiving variable loss circuit 14.
and an amplifier 16 constitute a receiving circuit, and the transmitting circuit and the receiving circuit are connected to the line by a side sound protection circuit 11.

The transmission signal from the microphone 1 is amplified by the amplifier 2 and applied to the amplifier 4 via the transmission variable loss circuit 3, and the amplified output signal is sent to the line via the side sound protection circuit 11, and is sent to the exchange ( (not shown) to the other party's telephone. Further, the signal transmitted from the other party's telephone via the exchange is applied to the amplifier 16 via the side sound protection circuit 11, and its amplified output signal is applied to the amplifier 13 via the receiving variable loss circuit 14, and its amplified output signal is The signal drives the speaker 12, and the voice from the other party's telephone is reproduced.

In the loudspeaker telephone, when the sound from the speaker 12 is input to the microphone 1, and the near-end loop gain and the far-end loop gain are 1 or more, howling will occur. Note that the near-end loop gain is the gain of the loop in which the speaker 12 and the microphone 1 are coupled via the side sound protection circuit 11 of the own telephone, and the far-end loop gain is the gain of the loop in which the speaker 12 and the microphone 1 are coupled via the side sound protection circuit 11 of the own telephone. is the gain of the loop containing the join.

In order to make the aforementioned near-end round-trip gain and far-end round-trip gain less than 1, the transmitting circuit has a transmitting variable loss circuit 3, and the receiving circuit has a receiving variable loss circuit 14. Then, the comparator 8 compares the transmitting level and the receiving level, and when transmitting, since the transmitting level is high, the loss of the transmitting variable loss circuit 3 is reduced, and conversely, the loss of the receiving variable loss circuit 14 is reduced. When receiving a call, the receiving level is high, so that the loss of the transmitting variable loss circuit 3 is increased, and conversely, the loss of the receiving variable loss circuit 14 is controlled to be small.

Further, either one of the amplifiers 13 and 16 is a variable gain amplifier so that the volume of the speaker 12 can be adjusted automatically or manually. Also, the sound during a call is not continuously at a constant level, but there are pauses in which there is no sound. Therefore, when the amplifier 16 is used as a variable gain amplifier and its gain is increased, a part of the transmitting signal at the time of transmitting a voice goes around to the receiver circuit side via the side sound protection circuit 11, is input to the amplifier 16, and is amplified. Then, even if the other party's telephone is in a silent period, the receiving level input may be higher than the transmitting level input compared by the comparator 8.

Since this state is the same as the receiving state, the control states of the transmitting variable loss circuit 3 and the receiving variable loss circuit 14 are reversed even during transmitting. That is, transmission blocking occurs. Conversely, when receiving a call, the voice from the speaker 12 or ambient noise is input to the microphone 1, and the comparator 8
The transmitting level input may be higher than the receiving level input compared in , and in that case as well, the control states of the transmitting variable loss circuit 3 and the receiving variable loss circuit 14 are reversed, resulting in receiving blocking.

In order to prevent the above-mentioned transmitting blocking and receiving blocking, an auxiliary transmitting variable loss circuit 5 and an auxiliary receiving variable loss circuit 15 are provided, and the output of the comparator 8 determines the auxiliary transmitting variable loss during transmitting. circuit 5
The loss of the auxiliary receiver variable loss circuit 15 is increased, and the transmitting level input applied to the comparator 8 is controlled to be relatively larger than the receiving level input. The configuration is such that control is performed so that the relationships between the two are opposite to each other. In this case, the loss of the auxiliary reception variable loss circuit 15 during transmission is selected so that transmission blocking does not occur even if the gain of the amplifier 16 as a variable gain amplifier is maximized.

The rectifier circuits 7 and 9 are used to perform DC level comparison in the comparator 8, and include a time constant circuit to prevent the output state of the comparator 8 from inverting during a short pause period during a call. The time constant is selected and the hangover time is e.g.
It is set to 200 to 300ms.

For calls over relatively short distances or calls in quiet conditions such as at night, it is common to reduce the gain of the amplifier 16, which serves as a variable gain amplifier, to reduce the volume of the speaker 12. In that case, since the loss of the auxiliary reception variable loss circuit 15 is controlled to be large when transmitting a call, even if a reception signal arrives from the other party's telephone, the reception level input applied to the comparator 8 will be affected by the auxiliary reception variable loss circuit 15. The signal is attenuated due to the large loss in the circuit 15, and there are cases where the level does not exceed the transmitting level input, which has the disadvantage that it is not possible to switch to the receiving state.

SUMMARY OF THE INVENTION It is an object of the present invention to smoothly switch from a transmitting state to a receiving state in the audio switch circuit of the above-mentioned loudspeaker telephone, regardless of the gain adjustment of the variable gain amplifier of the receiving circuit. be.
Examples will be described in detail below.

FIG. 2 is a block diagram of an embodiment of the present invention,
The same reference numerals as in FIG. 1 indicate the same parts, 15a is an auxiliary reception variable loss circuit, and 16a is a variable gain amplifier. This variable gain amplifier 16a and the auxiliary receiving variable loss circuit 15a to which this output is added are configured in an interlocking relationship, and when the gain of the variable gain amplifier 16a is reduced, the loss of the auxiliary receiving variable loss circuit 15a is also reduced in conjunction. It is something to do. For example, if the gain of the variable gain amplifier 16a is configured to be manually controlled by a variable resistor, the variable resistor and the variable resistor constituting the auxiliary receiver variable loss circuit 15a may be configured with a double variable resistor. Become.
Further, when the gain of the variable gain amplifier 16a is automatically controlled, the AGC voltage may be used to control the auxiliary reception variable loss circuit 15a.

FIG. 3 is an explanatory diagram of an example of the relationship between the gain of the variable gain amplifier 16a and the loss of the auxiliary receiver variable loss circuit 15a, and as shown by curve a, the loss of the auxiliary receiver variable loss circuit 15a during transmission. is increased or decreased in response to an increase or decrease in the gain of the variable gain amplifier 16a. Note that the dotted line b indicates the loss during transmission of the auxiliary receiving variable loss circuit 15 in the conventional example,
It is constant regardless of the gain adjustment of the variable gain amplifier.

FIG. 4 is a main part circuit diagram of an example of the auxiliary receiver variable loss circuit 15a, in which transistors Q1 and Q2 and a resistor R are connected in parallel, the output of the comparator 8 is applied to the gate of the transistor Q1, and the transistor AGC of variable gain amplifier 16a is connected to the gate of Q2.
Signals are applied according to voltage or manual gain control.

When receiving a call, the transistor Q1 is turned on and the resistor R is short-circuited, reducing the loss as the auxiliary receiver variable loss circuit 15a.When transmitting a call, the transistor Q1 is turned off and the resistor R is inserted, so that the auxiliary receiver Although the loss as the variable loss circuit 15a increases, if the gain of the variable gain amplifier 16a is controlled to be small, a corresponding signal is applied to the gate of the transistor Q2, and the transistor Q
The impedance of 2 is controlled to be small. Therefore, the loss of the auxiliary reception variable loss circuit 15a is controlled in accordance with the gain adjustment of the variable gain amplifier 16a.

As mentioned above, when the gain of the variable gain amplifier 16a is manually or automatically reduced, the loss of the auxiliary reception variable loss circuit 15a is correspondingly reduced, so that when a reception signal arrives, it is amplified by the variable gain amplifier 16a. Even if the level of the received reception signal is low, the loss of the auxiliary reception variable loss circuit 15a is also small, so the reception level input applied to the comparator 8 is larger than in the conventional example. Therefore, even if the volume of the speaker 12 is lowered, it is possible to reliably switch from the transmitting state to the receiving state.

As explained above, the present invention compares the transmitting level input and the receiving level input using the comparator 8,
An auxiliary transmitting variable loss circuit for controlling the transmitting variable loss circuit 3 of the transmitting circuit and the receiving variable loss circuit 14 of the receiving circuit reciprocally, and adding the transmitting level input and the receiving level input to the comparator 8. While controlling the loss circuit 5 and the auxiliary reception variable loss circuit 15a reciprocally,
The gain of the variable gain amplifier 16a for controlling the volume of the speaker 12 of the receiving circuit is controlled in conjunction with the loss of the auxiliary receiving variable loss circuit 15a to which the output of the variable gain amplifier 16a is added. Yes, so that the variable gain amplifier 16a
Even when the gain of the speaker 12 is reduced to reduce the volume from the speaker 12, it is possible to smoothly switch from the transmitting state to the receiving state when a receiving signal arrives from the other party's telephone. There is an advantage that the communication performance of the loudspeaker telephone can be improved.

[Brief explanation of the drawing]

Fig. 1 is a block diagram including the audio switch circuit of a conventional loudspeaker telephone, Fig. 2 is a block diagram of an embodiment of the present invention, and Fig. 3 shows the relationship between the gain of the variable gain amplifier and the loss of the auxiliary receiver variable loss circuit. A curve diagram showing the relationship, and FIG. 4 is a circuit diagram of a main part of an example of an auxiliary reception variable loss circuit. 1 is a microphone, 2, 4, 6, 10, 13,
16 is an amplifier, 16a is a variable gain amplifier, 3 is a transmission variable loss circuit, 5 is an auxiliary transmission variable loss circuit,
7 and 9 are rectifier circuits, 8 is a comparator, 11 is a side sound protection circuit, 12 is a speaker, and 15 and 15a are auxiliary reception variable loss circuits.

Claims (1)

[Claims] 1. The auxiliary transmitting level is determined by the comparison output of a comparator that compares the transmitting level input via the auxiliary transmitting variable loss circuit and the receiving level input via the auxiliary receiving variable loss circuit. In an audio switch circuit in which a variable loss circuit and an auxiliary reception variable loss circuit are controlled reciprocally, and a transmission variable loss circuit of a transmission circuit and a reception variable loss circuit of a reception circuit are controlled reciprocally, The gain of the variable gain amplifier for controlling the volume of the speaker of the receiving circuit and the loss of the auxiliary receiving variable loss circuit to which the output of the variable gain amplifier is added are controlled in conjunction. Audio switch circuit.