JPH0415653B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical field of the invention The present invention relates to a transmitter circuit for a telephone.

(2) Prior art and its problems Conventionally, the signal output from the transmitter of a telephone is often amplified by an amplifier, and the amplified signal is transmitted over the telephone line. In this case, the gain of the amplifier is constant regardless of the level of the input signal. Recent telephone sets use transmitters such as electric transmitters and ceramic transmitters that are more sensitive to minute sounds than carbon transmitters. For this reason, signals corresponding to minute sounds are also transmitted to the telephone line at a considerable level, and the sidetones are reflected in the reception of the call. Therefore, in a telephone that uses a conventional transmitting circuit, not only the caller's voice but also low-level ambient noise is amplified and sent to the telephone line, so when the caller's voice is not present, unnecessary ambient noise is amplified. The other party on the other end of the line could hear the noise and it was jarring.

In addition, ambient noise that circulates to the receiver as sidetone can be heard from the receiver, making it difficult for the caller to hear the voice of the other party.

A conventional technique for solving such problems is disclosed in Japanese Patent Publication No. 61-99451. This technique compares the input level with a reference level and switches the gain of the transmitter amplifier based on the comparison result. However, this conventional technique has the disadvantage that when the input level changes, a discontinuity occurs in the gain of the transmitter amplifier, and therefore the transmitter becomes unnatural.

(3) Purpose of the Invention In order to solve these drawbacks, the present invention provides a method of discontinuing the level of minute signals such as ambient noise that are sent to the line when there is no voice of the caller to the gain of the transmitting amplifier. By lowering the noise level without causing noise, only the voice of the caller can be heard clearly by the other party, and the sidetone level of the ambient noise that was being heard from the handset is lowered, making the voice of the other party clear. The present invention provides a transmitting circuit for a telephone that is audible.

(4) Structure of the Invention In order to achieve this object, the telephone transmitting circuit according to the present invention includes a logarithmic amplifier that amplifies the output signal of the transmitter, and a voltage that corresponds to the signal level of the output signal of the logarithmic amplifier. a rectifier circuit that has a first gain when the output voltage of the rectifier circuit is equal to or higher than the first level, and has a first gain when the output voltage of the rectifier circuit is lower than the first level; has a second gain smaller than the first gain, and when the output voltage of the rectifier circuit is between the first level and the second level, the output voltage of the rectifier circuit is The present invention has a configuration including a variable gain amplifier having a gain that smoothly changes between the first gain and the second gain.

The present invention will be explained in detail below with reference to the drawings.

A block diagram of a telephone transmitting circuit according to one embodiment of the present invention is shown in FIG. The transmitter 1 outputs an electrical signal according to the input sound pressure level. The electrical signal output from the transmitter 1 is input to a logarithmic amplifier 2 and a variable gain amplifier 3. In logarithmic amplifier 2,
The logarithmically amplified signal is input to the rectifier circuit 4.
The rectifier circuit 4 converts the input signal into a voltage having a level corresponding to the signal level, and outputs the voltage to the variable gain amplifier 3 via the control line 12. Further, the signal input to the variable gain amplifier 3 via the signal line 11 is amplified, and the signal is input to the output amplifier 16 via the signal line 13.
The output is outputted to the line 7 and receiver circuit 8 through the 2-wire to 4-wire conversion circuit 6. At this time, the gain of the variable gain amplifier 3 is outputted from the rectifier circuit 4 through the control line 12. It is made to change depending on the voltage level.

FIG. 2 shows a specific circuit configuration of the variable gain amplifier 3. When the output level of the transmitter 1 is low, the voltage level of the control line 12 is also low and the transistor 5 is cut off, so the gain of the amplifier 9 in the variable gain amplifier 3 is small. As the input level increases, the voltage level of the control line 12 also increases, transistor 5 enters the active region, and the gain of amplifier 9 increases as the input level increases. When the input level further increases, transistor 5 becomes saturated and the gain of amplifier 9 becomes constant. The gain control sensitivity of the variable gain amplifier 3 based on the voltage level of the control line 12 is adjusted by setting the values of the resistors 14, 15, and 16.
Also, the smaller the resistance 14, the greater the sensitivity. Since the output of the transmitter 1 is logarithmically amplified by the logarithmic amplifier 2, fluctuations in the voltage level of the control line 12 are very small compared to fluctuations in the output level of the transmitter 1. Therefore, the gain of the variable gain amplifier 3 with respect to ambient sound has characteristics as shown in FIG. 3, each having a cutoff region and a saturation region where the gain is approximately constant, and an active region where the gain changes continuously. At this time, it is desirable that the voice level of the caller is in the saturation region and the noise level is in the cutoff region. In FIG. 3, A and B are preferably set to about 10 dB. The output signal level characteristic with respect to ambient sound is a continuous characteristic as shown in FIG. Compared to the conventional linear relationship characteristic shown in FIG. 5, when the input signal level (ambient sound level) is low, the output signal level is lower. Therefore, when there is no voice from the caller, very little signals such as ambient noise are transmitted to the line, and hardly ever reach the receiving circuit. Therefore, since only the voice of the caller is amplified with a high gain, the call performance is improved.

(5) Effects of the Invention As described above, according to the present invention, when there is no voice of the caller, minute sounds such as ambient noise are transmitted to the line at an extremely low level, so that only the voice of the caller is heard by the other party. The voice of the person on the other end of the line can be heard clearly, and the sidetone of ambient noise heard from the handset is also extremely small.

[Brief explanation of drawings]

FIG. 1 is a block diagram of one embodiment of the present invention, and FIG.
The figure is a circuit diagram showing a configuration example of the variable gain amplifier used in the embodiment of Fig. 1, Fig. 3 is a characteristic diagram showing the gain characteristics of the variable gain amplifier used in the present invention, and Fig. 4 is a transmitting circuit according to the present invention. A characteristic diagram showing the input/output characteristics of
FIG. 5 is a characteristic diagram showing the input/output characteristics of a conventional transmitter circuit. DESCRIPTION OF SYMBOLS 1... Transmitter, 2... Logarithmic amplifier, 3... Variable gain amplifier, 4... Rectifier circuit, 5... Transistor, 6... 2-wire to 4-wire conversion circuit, 7... Telephone line, 8... ...Reception circuit, 9...Amplifier, 10,1
4, 15, 16...Resistor, 11, 13...Signal line, 12...Control line.

Claims (1)

[Claims] 1 a logarithmic amplifier that amplifies the output signal of the transmitter;
a rectifier circuit that converts the output signal of the logarithmic amplifier into a voltage according to the signal level; and a rectifier circuit that has a first gain when the output voltage of the rectifier circuit is equal to or higher than a first level; When the second level is lower than the first level, the first level is lower than the first level.
has a second gain smaller than the gain of the rectifier circuit, and when the output voltage of the rectifier circuit is between the first level and the second level, the output voltage of the rectifier circuit is A telephone transmitting circuit comprising a variable gain amplifier having a gain that smoothly changes between the gain and the second gain.