JP2636897B2 - Hands-free communication circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] A hand-free communication circuit used in a car phone or the like, which enables a smooth communication without giving the other party the feeling that the line is disconnected. In a hand-free communication circuit having variable insertion loss in the transmission path and the reception path, it is intended to enable bidirectional communication in a double talk state. Ambient noise generating means for generating a noise signal; adding means for adding the ambient noise signal generated by the ambient noise generating means to the output of the variable insertion loss on the transmission path side; Accordingly, an ambient noise generating means and a control means for controlling the variable insertion loss provided in the transmission path and the reception path are provided.

[Industrial applications]

The present invention relates to a hands-free communication circuit used for a car telephone or the like.

The hands-free communication circuit is used for a so-called loudspeaker. This loudspeaker phone uses a microphone and a speaker instead of a handset. The hands-free communication circuit has a microphone amplifier, a speaker amplifier, a howling prevention function, a voice switch function, and the like.

[Conventional technology]

FIG. 10 shows a block diagram of a conventionally used hands-free communication circuit. The hand-free communication circuit shown in the figure compares the transmission and reception voice signal levels detected by the voice detection circuits 2 and 4 composed of an AC-DC conversion circuit and the like with a comparison / control circuit 5 to obtain a variable insertion loss 1. And 3
Control the transmission loss. In this case, a large insertion loss is given to a communication path having a smaller signal level, and howling and reverberation are prevented. In the figure, 6 is a microphone, 7 is a speaker, 8 is a wireless transmitting / receiving circuit, 9 is a microphone amplifier, and 10 is a speaker amplifier.

[Problems to be solved by the invention]

However, the above-mentioned conventional hands-free communication circuit,
It has the following problems.

At the time of reception in a normal single talk state, the insertion loss of the variable insertion loss 1 provided on the microphone 1 side is set to be large as described above, and ambient noise entering from the microphone 6 is cut off. For this reason, there is a problem that the other party is given a feeling that the line is disconnected, and a smooth call is hindered. In particular, the above-mentioned problem is serious in hands-free communication using a car telephone because ambient noise such as running noise is large.

Further, since an insertion loss is given to one of the communication paths by the variable insertion loss 1 or 3, there is also a problem that a call in a double talk state is hindered.

Therefore, the present invention solves the above-mentioned problems, and enables a smooth call without giving the other party the feeling of disconnection, and enables a two-way call in a double talk state. The purpose is to.

[Means for solving the problem]

 FIG. 1 is a block diagram showing the principle of the present invention.

Ambient noise generation means 40 generates an ambient noise signal having predetermined characteristics based on a signal from the microphone.

The adding means 50 adds the ambient noise signal generated by the ambient noise generating means 40 and the output of the variable insertion loss 20 on the transmission path side.

The control means 60 controls the ambient noise generation means 40 and the variable insertion losses 20, 30 provided in the transmission path and the reception path according to the signal levels of the transmission path and the reception path.

[Action]

When the control means 60 determines that the receiving state is determined from the signal levels of the transmission path and the reception path, the attenuation of the variable insertion loss 20 on the transmission path side is increased, and the variable insertion loss 30 and the ambient noise generation means 40 on the reception path side increase the attenuation. The amount of attenuation of the signal level of the generated ambient noise signal is reduced. In this case, since there is no voice from the transmitter, the output of the adding means 50 is an ambient noise signal whose signal level is increasing. As described above, since the control means 60 changes the variable insertion loss 20 and the ambient noise signal level complementarily, when the state changes from the transmission state to the reception state, the ambient noise is not completely cut off and the smooth communication is performed. Becomes possible. In addition, the variable insertion loss 20 does not suddenly increase in the amount of attenuation but increases with the passage of time, so that bidirectional communication in a double talk state can be performed.

〔Example〕

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a block diagram of an embodiment of the present invention.
In FIG. 1, the ambient noise generating means 40 of FIG. 1 includes a low pass filter 41, a variable loss circuit 42, and an ON / OFF circuit 43. The low-pass filter 41 inputs a signal from the microphone 6. Preferably, as shown in FIG. 3, the microphone 6 has such a characteristic that the level of the high frequency band rises at, for example, +6 dB / octave. This improves the clarity. The low-pass filter 41 allows only the low-pass signal component of the signal from the microphone 6 to pass (attenuates the high-pass signal component). For example, as shown in FIG. Use what you have. Thereby, the low-pass filter 41
Output becomes almost flat as shown in FIG. This output signal is used as an ambient noise component, and a variable loss circuit 42 is used.
Give to. The variable loss circuit 42 sets the variable loss attenuation so as to complementarily change the ambient noise component attenuation in response to the change in the variable insertion loss 20 attenuation. This control is performed by the comparison / control circuit 62 of the control means 60. ON / OFF circuit 43
Block the output of the variable loss circuit 42 during transmission, and pass the output at other times (during no talk, receiving and double talk).

The control means 60 has volume detection circuits 61 and 63, and a comparison / control circuit 62. Volume detection circuits 61 and 63 detect the signal levels of the transmission path and the reception path, respectively. The comparison and control circuit 62 adjusts the variable insertion loss according to the detected signal level.
20 and 30, the variable loss circuit 42, and the ON / OFF circuit 43 are controlled as described later.

FIG. 6 is a more specific example of the block configuration of FIG. The low-pass filter 41 shown in FIG. 2 is, for example, a first-order low-pass filter (LPF) 41a having a cutoff frequency fc = 300 Hz and a gain G = -6 dB. The variable loss circuit 42 includes an electronic volume 42a. Similarly, variable insertion loss 20 and 30
Is also composed of electronic volumes 20a and 30a. ON / OFF circuit
43 is constituted by an analog switch 43a. The adding means 50 includes an adding amplifier 50a.

Next, the operation of this embodiment will be described with reference to the operation timing chart of FIG.

First, in the transmission state, the output A of the volume detection circuit 61 is at a high level (FIG. 7A), and the output B of the volume detection circuit 63 is at a low level. (FIG. 7 (b)). Comparison / control circuit 62
Detects this, and outputs control signals C, D, E and F shown in FIGS. 7C to 7F to the addition amplifier 50a, the electronic volume 20a, the electronic volume 42a, and the electronic volume 30a, respectively. Thus, the attenuation of the electronic volume 20a is minimum (0 dB), and the attenuation of the electronic volumes 30a and 42a is maximum (−2 dB).
Analog switch 43a is turned off. In this state, the audio signal from the microphone 6 passes through the microphone amplifier 9, the electronic volume 20a, and the analog switch 43a as it is, and is transmitted via the wireless transmission / reception circuit 8.

Next, the operation in the case where the state changes from the transmission state to the no-communication state and the case where the state changes from the non-communication state to the reception state will be described. In these states, as shown in FIG. 7, the control signals C to F output by the comparison / control circuit 62 are the same except that the levels of the outputs A and B of the audio detection circuits 61 and 63 are different. In these states, the comparison / control circuit 62 changes the control signals C to F as shown in FIGS. 7 (c) to 7 (f). Thereby, the attenuation of the electronic volume 20a changes from the minimum (0dB) to the maximum (-20dB), and the attenuation of the electronic volumes 30a and 42a changes from the maximum (-20dB) to the minimum (0dB).
, And the analog switch 43a changes from OFF to ON. In this state, the signal from the microphone 6 passes through the microphone amplifier 9, passes through the primary low-pass filter 41a, the electronic volume 42a, and the analog switch 43a, and is given to the addition amplifier 50a. As described above, the output of the primary low-pass filter 41a has a substantially flat frequency characteristic shown in FIG. 5, and this is used as an ambient noise component. Further, when the change in the amount of attenuation of the electronic volume 42a with respect to time is the same as that of the electronic volume 30a on the receiving path side, and when it is determined that the receiving state or the no-communication state is being performed and the signal level of the transmitting path is attenuated, the amount of attenuation is reduced. Make it smaller.

FIG. 8 is a diagram showing how the amount of attenuation changes with time. The solid line in the figure relates to the electronic volume 20a, and the broken lines relate to the electronic volumes 30a and 42a. In the receiving state or the non-talking state, since there is no voice from the transmitter, the signal passing through the first-order low-pass filter 41a is mostly occupied by ambient noise components. This is added to the transmission voice signal output from the electronic volume 20a by the addition amplifier 50a.
Since the attenuation amount of the electronic volume 20a and the attenuation amount of the electronic volume 42a change complementarily as shown in FIG. 8, when the state changes from the transmission state to the no-communication state or from the non-communication state to the reception state, the transmission voice And the switching of ambient noise is smooth.

Next, at the time of double talk, the outputs A and B of the voice detection circuits 61 and 63 are both at the high level as shown in FIGS. 7 (a) and 7 (b). Even in this state,
The control circuit 62 outputs the same control signals C to F as those in the above-mentioned non-talking state and receiving state. Therefore, the signal from the microphone 6 is output from the primary low-pass filter 41a through the electronic volume 42a, the analog switch 43a, and the addition amplifier 50a as described above. Therefore, two-way communication becomes possible to some extent (although the communication quality is degraded).

Here, the insertion loss in the transmission path is
a is the sum of the attenuation of the primary low-pass filter 41a and the electronic volume 42a. In a complete reception state, the attenuation of the electronic volume 42a is minimum, and the attenuation of the electronic volume 20a is maximum. In this state, since there is an insertion loss corresponding to the amount of attenuation of the primary low-pass filter 41a, a howling margin can be secured accordingly. In particular, since the low-frequency noise is small in a car telephone, the margin as shown is useful. This is indicated by the hatched portion in FIG. Since such a howling margin can be ensured, the reception can be performed without interrupting the ambient noise even in the reception single talk state. If a similar circuit is provided on the receiving side, the same effect can be obtained even in the transmission single talk state.

The embodiment of the present invention has been described above. In the above embodiment, the low-pass filter 41 is constituted by the primary low-pass filter 41a, but a secondary or higher-order low-pass filter can be used. FIG. 9 is a frequency characteristic diagram of a secondary low-pass filter. The hatched portion in FIG. 9 is a howling margin. Further, an equalizer having an arbitrary frequency characteristic may be used. Also in this case, a howling margin can be obtained according to the characteristics.

〔The invention's effect〕

As described above, according to the present invention, it is possible to perform a smooth call without giving the other party a feeling that the line is disconnected, and to achieve an effect that a two-way call in a double talk state can be realized. can get.

[Brief description of the drawings]

1 is a block diagram of the principle of the present invention, FIG. 2 is a block diagram of one embodiment of the present invention, FIG. 3 is a frequency characteristic diagram of a microphone used in the present embodiment, and FIG. FIG. 5 is a frequency characteristic diagram from the microphone input to the low band device in the present embodiment, FIG. 6 is a specific configuration diagram of the present embodiment, and FIG. FIG. 8 is a diagram showing the change of the attenuation amount with respect to time, FIG. 9 is a frequency characteristic diagram of a secondary low-pass filter that can be used in this embodiment, and FIG. It is a block diagram of a hands-free communication circuit. In the figure, 20 is a variable insertion loss, 30 is a variable insertion loss, 40 is an ambient noise generation means, 41 is a low-pass filter, 42 is a variable loss circuit, 43 is an ON / OFF circuit, 50 is an addition means, and 60 is a control. Means, 61 is a volume detection circuit, 62 is a comparison / control circuit, and 63 is a volume detection circuit.

Claims (1)

(57) [Claims] An ambient noise generating means for generating an ambient noise signal having predetermined characteristics based on a signal from a microphone in a hands-free communication circuit having variable insertion losses (20, 30) in a transmission path and a reception path, respectively. (40), adding means (50) for adding the ambient noise signal generated by the ambient noise generating means (40) and the output of the variable insertion loss (20) on the transmission path side, And a control means (60) for controlling the variable insertion loss (20, 30) provided in the transmission path and the reception path according to the signal level. Hand-free communication circuit.