JPS62269451A - Loudspeaker telephone set - Google Patents

Abstract

PURPOSE:To prevent the echo cancelling from being affected even if many distortion is caused from a speaker amplifier due to the increase in an input signal level by providing a means obtaining an input signal of a pseudo echo generating circuit from an output signal of the speaker amplifier. CONSTITUTION:In a loudspeaker telephone set provided with an echo canceller cancelling an echo signal produced from an acoustic signal outputted from a speaker from the acoustic signal inputted to a microphone, a pseudo echo generating circuit 4 is inserted between the output side of a speaker amplifier 2 and a substraction circuit 5 and the input signal of the circuit 4 is obtained from the output signal of the speaker amplifier 2. Since the pseudo echo is generated from the output of the speaker amplifier 2 having the possibility producing the largest nonlinear distortion in this way, even if the input signal level is increased and much distortion is caused by the speaker amplifier, the sending of noise due to the distortion is avoided to the opposite party without giving effect on the echo cancelling.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a public address telephone, a public address intercom, and a conference telephone using a microphone and a speaker, and particularly to a public address system using an echo canceller for housing suppression and echo suppression. It concerns telephones.

[Conventional technology]

Traditionally, echo cancellers have been mainly used to cancel echoes in long-distance communication lines such as satellite communications and submarine cable communication lines, but recently they have been used as LSIs, for example.
1981 IEICE University 600 Customized VLSI echo cancellers made it possible for Azuma to reduce size, lower power consumption, and lower prices, so conventional voice switches were also used for terminals such as loudspeaker telephones. Research is being carried out to put into practical use a loudspeaker telephone using an echo canceller in place of the 1979 Kenkyuho, Vol. 28, No. 3, 5-IP type loudspeaker telephone. For example, Adaptive Digital Echo Canceller (IEEE VTC1985)
A NEW ADAPTIVEI) IGITAL EC
HOCA, NCELL-ER'').

However, as shown in the block diagram in Figure 3, acoustic coupling is viewed as an echo bus from the same perspective as the conventional system installed at the terminal side of a transmission line to eliminate echoes generated in long-distance lines. Generally, a method is considered in which echo cancellation is performed on the transmission line side.

In FIG. 3 showing an example of a public address telephone using a conventional echo canceller, 1 is a transmission line, 2 speaker amplifiers, 3 are microphone amplifiers, 4 pseudo echo generation circuits, 5 is a subtraction circuit, 6 is a speaker, 7 is a microphone.

The input signal of the pseudo-echo generation circuit 4 is obtained from the reception signal from the transmission line 1.

Figure 4 is an operation explanatory diagram showing the waveforms of each part in Figure 3.
(a) shows the received signal Δ This shows the difference e between the output y of the Crophone amplifier 3 and the pseudo echo y.

Next, the operation of the loudspeaker telephone shown in FIG. 3 will be explained with reference to FIG. 4.

First, receive signal X from transmission path 1 (see Figure 4 (,))
is branched, and one side is power amplified in the speaker amplifier 2, and the output X' of the speaker amplifier 2 (Fig. 4(b)
) is output from the speaker 6 as a received audio signal. The other generates a pseudo echo (heating), and this pseudo echo 9 is input to the subtraction circuit 5.

Next, the echo signal generated by the transmitted voice and the received voice signal is converted into an electrical signal by the microphone 7 and amplified by the microphone 3. Then, the output y of the microphone 3 (see FIG. 4(C)) is input to the subtraction circuit 5, and the output y of the microphone 3 and the pseudo echo generated by the pseudo echo generation circuit 4 are sent to the transmission line 1. The difference e of y (see FIG. 4(e)) is output as a transmission signal.

As is clear from the signal waveform shown in FIG. 4(e),
When the input signal level to the speaker amplifier 2 becomes high, the distortion cannot be completely eliminated, and noise due to the distortion is transmitted to the other party.

[Problem that the invention seeks to solve]

However, unlike general communication lines, speaker amplifiers have a high possibility of causing distortion (clip) due to excessive input, and especially in loudspeaker telephones for car phones, the power supply voltage is limited by the car battery voltage. In some cases, there is a greater possibility that distortion will occur.

The nonlinear distortion of a speaker amplifier depends on the input level, and increases rapidly when the input level exceeds a certain value.

Therefore, in a public address telephone using an echo canceller based on the configuration shown in FIG. 3, as shown in FIG. 4, even if the echo is sufficiently suppressed when the input signal level to the speaker amplifier is low, When the input signal level becomes large, there is a problem in that the distortion cannot be completely eliminated and is transmitted to the other party as large noise.

[Means for solving problems]

A public address telephone according to the present invention is a public address telephone equipped with an echo canceller that cancels an echo signal generated by an acoustic signal input to a microphone and output from a speaker, and the input signal of a pseudo echo generation circuit is transmitted to a speaker amplifier. The apparatus further comprises means for obtaining an output signal from the output signal of the apparatus.

[Effect]

In the present invention, a pseudo echo signal is generated from the output of the speaker amplifier that is likely to cause the largest nonlinear distortion.

〔Example〕

Embodiments of the present invention will be described in detail below based on the drawings.

FIG. 1 is a block diagram showing an embodiment of a loudspeaker telephone according to the present invention.

In FIG. 1, the same reference numerals as in FIG. The output signal of the speaker amplifier 2 is configured to obtain a signal (the output signal of the speaker amplifier 2).

Next, the operation of the embodiment shown in FIG. 1 will be explained with reference to FIG. 2.

This figure 2 is an operation explanatory diagram showing the waveforms of each part of figure 1.
, ) shows the received signal X, (b) is the output of the speaker amplifier 2, is the difference e between the output y of the microphone amplifier 3 and the pseudo echo y.

First, receive signal X from transmission path 1 (see Figure 2 (,))
is power amplified in the speaker amplifier 2, and the output X' (see FIG. 2(b)) of the speaker amplifier 2 is outputted to the speaker 6 as a received audio signal.

On the other hand, the echo signal generated by the transmitted voice and the received voice signal is converted into an electric signal by the microphone 7, and amplified by the microphone amplifier 3.
On the output side, an output y of the microphone amplifier 3 having a waveform as shown in FIG. 2(e) is obtained.

Next, the output X' of the speaker amplifier 2 including the distortion caused by the speaker amplifier 2 (see Fig. 2 (b)) is input to the pseudo echo generation circuit 4, and a pseudo echo △ y (ω in Fig. 2) is inputted to the pseudo echo generation circuit 4. reference). Then, the output y of the microphone amplifier 3 (see FIG. 2(c)), in which the echo signal that went around to the microphone 7 is amplified, and the pseudo echo Δy generated by the pseudo echo generating circuit 4 (see FIG. 2(d) ) is input to the subtraction circuit 5, and the difference e (see Fig. 2 (e)) between the output y of the microphone amplifier 3 and the △ pseudo echo y is output to the transmission line 1 as a transmission signal. .

As is clear from the signal waveform shown in FIG. 2(e),
Even if the input level increases and a large amount of distortion occurs in the speaker amplifier 2, it will not affect the echo cancellation operation.
No distortion noise will be transmitted to the other party.

As described above, in the loudspeaker telephone according to the present invention, in order to prevent a pseudo echo from being significantly different from a true echo and being transmitted as large noise due to the nonlinear distortion caused by the speaker amplifier 2, the pseudo echo generating circuit 40 input signal is By providing means for obtaining y from the output signal of the speaker amplifier 2, a pseudo echo y including nonlinear distortion caused by the speaker amplifier 2 is generated, and regardless of the input level to the speaker amplifier 2, the echo consumption i is ′ is always the same.

〔Effect of the invention〕

As explained above, according to the present invention, a pseudo echo is generated from the output of the speaker amplifier that is likely to cause the largest nonlinear distortion, so the input signal level increases and the speaker amplifier produces a large amount of distortion. Even if this occurs, it will not affect the echo cancellation operation. Therefore, it is possible to realize a practical loudspeaker telephone without transmitting noise due to distortion to the other party, and the practical effect is extremely large.

Further, even when an adaptive echo canceller is used as an echo canceller, it is extremely effective in that it can minimize the adverse effects of distortion on adaptive operation.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of a loudspeaker telephone according to the present invention, FIG. 2 is an explanatory diagram of the operation of FIG. 1, FIG. 3 is a block diagram showing an example of a conventional loudspeaker telephone, and FIG. FIG. 3 is an explanatory diagram of the operation of FIG. 3; 1... Transmission line, 2... Competitive speaker amplifier, 3...
...Microphone amplifier, 4...pseudo echo generation circuit, 5...subtraction circuit, 6...-speaker, 7.φ
...Microphone.

Claims (1)

[Claims] A loudspeaker telephone equipped with an echo canceler that cancels an echo signal generated by an acoustic signal input to a microphone and output from a speaker, comprising means for obtaining an input signal of a pseudo echo generation circuit from an output signal of a speaker amplifier. A loudspeaker telephone that is characterized by the ability to