JPH0332142A - Hand-free telephone set - Google Patents

Abstract

PURPOSE:To improve the speech quality by providing a clip means clipping a reception signal level to a predetermined level when a reception signal level is at the outside of a prescribed level range on a pre-stage of an echo canceller. CONSTITUTION:A clip means 31 is provided between an echo canceller 19 and a variable resistor 23 and the clip means 31 clips a reception signal level to a prescribed level when the reception signal level of a reception signal RIN received by a radio equipment 21 and via the variable resistor 23 is at the outside of the prescribed level. Then since the linearity of the signal in the echo path is kept, the echo cancelling quantity in the echo canceller 19 is designated as the design and the speech quality is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) This invention relates to a hands-free telephone using an echo canceller.

(Conventional technique vF1) With the development of the information society, the importance of telephones is increasing. For this reason, a telephone with more functions and a telephone that is easier to use is desired. An example of an easy-to-use telephone is a hands-free telephone that allows you to make calls without having to hold a handset. However, in order to make good calls using a hands-free telephone, it is necessary to eliminate in-vehicle echo and howling if the telephone is used in a car, and indoor echo and howling if the telephone is used indoors. There are several known methods for removing these.
This method is widely used because it uses an echo canceller or allows simultaneous transmission and reception. (For example, literature: Showa 6
IEICE General National Conference Lecture No. 2667)
.

Hereinafter, a conventional example of a hands-free telephone using an echo canceller will be briefly explained using an example of a hands-free car telephone.

FIG. 3 is a block diagram schematically showing the most general configuration of a conventional hands-free car phone using an echo canceller, including those disclosed in the above-mentioned documents.

In FIG. 3, 11 is a microphone, 13 is a microphone amplifier, 15 is a speaker, 17 is a speaker amplifier, 19 is an echo canceller, 21 is a radio device, 21
a indicates an antenna, and 23 indicates a volume. The echo canceller 19 is usually composed of a digital signal processing processor (hereinafter referred to as DSP), and basically includes an adder 19a, an adaptive filter +
9b.

Further, the signal flow in a hands-free car telephone equipped with an echo canceller as shown in FIG. 3 is as follows.

The signal received by the radio 21 is a signal that changes positive and negative around zero. This received signal is volume 2
The signal is input to the echo canceller 19 via 3. The signal input to the echo canceller 19 is indicated by RIN in the figure.

A signal (indicated by ROUT in the figure) outputted from the echo canceller 19 to the single power amplifier 1f is amplified by this amplifier 17 and outputted from the speaker 15.

On the other hand, the signal input to the echo canceller 19 via the microphone 11 and the microphone amplifier 13 (including the echo signal) is subjected to predetermined processing in the echo canceller 19 and then transmitted to the outside via the radio device 21. Output as a signal.

Here, the level of the received signal input to the volume 23 via the radio 21 depends on various factors such as radio wave conditions IfIFFr
As a result, it fluctuates within a certain range overall. Therefore, if this signal is output from the speaker 15 as it is, the voice will be too low or too loud, and the call will not be good.
There may be cases where this is not done. The volume 23 is provided to avoid this, and the user of the telephone uses the volume 23 to adjust the volume to an appropriate level.

(Problems to be Solved by the Invention) However, even with a hands-free car phone as shown in FIG. 3, the phone user does not always adjust the volume using the volume 23, and while driving a car, Even if it is desired to adjust the volume using the volume 23, there may be cases where it is not possible. Therefore, even when the volume 23 is adjusted to a position where the volume is appropriate when the level of the received signal is low, a signal with a high level may not be received. In such a case, the output signal ROUT becomes excessive, so the speaker amplifier 17,
Either the speaker 15, the microphone 11, or the microphone amplifier 13 becomes saturated.

As a result, these components +7. l5i1. The signal becomes nonlinear within the echo bus portion (indicated by 25 in FIG. 3) composed of +3, etc. Therefore, since the echo signal also becomes non-linear, the problem arises that the echo cancellation by the echo canceller 19 is not performed as designed, resulting in deterioration of speech quality.

The present invention has been made in view of the above points, and therefore, an object of the present invention is to solve the above-mentioned problems and provide a hands-free telephone with excellent call quality.

(Means for Solving the Problems) In order to achieve this object IIi, according to the present invention, in a hands-free telephone using an echo canceller, a device is installed at a stage before the echo canceller to detect when the received signal level of the telephone is outside a predetermined level range. In this case, the present invention is characterized in that a clipping means is provided for clipping the received signal level to a predetermined level.

Here, the level outside the predetermined level range specifically refers to a received signal level at which the received signal level of the telephone is so excessive that the signal becomes nonlinear in the echo bus portion. Considering the case shown in FIG. 3, this means a received signal at a level that would cause the outputs of speakers, microphones, amplifiers, etc. to become saturated. These levels vary depending on the ratings of the speakers, microphones, and amplifiers used, and how the speakers and microphones are installed.

Therefore, it is preferable to determine it in advance by measurement or simulation.

Also, to clip the predetermined level 1 mentioned above,
For example, the meaning is as follows.

For example, suppose that the received signal is RIN, and when RIN is greater than a certain level +A on the positive side or -A on the negative side, the signal becomes nonlinear in the echo bus portion. In such a case, clipping to a predetermined level means that RIN greater on the positive side than +A will be clipped to +A, and RIN greater on the negative side than -A will be clipped to -A. However, the level to be clipped does not necessarily have to be -A or +A, and may be any other suitable value, such as +A, a value slightly closer to zero than -A, or a value slightly closer to zero than -A. It may be possible to set different values for the negative side and the negative side.

(Function) According to such a configuration, even if the received signal reaches an excessive level, this signal will be clipped to a predetermined value at the stage before the echo canceller.

Therefore, the signal is prevented from becoming non-linear in the echo bus portion. Furthermore, since excessive input is prevented, the output sound from the speaker will not be distorted.

(Example).

Embodiments of the hands-free telephone according to the present invention will be described below with reference to the drawings. The following embodiment is an example in which the present invention is applied to the hands-free car telephone shown in FIG.

FIG. 1 is a block diagram schematically showing the configuration of a hands-free car phone according to an embodiment.

In FIG. 1, 11 is a microphone, 13 is a microphone amplifier, 15 is a speaker, 17 is a speaker amplifier, 19 is an echo canceller, 21 is a radio device, 21
Components a and 23 are the same as those explained using FIG. 3, respectively. Therefore, since the functions of each of these constituent components are the same as those of the prior art, their explanation will be omitted here.

Briefly, in FIG. 1, reference numeral 31 indicates a clipping means provided before the echo canceller, which clips the received signal level to a predetermined level when the received signal level of the telephone is outside a predetermined level range. The clipping means 31 in this embodiment is provided between the echo canceller 19 and the volume 23. It should be noted that although this clipping means 31 can of course be constructed by hardware, in the case of this embodiment, it is realized using a DSP (digital signal processor).

Next, with reference to FIGS. 2(A) and 2(B), the operation of the clip means 31 of the hands-free car telephone according to the embodiment will be explained. Here, FIG. 2(A) shows the clip means 3
1 is a schematic flowchart of a program stored in a DSP to operate the DSP. FIG. 2(B) is a characteristic block diagram of the clipping means 31 of the embodiment, with the input level plotted on the horizontal axis and the output level plotted on the vertical axis.

The operation described below is explained using an example in which the reception signal of the telephone is the signal at the output terminal of the volume 23, that is, the RIN shown in FIG. 3. Additionally, +A and -A used in the following explanation refer to microphone 11. This is a predetermined value for clipping the input signal RIN I so that the signal does not become non-linear in the echo bus section composed of the microphone amplifier 13, the speaker 15, the speaker amplifier 17, etc. These +A and -A were determined in advance by measurements, etc., but they may occur if the speaker 15, microphone 11, amplifier 13, 17, etc. are replaced with other ones, or if the installation location of the speaker or microphone is changed. etc. may be subject to change.

First, the clipping means 31 stores the received signal RIN received by the radio 21 (thereby passing through the volume 23) in a register (register in the DS'P) as an output signal ROUT (step 41 in FIG. 2(A)). ).

Next, the clipping means 31 compares this ROUTt with a predetermined level +A (step 43).

In this comparison, if ROUT > 10, ROUT +
It clips it to A, that is, outputs it to the echo canceller 19 side as ROUT=+A (step 45).

In addition, in the comparison in step 43, ROtJT
If ≦+A, this ROUTl is compared with a predetermined oneA (step 51). In this comparison, ROUTl
If < -A, clip ROOT to -A, i.e. R
OUT=-A and output to the echo canceller 19 side (step 53).

Furthermore, in the comparison in step 51, ROUT≧
In the case of -A, the input signal is at an appropriate level so that the signal does not become non-linear in the echo path, so it remains as it is (RO
OT= ROOT i.e. ROOT=RIN (7) Mom)
Output to the echo canceller 19 side (step 61)
.

Thereafter, the ROUT signal output from the clipping means 31 to the echo canceller 19 side is processed in the same manner as the conventional hands-free telephone shown in FIG.

The above is a detailed explanation of this invention. However, the present invention is not limited to the above-described embodiments, and various modifications can be made, for example, as described below.

In the embodiment described above, first the comparison of ROOT>+A is performed, and then the comparison of ROOT<-A is performed. However, this order may of course be reversed.

In addition, in the example, the appropriate level range is −A<RIN<
Although the explanation is given as a range of +A, it should be understood that this is not a strict meaning and that -A≦RIN≦+Δ may also be satisfied.

Further, in the above embodiment, ROtlT is compared with the predetermined levels of +A and -A, but it is of course possible to first obtain the absolute value of ROUT and compare it with A.

Furthermore, in the above-described embodiments, the present invention is explained as an example applied to the conventional hands-free car telephone shown in FIG. However, it is clear that the invention can also be applied to other hands-free telephones with echo cancellers.

(Effects of the Invention) As is clear from the above description, according to the hands-free telephone of the present invention, the linearity of the signal is maintained even in the echo bus portion, so that the amount of echo cancellation in the echo canceller is as designed. Therefore, it is possible to provide the hands-free telephone 7& with good call quality.

Roughly speaking, the desired speech quality can be obtained by performing a very simple process of clipping the excessive level to a predetermined level.

Therefore, whether the clipping means is constructed of hardware or software, the scale of the telephone set can be small.

[Brief explanation of drawings]

FIG. 1 is a block diagram schematically showing a hands-free telephone according to an embodiment; FIG. 2A is a flowchart schematically showing the operation of the clip means; FIG. CB) is a characteristic block diagram of the clip means of the embodiment, and FIG. 3 is an explanatory diagram of a conventional hands-free telephone as an example of a car telephone. 1... Microphone 3... Microphone amplifier 5... Speaker 7... Speaker amplifier 9... Echo canceller 9a... Adder 9b... Adaptive filter 21... Radio device, 21a ”・Antenna 23
...Volume 31...Clip means. L== ==--==-J Diagram for explaining the clipping means FIG. 2 (A) Characteristic block diagram of the clipping means of the embodiment FIG. 2 (B >

Claims (1)

[Claims] (1) In a hands-free telephone using an echo canceller, clipping means is provided before the echo canceller for clipping the received signal level to a predetermined level when the received signal level of the telephone is outside a predetermined level range. A hands-free phone featuring