JPH088535B2 - Mode switching back method - Google Patents

Description

[Detailed Description of the Invention] [Table of Contents] Outline Industrial field of use Conventional technology Problems to be solved by the invention Means for solving the problem Action Example I. Correspondence between Example and FIG. 1 II. Configuration of Embodiment (i) Overall Configuration (ii) Decoder Configuration (iii) Configuration of Mode Switchback Judgment Section III. Operation of Example (i) Overall Operation (ii) Mode Switchback Determination Operation IV. Example Summary V. Modifications of the invention [Advantages of the invention] [Outline] A data communication system having a coder and a decoder which operate by automatically switching and switching back and forth between two different transmission modes to which an adaptive differential pulse code modulation method is applied. And a first end judging means for judging whether or not the signal transmission is completed, based on a silence threshold corresponding to a silent state, for the purpose of surely performing the switching back of the transmission mode. ,normal Based on the maximum threshold value corresponding to the maximum reproduction signal power in the transmission state, a coincidence determination means for determining whether or not the transmission mode of the device facing the coder or the decoder coincides with the reproduction signal power. A second end judging means for judging whether or not the signal transmission is completed based on the predetermined threshold value, a first end judging means, a coincidence judging means, and a second
A mode control signal generation unit that generates a mode control signal based on the determination result of the end determination unit is configured.

[Industrial application field]

The present invention comprises, for example, a coder and a decoder which operate by automatically switching and switching back and forth between a general mode for voice data transmission and a modem mode for modem data transmission, and an adaptive differential pulse code modulation (Adaptive Different
The present invention relates to a mode switching back method of a data communication system to which the ial pulse code modulation (ADPCM) method is applied.

[Conventional technology]

For example, in a data communication system in which a modem signal of a facsimile machine and a voice signal of a telephone machine are communicated using the same line, a 32 kbps ADPCM system is adopted.

The international standard of 32kbps ADPCM system is CCITT Recommendation G721.
Is based on. In the data communication system to which this international standard method is applied, it is possible to transmit the modem data up to 4800bps.

On the other hand, some facsimile machines use a device that uses a 9600bps modem signal (for example, GIIIFAX), so 32kbps ADPCM that enables transmission of a 9600bps modem signal.
The method is proposed by each company.

Hereinafter, the transmission mode to which the 32kbps ADPCM method that enables transmission of such a 9600bps modem signal is applied is referred to as a 9600bps modem mode, and the transmission mode to which the international standard method is applied is referred to as a general mode.

Even in the data communication system corresponding to the 9600bps modem mode, the general mode is applied when transmitting the voice signal and when transmitting the modem signal of 4800bps or less. In such a data communication system, 9600bps
9600bps from general mode when transmitting the modem signal
The mode is automatically switched to the modem mode, and when the transmission of the 9600 bps modem signal is completed, the mode is automatically switched back to the general mode.

The determination of switching from the general mode to the 9600 bps modem mode is made by detecting a characteristic change point appearing in the training signal of the 9600 bps modem signal.

On the other hand, a switchback determination from the 9600bps modem mode to the general mode is performed by detecting a silent section accompanying the end of transmission of the 9600bps modem signal. For example, the power of the reproduction signal generated in the coder or the decoder is compared with the silence threshold corresponding to the reproduction signal power in the silent state, and the reproduction signal power is lower than this silence threshold for a certain time (for example, 5 ms). Such a section is detected. When such a silent section is detected, it is determined that the transmission of the modem signal is completed. As a concrete example of such a method, the present inventor has proposed Japanese Patent Application No. 62-336585.
No. "Mode switchback method in data communication system" has already been proposed.

[Problems to be Solved by the Invention]

By the way, in a data communication system, a coder compatible only with the general mode and a decoder compatible with the 9600 bps modem mode may face each other. In such a case, when the 9600 bps modem signal is input, the decoder is switched to the 9600 bps modem mode by the above-described switching judgment, so that the transmission modes of the opposite coder and the coder do not match. In this case, since the training is not normally performed between the transmitting modem and the receiving modem, the transmission is terminated and a silent state occurs.

At this time, if there is noise above a certain level on the transmission path, the noise may cause the reproduced signal power of the decoder to exceed the silence threshold.

In the above-mentioned conventional method, since the silent section cannot be detected in such a case, the transmission mode is 96
There was a problem that the 00bps modem mode was not switched back to the general mode, and a deadlock state in which data transmission could not be performed occurred.

The present invention has been made in view of such a point, and an object thereof is to provide a mode reverting method that surely reverts from one transmission mode to the other transmission mode. .

[Means for solving the problem]

FIG. 1 is a block diagram showing the principle of the mode switching method according to the present invention.

In the figure, a first end judging means in a mode switching back system of a data communication system having a coder or a decoder which automatically switches and switches back between two different transmission modes to which the adaptive differential pulse code modulation system is applied. The reproduction signal power is supplied from the coder or the decoder 141, and 141 determines whether or not the signal transmission is completed based on the silence threshold corresponding to the silence state.

The coincidence determining means 151 is supplied with the reproduction signal power, and based on the maximum threshold value corresponding to the maximum reproduction signal power allowed in the normal transmission state, the transmission mode with the device facing the coder or the decoder is set to be the same. Determine whether you are doing.

The second end determination means 161 is supplied with the reproduction signal power,
Based on a predetermined threshold value according to the reproduction signal power, it is determined whether or not the signal transmission is completed.

The mode control signal generation means 171 includes the first end determination means 14
1, a mode control signal for switching back from one transmission mode to the other transmission mode is generated for the coder or the decoder based on the determination results by the coincidence determination means 151 and the second end determination means 161.

[Action]

A reproduction signal is supplied from a coder or a decoder, and the first end determination means 141 compares the reproduction signal power corresponding to this reproduction signal with a silence threshold to determine whether or not the signal transmission is completed.

Further, the coincidence determining means 151 compares the reproduction signal power with the maximum threshold to determine whether or not the transmission modes of the coder or the decoder and the opposing device are in agreement.

Further, the second end determination means 161 compares the reproduction signal power with a predetermined threshold value corresponding to the reproduction signal power to determine whether or not the signal transmission is completed.

A mode control signal generation unit 171 generates a mode control signal based on the detection results of the first end determination unit 141, the coincidence determination unit 151, and the second end determination unit 161, and the transmission mode control signal causes a coder or a decoder. On the other hand, switching back from one transmission mode to the other transmission mode is instructed.

In the present invention, the first end determination means 141 and the second end determination means 161 determine whether or not the signal transmission is completed, and the coincidence determination means 151 determines whether or not the transmission modes match. To be judged. In this way, it is necessary to switch back the transmission mode by the first end determination means 141, the coincidence determination means 151, and the second end determination means 161, even when the transmission modes do not match in the data communication system. The time point is detected. As a result, it is possible to reliably switch back from one transmission mode to the other transmission mode.

〔Example〕

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

FIG. 2 shows the configuration of the mode switchback system in one embodiment of the present invention.

I. Correspondence between Embodiment and FIG. 1 Here, the correspondence between the embodiment of the present invention and FIG. 1 will be described.

The first end determination unit 141 corresponds to the first end determination unit 241.

 The match determining means 151 corresponds to the match determining unit 242.

The second end determination unit 161 corresponds to the second end determination unit 243.

The mode control signal generation means 171 corresponds to the mode control signal generation unit 250.

An embodiment of the present invention will be described below on the basis of the above correspondence.

II. Configuration of Embodiment (i) Overall Configuration Referring to FIG. 2, the data communication system using the mode switching-back method according to the embodiment includes a coder 210 compatible only with a general mode and a transmission side such as a facsimile machine. A modem 201s that transmits data from a device (not shown) as a modem signal through an analog line, a decoder 220 that supports a 9600bps modem mode, and a mode switch that determines the transmission mode switching based on the output of the decoder 220. The reproduction signal S supplied from the determination unit 230 and the decoder 220
_The decoder 2 based on the determination results of the mode switchback determination unit 240 that performs switchback determination of the transmission mode based on _R (described later), and the mode switching determination unit 230 and the mode switchback determination unit 240
The output of the mode control signal generator 250 for generating the mode control signal S _m for controlling the transmission mode of 20 and the output of the decoder 220 introduced via the digital line are decoded and transmitted to the receiving side device (not shown). And a modem 201r that operates.

The coder 210 encodes the modem signal supplied from the modem 201s by a coding algorithm based on the general mode to generate a 32kbps ADPCM signal, and supplies the 32kbps ADPCM signal to the decoder 220 via a digital line.

Further, the mode control signal S _m is supplied to the control terminal S of the decoder 220.

(Ii) Configuration of Decoder FIG. 3 shows the configuration of the decoder 220. Here, the decoder 220 includes a decoding unit 221a that performs the decoding process based on the 9600bps modem mode, and a decoding unit 221b that performs the decoding process based on the general mode, and the decoding unit 221a and the decoding unit 221a.
Each of the 221b has a coder 210 via a digital line.
The 32 kbps ADPCM signal D _m generated by is supplied.

A control terminal supplied mode control signal to the S S _m of the decoder 220 is introduced into the control terminal S _B of the control terminal S _A a decoder 221b of the decoding unit 221a. The decoding unit 221a operates, for example, when "0" is input to the control terminal S _A , and the decoding unit 221b
Are configured to operate when “1” is input to the control terminal S _B.

The decoding unit 221a includes an inverse quantization circuit 222a that inversely quantizes the introduced 32kbps ADPCM signal D _m , a prediction circuit 223a that generates a prediction signal S _E , an output of the inverse quantization circuit 222a, and this prediction signal.
It is formed by an adder circuit 224a that adds S _E and a compression circuit 225a that compresses and outputs the output of the adder circuit 224a.

Each of the inverse quantization circuit 222a, the prediction circuit 223a, the addition circuit 224a, and the compression circuit 225a is configured to operate based on a decoding algorithm compatible with the 9600bps modem mode.

Here, the output of the adder circuit 224a is a reproduction of the 32 kbps ADPCM signal D _m compressed to 4 bits as a 16-bit linear signal. The output of this adder circuit 224a is the reproduction signal.
It is supplied to the mode switchback determination unit 240 as S _R.

Numerical binary represented by this reproduced signal S _R corresponds to the reproduced signal S _R of the power (reproduced signal power P _W).

Also, if the encoding algorithm on the transmitting side and the decoding algorithm on the receiving side match, this reproduced signal power
P _W never exceeds a predetermined value (maximum threshold Th _M ).
For example, if the dynamic range is 14 bits, this maximum threshold value Th _M is a maximum value of 14 bits. Further, in this case, when the transmission of the modem signal is completed, the reproduction signal power P _W becomes equal to or lower than the predetermined silence threshold Th ₁ .

On the other hand, there is a case where the encoding algorithm and the decoding algorithm do not match, and this reproduction signal power P _W may be several times that in the normal state. In this case, even after the transmission of the modem signal is finished, the reproduced signal power P _W may not be less than the silence threshold Th ₁ due to noise mixed in the analog line connecting the modem 201s and the coder 210, for example. .

The decoding unit 221b is formed similarly to the decoding unit 221a,
It is configured to operate based on a decoding algorithm corresponding to the general mode.

The outputs of the decoding units 221a and 221b are supplied to the modem 201r and the mode switching determination unit 230 shown in FIG. 2 as output signals of the decoder 220.

(Iii) Configuration of Mode Switchback Determination Unit The mode switchback determination unit 240 determines whether the transmission of the modem signal is completed based on the silence threshold Th ₁ described above.
The end determination unit 241, based on the above-mentioned maximum threshold value Th _M , a match determination unit 242 that determines whether or not the algorithms for encoding and decoding match, and a predetermined threshold value according to the reproduction signal S _R. And a second end determination unit 243 that determines whether or not the signal transmission is completed based on the above.

III. Operation of Embodiment Hereinafter, the operation of the data communication system using the embodiment will be described separately for the overall operation and the mode switchback determination operation.

(I) Overall Operation FIG. 4 is a flowchart showing the overall operation of the digital communication system.

The modem signal from the modem 201s encoded by the coder 210 based on the general mode is supplied to the decoder 220 via the digital line (step 401).

The decoder 220 decodes the introduced 32 kbps ADPCM signal D _m on the basis of an algorithm corresponding to the mode control signal S _m and supplies it to the modem 201r and the mode switching determination unit 230.
In addition, the decoder 220 outputs the reproduction signal S _R to the mode switchback determination unit 240
First end determination unit 241, coincidence determination unit 242, second end determination unit 24
Supply to each of 3 (step 402, step 403).

In step 402, if the decoding process is performed based on the general mode (when the mode control signal S _m is “1”), the affirmative determination is made in step 404. At this time, the mode switching determination unit 230 determines whether to switch the transmission mode based on the output of the introduced decoder 220. For example, the modem 201s sends out a training signal when sending a 9600 bps modem signal. The mode switching determination unit 230 detects a characteristic change point of the training signal based on the output of the decoder 220 and determines that the transmission mode needs to be switched (step 405).

In the case of a positive determination in step 405, the mode control signal generation unit 250 outputs “0” as the mode control signal S _m , and the decoding algorithm of the decoder 220 is switched from the general mode to the algorithm corresponding to the 9600 bps modem mode ( Step 406).

On the other hand, if a negative determination in step 405, the mode control signal S _m remains "1".

By the way, in the data communication system shown in FIG. 2, the transmission mode of the decoder 220 is set to 9600 in step 406.
When the mode is switched to the bps modem mode, the coder 210 and the decoder 220 have an algorithm mismatch. Therefore,
Since the subsequent transmission / reception of the training signal is not normally performed, the modem 201s ends the transmission.

On the other hand, in the case of a negative determination in step 404, the mode switchback determination unit 240 determines whether to switch back the decoding algorithm described later (step 407).

For the affirmative determination in step 407, the mode control signal generation unit 250 outputs "1" as the mode control signal S _m (step 408).

On the other hand, in the case of negative determination of step 407, the mode control signal S _m remains "0".

The processing of steps 401 to 408 described above is repeated at regular intervals (for example, 125 μs), and transmission and reception operations are performed between the modem 201s and the modem 201r.

Therefore, when the decoder 220 is operating in the general mode, the mode switching determination processing in step 405 is performed every 125 μs, while when operating in the 9600 bps modem mode, similarly in step 407 every 125 μs. Mode switching back determination processing is performed.

(Ii) Mode switching back determination operation FIG. 5 is a flowchart showing the mode switching back determination operation. Further, FIG. 6 shows an example of the reproduction signal power.

(Ii-1) Operation of first end determination unit FIG. 5 (a) shows the operation of the first end determination unit 241.

The first end determination unit 241 compares the reproduction signal power P _W corresponding to the introduced reproduction signal S _R with the silence threshold Th ₁ described above (step 511).

If it is determined in step 511 that P _W <Th ₁ , then “1” is added to the variable N and step 514 is performed.
Here, it is assumed that this variable N is set to an initial value "0" when the decoding algorithm is switched from the general mode to the 9600 bps modem mode in step 406 described above (step 512).

On the other hand, if it is determined in step 511 that P _W ≧ Th ₁ , then an initial value “0” is set in this variable (step 513).

The first end determination unit 241 sets the variable N to a constant K (for example, 40).
(Step 514).

When it is determined in step 514 that N <K, the first end determination unit 241 determines that switching back to the transmission mode is not necessary.

On the other hand, for example, as shown in FIG.
In the section B, the reproduction signal power P _W is below the silence threshold Th _s1 . Therefore, in this section AB,
Each time the switchback determination is performed, "1" is added to the variable N by step 512, and at step B, step 51
From 4, it is determined that N ≧ K.

As described above, the first end determination unit 241 determines that the reproduction signal power P _W is 5 ms (125 μs × 4), as in the area A-B in FIG. 6A.
During (0), a section (silent section) that continuously falls below the silent threshold Th _s1 is detected, and it is determined that the transmission of the modem signal is completed when the silent section is detected.

As described above, the first end determination unit 241 causes the step 514
When it is determined that N ≧ K, it is determined that the switching back process of the transmission mode from the 9600 bps modem mode to the general mode is necessary.

(Ii-2) Operation of Matching Judgment Unit FIG. 5 (b) is a flowchart showing the operation of the matching judgment unit 242.

The coincidence determination unit 242 compares the reproduction signal power P _W with the above-described maximum reproduction signal power Th _M (step 521).

When it is determined in step 521 that P _W > Th _M , “1” is added to the variable M, and then step 524 is performed. Here, it is assumed that the variable M is set to the initial value "0" when the decoding algorithm is switched from the general mode to the 9600 bps modem mode in step 405 described above (step 522).

On the other hand, when it is determined in step 521 that P _W ≦ Th _M , the variable M is set to the initial value “0” (step
523).

The coincidence determination unit 242 compares the variable M with a constant K (for example, 40) (step 524).

When it is determined in step 524 that M <K, the coincidence determination unit 242 determines that switching back of the transmission mode is not necessary.

On the other hand, for example, as shown in FIG.
In the section D, the reproduction signal power P _W exceeds the maximum threshold Th _M. Therefore, in this CD section,
Every time the switchback determination is performed, "1" is added to the variable M by step 522, and at step D52, step 52 is performed.
From 4, it is determined that M ≧ K.

As described above, the match determination unit 242 determines that the C- of FIG.
A section in which the reproduction signal power P _W is continuously lower than the silence threshold Th _s1 for 5 ms (125 μs × 40), such as between D, is detected, and the transmission mode of the decoder 220 and the coder 210 is detected when this section is detected. Is determined not to match.

As described above, when it is determined in step 524 that M ≧ K, the match determination unit 242 determines that the transmission mode switchback process from the 9600 bps modem mode to the general mode is necessary.

(Ii-3) Operation of Second End Judgment Unit FIG. 5C is a flowchart showing the operation of the second end judgment unit 243.

The second end determination unit 243 compares the reproduction signal power P _W with the variable MAX. It is assumed that this variable MAX is set to an initial value “0” when the decoding algorithm of the decoder 220 is switched to the 9600 bps modem mode in step 405 described above. When it is determined that the reproduction signal power P _W is smaller than the variable MAX, the process proceeds to step 533 (step 5
31).

When it is determined in step 521 that the reproduction signal power P _W is larger than the variable MAX, the reproduction signal power P _W is set in the variable MAX.
Is substituted. Therefore, the value of the variable MAX becomes the maximum value of the reproduction signal power P _W introduced so far (step 53).
2).

Next, the second end determination unit 243 determines the reproduction signal power P _W and the variable P
Compare with MAX. It is assumed that this variable PMAX is set to an initial value "0" when the transmission mode of the decoder 220 is switched to the 9600bps modem mode. When it is determined that the reproduction signal power P _W is smaller than the variable P MAX,
Proceed to step 535 (step 533).

If it is determined in step 521 that the reproduction signal power P _W is larger than the variable PMAX, the reproduction signal power P
Substitute P _W and add “1” to the variable L. Here, it is assumed that this variable L is set to the initial value "0" when the transmission mode of the decoder 220 is switched to the 9600 bps modem mode (steps 534, 435).

The second end determination unit 243 compares the variable L and the constant J (for example, 256) described above, and when L <J, the variable
The values of L, MAX and PMAX are held and the switchback determination operation is ended (step 536).

If L ≧ J in step 536, MAX / 4 is compared with the variable PMAX, and if it is determined that the variable PMAX is larger than MAX / 4, the process proceeds to step 538 (step 53).
7).

When it is determined in step 537 that the variable PMAX is smaller than MAX / 4, the second end determination unit 243 determines that the transmission mode needs to be switched back.

On the other hand, when it is determined in step 537 that the variable PMAX is larger than MAX / 4, the second end determination unit 243 determines that the variable PMAX
The initial value "0" is set in MAX and the variable L, and the switchback determination processing is ended (step 538).

The variable MAX described above is used when the transmission mode of the decoder 220 is 9600.
After switching to the bps modem mode, the maximum value M _{TOTAL of} the reproduction signal power P _W introduced into the second end determination unit 243 is _reached . On the other hand, the variable PMAX is set to L> J in step 536.
The initial value “0” is set each time it is determined that
Therefore, the section maximum value M _SECTION of the reproduction signal power P _W introduced into the second end determination unit 243 during the time indicated by the constant J (for example, 32 ms) is obtained.

For example, as shown in FIG. 6 (C), the second end determination unit
243 determines the maximum section value of the reproduced signal power P _W every 32 ms. Further, the reproduction signal power P _W introduced at time E in the figure is held as the maximum value M _TOTAL of the reproduction signal power P _W.

The section maximum value M _SECTION obtained in the section between F and G in FIG. 6C is the maximum value M _TOTAL of the reproduction signal power P _W.
It is less than 1/4. Therefore, if the comparison in step 537 described above is made at the time of point G, the variable PMAX becomes M.
It is judged to be smaller than AX / 4. As a result, the second end determination unit 243 determines that the transmission mode switchback processing is necessary.

The reproduction signal power P _W shown in FIG. 6 (C) does not fall below the silence threshold Th ₁ due to noise on the transmission path even after the transmission of the modem signal is completed.

Also in this case, 1/4 of the maximum value of the reproduced signal power P _W as the threshold value Th _2, by interval maximum value of the reproduced signal power P _W detects becomes smaller interval than the threshold value Th _2, It is possible to detect a silent section associated with the end of transmission of the modem signal.

In this way, according to the reproduction signal S _R introduced into the second end determination unit 243, it is larger than the silence threshold Th ₁ , and
A threshold Th ₂ that is smaller than the reproduction signal power P _W of the minimum modem signal to be transmitted is set.

IV. Summary of Examples As described above, the coincidence determination unit 242 determines the reproduction signal power P _W.
The coder 21 based on whether there is a section in which the value exceeds the maximum threshold Th _M for a predetermined time (for example, 5 ms).
It is determined whether 0 and the transmission mode of the decoder 220 match. Also, the second end determination unit 243 determines that the reproduction signal power P
Whether or not the transmission of the modem signal is completed is determined based on whether or not there is a section in which _W is below the threshold Th ₂ corresponding to the reproduction signal S _R for a predetermined time (for example, 32 ms). .

In this way, a transmission mode mismatch occurs between the coder 210 and the decoder 220, and the reproduction signal power P _{W of the} decoder 220 is increased.
Even when no silent section is detected by the conventional method because the value does not fall below the silent threshold Th ₁ , it is possible to reliably perform the switching back of the transmission mode.

This can prevent the data communication system from entering a deadlock state.

V. Modified Embodiment of the Invention In the above-described embodiment of the present invention, the data communication system includes the coders 210 and 9600b compatible with the general mode.
A case was considered in which the decoder 220 corresponding to the ps modem mode was provided, but the present invention is not limited to this, and it is applicable as long as the transmission mode of the coder and the decoder is switched back from the 9600 bps modem mode to the general mode. The value of the threshold value Th ₂ set according to the reproduction signal power P _W is not limited to 1/4 of the maximum value M _TOTAL of the reproduction signal power described above.

Furthermore, in “I. Correspondence between Example and FIG. 1”,
Although the correspondence between the present invention and the embodiments has been described, the present invention is not limited to this, and those skilled in the art can easily contemplate that the present invention has various modifications.

〔The invention's effect〕

As described above, according to the present invention, even when the transmission modes are mismatched in the data communication system by the first end determination means, the coincidence determination means, and the second end determination means,
The point in time when the transmission mode needs to be switched back is detected.
This makes it possible to reliably switch back from one transmission mode to the other transmission mode and prevent a deadlock state, which is extremely useful in practice.

[Brief description of drawings]

FIG. 1 is a block diagram showing the principle of the mode switching back method of the present invention, FIG. 2 is a block diagram of a data communication system using the mode switching back method according to an embodiment of the present invention, and FIG. 3 is a detailed configuration of a decoder. 4 and 5 are flow charts showing the overall operation, FIG. 5 is a flow chart showing the mode switching back determination operation, and FIG. 6 is a view showing an example of reproduction signal power. In the figure, 141 is a first end determination means, 151 is a match determination means, 161 is a second end determination means, 171 is a mode control signal generation means, 201 is a modem, 210 is a coder, 220 is a decoder, 221 is a decoding section, 2
22 is an inverse quantization circuit, 223 is a prediction circuit, 224 is an addition circuit, 22
5 is a compression circuit, 230 is a mode switching determination unit, 240 is a mode switching back determination unit, 241 is a first end determination unit, 242 is a match determination unit, and 24
3 is a second end determination unit, and 250 is a mode control signal generation unit.

Claims (1)

[Claims] 1. A mode switching back system of a data communication system having a coder or a decoder, which is adapted to automatically switch and switch back between two different transmission modes to which an adaptive differential pulse code modulation system is applied. Alternatively, reproduction signal power is supplied from the decoder, and a first end determination means (141) for determining whether or not signal transmission is completed based on a silence threshold corresponding to a silence state, and the reproduction signal power is supplied. Based on the maximum threshold value corresponding to the maximum reproduction signal power allowed in the normal transmission state, it is determined whether or not the transmission mode of the device facing the coder or decoder is the same. Means (151), the reproduction signal power is supplied, and whether the signal transmission is completed based on a predetermined threshold value according to the reproduction signal power Or the second end determining unit determines (161), said first end determining unit (141), match determining means (151),
Based on the determination result by the second end determination means (161),
Mode control signal generation means (171) for generating a mode control signal for switching back from one transmission mode to the other transmission mode for the coder or decoder.
A mode switchback method characterized by being configured so that