JP3463688B2 - Communication system and communication method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention reduces the influence of mutual communication on the other in the presence of the first wireless system and the second wireless system, and, if necessary, that of one wireless system. The present invention relates to a communication system and a communication method that enable a receiving device to receive a signal from a transmitting device of the other wireless system.

[0002]

2. Description of the Related Art When wireless communication is performed between a transmitter and a receiver, a bit synchronization signal and a frame synchronization signal are transmitted first, and then an identification signal for identifying a communication partner and a data signal to be transmitted are transmitted. Is common. FIG. 8 shows a communication message format of low power wireless.

The bit synchronization signal 51 is a signal for synchronizing the bit timing between the transmitting device and the receiving device.
"01 ..." is repeated for several tens to hundreds of bits.
The frame synchronization signal 52 is a signal for transmitting the start position of the next identification signal to the receiving device, and is, for example, a 31-bit pseudo random signal “00011011101010000.
10010110011111 "is used. The identification signal 53 is a signal for designating a receiving device, and the receiving device confirms that the identification signal stored in advance by the receiving device matches the received identification signal and accepts the next transmission data signal 54. .

Therefore, the receiving process performed by the receiving device first receives the bit synchronizing signal 51 and establishes synchronization with the transmitting device, and then waits for the frame synchronizing signal 52. If the frame synchronization signal 52 is detected in the received signal, the next signal is regarded as an identification signal and compared with its own identification signal. If the identification signals match, the transmission data signal is accepted. If the frame synchronization signal 52 is not detected within a certain time after receiving the bit synchronization signal 51, the reception is interrupted.

As a method of detecting the frame synchronization signal 52, the above-mentioned 31-bit pseudo-random signal is stored in the receiving device in advance, and the reception signal which completely matches the random signal by 31 bits is regarded as the frame synchronization signal. Alternatively, there is also a design in which a 1-bit error is recognized in consideration of a reception error due to noise in a communication path and the like, even when 30-bit coincidence is considered as a frame synchronization signal.

[0006]

When there are a plurality of wireless systems in the above communication system, the influence of the mutual communication on the other must be reduced. If necessary, the receiving device of one wireless system can easily receive the signal from the transmitting device of the other wireless system.

For simplicity, the first wireless system will be described as an existing system and the second wireless system will be described as a new system.

The first wireless system is, for example, a gas meter reading system, which comprises a transmitter / receiver on the gas meter side and a transmitter / receiver on the telephone line side. The purpose is to notify the meter usage amount from the gas meter to the meter reading center device through the telephone line, or to send the status request command to the gas meter from the meter reading center side to grasp the status of the gas meter on the meter reading center side. When this transmitter / receiver is installed, it cannot be predicted what kind of transmitter / receiver will be newly installed in the same building later.

The second wireless system is, for example, a remote control system for a gas water heater. The transmitter / receiver on the remote control side sends a control command to the transmitter / receiver on the gas water heater side to remotely operate the gas water heater, and the transmitter / receiver on the gas water heater side transmits the operating status of the gas water heater to the transmitter / receiver on the remote controller side. The purpose is to display it on the remote control.

In such a case, it is necessary to reduce the influence of the communication of the gas meter reading system and the communication of the remote control system of the gas water heater on each other. In some cases, it is desired that the remote controller receives a communication message for notifying the gas usage amount from the gas meter side to the telephone line side and displays the gas usage amount on the display unit.

The gas meter reading system and the remote control system have different contents of data to be transmitted and received. Although the communication message format may be slightly different and the transmission / reception processing may be different, the communication message format is the same here.

[0012] A communication telegram for communication between the existing transmitting device and the receiving device, which is the first wireless system, is transmitted between the communication telegram A and the newly installed transmitting device and the receiving device, which is the second wireless system. The communication message to be communicated is referred to as communication message B. The communication message format is the same as that in FIG.

In both the communication telegrams A and B, the frame synchronization signal is the above-mentioned 31-bit pseudo random code "00011011".
1010000100010010110011111 ". When the new receiving device receives the communication message A, the frame synchronization signal is the same, so the frame synchronization signal is detected, and the next received signal is regarded as an identification signal and compared with its own identification signal. If the existing receiver and the new receiver each have their own identification signals, the identification signals may overlap. In this case, the new receiving device cannot distinguish between the existing transmitting device and the new transmitting device, and the data is erroneously accepted as the transmission data from the new transmitting device.

Therefore, if the frame synchronization signals are the same, it is necessary to manage and adjust so that the identification signals do not overlap. Since there is a wide communication range for low-power wireless, there was a system to unify this. However, in recent years, if this system disappears and manufacturers decide their own identification signals, it is difficult to manage and adjust the identification signals, and there is a possibility that existing communication and new communication will cause mutual interference.

Similarly, when an existing receiver receives the communication message B, a frame synchronization signal is detected, and the next received signal is regarded as an identification signal. If the identification signals happen to coincide with each other, the existing receiving device cannot distinguish the existing transmitting device and the new transmitting device, and accepts them as transmission data from the existing transmitting device.

[0016]

In order to solve the above problems, the present invention is directed to a communication system in which a transmitter and a receiver communicate with each other.
A transmitter of the first wireless system for transmitting an identification code;
A bit inversion signal of the first frame synchronization signal
Second frame sync signal and N bit (not M = N)
A transmitter of the second wireless system for transmitting another code;
The receiving device of the first wireless system is the first wireless system.
The first frame synchronization for performing system reception processing
To perform signal and reception processing of the second wireless system
Waiting at the same time with the second frame synchronization signal of
While receiving the signals from each transmitter,
Identification received based on previously detected frame sync signal
Switch the number of bits of the code to M bits or N bits
And a receiver.

According to the above invention, since the two frame synchronization signals are bit inversion signals and all the bits are different, they can be easily and surely identified. Therefore, the possibility that the receiving device of the second wireless system receives the communication of the first wireless system by mistake can be extremely reduced.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention can be implemented in the modes described in each claim.

That is, like the invention according to claim 1,
In a communication system in which a transmitter and a receiver communicate with each other, a first frame synchronization signal and an M-bit identification code are transmitted.
A transmitting device of the first wireless system that receives
The second frame, which is the bit inversion of the ram sync signal
Send sync signal and N-bit (not M = N) identification code
A second wireless system transmitting device, and the first wireless system
The receiving device of the system is the receiving device of the first wireless system.
The first frame synchronization signal for performing processing, and
The second for performing reception processing of the second wireless system
At the same time waiting for the frame sync signal,
Received the signal from the transmitter and detected it first
Bit of the identification code received based on the frame sync signal
With a receiving device that switches the number to M bits or N bits
Prepare Since the two frame synchronization signals are bit inversion signals and all the bits are different, they can be easily and surely identified. Therefore, the possibility that the receiving device of the second wireless system receives the communication of the first wireless system by mistake can be extremely reduced. Furthermore, when the receiving device of the first wireless system can also receive communication from the transmitting device of the second wireless system, it waits with two frame synchronization signals, and when the first frame synchronization signal is detected, Reception processing for receiving communication of the first wireless system (M bit
Receiving the communication code of the second wireless system when the second frame synchronization signal is detected ( reception of the N-bit identification code) is performed.

According to a second aspect of the invention, in the receiving device that communicates with the transmitting device, the first wireless system
First frame synchronization signal transmitted from the transmitter of the system
And from the transmitter of the second wireless system and before
The second frame that is the bit inversion of the first frame sync signal.
The frame sync signal and
Number of bits of the identification code received based on the frame synchronization signal
Is the number of bits of the identification code of the first wireless system.
Or the number of bits of the identification code of the second wireless system
Switch to some N bits (not M = N). And
Since the two frame synchronization signals are bit inversion signals and all bits are different, they can be easily and surely identified. Therefore, the possibility that the receiving device of the second wireless system receives the communication of the first wireless system by mistake can be extremely reduced. Furthermore, when the receiving device of the first wireless system can also receive communication from the transmitting device of the second wireless system, it waits with two frame synchronization signals, and when the first frame synchronization signal is detected, Reception processing for receiving communication of the first wireless system ( receiving an M-bit identification code
Signal ) , the receiving process (N-bit of the second wireless system receiving the communication of the second wireless system when the second frame synchronization signal is detected )
Receive the identification code) .

Further, according to the invention of claim 3, the transmission is performed.
In the communication method of communicating between the device and the receiving device,first
First frame synchronization signal from the transmitter of the wireless system
And a M-bit identification code are transmitted, and a second wireless system
From the transmitter of the first frame sync signal
The second frame synchronization signal, which is an inverted signal, and N bits (M =
An identification code (not N) is transmitted, and the first wireless system
System receiving device, the receiving process of the first wireless system
The first frame synchronization signal for performing
The second frame for performing reception processing of the wireless system of
And waits for the frame synchronization signal at the same time, and transmits each of the above.
While receiving the signal from the device,
The number of bits of the identification code received based on the home sync signal.
Switch to M bit or N bit.And two
The frame sync signal is a bit inversion signal and all bits are different.
Can be identified easily and reliably. Therefore the first radio
Incorrect communication with the second wireless system receiver
The probability of receiving
It In addition, the receiving device of the first wireless system is
When communication from the transmitter of the line system can also be received
Waits for two frame sync signals, and the first frame
When the home sync signal is detected, the communication of the first wireless system is detected.
Receive processing to receive the message(Receive M-bit identification code)
When the second frame sync signal is detected, the second
Processing to receive line system communication(Identification of N bits
Receive the code)I do.

According to a fourth aspect of the present invention, there is provided a recording medium on which a program for causing a computer to function as at least a part of the means according to any one of the first and second aspects is recorded. Further, since the program is a recording medium, at least a part of the communication system and the receiving device of the present invention can be easily realized by using a computer or a microcomputer. You can easily distribute and install the program.

[0023]

Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 1 is a block diagram showing the configuration of a transmitting / receiving apparatus according to Embodiment 1 of the present invention. In the figure, in order to communicate with a transmitter / receiver of a partner, a receiving means 3 for receiving a radio signal 2 from the transmitter / receiver of a partner via an antenna 1 and demodulating into a received message, turning on / off of the receiver 3 and receiving channel A receiving circuit control means 4 for controlling the above, a transmitting means 5 for modulating a transmission message and transmitting a radio signal 2 to a transmitting / receiving device of the other party through the antenna 1, a power on / off of the transmitting means 5, a transmission channel, etc. The transmission circuit control means 6 for controlling, and the communication control means 7 composed of, for example, a microcomputer for decoding the received message from the receiving means 3 and performing reception processing and for assembling the transmitted message and passing it to the transmitting means 5 control the entire communication. It

Further, a bit synchronization processing means 8 for detecting a bit synchronization signal in a reception message from the receiving means 3 and synchronizing with the transmitting device, and a frame synchronization signal for a predetermined time based on the synchronization. There is a frame sync signal detecting means 9 for searching / detecting in the meantime. For searching / detection, the frame synchronization signal stored in advance in the frame synchronization signal storage means 11 is compared with the received signal, and when they match, it is determined to be the frame synchronization signal.

When the frame synchronization signal is detected, the communication control means 7 carries out the following receiving process. That is, the head of the identification signal is determined by the frame synchronization signal, the identification signal confirms that it is addressed to itself, and the transmission data signal is received. The identification signal uses a unique code for each transmission / reception device to identify the communication partner, and the identification signal storage means 1 stores the identification signal of the transmission / reception device itself and the identification signal of the communication partner at the time of initial setting.
Store in 3. The communication control means 7 is an identification signal storage means 11
The identification signal is read from and the identification signal of the communication partner is inserted into the transmission message during transmission. At the time of reception, only the telegram in which the identification signal in the received telegram matches the own identification signal is decoded.

Two types of transmitting / receiving devices will be considered here. One is an existing transmission / reception device that constitutes the first wireless system. The other is a new transmission / reception device that constitutes the second wireless system. The two types of transceivers are at different times,
They may be installed for different purposes, and when installing existing transceivers, it may not be possible to predict what transceivers will be installed later in the same building. Also,
The contents of data to be transmitted and received are different between the existing transmission / reception device and the new transmission / reception device. Also, the communication message format is slightly different, and the transmission / reception processing is often different.

The communication message format of the transmitter / receiver is shown in FIG.
Shown in. In the figure, (1) is a communication message of an existing transceiver, and (2) is a communication message of a new transceiver.

The same points of (1) and (2) are that the bit synchronization signal, the frame synchronization signal and the identification signal for identifying the communication partner are transmitted first, and then the transmission data signal is transmitted. Also, the number of bits of the bit synchronization signal, the number of bits of the frame synchronization signal, and the number of identification signals are different. Here, the frame synchronization signal has a bit inversion relationship.

As the receiving process, the bit synchronizing signal is received to synchronize with the transmitting device, and subsequently the frame synchronizing signal is detected in the bit string to be received to determine the head of the identification signal. The case where the existing communication message is received by the existing transmission / reception device and the new transmission / reception device is as shown in FIG. When the existing communication message (1) is processed by the existing receiver (2)
As described above, the first frame synchronization signal is detected, the identification code is confirmed, and the transmission data signal is accepted. However, since the new receiver cannot detect the second frame synchronization signal as in (3), reception is interrupted.

When a new communication message is received by an existing transmitting / receiving device or a new transmitting / receiving device, it is as shown in FIG. Figure 3
On the contrary, when the new communication message (1) is received by the new receiving device, the second frame synchronization signal is detected as shown in (3), the identification code is confirmed, and the transmission data signal is accepted. To do. However, since the existing receiver cannot detect the first frame synchronization signal as in (2), reception is interrupted.

As described above, the existing transmission / reception device only needs to search / detect the first frame synchronization signal, and the new transmission / reception device only needs to search / detect the second frame synchronization signal. Therefore, it is not necessary to prepare a special means or a receiving process for preventing the new reception device from erroneously receiving the existing communication or the new reception device from erroneously receiving the new communication.

In the present invention, the first frame synchronization signal and the second frame synchronization signal are bit inversion signals and all bits are different, and the bit difference is the maximum. The possibility of receiving this by mistake can be extremely reduced.

A pseudo random code is used as the frame synchronization signal. This is a code with high randomness,
In FIG. 3C, a signal similar to the second frame sync signal does not appear in the first frame sync signal from the bit sync signal. In FIG. 4B, a signal similar to the first frame sync signal does not appear in the second frame sync signal from the bit sync signal.

First, the inconvenience when the pseudo random code is not used will be shown. For example, if the first frame synchronization signal is “00110
If “0”, the second frame synchronization signal is “11001”.
1 ”. The 4th to 6th bits of the first frame synchronization signal and the 1st to 4th bits of the second frame synchronization signal are "110".
Since it is the same for "0", if "00" is present in the 2 bits immediately before the second frame synchronization signal, the first frame synchronization signal will be detected in this portion. That is, unless an appropriate first frame synchronization signal is selected, a signal very similar to the first frame synchronization signal will be included in the second frame synchronization signal which is the inverted bit signal thereof. Further, the first frame synchronization signal contains a signal very similar to the second frame synchronization signal.

Therefore, if the first frame synchronization signal is a pseudo random code, its inverted signal does not include a signal very similar to the first frame synchronization signal. FIG. 5 shows an example of the bit difference between the first frame synchronization signal and the bit synchronization signal to the second frame synchronization signal. Although the bit difference changes depending on the comparison position of the first frame sync signal,
~ 31 bits is a sufficient size.

As described above, when the first frame synchronization signal is the pseudo random code, the bit inversion of the second frame synchronization signal is the pseudo random code. Since both of them are codes having high randomness, it is difficult for a frame synchronization signal to appear in a random bit string by accident, and the possibility of erroneous reception processing can be reduced.

Particularly in the case of 31-bit pseudo-random code,
It is convenient that the probability that the random bit string happens to be the same as this frame synchronization signal is 2 31. This probability is too small for a pseudo-random code having a smaller number of bits than this, for example, 15 bits. A pseudo-random code having a large number of bits, for example 63 bits, has a large number of bits and is difficult to handle when searching and detecting a frame synchronization signal.

By the way, if the first frame synchronization signal and the second frame synchronization signal are the same, it is impossible to distinguish between the two. The existing receiving device receives and processes the new communication as the existing communication, and the identification signal for confirming the communication partner ceases to function. If the identification signals happen to coincide with each other, the existing receiving device cannot distinguish the existing transmitting device and the new transmitting device and accepts the transmission data.

If the first frame synchronization signal and the second frame synchronization signal are similar to each other, it becomes difficult to distinguish the two when a reception error occurs. In this case as well, the existing receiver receives the new communication as the existing communication and the identification signal does not function.

As described above, when the first wireless system and the second wireless system exist, the two frame synchronization signals are bit inversion signals and all bits are different, so that they can be easily and surely identified, and the first wireless system can be identified. The possibility that the receiving device of the second wireless system receives the communication of the system by mistake can be extremely reduced.

(Embodiment 2) Next, as Embodiment 2 of the present invention, communication in which a new transmitting / receiving apparatus which is a second wireless system can receive a signal from an existing transmitting / receiving apparatus which is a first wireless system. The method will be described.

Since the purpose of the existing transmission / reception device is different from that of the new transmission / reception device, the communication message format is different as shown in FIG. At this time, it is desired to realize this only by adapting a new transmission / reception device without changing the existing transmission / reception device.

In the second embodiment of the present invention, the newly installed receiver waits for the first frame synchronization signal and the second frame synchronization signal at the same time after receiving the bit synchronization signal, and detects the previously detected frame. The receiving process is switched based on the synchronization signal. That is, as shown in FIG. 6, after bit synchronization, two frame synchronization signals are waited for, and when the frame synchronization signal of the existing communication is detected, the reception processing of receiving the existing communication is performed, and the frame synchronization signal of the new communication is detected. When it does, the receiving process which receives new communication is performed.

At this time, since the two frame synchronization signals are bit inversion signals and are different in all bits, the received signal and the second frame synchronization signal are compared, and when the number of mismatched bits is N bits or less, the second If the number of matching bits is less than N bits, it can be determined as the first frame synchronizing signal. For example, when the frame sync signal is 31 bits, it is 3 when compared with the second frame sync signal.
When 1 bit matches, the second frame synchronization signal, 31
When the bits do not match, it can be determined as the first frame synchronization signal which is a bit inversion signal.

When considering a reception error up to 2 bits,
When the number of mismatched bits is 2 bits or less as compared with the second frame synchronization signal, it can be determined as the second frame synchronization signal, and when the number of matching bits is 2 bits or less, it can be determined as the first frame synchronization signal.

That is, the first frame sync signal can be determined at the same time simply by comparing with the second frame sync signal.
Easy and reliable. This method is easier and more reliable than the method of switching the reception processing by providing a flag of about 1 bit in the identification code or the transmission data.

FIG. 7 is a block diagram showing the configuration of the transmitting / receiving apparatus according to the second embodiment of the present invention. The description of the same components as those in FIG. 1 will be omitted.

First and second frame synchronization signal detection means 10 for searching / detecting the first frame synchronization signal and the second frame synchronization signal for a fixed time with reference to the bit synchronization by the bit synchronization processing means 8. Equipped with. search·
For detection, the second frame synchronization signal stored in the second frame synchronization signal storage means 12 in advance is compared with the received signal to determine the frame synchronization signal.

When the first frame synchronization signal is detected, the communication control means 7 performs the first reception process. That is, FIG.
, The head of the first identification signal is discriminated, the 63-bit first identification signal confirms that it is addressed to itself, and the transmission data signal is received. When the second frame synchronization signal is detected, the head of the second identification signal is determined and
The second identification signal of the bit confirms that it is addressed to itself and receives the transmission data signal.

Although the first wireless system has been described as an existing system and the second wireless system as a new system, the first and second wireless systems are not limited to this. A wireless system that has different purposes, communication contents, and communication message formats.

The 31-bit pseudo random code "0
0011011101010000100101100
As a frame synchronization signal using "11111", "001101110101" in which the first 1 bit is shifted at the end
000010100101100111110 "or" 1111100110100100100 "in which the bit string is reversed.
001010111011000 ".

Although the second frame synchronization signal has been described as an inversion signal of all bits of the first frame synchronization signal, it may be a part of the bit inversion signal of the first frame synchronization signal. For example, for the sake of explanation, a 31-bit pseudo-random code “000110111010100001” is used.
First half 15 bits of "0010110011111""00"
0110111010100 ”= A block, latter half 16
Bit “0010010110011111” = B block. The first frame sync signal is A block + B
Block + A block "00011011101
0100 0010010110011111 000
110111010100 ", the second frame synchronization signal is an inverted signal of (A block + B block)" 111001000101011 110110 ".
1001100000 ", or (B block + A
Block) inverted signal "11011010011
0000 0111001000101011 ". When the first frame sync signal is long like this,
A signal obtained by extracting a part of the extracted bit and inverting the bit can be used as the second frame synchronization signal.

[0054]

As is apparent from the above description, according to the communication system and the communication method of the present invention, the transmitter of the first wireless system transmits the first frame synchronization signal and the transmitter of the second wireless system. The transmitter transmits a second frame synchronization signal which is a bit inversion signal of the first frame synchronization signal. Since the two frame synchronization signals are bit inversion signals and all bits are different, they can be easily and surely identified.

When the receiving device of the first wireless system can receive the communication from the transmitting device of the second wireless system, it waits with two frame synchronization signals and detects the first frame synchronization signal. When receiving, the receiving process for receiving the communication of the first wireless system ( receiving the M-bit identification code
Signal ) , the receiving process (N-bit of the second wireless system receiving the communication of the second wireless system when the second frame synchronization signal is detected )
Receive the identification code) .

[Brief description of drawings]

FIG. 1 is a block diagram of a transmission / reception device in a communication method according to a first embodiment of the present invention.

FIG. 2 is a diagram showing existing and new communication message formats in the same device.

FIG. 3 is a diagram showing a method of receiving an existing communication message in the same device.

FIG. 4 is a diagram showing a method of receiving a new communication message in the same device.

FIG. 5 is a diagram for explaining detection of a frame synchronization signal and a bit difference in the same device.

FIG. 6 is a diagram showing a method for receiving an existing communication message in the communication method according to the second embodiment of the present invention.

FIG. 7 is a block diagram of a transmission / reception device in the same device.

FIG. 8 is a diagram showing a general communication message format.

[Explanation of symbols]

1 wireless antenna 2 radio signals 3 Receiving means 4 Receiver circuit control means 5 Transmission means 6 Transmission circuit control means 7 Communication control means 8-bit synchronization processing means 9 frame synchronization signal detecting means 10 First and second frame synchronization signal detecting means 11 First frame synchronization signal storage means 12 Second frame synchronization signal storage means 13 identification signal storage means

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-56016 (JP, A) JP-B 6-83168 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) H04L 7/08 H04L 7/10

Claims (4)

(57) [Claims] 1. A first frame synchronization signal and M bits in a communication system in which a transmitter and a receiver communicate with each other.
Of the first wireless system for transmitting the identification code of the
And a second frame synchronization signal which is a bit inversion signal of the first frame synchronization signal and N bits (not M = N)
Device of the second wireless system for transmitting the identification code of the
If the receiving device of the first radio system, said first frame synchronization signal for performing the receiving process of the first wireless system, wherein for performing reception processing of the second wireless system first When waiting for the second frame synchronization signal at the same time and receiving the signals from the respective transmitting devices,
In addition, the signal is received based on the frame sync signal detected earlier .
And a receiving device that switches the number of bits of the identification code to M bits or N bits . 2. Transmission from a transmitter of the first wireless system
First frame synchronization signal and second wireless system
Of the first frame synchronization signal transmitted from the transmitter of
Receive the second frame sync signal which is the bit inversion of the No.
The number of bits of the identification code received based on the previously detected frame synchronization signal
The number of bits of the identification code is M bits or the second radio sequence.
N bits, which is the number of bits of the stem identification code (when M = N
Receiver ) to switch to ( no) . 3. A communication method in which a transmitter and a receiver communicate with each other, wherein a first frame synchronization signal and an M-bit identification code are transmitted from the transmitter of the first wireless system,
A second frame synchronization signal, which is a bit inversion signal of the first frame synchronization signal, and an N-bit (not M = N) identification code are transmitted from a transmitter of the second wireless system ,
Said first receiving device of a wireless system, the said first frame synchronization signal for performing reception processing of the first wireless system, for performing a reception processing of the second wireless system the second Wait for the frame sync signal at the same time,
When receiving signals from the respective transmitters,
To receive based on the previously detected frame sync signal
A communication method for switching the number of bits of an identification code to M bits or N bits . 4. A recording medium recording a program for causing a computer to function as at least a part of the means according to claim 1.