Abstract:
A device divides a sampled sound signal into a high frequency signal and a low frequency signal, individually encodes the high frequency signal and the low frequency signal, and generates error detection code pertinent to the high frequency ADPCM data and the low frequency ADPCM data. The device replaces data pertinent to some of a plurality of bits which configure the low frequency ADPCM data with the error detection code and transmits them. A receiver side receives the high frequency ADPCM data and the low frequency ADPCM data, and individually processes the high frequency ADPCM data and the low frequency ADPCM data in accordance with a value of the error detection code.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a wireless communication device and a communication terminal that perform a communication by use of sound packets. 
         [0003]    2. Description of the Related Art 
         [0004]    When a user carries a wireless voice communication system in one&#39;s hand, such as a cellular phone and a cordless phone, or uses the system at a location where ambient circumstances change, variations occur in received signal strength indicator, a distance between devices, and an interference radio wave, which greatly affects communication quality. A known countermeasure against the problems is; for instance, a technique described in relation to a wireless telephone system as disclosed in JP-T-2002-509387. 
         [0005]    The wireless telephone system of JP-T-2002-509387 is made up of a base unit that has a base transceiver and at least one wireless handset and that is connectable with one external phone line. The wireless handset and the base unit implement communication by means of fixed-size audio packets, each of which is composed of a plurality of audio data samples and error correction bits. Moreover, JP-T-2002-509387 includes descriptions about changing a relative number of bits assigned to an audio data sample and an error correction bit in connection with a packet configuration of a subsequent audio packet by means of monitoring the quality of a digital link set on a wireless channel and determining whether or not a change has occurred in error rate. 
         [0006]    Specifically, when the error rate becomes worse, the wireless telephone system performs processing for decreasing an amount of data pertinent to an audio data sample contained in a packet and increasing an amount of data pertinent to error correction bits to thus assign a larger number of bits to error correction, thereby broadening a distance range and enhancing a tolerance to the interference radio wave. 
         [0007]    JP-A-2010-154163 discloses a technique of reversing least significant bits in “n” bits in accordance with the number of “1s” in the “n” bits of n-bit ADPCM data, thereby letting the least significant bits act like a parity signal and enabling detection of an error without involvement of an increase in the number of bits. As a result, when a communication environment becomes worse, sound quality is somewhat distorted, but accuracy in error detection is increased. When an error occurs in ADPCM data, the ADPCM data themselves can be switched to a muted or attenuated state. 
         [0008]    In addition, JP-A-7-143074 discloses a technique of converting an ADPCM code for which an error is detected into an ADPCM code which may cause a much smaller amplitude change in a decoded signal, thereby preventing occurrence of high-level unusual noise and avoiding a situation in which a level of a normal decoded sound will fall more than necessary to thus prevent a listener from feeling unpleasant sensation. 
       SUMMARY 
       [0009]    When the error rate is increased as a result of deterioration of the communication environment, the wireless telephone system described in connection with JP-T-2002-509387 switches a quality level from the highest quality level to an intermediate quality level or a low quality level by decreasing a data rate of sound in order to increase the amount of data pertinent to the error correction bits. For this reason, sound quality is considered to become deteriorated. However, if the sound quality changes for the worse in sequence as the communication environment becomes worse, the user will experience inconvenience in using the wireless telephone system. 
         [0010]    Moreover, the techniques described in connection with JP-A-2010-154163 and JP-A-7-143074 make it possible to perform audio processing that inhibits muting of a sound or occurrence of unpleasant unusual noise, which would otherwise arise at the time of occurrence of an error, without greatly degrading sound quality even when the communication environments become worse, thereby preventing the listener from feeling unpleasant sensation. In the meantime, these techniques are based on the premise that the system uses a signal band employed in a popular telephone line. However, even in the field of wireless communication, a demand exists for extending a frequency band to a broader band to implement communication of more natural sound quality. 
         [0011]    Accordingly, an object of the present invention is to provide a wireless communication device and a communication terminal that can maintain sound quality without changing a sound data rate through use of a fixed-size packet even when the communication environments become worse and, hence, can enhance accuracy in error detection. 
         [0012]    A wireless communication device according to an aspect of the present invention is configured to include: an ADPCM encoding unit that divides a sampled sound signal into a high frequency signal and a low frequency signal and individually encodes the high frequency signal and the low frequency signal by means of adaptive differential pulse code modulation to thereby convert the high frequency signal into high frequency ADPCM data having a first number of bits, and convert the low frequency signal into low frequency ADPCM data having a second number of bits; an error detection code generation unit that generates an error detection code that is pertinent to the high frequency ADPCM data having the first number of bits and the low frequency ADPCM data having the second number of bits; a transmission data conversion unit replaces data pertinent to some of a plurality of bits which configure the low frequency ADPCM data with the error detection code formed from the error detection code generation unit; and a received data processing unit that receives the high frequency ADPCM data and the low frequency ADPCM data, which are sent along with the error detection code, and that individually processes the high frequency ADPCM data and the low frequency ADPCM data in accordance with a value of the error detection code. 
         [0013]    The wireless communication device according may be configured so that the error detection code generation unit generates a first error detection code pertaining to the high frequent ADPCM data and a second error detection code pertaining to the low frequency ADPCM data, and the transmission data conversion unit replaces at least two bits of data among the plurality of bits that configure the low frequency ADPCM data with at least two bits of an error detection code including the first error detection code and the second error detection code. 
         [0014]    The wireless communication device may be configured so that the error detection code generation unit generates a third error detection code that acts as a parity signal for reversing bits in accordance with the number of “1s” in a series of data that include the high frequency ADPCM data and the low frequency ADPCM data. 
         [0015]    The wireless communication device may be configured so that when an error is detected by means of the received error detection code, the received data processing unit performs first signal processing on the high frequency ADPCM data that are received along with the error detection code and performs second signal processing on the low frequency ADPCM data that are received along with the error detection code. 
         [0016]    The wireless communication device may be configured so that the received data processing unit performs third signal processing on the high frequency ADPCM data received along with the error detection code and fourth signal processing on the low frequency ADPCM data received along with the error detection code when an error in a high frequency signal is detected by means of the received first error detection code, and the received data processing unit performs fifth signal processing on the high frequency ADPCM data received along with the error detection code and sixth signal processing on the low frequency ADPCM data received along with the error detection code when an error in a low frequency signal is detected by means of the received second error detection code. 
         [0017]    According to the present invention, in a wireless wideband voice communication, high frequency ADPCM data and low frequency ADPCM data are individually processed by means of an error detection code that is transmitted after replacement of some of bits of the low range ADPCM data. Therefore, even when the communication environments become worse, wideband sound quality can be maintained by a fixed-size packet without changing a data rate of sound. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a block diagram showing a cordless telephone according to an embodiment of the present invention. 
           [0019]      FIG. 2  is a block diagram for explaining a low frequency ADPCM encoder of the cordless telephone according to the embodiment of the present invention. 
           [0020]      FIG. 3  is a diagram showing a layout of sound data in a sound packet of the cordless telephone according to the embodiment of the present invention. 
           [0021]      FIG. 4  is a diagram for explaining a transmission conversion table of the cordless telephone according to the embodiment of the present invention. 
           [0022]      FIG. 5  is a diagram for explaining a transmission conversion table switching unit of the cordless telephone according to the embodiment of the present invention. 
           [0023]      FIG. 6  is a diagram showing a format of the sound packet and the layout of the sound data in the sound packet of the cordless telephone according to the embodiment of the present invention. 
           [0024]      FIG. 7  is a diagram for explaining tables pertinent to a low frequency signal in a reception conversion table of the cordless telephone according to the embodiment of the present invention. 
           [0025]      FIG. 8  is a diagram for explaining tables pertinent to a high frequency signal in the reception conversion table of the cordless telephone according to the embodiment of the present invention. 
           [0026]      FIG. 9  is a diagram for explaining a reception conversion table switching unit of the cordless telephone according to the embodiment of the present invention. 
           [0027]      FIG. 10  is a block diagram for explaining a low frequency ADPCM decoder of the cordless telephone according to the embodiment of the present invention. 
           [0028]      FIG. 11  is a diagram for explaining sound processing using a related-art sound packet. 
           [0029]      FIG. 12  is a diagram for explaining sound processing using a sound packet of the cordless telephone according to the embodiment of the present invention. 
           [0030]      FIGS. 13A and 13B  are diagrams showing counters disposed in a receiving error processing unit of the cordless telephone according to the embodiment of the present invention. 
           [0031]      FIG. 14  is a flowchart for explaining reception conversion table switching operation of the cordless telephone according to the embodiment of the present invention. 
           [0032]      FIG. 15  is a flowchart for explaining the reception conversion table switching continued from  FIG. 14 . 
           [0033]      FIG. 16  is a diagram for explaining reception table switching operation that is determined by means of received signal strength. 
           [0034]      FIG. 17  is a diagram for explaining reception conversion table switching that is determined by a frame error counter. 
           [0035]      FIGS. 18A and 18B  are diagrams showing an example of received data processing of the cordless telephone according to the embodiment of the present invention. 
           [0036]      FIG. 19  is a diagram showing the example of received data processing of the cordless telephone according to the embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
     Embodiment 
       [0037]    By reference to the drawings, a wireless communication device according to an embodiment of the present invention is described by taking a cordless telephone as an example.  FIG. 1  is a block diagram showing a cordless telephone according to an embodiment of the present invention. In  FIG. 1 , (A) is a block diagram showing a transmission function of a master device (a first communication terminal), and (B) is a block diagram showing a receiving function of a slave device (a second communication terminal). For the sake of convenience,  FIG. 1  illustrates only the transmission function of the master device and the receiving function of the slave device. However, each of the master device and the slave device has both functions. 
         [0038]    The cordless telephone includes the master device  10  and one or more slave devices  20 , as shown in  FIG. 1 . The cordless telephone is a wireless communication device that establishes a digital link on a wireless channel between the master device  10  and the slave device  20 ; that compresses a sound signal by adaptive pulse code modulation; and that performs a communication while containing the thus-compressed sound signal in a sound packet by means of TDMA (Time Division Multiple Access)/TDD (Time Division Duplex). G.722 that is the ITU-T (International Telecommunication Union Telecommunication Standardization Sector) recommendation is adopted for the codec. 
         [0039]    The master device  10  includes a sound input unit  11 , a PCM conversion unit  12 , an ADPCM encoding unit  13 , a transmission conversion table  14 , a transmission conversion table switching unit  15 , a transmission packet generation unit  16 , and a wireless transmission circuit  17 . 
         [0040]    The sound input unit  11  inputs a sound signal formed from signal that is delivered by way of a telephone line network or an IP network. The sound input unit  11  corresponds to a microphone that is built in a handset if the master device  10  is provided with the handset. 
         [0041]    The PCM conversion unit  12  samples the sound signal at a predetermined cycle and quantizes the thus-sampled signal into an integral value including a predetermined number of bits. 
         [0042]    The ADPCM encoding unit  13  generates digital sound data (hereinafter referred to simply as “sound data”) by means of G.722 wideband ADPCM (Adaptive Differential Pulse Code Modulation). The ADPCM encoding unit  13  first separates input data into a high frequency signal and a low frequency signal by means of a quadrature mirror filter and performs ADPCM encoding on the high frequency signal and the low frequency signal, respectively. 
         [0043]      FIG. 2  is a diagram for explaining a low frequency ADPCM encoder of the ADPCM encoding unit  13 . The low frequency ADPCM encoder shown in  FIG. 2  includes a 60-level adaptive quantizer  13   a , a bit mask unit  13   b , and a 15-level adaptive dequantizer  13   c , and an adaptive predictor  13   d . The bit mask unit  13   b  extracts bits which are set as core bits, from a 6-bit low frequency ADPCM code generated by the adaptive quantizer  13   a . In the embodiment, four higher order bits are set as core bits and input to a feedback loop. 
         [0044]    The 15-level adaptive dequantizer  13   c  calculates a quantized differential signal from data pertinent to the core bits (four bits), outputting a calculation result. The differential signal output from the 15-level adaptive dequantizer  13   c  is delivered to the adaptive predictor  13   d  and an adder  13   f . The adder  13   f  adds a prediction signal generated in the encoder to the differential signal, to thus generate a regenerative signal. The adaptive predictor  13   d  generates a prediction signal from the differential signal originating from the 15-level adaptive dequantizer  13   c  and the regenerative signal originating from the adder  13   f.    
         [0045]    An input signal sent from the PCM conversion unit  12  is delivered to the adder  13   e , and the adder  13   e  calculates a difference between the input signal sent from the PCM conversion unit  12  and the prediction signal sent from the adaptive predictor  13   d . A resultant difference signal generated by the adder  13   e  is delivered to the 60-level adaptive quantizer  13   a , and the 60-level adaptive quantizer  13   a  generates a 6-bit low frequency ADPCM code. 
         [0046]    In the meantime, the high frequency ADPCM encoder performs 2-bit high frequency ADPCM encoding on an input high frequency signal according to G.722 standards. The high frequency ADPCM encoder does not have a bit mask unit and is configured so as to input all bits into the adaptive dequantizer. The high frequency ADPCM encoder can be analogous to the low frequency ADPCM encoder except this configuration, and hence its detailed explanations using the drawings are omitted. The ADPCM encoding unit  13  multiplexes the thus-generated 6-bit low frequency ADPCM code and the 2-bit high frequency ADPCM code by use of a multiplexer as shown in  FIG. 1 , thereby generating an 8-bit wideband ADPCM code. 
         [0047]    In the embodiment, the ADPCM encoding unit  13  of the master device  10  generates ADPCM data that are a low frequency signal having a data rate of 48 kbps. In the ADPCM data, low frequency sound data are assigned six bits, and highest order bits include a positive code bit and a negative code bit. Accordingly, as shown in  FIG. 3 , the 6-bit low frequency ADPCM data are represented as 000000 to 111111. Since the ADPCM data represent a difference between data that have been digitized immediately before and the current data. Hence, 000000 designates the positive minimum value, and 111111 designates the negative minimum value. In addition, 011111 denotes the positive maximum value, and 100000 designates the negative maximum value. 
         [0048]    In conformance with G.722 standards, four bits of the 6-bit low frequency ADPCM code are set as core bits, and remaining two bits are set as enhancement bits in the embodiment. Specifically, the ADPCM encoding unit  13  generates ADPCM data while taking four higher order bits as core bits, and the decoding unit of the receiving side also performs decoding operation while taking the four higher order bits as core bits. As above, so long as the same number of core bits is set on the encoder and the decoder, respectively, a prediction signal generated by the adaptive predictor  13   d  assumes the same value at both the encoder and the decoder. Accordingly, even when enhancement bits are used in another application, like a data communication, great degradation of sound quality cannot be avoided. 
         [0049]    In  FIG. 1 , the wideband ADPCM code generated by the ADPCM encoding unit  13  is delivered to the transmission conversion table  14 , and the transmission conversion table  14  converts the 4-bit sound data output from the ADPCM encoding unit  13  into 4-bit transmission data to be output to the slave device  20 . 
         [0050]      FIG. 4  is a diagram for explaining the transmission conversion table  14 . The transmission conversion table  14  is now described in detail by reference to  FIG. 4 . The transmission conversion table  14  contains a table T 1  and a table T 2 . When receiving condition is excellent, the table T 1  is employed, and input sound data are output while assuming exactly the same value. When receiving error information on the receiving side reaches a predetermined level, the table T 2  is used. The table T 2  is configured such that two bits of the high frequency ADPCM data and four higher order bits among six bits of the low frequency ADPCM data respectively assume exactly the same values as their originally input values and that two lower order bits act as even parity bits for the two bits of the high frequency ADPCM data and the four higher order bits of the low frequency ADPCM data, respectively. 
         [0051]    In short, in relation to the two lower order bits of the table T 2 ; namely, b 6  (the next least significant bit (LSB)) and b 7  (the least significant bit), the “next least significant bit” is reversed such that the number of “is” in the two bits becomes even according to the number of “is” in the two bits of the high frequency ADPCM data, and the “least significant bit” is reversed such that the number of “is” in the four bits become even according to the number of “1s” in the four higher order bits of the low frequency ADPCM data, thereby letting the two bits act as a parity signal. By means of converting the sound data by use of the table T 2 , the two lower order bits b 6  and b 7  of eight bits per one sample value of a sound data sequence to be transmitted; for instance, eight bits b 0 , b 1 , b 7  shown in  FIG. 3 , act as a parity signal. 
         [0052]    In  FIG. 1 , the transmission conversion table switching unit  15  performs switching between two tables (the table T 1  and the table T 2 ) of the transmission conversion table  14  according to the receiving error information sent from the slave device  20 .  FIG. 5  is a diagram for explaining the transmission conversion table switching unit  15 . A function of the transmission conversion table switching unit  15  is now described by reference to  FIG. 5 . 
         [0053]    By use of a changeover switch  15   a  intended for connection with the ADPCM encoding unit  13  and a changeover switch  15   b  intended for connection with the transmission packet generation unit  16 , the transmission conversion table switching unit  15  toggles between the table T 1  and the table T 2  in accordance with the receiving error information so as to apply any one of the tables to the ADPCM encoding unit  13  and the transmission packet generation unit  16 . When an excellent communication environment is maintained and when there is no need for transmission conversion, the changeover switches  15   a  and  15   b  are switched to the table T 1 . When transmission conversion is required as a result of deterioration of the communication environment, the changeover switches  15   a  and  15   b  are switched to the table T 2 , thereby converting transmission data such that two lower order bits of the transmission data act as a parity signal. 
         [0054]    In  FIG. 1 , the transmission packet generation unit  16  accumulates sound data output from the transmission table  14  by an amount corresponding to 10 msec, thereby generating a sound packet. By reference to  FIG. 5 , the sound packet is described.  FIG. 6  is a diagram showing a format of the sound packet and a configuration of a field for storing sound data in the embodiment. 
         [0055]    The sound packet shown in  FIG. 6  is made up of a sync field (16 bits) where sync data (a sync word) are to be stored, a field A (48 bits) where control signal data are to be stored, a field-A CRC (16 bits) where a CRC (Cyclic Redundancy Check) for the field A is to be stored, a field B (640 bits) where sound data are to be stored, and a field-B CRC (four bits) where a CRC for the field B is to be stored. 
         [0056]    In the embodiment, wideband ADPCM sound data to be stored in the field B are assigned eight bits per sample value, and two lower order bits (b 6 , b 7 ) of the eight bits are allocated for a parity signal. In addition, two higher order bits (b 0 , b 1 ) are assigned for high frequency ADPCM data, and subsequent four bits (b 2 , b 5 ) are assigned for four core bits of the lower frequency ADPCM code. 
         [0057]    Further, in the embodiment, when conversion is carried out by reference to the table T 2 , the transmission side sends per sample, as low frequency ADPCM data, 5-bit data that are made up of four bits of ADPCM data (core bits) and one least significant bit that is to act as a parity bit. The transmission side also sends per sample, as high frequency ADPCM data, 3-bit data that are made up of two bits of ADPCM data and one next least significant bit that is to act as a parity bit. Thus, the transmission side transmits both the parity bit for a low frequency signal and the parity bit for a high frequency signal without changing the number of bits (eight bits) per sample. 
         [0058]    The CRC for the field B does not take the entirety of 320-bit field B as a target and partially takes only data pertinent to predetermined bit positions as a target. To be specific, the field-B CRC takes, as a target, sound data that are distributed in ten locations every 16 bits. The sound data are only a total of 160 bits that are represented by bit numbers: b 48  to b 63 , b 112  to b 127 , b 176  to b 191 , . . . , b 560  to b 575 , and b 624  to b 639 . 
         [0059]    In  FIG. 1 , the wireless transmission circuit  17  functions as a transmission circuit unit that modulates the sound packet output from the transmission packet generation unit  16  into a wireless signal and that transmits the wireless signal from the antenna  17   a.    
         [0060]    As above, in the master device  10 , data transmission section is configured by the transmission conversion table  14  that converts a portion of the ADPCM sound data into a parity bit, the transmission packet generation unit  16  that generates a sound packet including the thus-converted sound data, and the wireless transmission circuit  17  that transmits the sound packet as a wireless signal to the slave device  20 . 
         [0061]    The slave device  20  is now described by reference to (B) in  FIG. 1 . The slave device  20  includes a wireless reception circuit  21  and an antenna  21   a . A signal received by the wireless reception circuit  21  is processed by a received data processing unit  30 , whereupon the thus-processed signal is output as high frequency ADPCM data and low frequency ADPCM data. The received data processing unit  30  is provided with a received packet processing unit  22 , a reception conversion table  23 , a receiving error processing unit  24 , and a reception conversion table switching unit  25 . Further, the slave device  20  is provided with the ADPCM decoding unit  26 , a PCM conversion unit  27 , a sound output unit  28 , and a received signal strength indicator processing unit  29 . 
         [0062]    The wireless reception circuit  21  acts as a reception circuit unit that receives by way of an antenna  21   a  the wireless signal output from the master device  10 , demodulates the thus-received signal, and outputs the thus-demodulated signal as a sound packet to the received packet processing unit  22 . The wireless reception circuit  21  measures a received strength indicator (RSSI) of the received sound packet, outputting the thus-measured indicator to received field strength processing. 
         [0063]    The received packet processing unit  22  detects a sync error when a predetermined sync word is not acquired, a CRC error for the field A or the field B, and a parity error in sound data, sending the error to the receiving error processing unit  24  and extracting the sound data and outputting the thus-extracted sound data to the reception conversion table  23 . 
         [0064]    The reception conversion table  23  converts the 4-bit sound data received from the master device  10 , outputting a conversion result. By reference to  FIG. 7  and  FIG. 8 , the reception conversion table  23  is now described in detail. 
         [0065]      FIG. 7  is a diagram for explaining a table pertaining to a low frequency signal in the reception conversion table  23 , and  FIG. 8  is a diagram for explaining a table pertaining to a high frequency signal. As shown in  FIG. 7  and  FIG. 8 , the reception conversion table  23  includes tables R 1  to R 4  for each of a low frequency signal and a high frequency signal. In any event, the table R 1  is configured so as to output input sound data in exactly the same values. 
         [0066]    In  FIG. 7  and  FIG. 8 , the table R 2  is configured so as to replace the sound data with mute data when results on the parity check of the two bits of the high frequency ADPCM data and the six bits of the low frequency ADPCM data show occurrence of a parity error. In conformance with an even parity rule, the transmission side (the master device  10 ) identifies occurrence of a high frequency parity error by means of the number of “is” in a total of three bits; namely, the two bits of the high frequency ADPCM data and the “next least significant bit P 1 .” Specifically, a parity error is identified by means of “001,” “010,” and “111” that include an odd number of is. Likewise, occurrence of a low frequency parity error is identified by means of the number of “is” in a total of five bits; that is, four higher order bits of the low frequency ADPCM data and the “least significant bit P 2 .” 
         [0067]    During the conversion performed by the table R 2  of the reception conversion table  23 , sound data are exactly output without change unless a parity error occurs. In contrast, if occurrence of a parity error is identified (when the number of “is” in target bits is determined to be odd), sound data are replaced with mute data. In the embodiment, high frequency signal mute data are assigned “11,” and low frequency signal mute data are assigned “111111” or “000000.” However, the mute data are not limited to them, and another mute data can also be used. As above, in response to occurrence of a parity error, the reception conversion table  23  converts the sound data including the error with mute data, thereby preventing reproduced sound from being affected by the error. 
         [0068]    In connection with high frequency data, the table R 3  converts data including a parity error with mute data as does the table R 2 . In addition to this, in connection with low frequency data, the table R 3  is configured so as to add one to four higher order bits of data that are free of an error, thereby replacing the data with sound data that will attenuate a sound. In the case of; for instance, “1001,” “1010,” and “1100,” they are converted into “1010,” “1011,” and “1101” by addition of one. 
         [0069]    The table R 4  is configured so as to replace all of the sound data with mute data regardless of occurrence of the parity error; in other words, the high frequency data with “11” and the low frequency data with “111111” or “000000.” 
         [0070]    In (B) of  FIG. 1 , the receiving error processing unit  24  is an error determination section that has a counter (to be described in detail later) which performs count-up operation at the time of occurrence of a reception error, such as a sync error detected by the receiving packet processing unit  22 , a field A error, and a field B error and which performs count-down operation at the time of occurrence of no error. A value of the counter is sent to the reception conversion table  25 , whereupon the reception conversion table  23  is switched in accordance with the counter value. 
         [0071]    The reception conversion table switching unit  25  acts as a data conversion section (a receiving side processing unit) along with the reception conversion table  23  by switching among four tables (the tables R 1  to R 4 ) of the reception conversion table  23  in accordance with a command from the slave device  20  by way of the receiving error processing unit  24  or a received signal strength indicator signal originating from the received signal strength indicator processing unit  29 . The reception conversion table switching unit  25  is now described by reference to  FIG. 9 .  FIG. 9  is a diagram for explaining the reception conversion table switching unit  25 . 
         [0072]    By means of changeover switches  25   a  and  25   b , the reception conversion table switching unit  25  switches among the tables (R 1  to R 4 ) used in the reception conversion table  23 . For instance, when conversion processing is not performed, the received packet processing unit  22  and the ADPCM decoding unit  26  are switched to the table R 1 . In the meantime, when parity processing is practiced, the received packet processing unit  22  and the ADPCM decoding unit  26  are switched to the table R 2 . In addition, when parity processing and attenuation processing are performed, the received packet processing unit  22  and the ADPCM decoding unit  26  are switched to the table R 3 . 
         [0073]    The ADPCM decoding unit  26  shown in (B) of  FIG. 1  separates 8-bit input data sent from the reception conversion table  23  into a high frequency signal and a low frequency signal by means of a separator, subsequently performing ADPCM decoding on the respective signals. In other words, even when the 8-bit data include two lower order bits of parity bits and the mute data, all of the eight bits are decoded into sound data. The ADPCM decoding unit  26  is now described by reference to  FIG. 10 . 
         [0074]      FIG. 10  is a diagram for explaining a low frequency ADPCM decoder of the ADPCM decoding unit  26 . The low frequency ADPCM decoder shown in  FIG. 10  is now described. The low frequency ADPCM decoder includes a feedback adaptive dequantizer  26   a , a feedforward adaptive dequantizer  26   b , a bitmask unit  26   c , and an adaptive predictor  26   d . The bitmask unit  26   c  extracts core bits from the ADPCM code input, and only the thus-extracted core bits are input to the feedback adaptive dequantizer  26   a . The feedback adaptive dequantizer  26   a  calculates a quantized differential signal, outputting the thus-calculated, quantized differential signal. The thus-output, quantized differential signal is added to the prediction signal by the adder  26   e , and a result of addition is input to the adaptive predictor  26   d , where a prediction signal is generated. 
         [0075]    The feedforward adaptive quantizer  26   b  calculates a quantized differential signal by use of all bits in the ADPCM data, outputting a calculation result. When the low frequency sound data are 48 kbps, a 6-bit ADPCM code input is output. The adder  26   f  adds the prediction signal calculated only from the core bits to the quantized differential signal calculated from all six bits, whereby a low frequency regenerative signal is output. 
         [0076]    In the meantime, the essential requirement for the high frequency ADPCM decoder of the ADPCM decoding unit  26  is to perform processing conforming to the known G722 standard. The bitmask unit is not necessary, and all of the bits are input to the adaptive dequantizer. The high frequency ADPCM decoder is analogous to the low frequency ADPCM decoder except absence of the bitmask unit, and hence its detailed explanations are unnecessary. 
         [0077]    The ADPCM decoding unit  26  combines the thus-decoded low frequency signal and the high frequency signal together by means of a receiving rectangular mirror filter, to thus generate a wideband sound signal. Even when a parity bit is transmitted without changing the number of bits per sample (eight bits), the receiving side uses the low frequency ADPCM data as they are without changing four core bits thereof to generate a prediction signal. The receiving side also processes the high frequency ADPCM data while taking them as two bits, so that deterioration of sound quality is small, and a conversation can be performed while a certain degree of sound quality is maintained. 
         [0078]    In (B) of  FIG. 1 , the PCM conversion unit  27  generates an analog sound signal from the reproduction signal. The sound output unit  28  may be a speaker that reproduces the sound signal, for example. 
         [0079]    The received electric field strength processing unit  29  functions as the received electric field strength level determination means that determines the change of the received electric field strength measured by the wireless reception circuit  21  and outputs the result of the determination to the reception conversion table switching unit  25 . This determination is made in a manner such that, in the case where the master device  10  and the slave device  20  are spaced apart from each other, the received electric field strength is lowered, and if the received electric field strength is lower than a threshold value A (a first threshold value), the communication environment is inferior. Further, in the case where the master device  10  and the slave device  20  are close to each other, the communication environment becomes good and the received electric field strength is elevated. If the received electric field strength exceeds a threshold value B (a second threshold value), the communication environment becomes good. However, in determination, the threshold value B is set to be higher than the threshold value A. 
         [0080]    The received electric field strength processing unit  29  outputs information on the communication environment to the reception conversion table switching unit  25 , and the reception conversion table switching unit  25  selects the table R 1  that does not perform the parity check if the communication environment is good. Further, if the communication environment is inferior, the reception conversion table switching unit  25  performs the parity check (the transmission side: table T 2 ), the reception side selects any one of other tables (tables R 2  to R 4 ) to be sound-processed. The received electric field strength processing unit  29  performs the synchronization between the master device  10  and the conversion table by transferring the determination result information that is obtained by determining the change of the received electric field strength to the master device  10  using a control packet. 
         [0081]    By setting the threshold value B to be larger than the threshold value A, switching is performed from the table R 1  in which the parity check is not performed to the tables R 2  to R 4  in which the parity check is performed and the sound process is performed when the communication environment is deteriorated, and even if the communication environment becomes good thereafter, the switching is not performed at the same electric field strength as that switched by the reception conversion table  23 . Since the parity check is stopped after the communication environment reaches a sufficiently good level, the reception conversion table  23  and the transmission conversion table  14  are prevented from being frequently switched. 
         [0082]    A communication method of the cordless telephone as configured above according to the embodiment of the present invention will be described based on the drawings. First, in communication between the master device  10  and the slave device  20 , a case where the communication environment is good and no reception error occurs will be described. In this case, it is assumed that the table T 1  illustrated in  FIG. 4  is selected in the transmission conversion table  14 , and the table R 1  illustrated in  FIGS. 7 and 8  is selected in the reception conversion table  23 . 
         [0083]    The sound signal from the sound input unit  11  is quantized by the PCM conversion unit  12 , and one code is compressed into 8-bit sound data through ADPCM by the ADPCM encoding unit  13 . 
         [0084]    This 8-bit sound data is input to the table T 1  of the transmission conversion table  14 , and then the sound data having the same value as the input is output from the table T 1  as the transmission data. The sound data output from the transmission conversion table  14  is included in the sound packet by the transmission packet generation unit  16 , and is transmitted to the slave device  20  through the antenna  17   a  by the wireless transmission circuit  17  as the wireless signal. 
         [0085]    In the slave device  20 , the wireless signal from the master device  10  is received in the wireless reception circuit  21  through the antenna  21   a . The wireless signal received in the wireless reception circuit  21  is demodulated and output to the received packet processing unit  22  as the sound packet. 
         [0086]    The received packet processing unit  22  checks the occurrence of the reception error of the sound packet, extracts the 8-bit sound data included in the sound packet, and outputs the extracted sound data to the reception conversion table  23 . 
         [0087]    If the sound data is input to the table R 1  of the reception conversion table  23 , 8-bit sound data having the same value as the input is output from the table T 1 . The sound data output from the reception conversion table  23  is input to and expanded by the ADPCM decoding unit  26 , converted into a sound signal by the PCM conversion unit  27 , and is reproduced by the sound output unit  28 . 
         [0088]    In this case, since the master device  10  that is the transmission side transmits the sound to the slave device  20  as it is without processing all the 8-bit ADPCM sound data, high-quality sound can be transmitted. 
         [0089]    Next, a case where the slave device  20  detects the reception error will be described. 
         [0090]    If the received packet processing unit  22  of the slave device  20  detects the reception error such as a sync error or a CRC error, it transmits reception error information regarding the effect that the reception error has occurred to the master device  10  using a transmission function (not illustrated). The master device  10  can recognize that the communication environment is deteriorated through the notification of the reception error information. Accordingly, the transmission conversion table switching unit  15  performs switching of the connections of the transmission conversion table  14  to be applied between the ADPCM encoding unit  13  and the transmission packet generation unit  16  from the table T 1  to the table T 2 . By doing so, the two lower order bits of the 8-bit sound data is converted into the parity bit (see  FIG. 3 ). The parity bit generation using the transmission conversion table  14  is simpler than the parity bit calculation through operation. 
         [0091]    The slave device  20  instructs, in synchronization with the notification of the reception error to the master device  10 , the reception conversion table switching unit  25  to perform switching of the reception conversion table  23  from the table R 1  to the table R 2 . That is, the reception conversion table  23  is switched to the table R 2  as a reception conversion table pertinent to the low frequency signal as shown in  FIG. 7 , and switched to the table R 2  as a reception conversion table pertinent to the high frequency signal as shown in  FIG. 8 . As described above, the conversion by the table R 2  is performed in a manner such that the sound data is output as it is if the parity error has not occurred, and the sound data is converted into the mute data if the parity error has occurred. 
         [0092]    Next, the influence on the sound packet in the related art and the influence on the sound of the sound packet according to the embodiment of the present invention will be described based on  FIGS. 11 to 12 .  FIG. 11  is a diagram illustrating sound processing in a sound packet in the related art.  FIG. 12  is a diagram illustrating sound processing in a sound packet according to the embodiment of the present invention. 
         [0093]    In the sound packet in the related art as illustrated in  FIG. 11 , since a CRC of a B-field is added to 16-bit sound data that is distributed in ten locations in the B-field, sound data that is not the subject of the CRC is present, and the reception error for all the sound data may not be detected. Accordingly, even in the case where not only the CRC error of the B-field but also the error of data for synchronization and the CRC error (CRC error of an A-field) added to data for a control signal are detected, it is necessary to perform the sound process such as mute with respect to the entire one frame on the assumption that there is a high possibility that the error has occurred even in the sound data that is not the subject of the CRC of the B-field. As illustrated in  FIG. 11 , sound data of about 10 msec is included in one frame, and if the sound data for one frame is processed, it exerts a great influence on the sound. 
         [0094]    In the sound packet according to the embodiment, a parity bit is added for each 8-bit sound data, and thus the error can be detected every 8 bits. Accordingly, as illustrated in  FIG. 12 , by converting only the sound data in which the parity error has occurred into the mute data, it is not required to perform the sound process with respect to the whole one frame. In a conventional method in which a CRC is added to 16-bit sound data that is distributed in about ten locations in the B-field of one frame of a sound packet, it is required to perform sound processing such as muting with respect to the whole of one frame if the CRC error is detected. On the other hand, according to the embodiment, it is possible to detect an error in a minute unit in a sound packet and replace the sound packet. Thus, since the influence range corresponds to only the replaced sound data, it exerts a low influence on the sound. 
         [0095]    The sound data output from the reception conversion table  23  is input to and expanded by the ADPCM decoding unit  26 , converted into a sound signal by the PCM conversion unit  27 , and is reproduced by the sound output unit  28 . In reproducing the sound, since the two lower order bits are used as the parity bit while the data rate is maintained, the sound quality is somewhat deteriorated in comparison to the case where all the 8 bits are used as the sound data, but high sound quality can be secured in comparison to the case where sound data for one frame is processed in a state of the sync word error or the CRC error due to the deterioration of the communication environment. 
         [0096]    Next, a method of switching transmission and reception conversion tables that is performed by the reception error processing unit  24  will be described based on  FIGS. 13A to 17 .  FIGS. 13A and 13B  are diagrams illustrating a counter installed in the reception error processing unit  24 .  FIG. 13A  is a diagram illustrating a frame error counter, and  FIG. 13B  is a diagram illustrating a sound data error counter.  FIG. 14  is a flowchart illustrating a reception conversion table switching process, and  FIG. 15  is a flowchart illustrating a reception conversion table switching process continuously performed from  FIG. 14 .  FIG. 16  is a diagram illustrating a reception conversion table switching process that is determined by the received electric field strength, and  FIG. 17  is a diagram illustrating a reception conversion table switching process that is determined by a frame error counter. 
         [0097]    As shown in  FIGS. 13A and 13B , the reception error processing unit  24  has two counters that count number according to the contents of error notification from the received packet processing unit  22 . These counters include, as illustrated in  FIG. 13A , a frame error counter C 1  that counts up by the sync word error, A-field CRC error, or B-field CRC error and counts down by no error, and, as illustrated in  FIG. 13B , a sound data error counter C 2  that counts up when the 8-bit sound data is the parity error and counts down by no error. In this embodiment of the present invention, count-up corresponds to +1 and count-down corresponds to −1. However, the count-up/down may be set to have different values in order to change the weight. It is possible to appropriately determine these values according to the communication environment where the cordless telephone is installed. 
         [0098]    As illustrated in  FIG. 14 , the reception error processing unit  24  determines whether or not the frame system error, such as the sync word error, A-field CRC error, or B-field CRC error, has occurred in the received packet processing unit  22  (S 100 ). If the error has occurred, the reception error processing unit  24  increases the frame error counter C 1  by +1 (S 110 ), while if the error has not occurred, the reception error processing unit  24  increases the frame error counter C 1  by −1 (S 120 ). 
         [0099]    Next, the reception error processing unit  24  determines whether or not the reception conversion table  23  that is currently used is the table R 1  (S 125 ). If the reception conversion table  23  is the table R 1 , the reception error processing unit  24  determines whether or not the frame error counter value is equal to or larger than a threshold value C (S 130 ). If the frame error counter value is equal to or larger than the threshold value C, the reception error processing unit  24  invalidates a flag of the table R 1  (S 135 ). That is, as illustrated in  FIG. 17 , since the error is increased due to an interference radio wave or the like, the reception error processing unit  24  invalidates the flag of the table R 1 , and performs switching of the reception conversion table  23  from the table R 1  that does not perform the parity check to the tables R 2  to R 4  that perform the parity check or the sound processing. By doing so, the reception error processing unit  24  can detect the error of the sound data that occurs according to the deterioration of the communication environment at high accuracy. If the frame error counter value is smaller than the threshold value C in S 130 , the table R 1  of the reception conversion table  23  is used as the current state, and the flag of the table R 1  is in a valid state. 
         [0100]    Further, if it is determined that the reception conversion table  23  that is currently used is not the table R 1  in S 125  as illustrated in  FIG. 14 , the reception error processing unit  24  determines whether or not the frame error counter value is smaller than a threshold value D (S 140 ). If the frame error counter value is smaller than the threshold value D, the reception error processing unit  24  validates the flag of the table R 1  (S 150 ). That is, as illustrated in  FIG. 17 , in the case where no error has occurred due to the nonexistence of the interference radio wave, the reception error processing unit  24  validates the flag of the table R 1 , and performs switching of the reception conversion table  23  from the tables R 2  to R 4  that perform the parity check or the sound processing to the table R 1  that does not perform the parity check. By doing so, the communication environment becomes good, and thus communication of the sound data can be performed with a better sound quality. If the frame error counter value is not smaller than the threshold value Din S 140 , the tables R 2  to R 4  of the reception conversion table  23  are used as the current state, and the flag of the table R 1  is in an invalid state. 
         [0101]    Next, the received electric field strength processing unit  29  determines whether or not the reception conversion table  23  currently used is the table R 1  (S 160 ). If the reception conversion table  23  is the table R 1 , the received electric field strength processing unit  29  determines whether or not the received electric field strength measured by the wireless reception circuit  21  is lower than the threshold value A (S 170 ). If the received electric field strength is lower than the threshold value A, the received electric field strength processing unit  29  invalidates the flag of the table R 1  (S 180 ). That is, as illustrated in  FIG. 16 , since the received electric field strength is lower than the threshold value A due to the long distance between the master device  10  and the slave device  20  or the like, the received electric field strength processing unit  29  invalidates the flag of the table R 1 , and performs the switching of the reception conversion table  23  from the table R 1  that does not perform the parity check to the tables R 2  to R 4  that perform the parity check or the sound processing. By doing so, the received electric field strength processing unit  29  can detect the error of the sound data that occurs according to the deterioration of the communication environment at high accuracy. If the received electric field strength is not smaller than the threshold value A in S 170 , the table R 1  of the reception conversion table  23  is used as the current state, and the flag of the table R 1  is in a valid state. 
         [0102]    Further, if it is determined that the reception conversion table  23  currently used is not the table R 1  in S 160  shown in  FIG. 14 , the received electric field strength processing unit  29  determines whether or not the received electric field strength is equal to or higher than the threshold value B (S 190 ). If the received electric field strength is equal to or higher than the threshold value B, the received electric field strength processing unit  29  validates the flag of the table R 1  (S 200 ). That is, as illustrated in  FIG. 16 , since the received electric field strength is equal to or higher than the threshold value B due to the short distance between the master device  10  and the slave device  20  or the like, the received electric field strength processing unit  29  validates the flag of the table R 1 , and performs the switching of the reception conversion table  23  from the tables R 2  to R 4  that perform the parity check or the sound processing to the table R 1  that does not perform the parity check. By doing so, the communication environment becomes good, and thus communication of the sound data can be performed with a better sound quality. If the received electric field strength is smaller than the threshold value B in S 190 , the tables R 2  to R 4  of the reception conversion table  23  are used as the current state, and the flag of the table R 1  is in an invalid state. 
         [0103]    Next, as illustrated in  FIG. 15 , the reception error processing unit  24  determines whether or not the sound data error that is the parity error of the sound data has occurred (S 210 ). If the error has occurred, the reception error processing unit  24  increases the sound data error counter C 2  by +1 (S 220 ), while if the error has not occurred, the reception error processing unit  24  increases the sound data error counter C 2  by −1 (S 230 ). 
         [0104]    Next, the reception error processing unit  24  determines whether or not the flag of the table R 1  is valid (S 240 ). If the flag of the table R 1  is valid, it means that the frame error rate is low and the communication environment is good, and thus regardless of the count value of the sound data error counter C 2 , the side of the master device  10  is switched to the table T 1  and the side of the slave device  20  proceeds to S 300  to be switched to the table R 1 . 
         [0105]    Next, the reception error processing unit  24  determines whether or not the sound data error counter C 2  is within a range of the level B (S 250 ). This level B is in a range where the frame system error rate has been elevated, but the sound data error rate is determined to be still low. Accordingly, in order to perform the parity check of the sound data, the reception conversion table  23  validates the flag of the table R 2  that selects the table R 2  (S 260 ), and then proceeds to S 300 . 
         [0106]    If the sound data error counter C 2  is not within the range of the level B, the reception error processing unit  24  then determines whether or not the sound data error counter C 2  is within the range of the level C (S 270 ). This level C is in a range where it is determined that the sound data error rate has been gradually elevated. Accordingly, the reception error processing unit  24  validates the flag of the table R 3 , which selects the table R 3  that not only replaces the sound data with the mute data in the case where the parity error of the sound data has occurred but also replaces the sound data with the sound data that mutes the high frequency sound and attenuates the low frequency sound even in the case where the parity error has not occurred (S 280 ), and then proceeds to S 300 . 
         [0107]    If the sound data error counter C 2  is not within the range of the level C, it means that the sound data error counter C 2  is within the level D, and the reception error processing unit  24  validates the flag of the table R 4 . This level D is in the range where the communication environment is worst. Accordingly, the table R 4  which replaces all the sound data with the mute data is selected in the reception conversion table  23  (S 290 ). 
         [0108]    In S 300 , the reception error processing unit  24  performs the switching of the reception conversion table  23  according to the flag. For example, if the flag of the table R 1  is valid, the reception error processing unit  24  instructs the reception conversion table switching unit  25  to perform switching of the reception conversion table  23  to the table R 1 . Further, the reception error processing unit  24  transmits a control packet to the master device  10  so that the master device  10  switches the transmission conversion table  14  to the table T 1 . 
         [0109]    Further, if any one of flags of the tables R 2  to R 4  is valid, the reception error processing unit  24  instructs the reception conversion table switching unit  25  to perform switching of the reception conversion table  23  to any one of the tables R 2  to R 4 . Further, the reception error processing unit  24  notifies the master device  10  of the error information, and instructs the master device  10  to perform switching of the transmission conversion table  14  to the table T 2 . 
         [0110]    As described above, since the reception error processing unit  24  determines the increase/decrease of the error rate by adding the frame system error, such as the sync word error, the A-field CRC error, or the B-field CRC error, to the parity error of the sound data, it can cope with the occurrence of the error with higher accuracy. 
         [0111]    Further, if it is determined that the error has occurred by the parity bit value of the sound data, the reception error processing unit  24  makes the sound data error counter C 2  count up, while if it is determined that the error has not occurred, the reception error processing unit  24  makes the sound data error counter C 2  count down. By increasing/decreasing the error rate by the sound data error counter C 2 , the reception error processing unit  24  can cope with the communication environment in which the reception level is deteriorated or becomes good. 
         [0112]    Further, in the table R 2  of the reception conversion table  23 , the sound data in which the parity error has occurred is converted into the mute data. However, a click noise may occur depending on the sound data. Accordingly, by performing switching of the reception conversion table  23  to the table R 3 , the sound data for which the error does not occur in the same frame is converted so that the sound is attenuated, and thus the influence of the click noise can be suppressed. 
         [0113]    Further, in the case where the communication environment is further deteriorated, by performing switching of the reception conversion table  23 , which converts the whole sound data in the same frame into the mute data, to the table R 4 , the click noise can be suppressed more effectively. 
         [0114]    As described above, the embodiment of the present invention has been described thus far, but the present invention is not limited to the embodiment. For instance, in the embodiment, the high frequency signal is muted by means of a parity error for a high frequency signal, and the low frequency signal is muted by means of a parity error for a low frequency signal. However, both the high frequency signal and the low frequency signal can be muted by means of either of the two parity errors. 
         [0115]      FIGS. 18A and 18B  are diagrams collectively showing an example of received data processing used for setting the reception conversion table  23 . Explanations are hereunder provided along  FIGS. 18A and 18B . 
         [0116]    In an example shown in  FIG. 18A , when a sound data error counter C 2  in the receiving error processing unit  24  shows “low” or “level B” that is below a threshold value of one, “mute” processing is performed for a high frequency sound when the high frequency signal parity bit P 1  represents “NG.” “Mute” processing is performed for a low frequency sound when a low frequency signal parity bit P 2  represents “NG.” 
         [0117]    In the example shown in  FIG. 18A , when the sound data error counter C 2  shows “level C” or a threshold value which is more than one to less than two and when the high frequency signal parity bit P 1  represents “NG,” “mute” processing is performed. Further, in connection with a low frequency sound achieved when the sound data error counter C 2  represents “level C,” even when the low frequency signal parity bit and the high frequency signal parity bit are normal; namely, “OK,” “attenuation” is performed. If the high frequency signal parity bit P 1  represents “NG,” “attenuation” will be performed. Further, if the low frequency signal parity bit P 2  represents “NG,” “mute” processing will be performed. 
         [0118]    In the example shown in  FIG. 18A , when the sound data error counter C 2  shows a threshold value of more than two, or deterioration proceeding up to; namely, “level D,” both the high frequency signal and the low frequency signal are muted regardless of whether both the low frequency signal parity bit and the high frequency signal parity bit are “OK” or “NG.” 
         [0119]    In the examples shown in  FIGS. 18A and 18B , first signal processing corresponds to high frequency sound processing that is practiced when the high frequency signal parity bit P 1  represents “NG” or when the low frequency signal parity bit P 2  represents “NG.” The second signal processing corresponds to low frequency sound processing that is practiced when the high frequency signal parity bit P 1  represents “NG” or when the low frequency signal parity bit P 2  represents “NG.” 
         [0120]    In the example shown in  FIG. 18B , in a case where the sound data error counter C 2  in the receiving error processing unit  24  shows “low” or “level B” that is below a threshold value of one, “mute” processing is performed for a high frequency sound when the high frequency signal parity bit P 1  represents “NG” or when the low frequency signal parity bit P 2  represents “NG.” In a case where the sound data error counter C 2  represents “level B,” “mute” processing is performed for the low frequency sound when the high frequency signal parity bit P 1  represents “NG” or when the low frequency signal parity bit P 2  represents “NG.” 
         [0121]    In the example shown in  FIG. 18B , when the sound data error counter C 2  shows “level C,” or a threshold value which is more than one to less than two, “mute” processing is performed for the high frequency sound if the high frequency signal parity bit P 1  represents “NG” or if the low frequency signal parity bit P 2  represents “NG.” Further, when the sound data error counter C 2  represents “level C,” “attenuation” processing is performed even when both the low frequency signal parity bit and the high frequency signal parity bit are normal; namely, “OK.” In addition, when the low frequency signal parity bit P 2  represents “level C,” “mute” processing is performed for a low frequency sound if the high frequency signal parity bit P 1  represents “NG,” and “mute” processing is performed for a low frequency sound if the low frequency signal parity bit P 2  represents “NG.” 
         [0122]    In the example shown in  FIG. 18B , when the sound data error counter C 2  shows a threshold value of more than two, or deterioration proceeding up to; namely, “level D,” both the high frequency signal and the low frequency signal are muted regardless of whether both the low frequency signal parity bit and the high frequency signal parity bit are “OK” or “NG.” 
         [0123]    Third to fifth signal processing corresponds to the following processing in the example shown in  FIG. 18B . The third signal processing corresponds to high frequency sound processing that is performed when the high frequency signal parity bit P 1  represents “NG.” The fifth signal processing corresponds to high frequency sound processing that is performed when the low frequency signal parity bit P 2  represents “NG.” The fourth signal processing corresponds to low frequency sound processing that is performed when the high frequency signal parity bit P 1  represents “NG.” In addition, the sixth signal processing corresponds to low frequency sound processing that is performed when the low frequency signal parity bit P 2  represents “NG.” 
         [0124]    In the embodiment, lower order bits of sound data are employed as error detection bits; namely, parity bits. However, the least significant bit can also be generated by means of another error detection method. 
         [0125]    In the above embodiment, an error detection code including a total of two bits; namely, the high frequency signal parity bit P 1  and the low frequency signal parity bit P 2 , is generated, and the error detection codes are assigned to two lower order bits of the sound data. However, in another embodiment, only one parity bit of entire data that include both a high frequency signal and a low frequency signal is assigned to the error detection code, and both the high frequency signal and the low frequency signal can also be muted in accordance with the parity error code. Specifically, there may be generated a parity bit (the third parity bit: P 3 ) that reverses bits in accordance with the number of “1s” in a series of data including both high frequency ADPCM data and low frequency ADPCM data, and a least significant bit of the low frequency ADPCM data can be also replaced with the third parity bit P 3 . 
         [0126]    In the embodiment shown in  FIG. 19 , processing is performed by use of only the parity bit P 3 . To be specific, if the sound data error counter C 2  represents “small,” or “level B” which is a threshold value below one, “mute” processing will be performed for high frequency sound if the parity bit P 3  represents “NG.” Further, “mute” processing will be performed even for a low frequency sound if the parity bit P 3  represents “NG.” 
         [0127]    Moreover, in the embodiment, when the sound data error counter C 2  shows “level C,” or a threshold value which is more than one to less than two, “mute” processing is performed for the high frequency sound when the high frequency signal parity bit P 1  represents “NG.” Further, when the sound data error counter C 2  represents “level C,” “attenuation” processing is performed for the low frequency sound regardless of whether the parity bit P 3  is normal, that is, “OK,” or “NG.” 
         [0128]    Furthermore, in the example, when the sound data error counter C 2  shows a threshold value of more than two, or deterioration proceeding up to “level D,” both the high frequency signal and the low frequency signal are muted regardless of whether the parity bit P 3  represents “OK” or “NG.” 
         [0129]    Since the system that is based on a wideband fixed-size sound packet can enhance accuracy in error detection while assuring a certain degree of sound quality when a communication environment becomes worse, the present invention is preferable for a wireless communication device and a communication terminal that practice a communication by means of a wideband sound packet. 
         [0130]    The present application is a continuation-in-part of international patent application No. PCT/JP2013/077016 filed on Sep. 27, 2013 claiming the priority of Japanese Patent Application No. 2012-215890 filed on Sep. 28, 2012, the contents of which are incorporated herein by reference in its entirety.