Digital communication device

Abstract: A digital communication device of the present invention is provided with a modulation circuit for modulating a digital transmit signal, a first interpolater for converting the modulated signal in frequency, a coding circuit for coding the signal converted in frequency into an audio PCM transmission code with reference to a voice companding code table, a decoding circuit for decoding a coded audio PCM receive code into a digital signal with reference to the voice companding code table, a second interpolater for converting the decoded digital signal in frequency, and a demodulation circuit for demodulating the converted signal, and digitally performs modem modulation/demodulation and voice codec processing.

Description: The present invention relates to a digital communication device for communicating a modulated signal through a digital line. A typical digital modem for performing modem/voice codec processes is mounted in a communication control unit for a facsimile having G3/G4 functions connected to a digital line. The communication control unit of the G3/G4 facsimile is constructed as shown in FIG. 9. Referring to FIG.9, numeral 817 denotes a processing section having G3/G4 functions. On the other hand, numeral 801 denotes a communication control section. In this example, an Integrated Services Digital Network (ISDN) is used as a digital line, and a part corresponding to the subscriber's line is, herein, expressed as a S interface. A facsimile communication operation of the communication control unit according to the prior art will now be described. First, the case in which the communication control unit functions as a G4 facsimile will be explained. In this case, it is assumed that switches SW1 and SW2 shown in FIG. 9 each are connected (turned ON) to the side of G4. In the above processing section 817, a facsimile control portion 814 executes various kinds of control operations in order to operate the whole unit as a facsimile. The facsimile control portion 814 operates a network control portion 813 and carries out a call-out procedure or a call-in procedure by using a G4 terminal number 816, prior to facsimile transmission. In the ISDN, these procedures are carried out by using a D channel, and thus the network control portion 813 is connected to the S interface through a D channel control circuit 808 and a line interface circuit 802. After the connection of the unit to the network is thus completed, facsimile transmission is started. The facsimile control portion 814 carries out a procedure b 812 previous to image transmission. The procedure b 812 is a G4 communication procedure according to T. 62, T. 73 and T. 70 of the CCITT Recommendation. These procedures are connected to the switch SW1 through a connecting circuit 807. The connecting circuit 807 is a connecting circuit for digital communication according to X. 21bis of the CCITT Recommendation. These procedures and the image transmission are normally executed by using a B channel in the ISDN, and thus the connecting circuit 807 is connected to the S interface through a B channel control circuit 803 and the line interface circuit 802. Subsequently, when the connection is completed, the facsimile control portion 814 operates a MMR codec 811, thereby starting to transmit image data. The MMR codec 811 executes coding and decoding of image data according to T. 6 of the CCITT Recommendation, and compresses and expands the data. A codec for executing coding/decoding processes according to T. 6 will be referred to as a Modified Modified READ (MMR) codec hereinafter. The image data is also generally transmitted by using the B channel in the ISDN, and thus the MMR codec 811 is connected to the S interface through the connecting circuit 807, the B-channel control circuit 803 and the line interface circuit 802. Image transmission with the G4 facsimile function is carried out according to the above-mentioned operations. The case in which the communication control unit functions as a G3 facsimile will now be described. In this case, it is assumed that the switches SW1 and SW2 shown in FIG. 9 are connected (turned ON) to G3. First, the facsimile control portion 814 operates the network control portion 813 and carries out a call-out procedure or a call-in procedure by using a G3 phone number 815, prior to facsimile transmission. The G3 phone number 815 is actually the number of a digital telephone. When the procedures are completed through the network control portion 813, the D-channel control circuit 808 and the line interface circuit 802, a line of audio communication between a terminal of the subscriber and the other terminal is established. Although not shown in FIG. 9, a voice call by a digital telephone through the B channel is possible in this state. After the connection of the the terminal of the subscriber to the network and the other terminal is established, facsimile transmission is started. The facsimile control portion 814 operates a procedure a 810 previous to image transmission. The procedure a 810 is a G3 and G2 communication control procedure according to T. 30 of the CCITT Recommendation. This communication procedure is carried out by analog signals (in the voice grade) through a modem 805. In the case of a conventional G3 facsimile, since its communication line is an analog line, communication is possible in this state. However, in the ISDN, the analog signals are converted into digital signals through a voice codec 806 for a digital telephone, and audio communication is performed through the B-channel control circuit 803 and the line interface circuit 802. The modem 805 is a modem (V21) for a communication control procedure according to the CCITT Recommendation. After carrying out the communication procedure with the other terminal, the facsimile control portion 814 operates a MH/MR codec 809 so as to start image transmission. The MH/MR codec 809 performs a G3 coding/decoding processes of image data according to T. 4 of the CCITT Recommendation and compresses and expands the image data. The image data is converted into analog signals (in the voice grade) through the modem 805. The modem 805 is a modem (V27ter or V29) for G3 image transmission according to the CCITT Recommendation. These image signals are also connected to the S interface through the voice codec 806, the B-channel control circuit 803 and the line interface circuit 802 in the same manner as above. Image transmission with the G3 facsimile function is performed according to the above-mentioned operations. The components (the modem 805 and the voice codec 806) enclosed by the line 804 shown in FIG. 9 will now be described in detail with reference to FIG. 10. The modem 805 performs modulation or demodulation on the basis of a digital operation, and executes D/A or A/D conversion in each sampling clock which is normally 9.6KHz. The voice codec 806 is an audio PCM codec for converting analog signals in the voice grade into digital signals and performing compression coding, or performing the operations in reverse order, and generally executes A/D or D/A conversion and corresponding coding/decoding in each sampling clock which is 8.0KHz. In transmission, a digital transmit signal (a) is input to a modulation circuit 901, and modulated by a carrier frequency prescribed in the CCITT Recommendation. The digital output signal from the modulation circuit 901 is converted into an analog transmit signal (b) in the voice grade by a first D/A converter 902 and a first lowpass filter (LPF) 903. The signal (b) is input to the voice codec 806, and converted into a digital signal again by a first A/D converter 904, and furthermore, coded into an audio PCM transmission code (c) by a coding circuit 905 with reference to a voice companding code table 906 and output. The first lowpass filter 903 and a second lowpass filter 909 remove harmonic noise included in analog signals output from the first D/A converter 902 and a second D/A converter 908. Receiving operations will now be described. As illustrated, an audio PCM receive code (d) is input to a decoding circuit 907 and decoded with reference to the voice companding code table 906. The decoded digital signal is converted into an analog signal (e) in the voice grade by the second D/A converter 908 and the second LPF 909. The analog signal (e) is input to a second A/D converter 910 in the modem 805 and converted into a digital signal again. Then, the digital signal is demodulated and output as a digital receive signal (f) by a demodulation circuit 911. However, in the above prior art shown in FIG. 10, since the sampling frequency of the D/A and A/D converters 902 and 910 used in the modem 805 and the sampling frequency of the A/D and D/A converters 904 and 908 used in the voice codec 806 are different, the conversion, a digital signal ' an analog signal ' a digital signal, is necessary between the modem 805 and the voice codec 806, thereby causing a quantitizing error which results in the deteriorated transmission ability. Since both sampling frequencies of 9.6KHz and 8.0KHz are used, the modem 805 and the voice codec 806 constitutes an extremely large circuitry. It is necessary to design a LSI for the modem 805 and a LSI for the voice codec 806 as separate chips. This needs a wide space and increases costs. A conventional system as described herein above is for example known from the publication JP-A-1 144 762. From the textbook "Digitate Filter in der Videotechnik", pp. 121-124, Drei-R-Verlag, Berlin, 1988, edited by H. Schönfelder, principles of interpolation are known. The present invention aims to solve the above problems. Therefore, an object of the present invention is to provide a digital communication device capable of digitally processing modem modulation and demodulation and voice codec processes with high transmission quality. This object is achieved by a digital device according to claim 1 and a digital reception device according to claim 4. Furthermore, this object is achieved by a method as defined in claim 9, and by a method as defined in claim 10. Advantageous further developments are as set out in the respective dependent claims. Additional and other objects of the present invention will become apparent from the detailed description in conjunction with the accompanying drawings. FIG. 1 is a schematic block diagram showing the construction of a digital modem 101 according to a embodiment; FIG. 2 is a flowchart showing operations of a transmitting side in the embodiment; FIG. 3 is a flowchart showing operations of a receiving side in the embodiment; FIGS. 4 and 5 are views explaining processing of a first interpolater; FIGS. 6 and 7 are views explaining processing of a second interpolater; FIG. 8 is a schematic block diagram showing the construction of a facsimile according to the embodiment; FIG. 9 is a schematic block diagram showing the construction of a communication control unit for a conventional facsimile; FIG. 10 is a detailed block diagram showing the constructions of a modem and a voice codec shown in FIG. 9. A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings. [Embodiment] FIG. 1 is a schematic block diagram showing the construction of a digital modem (codec/modem) 101 according to a first embodiment. FIG. 8 is a schematic block diagram showing the construction of a facsimile according to the embodiment. As illustrated, in this embodiment, the D/A and A/D converters in the modem and the D/A and A/D converters in the voice codec are removed from the conventional modem/voice codec processing basic composition shown in FIG. 10, the LPF in the modem and the LPF in the voice codec, which are unnecessary due to the removal of the above converters, are also taken away, and interpolaters 103 and 107 are substituted for the converters and the LPFs. In the above construction, when a transmission operation is performed, a digital transmit signal (a) input to a modulation circuit 102 is modulated by a carrier frequency prescribed according to the CCITT Recommendation. The modulated output signal is a digital signal, and output in each sampling clock which is normally 9.6KHz. The output signal is input to a first interpolater 103 and sampled by a sampling clock of 8.0KHz. In other words, the sampling conversion from 9.6KHz to 8.0KHz is carried out. The converted signal is furthermore input to a coding circuit 104, converted to an audio PCM transmission code (b) with reference to a voice companding code table 105, and output in a sampling clock of 8.0KHz. A receiving operation will now be described. An input audio PCM receive code (c) is decoded by a decoding circuit 106 with reference to the voice companding code table 105. The decoded digital signal is output in the sampling clock of 8.0KHz. Then, the output signal is input to a second interpolater 107 and sampled by a sampling clock which is normally 9.6KHz. In other words, the sampling conversion from 8.0KHz to 9.6KHz is carried out. The converted signal is furthermore demodulated by a demodulation circuit 108 and output as a digital receive signal (d) in a sampling clock of 9.6KHz. Operations of a transmitting side of the digital modem 101 according this embodiment will be described with reference to the flowchart shown in FIG. 2. The case that a V27ter modem is used and the transmission rate is 4800bps is given as an example. The modulation circuit 102 shown in FIG. 1 repeats processes in each transmit data symbol (1/1600"). Therefore, one transmit data symbol (= 3 bit) is read from a transmitting facsimile (FAX) in Step S200 shown in FIG.2, and modulation for a symbol cycle (1/1600") is conducted on the data by a basic frequency having a cycle of 1/9600" in Step S201. In other words, six (= 1/1600 ÷ 1/9600) modulation results can be obtained by executing processing by the basic frequency having a cycle of 1/9600" for 1/1600" In Step S202, the six modulation results are temporarily saved in an unillustrated buffer in preparation for the subsequent interpolation. Then, in Step S203, the interpolation using the saved six data is performed by the first interpolater 103, the conversion of the sampling frequency from 9.6KHz to 8.0KHz is achieved. This sampling frequency conversion can be performed by conducting interpolation by using the six data in a symbol interval ( = 1/1600"), obtaining five interpolation results, and sampling the results at regular intervals (1/8000 = 1/1600 ÷ 5). When the above interpolation is completed, the interpolation results are temporarily saved in Step S204 in preparation for the subsequent PCM coding. The coding circuit 104 performs coding according to G. 711 of the CCITT Recommendation in Step S205, and a PCM code is output in Step S206. The above-mentioned processes in Steps S200 to S206 are repeated in each symbol interval (1/1600"). The above interpolation will now be described in detail with reference to FIGS. 4 and 5. FIG. 4 shows a discrete modulation waveform obtained by the process in Step S201 shown in FIG. 2, and a dotted line expresses an envelope. As shown in FIG. 4, six discrete data are arranged at regular intervals (= 1/9600") in a symbol interval (= 1/1600"), and the envelope expressed in the dotted line can be formed by performing interpolation processing based on the six data. FIG. 5 shows that five discrete data are arranged at regular intervals (= 1/8000") in a symbol interval (= 1/1600") after the sampling frequency conversion from 9.6KHz to 8.0KHz. The five data arranged at regular intervals are obtained by calculating values of an envelope at intervals of 1/8000". Operations of a receiving side of the digital modem 101 in this embodiment will now be described with reference to the flowchart shown in FIG. 3. The processes are repeated in each receive data symbol (1/1600 ") on the receiving side in the same manner as in the transmitting side. Therefore, one receive data symbol, that is, five PCM codes (= 1/1600 ÷ 1/8000) are input in Step S300 shown in FIG. 3. Subsequently, the decoding circuit 106 decodes the input five PCM codes according to G. 711 of the CCITT Recommendation in Step S301. Then, in Step S302, the decoded results are temporarily saved in preparation for the subsequent interpolation. Next, interpolation using the saved five data is executed by the second interpolater 107 in Step S303, and the sampling frequency conversion from 8.0KHz to 9.6KHz is achieved. This sampling conversion can be carried out by performing interpolation based on the five data in the symbol interval (= 1/1600"), obtaining six interpolation results, and sampling the results at regular intervals (1/1600 ÷ 6 = 1/9600). When the above interpolation is completed, the interpolation results are temporarily saved in Step S304 in preparation for the subsequent demodulation. In Step S305, the six interpolation results are demodulated by the demodulation circuit 108 in a symbol cycle (1/1600") by a basic frequency having a cycle of 1/9600". Then, one symbol data (= 3 bit) is obtained as a result and output to the FAX in Step S306. The above interpolation on the receiving side will be explained with reference to FIGS. 6 and 7. FIG. 6 shows a discrete receive waveform obtained by the process in Step S301 shown in FIG. 3, and a dotted line expresses an envelope. As shown in FIG. 6, five discrete data are arranged at regular intervals (= 1/8000") in a symbol interval (= 1/1600"). The envelope expressed in the dotted line can be formed by interpolation based on the five data. FIG. 7 shows that six discrete data are arranged at regular intervals (= 1/9600") in a symbol interval (= 1/1600") after the sampling frequency conversion from 8.0KHz to 9.6KHz. The six data arranged at regular intervals are obtained by calculating values of the envelope at intervals of 1/9600". As described above, -according to this embodiment, the following effects can be obtained by digitally performing the modem/PCM codec processing. The A/D and D/A converters mounted in the modem which normally operate at a sample rate of 9.6KHz and the A/D and D/A converters mounted in the PCM codec which normally operate at a sample rate of 8.0KHz are taken away from the conventional communication control unit, the LPF in the modem and the LPF in the PCM codec are also taken away, and a sample rate conversion processing circuit is disposed between the modem and the PCM codec instead of the above converters and the LPFs. It is thereby possible to digitally achieve the modem modulation/demodulation processing and the PCM codec processing, and thus to reduce the quantitizing errors. Furthermore, since two sets of A/D and D/A converters and two LPFs can be removed, the size of the circuit is made small and costs are lowered. In addition, it is easy to merge the modem and the PCM codec into a chip instead of the conventional composition of the modem, the PCM code, and the LSIs, and thus it can be greatly expected to save the space. As described above, according to this embodiment, it is possible to provide a digital modem 101 having a simple structure and capable of preventing quantitizing errors by digitally executing the modem modulation/demodulation and the voice codec processing. Although the sampling frequencies for modulation/demodulation and for the PCM codec processing are set at 9.6KHz and 8.0KHz, respectively, the sampling frequencies of the present invention are not limited to these values. Furthermore, although the case in which a V27ter modem is used and the transmission rate is 4800bps is described, it is obvious that the present invention can be easily applied to other modems. Although a simple interpolation is performed as sampling frequency conversion, the same effect can be obtained even by using splined interpolation, interpolation with a polynomial, or the like. The interpolation of the present invention is not limited to the above interpolating methods. In addition, although the description is given with a consciousness that the sampling frequency conversion is realized by DSP, the realization of the conversion in the present invention is not limited to the realization by hardware and software. The present invention is applicable to a modulating device without any demodulating function and a demodulating device without any modulating function, besides a modem having both modulating and demodulating functions.

Claims: A digital device connected to a digital network for transmitting a digital signal (a) consisting of a plurality of data symbols each symbol being represented by a plurality of bits comprising: first conversion means (102) for converting, data symbol per data symbol, the respective data symbol into first data, the first data representing for a period of time of the symbol interval an envelope of an analog signal modulated based on the data symbol the first data representing the analog signal at a first sampling rate for modulation, and the number of the first data being determined by the symbol interval and the first sampling rate for modulation; buffer means for temporarily storing the first data and second conversion means ( 103, 104 ) for converting, data symbol per data symbol, the first data into second data by interpolation using only the first data stored in said buffer means, and for transmitting the second data to the digital network at a second sampling rate for the digital network, the second data representing for the period of time of the symbol interval the envelope of the analog signal the second data representing the analog signal at the second sampling rate for the digital network, and the number of the second data being determined by the symbol interval and the second sampling rate for the digiatal network. A digital device according to claim 1, wherein said second conversion means ( 103, 104 ) comprises coding means ( 104 ) ; and the second sampling rate is equal to a sampling rate for said coding means ( 104 ). A digital device according to claim 1, wherein the first sampling rate is faster than the second sampling rate. A digital reception device connected to a digital network for receiving a digital signal (c) consisting of a plurality of data symbols, comprising: buffer means for temporarily storing second data derived from the digital signal received from the digital network, second conversion means ( 106, 107 ) for converting, data symbol per data symbol, the second data into first data by interpolation using only the second data stored in said buffer means, the second data representing an envelope of an analog signal at a second sampling rate for the digital network, the analog signal having been modulated based on transmission data, the number of the second data being determined by a symbol interval and the second sampling rate, and first conversion means ( 108 ) for converting, data symbol per data symbol, the first data representing for a period of time of the symbol interval the envelope of the analog signal at a first sampling rate for demodulation, into the transmission data, the number of the first data being determined by the symbol interval and the first sampling rate, wherein the symbol interval is a period of time required to output one symbol of transmission data comprising a plurality of bits. A digital reception device according to claim 4, wherein said second conversion means ( 106, 107 ) comprises decoding means ( 106 ) for decoding the received digital signal (c). A digital reception device according to claim 5, wherein the second sampling rate as equal to a sampling raid for said decoding means. A digital reception device according to claim 4 or 5, wherein the first sampling rate is faster than the second sampling rate. A system comprising a digital device according to any one of claims 1 to 3 and a digital reception device according to any one of claims 4 to 7. A method for transmitting a digital signal (a) consisting of a plurality of data symbols each being represented by a plurality of bits, comprising the steps of: converting, data symbol per data symbol, the respective data symbol into first data, the first data representing for a period of time of a symbol interval an envelope of an analog signal modulated based on the data symbol the first data representing the analog signal at a first sampling rate for modulation, and the number of the first data being determined by the symbol interval and the first sampling rate for modulation; temporarily storing the first data, and converting, data symbol per data symbol, the first data into second data by interpolation using only the first data temporarily stored, and for transmitting the second data to a digital network at a second sampling rate for the digital network, the second data representing for the period of time of the symbol interval the envelope of the analog signal the second data representing the analog signal at the second sampling rate for the digital network, and the number of the second data being determined by the symbol interval and the second sampling rate for the digital network. A method for receiving a digital signal (c) consisting of a plurality of data symbols, comprising the steps of: temporarily storing second data derived from the digital signal received from a digital network, converting, data symbol per data symbol, the stored second data into first data by interpolation using only the second data temporarily stored, the second data representing an envelope of an analog signal at a second sampling rate for the digital network, the analog signal having been modulated based on transmission data, the number of the second data being determined by a symbol interval and the second sampling rate, and converting, data symbol per data symbol, the first data representing for a period of time of the symbol interval the envelope of the analog signal at a first sampling rate for demodulation, into the transmission data, the number of the first data being determined by the symbol interval and the first sampling rate, wherein the symbol interval is a period of time required to output one symbol of transmission data comprising a plurality of bits.

Inventors: YAGUCHI TATSUYA, YAGUCHI, TATSUYA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H04N1
IPC codes: H04Q11, H04N1