Abstract:
Transceiver for transmitting and receiving voice and data signals includes a voice signal driver configured to operate in idle or working operating modes, during which it has a high or low output impedances respectively. A data signal driver drives an analog data transmission signal via a channel to the subscriber. A data reception circuit that converts a received analog data reception signal into digital reception data includes a multiplication circuit for multiplying the digital reception data by compensation coefficients selected to compensate for channel distortions, an identification circuit connected to the channel for providing an identification signal indicative of whether the subscriber terminal is on or off-hook, and a control circuit to select the operating mode in response to the identification signal and to determine compensation coefficients to compensate for a change in the channel resulting from a change in the output impedance of the voice signal driver.

Description:
FIELD OF INVENTION 
     The invention relates to a transceiver for transmitting and receiving voice and data signals, and in particular to an xDSL transceiver. 
     RELATED APPLICATIONS 
     This application claims the benefit of the priority date of German patent application 100 59 135.3 filed on Nov. 29, 2000, the contents of which are herein incorporated by reference. 
       FIG. 1  shows a subscriber connected via a telephone subscriber line to a transceiver situated within a switching center. The telephone subscriber line forms a transmission channel having specific transmission properties. At the subscriber end, a splitter for separating voice and data signals is provided at the transmission channel. The splitter comprises a low-pass filter TP and a high-pass filter HP. An analog telephone terminal is connected to the low-pass filter TP. The high-pass filter HP is connected via a line to a data modem of the subscriber. At the switching end, a transceiver for transmitting and receiving voice and data signals is connected to the transmission channel. When the terminal is taken off-hook by the subscriber, the impedance of the terminal changes and the resultant current rise in the transmission channel is detected by an identification circuit integrated in the transceiver. The identification circuit outputs an off-hook identification signal, with the result that the transceiver changes over from an idle operating mode (on-hook) to a working operating mode (off-mode). The change over in the operating mode causes the output impedance of the transceiver to change, with the result that the transmission properties of the transmission channel change. This can lead to errors in the data transmission between the transceiver and the subscriber modem. If the bit error rate exceeds a specific threshold value, according to the prior art a re-initialization and a renewed setup of the data transmission connection are effected after an interruption of the data transmission. Such a re-initialization may become necessary each time the handset is taken off-hook and each time the handset is placed on-hook. Since the initialization phase for the renewed setup of the data transmission connection requires a relatively long time, the duration of the data transmission is correspondingly delayed. 
     SUMMARY 
     The object of the present invention, therefore, is to provide a transceiver for data and voice signal transmission in which the data transmission does not lead to an interruption of the data transmission when the telephone handset is taken off-hook or placed on-hook by the subscriber. 
     The invention provides a transceiver for transmitting and receiving voice signals and data signals having a voice signal driver, which can be changed over between an idle operating mode and a working operating mode, the voice signal driver having a high output impedance in the idle operating mode and, in the working operating mode with a low output impedance, driving an analog transmission voice signal via a transmission channel to a subscriber terminal, a data signal driver for driving an analog data transmission signal via the transmission channel to the subscriber terminal, a data reception circuit, which converts a received analog data reception signal into digital reception data which are multiplied, by means of a multiplication circuit, by adjustable channel distortion compensation coefficients for the compensation of channel distortions of the transmission channel, and having an identification circuit connected to the transmission channel, which circuit identifies when the subscriber terminal is taken off-hook or placed on-hook and outputs an identification signal to a control circuit, which switches the voice signal driver into the working operating mode when the subscriber terminal is taken off-hook and into the idle operating mode when the subscriber terminal is placed on-hook, the control circuit setting the channel distortion compensation coefficients for the compensation of the transmission channel changed by the output impedance change of the voice signal driver. 
     The data signals are preferably modulated by a discrete multitone modulation method. 
     The control circuit preferably applies a first channel distortion compensation coefficient set to the multiplication circuit in the idle operating mode and a second channel distortion compensation coefficient set is preferably applied to the multiplication circuit in the working operating mode. 
     The channel distortion compensation coefficient sets preferably comprise a multiplicity of complex coefficients. 
     The channel distortion compensation coefficients of the second channel distortion compensation coefficient set are preferably higher or lower in each case by a constant complex value than the channel distortion compensation coefficients of the first channel distortion compensation coefficient set. 
     In a first embodiment of the transceiver according to the invention, the coefficients of the second channel distortion compensation coefficient set are calculated by a calculation unit as a function of the coefficients of the first channel distortion compensation coefficients within the control circuit. 
     In an alternative embodiment of the transceiver according to the invention, the coefficients of the two channel distortion compensation coefficient sets are stored in a memory unit within the control circuit 
     The control circuit of the transceiver according to the invention preferably isolates the identification circuit from the transmission channel by opening a switching unit when the subscriber terminal is taken off-hook and switches the identification circuit to the transmission channel by closing the switching device when the subscriber terminal is placed on-hook. 
     The transmission channel is preferably formed by a two-wire telephone subscriber line. 
     In a preferred embodiment, the data signals are multi-tone-modulated signals. 
     Preferably, there are connected upstream of the data signal driver of the transceiver according to the invention a coder for coding the digital transmission data to be transmitted, 
     an IFFT calculation circuit for IFFT transformation of the digital transmission data and 
     a digital/analog converter for converting the digital transmission data into the analog data transmission signal. 
     Preferably, there are connected upstream of the multiplication circuit of the transceiver according to the invention 
     an analogue/digital converter for converting the analog data reception signal into digital reception data, 
     a digital FIR filter with adjustable coefficients and an FFT calculation circuit for FFT transformation of the filtered digital reception data. 
     Preferably, there are connected downstream of the multiplication circuit of the transceiver according to the invention a decision circuit and a decoder for decoding the equalized digital reception data. 
     The channel distortion compensation coefficients of the first channel distortion compensation coefficient set for the idle operating mode are preferably determined by measuring the transmission channel during an initialization operation for initializing the transceiver 
     The transceiver according to the invention is preferably an xDSL transceiver, and in particular an ADSL transceiver. 
     A preferred embodiment of the transceiver according to the invention for transmitting and receiving voice and data signals is described below with reference to the accompanying figures for the purpose of explaining features that are essential to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       In the figures: 
         FIG. 1  shows a data transmission system with a transceiver according to the prior art. 
         FIG. 2  shows a block diagram of a transceiver according to the invention; 
         FIG. 3  shows a circuit diagram of a multiplication circuit provided within the transceiver according to the invention; and 
         FIG. 4  shows a flow diagram for explaining the method of operation of the transceiver according to the invention which is illustrated in FIG.  2 . 
     
    
    
     DESCRIPTION 
       FIG. 2  shows the circuitry construction of a preferred embodiment of a transceiver  1  according to the invention. The transceiver  1  receives from a data DSP  2 , via a data line  3 , the data to be transmitted and writes said data to a register  4 . The data to be transmitted are combined into data blocks in the register  4  and forwarded to a coder  6  via data lines  5 . The coder  6  codes the data present using an arbitrary coding method, for example a Reed-Solomon coding method. The coded transmission data are output from the coder  6  via parallel data lines  7  to a transformation calculation circuit  8 . In the calculation circuit  8 , the coded data to be transmitted are converted by means of inverse Fast Fourier transformation. The transformed data are written via data lines  9  to a further register  10  and buffer-stored there. The buffer-stored data are applied via data lines  11  to a digital/analog converter  12 , which converts the transformed data into an analog data transmission signal. The transmission signal is composed of a multiplicity of sinusoidal signals, each sinusoidal transmission signal being modulated both in terms of amplitude and in terms of phase. Consequently, the transmission signal comprises a multiplicity of quadrature-amplitude-modulated transmission signals. The quadrature-amplitude-modulated transmission signal is applied via a line  13  to a data driver circuit  14  for signal level matching. The data driver circuit  14  amplifies the data transmission signal present and outputs it via a line  15  to a signal output connection  16  of the transceiver  1 . The output connection  16  can be connected to an arbitrary transmission channel  17 . The transmission channel  17  is preferably a two-wire telephone line with connected subscriber terminal. 
     The register  4 , the coder  6 , the transformation calculation circuit  8 , the register  10 , the digital/analog converter  12  and the analog data signal driver  14  form a data transmission signal path  18  within the transceiver  1  according to the invention. 
     In addition to the data transmission signal path  8  the transceiver  1  according to the invention contains a data reception signal path  19 . A data signal received at the connection  16  pass via a line  20  to an analog/digital converter  21 , which converts the received data signal into reception data. The converted reception data are fed via a line  22  to an FIR filter  23 , which serves to shorten the impulse response of the entire transmission system. The FIR filter  23  is a transverse filter whose coefficients are adaptively adjustable. The provision of the FIR filter  23  shortens the transient process of the transceiver  1  when receiving data. The filtered reception data are output from the FIR filter  23  via a line  27  to a register  25 , where they are buffer-stored. The buffer-stored reception data are output via parallel data lines  26  to an FFT calculation circuit  27 , which carries out Fast Fourier transformation of the reception data present. In a preferred embodiment of the transceiver according to the invention, five hundred and twelve samples are present at the FFT calculation circuit  27  and are transformed into two hundred and fifty-six complex numerical values. The transformed reception data are output via parallel data lines  28  to a multiplication circuit  29 . The multiplication circuit  29  multiplies the digital reception data present on the lines  28  by adjustable channel distortion compensation coefficients which are output by a control circuit  31  via parallel lines  30 . The multiplied or weighted output data of the multiplication circuit  29  are output via data lines  32  to a QAM decision circuit  33 . The received data are a multitone reception signal. On the output side, the decision circuit  33  is connected via data lines  34  to a decoder  35 , which decodes the digital reception data and writes them via data lines  36  to a register  37 . The register  37  outputs the buffer-stored decoded reception data via a data line  38  to the data DSP  2  for further processing of the data. 
     Via a line  40 , a voice DSP  39  outputs the voice transmission data via a digital/analog converter  41  of the transceiver  1  according to the invention. The analog voice transmission signal is output from the digital/analog converter  41  via a line  42  to an analog signal conditioning stage  43 , which applies the conditioned voice transmission signal via a line  44  to a voice signal driver  45 . The voice signal driver  45  amplifies the voice transmission signal present in a working operating mode of the transceiver  1  and outputs the amplified voice transmission signal via a line  46  to the transmission channel connection  16  of the transceiver  1 . The amplified voice transmission signal is transmitted via the transmission channel  17  to the analog subscriber terminal. The voice signal driver  45  is connected via a control line  47  to the control circuit  31 . The control circuit  31  changes over the voice signal driver  45  between an idle operating mode and a working operating mode via the control line  47 . The voice signal driver  45  has a high output impedance in the idle operating mode, while it has a low output impedance in the working operating mode. Furthermore, an identification circuit  48  is connected to the line  46  via a switching device  47 . The identification circuit  48  identifies when the telephone handset is taken off-hook and placed on-hook by the subscriber through a current rise and current fall respectively, on the subscriber line  17 . In the idle operating mode of the transceiver  1 , the identification circuit  48  is connected to the line  46 , i.e. the controllable switch  47  is kept in the closed position by the control circuit  31  via a control line  49 . The identification circuit  48  identifies when the subscriber terminal is taken off-hook by the subscriber from a current rise on the line  46  and outputs a corresponding identification signal via a line  50  to the control circuit  31 . When the subscriber terminal is taken off-hook, the control circuit  31  changes over the voice signal driver  45  to the working operating mode via the control line  47  and sets the channel distortion compensation coefficients, which are applied via the lines  30  to the multiplication circuit  29 , for the compensation of the transmission channel changed by the output impedance change of the voice signal driver. Preferably, furthermore, the coefficients of the FIR filter  23  are adaptively set anew by the control circuit  31  via lines  51  in order to compensate for the output impedance change. Furthermore, in the working operating mode, the controllable switch  47  is opened by the control circuit  31  by the application of a control signal to the control line  49 , with the result that the identification circuit  48  is isolated from the transmission channel  17 . 
     The digital/analog converter  41 , signal conditioning circuit  43   a  and the voice signal driver  45  form a voice transmission signal path within the transceiver  1  according to the invention. 
     In addition to the voice signal transmission path, the transceiver  1  has a voice reception signal path. The voice signal reception path has a low-pass filter  51   a  connected to the connection  16 . The low-pass filter  51   a  filters out the low-frequency analog voice signal transmitted from the subscriber via the transmission channel  17  and outputs the filtered voice signal via a line  52  to the signal conditioning circuit  43 . From the voice signal conditioning circuit  43 , the analog conditioned voice reception signal passes via a line  53  to an analog/digital converter  54 , which converts the analog voice reception signal into digital voice reception data and outputs them via a line  55  to the voice DSP  39 . 
       FIG. 3  shows the circuitry construction of the multiplication circuit  29 . The digital tone data output by the FFT transformation circuit  27  are applied via n line  28  to n multiplication circuits  56  within the multiplication circuit  29 , where they are multiplied by channel distortion compensation coefficients which are applied to the multiplication circuit  29  by the control circuit  31  via control lines  30 - 1  to  30 -n. The received multitone data of the data reception signal which are weighted in this way are applied via n data lines  32  to the QAM decision circuit  33 . 
     In the idle operating mode of the transceiver  1 , the control circuit  31  applies signal coefficients of a first channel distortion compensation coefficient set to the multiplication circuit  29  via the control lines  30 . If the control circuit  31  receives from the identification circuit  48 , via the line  50 , an identification signal indicating that the handset has been taken off-hook by the subscriber, the control circuit  31  changes over the voice signal driver  45  to the working operating mode and simultaneously applies signal coefficients of a second channel distortion compensation coefficient set to the multiplication circuit  29 . 
     The channel coefficients of the first channel distortion compensation coefficient set are determined during a single initialization operation by measurement of the transmission channel  17  and calculation by means of an LMS method by the control circuit  31 . 
     In a first embodiment of the transceiver  1  according to the invention, the control circuit  31  furthermore contains a calculation unit which calculates the coefficients of the second channel distortion compensation coefficient set for the working operating mode as a function of the first channel distortion compensation coefficient set determined during the initialization operation. The coefficients of the two channel distortion compensation coefficient sets are preferably complex coefficients. The channel distortion compensation coefficients of the second channel distortion compensation coefficient set for the working operating mode are preferably higher in each case by a constant complex value than the channel distortion compensation coefficients of the first channel distortion compensation coefficient set for the idle operating mode. 
     In an alternative embodiment, the control circuit  31  contains a memory device in which are stored at least two different channel coefficient sets for the idle operating mode and the working operating mode. 
       FIG. 4  shows a flow diagram for explaining the method of operation of the transceiver  1  which is illustrated in FIG.  2 . 
     After a Step S 0 , in a Step S 1  a check is made to determine whether or not the bit error rate during the data transmission has exceeded a specific threshold value. If the bit error rate is too large, then in a step S 2  in a standardized initialization phase, the transmission channel  17  is measured and the channel distortion compensation coefficients for compensating channel distortions of the transmission channel  17  are determined by means of an LMS calculation method. 
     In a Step S 3 , the control circuit  31  checks whether or not the subscriber terminal has been taken off-hook by the subscriber. If the subscriber terminal has been taken off-hook, in a Step S 4  the coefficients of the second channel distribution compensation coefficient set for the working operating mode are applied to the multiplication circuit  29  by the control circuit  31  in order to compensate the changed transmission properties of the transmission channel  17 , said transmission properties having been changed as a result of the output impedance change of the voice signal driver  45 . The process subsequently returns to Step S 1 . As a result of the compensation, the bit error rate decreases and, in a Step S 5 , the normal data transmission takes place between the transceiver  1  and the terminating subscriber. If it is ascertained in Step S 3  that the telephone handset has been placed on-hook again by the terminating subscriber, in Step S 4  the original first channel coefficient set for the idle operating mode is set again. 
     In the transceiver  1  according to the invention, a renewed initialization operation does not have to be carried out when the handset is taken off-hook or placed on-hook by the subscriber, so that the data transmission between the subscriber modem and the transceiver  1  can be continued without interruption. The change in the transmission properties of the transmission channel  17  as a result of the changed output impedance of the voice signal driver  45  in the event of changeover between the two operating modes is compensated by the changeover between the two signal distortion compensating coefficient sets of the control circuit  31 .