Abstract:
A codec circuit having a programmable digital bandpass filter is disclosed. According to one aspect, a bandpass filter matches the filter characteristics of the codec circuit to a transmitted pulse code modulation (PCM) signal. The codec circuit includes at least one programmable digital high-pass filter connected in series to at least one programmable digital low-pass filter. The setting filter characteristics for the programmable digital high-pass and low pass filters are each set by means of a signal identification device for identifying a PCM signal transmitted through the codec circuit. The filters are set as a function of the transmitted PCM signal in order to vary a bandpass filter characteristic for the programmable digital bandpass filter.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a codec circuit having a programmable digital bandpass filter for matching the filter characteristics of the codec circuit to a transmitted PCM signal. 
     2. Description of Related Art 
     U.S. Pat. No. 5,212,817 discloses a programmable digital bandpass filter for a codec circuit for adaptation of the filter characteristics, in which case the setting filter coefficients can be set by changing the bandpass filter characteristic of the programmable digital bandpass filter. 
     A codec circuit is a circuit which intrinsically combines the functions of a coding switching device and a decoding switching device. When transmitting information, information is in many cases transmitted in both directions between two connections. In this situation, the codec circuit would carry out PCM (Pulse Code Modulation) signal coding in the transmission direction and PCM signal decoding in the reception direction. 
     In practice, codec circuits are normally in the form of digital signal processors (DSP), which are highly specialized, but nevertheless standardized, integrated circuits produced in large quantities for high-speed processing of a narrowly constrained set of input signals in real time. Digital signal processors such as these are typically used for data transmission by means of modems. A modem is a terminal which modulates outgoing signals and demodulates received signals. 
     For signal transmission of voice signals, the filter characteristics of the codec circuit have to comply with specific, predetermined specifications. There are no such specifications for the filter characteristics for signal transmission of modem signals. 
     Since the transmission rate of modems is directly proportional to the transmission frequency bandwidth, the data transmission rate increases with increasing frequency bandwidth of the digital filters integrated in the codec circuit. The digital filters which have been used in known codec circuits until now have filter characteristics matched to the specifications which exist for transmission of voice signals, and are not programmable. It is thus impossible when using such digital filters for codec circuits according to the prior art to widen the filter bandwidth, or to reset it, for data transmission when modem signals are being transmitted from a first modem to a second modem instead of the normal voice signals which originate, for example, from a telephone. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is thus to provide a codec circuit, in which the data transmission frequency range can be matched to the transmitted PCM signal. 
     According to the invention, this object is achieved by a programmable digital bandpass filter having the features specified in patent claim  1 . 
     Further advantageous refinements are specified in the dependent claims. The invention is in this case based on the idea of providing the programmable digital bandpass filter in addition to the digital filters with fixed settings in the codec circuit, in order that the filter bandwidth, and hence the data transmission rate, can be matched to the transmitted PCM signal. 
     In one advantageous refinement of the codec circuit according to the invention, the setting filter coefficients can be stored in coefficient memory devices, which are associated with the digital high-pass filter and the digital low-pass filter. 
     This offers the particular advantage that the coefficients of the digital filters can be matched to or reprogrammed for technical requirements of the transmission channel and to and for the transmitted PCM signal at any time. 
     In a further advantageous refinement, the memory devices are random access memories (RAMs). 
     The memory devices are preferably connected via coefficient setting lines to a signal identification device for identification of the PCM signal transmitted through the codec circuit. 
     This offers the particular advantage that the setting of the setting filter coefficients, and hence the matching of the filter characteristics of the codec circuit to the transmitted PCM signal, can be carried out automatically by identifying the nature of the transmitted PCM signal, without any need for manual programming for each specific case. The programmable digital filters are preferably seventh-order filters. 
     In one preferred embodiment, the upper and the lower signal transmission cut-off frequencies for the digital bandpass filter and the gradient of the bandpass filter flanks can be set separately by means of the setting filter coefficients. 
     This offers the particular advantage that the two signal transmission cut-off frequencies of the bandpass filter can be set independently of one another. Furthermore, the gradient of the bandpass filter flanks can be programmed in accordance with the requirements for the transmission channel. 
     The lower signal transmission cut-off frequency of the bandpass filter can preferably be set by setting the setting filter coefficients of the digital high-pass filter. 
     The upper signal transmission cut-off frequency of the bandpass filter can preferably be set by setting the setting filter coefficients of the digital low-pass filter. In one preferred embodiment, a frequency response correction filter is also connected in series with the digital low-pass filter and the digital high-pass filter in order to compensate for the ripple in the frequency passband of the bandpass filter. 
     This offers the particular advantage that the transmitted PCM signal is filtered uniformly, independently of the frequency, throughout the frequency passband of the bandpass filter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       One preferred embodiment of a codec circuit is described in the following text, in order to explain the features that are essential to the invention, with reference to the attached drawings, in which: 
         FIG. 1  shows a codec circuit, in which programmable digital bandpass filters according to the invention are used both in the transmission signal path and in the reception signal path of the codec circuit; 
         FIG. 2  shows a block diagram of the codec circuit according to the invention; 
         FIG. 3  shows a typical filter characteristic of a codec circuit as a function of the filters that are used in the transmission signal path of the codec circuit; 
         FIG. 4  shows a filter transmission characteristic of a codec circuit in the region of the lower cut-off frequency for various filter coefficient settings; 
         FIG. 5  shows a filter transmission characteristic of a codec circuit in the region of the upper cut-off frequency. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a block diagram of a codec circuit, in which a transmission signal is PCM-coded and a received PCM signal is decoded. The codec circuit has a transmission signal path and a reception signal path for this purpose. 
     The analog input filter is preferably an analog high-pass filter for outputting alternating signals. On the output side, the analog input filter  2  is connected via a line  3  to an analog/digital converter  4 . The analog/digital converter  4  samples the filtered analog signal supplied to it via the line  3 , and produces a digital output signal, which is passed via the line  5  to the programmable digital bandpass filter  6  according to the invention. The digital bandpass filter  6  has a bandpass filter characteristic and filters the digital signal applied on the line  5  such that only signals in the frequency passband of the bandpass filter are passed via a line  7  to a digital high-pass filter  8 . The digital high-pass filter has a high-pass filter characteristic which is permanently set and cannot be varied. 
     On the output side, the digital high-pass filter  8  is connected via a line  9  to a frequency response correction filter  10 . The frequency response correction filter  10  compensates for ripple in the passband of the bandpass filter characteristic. 
     The transmission signal path in the codec circuit is formed, as is shown in  FIG. 1 , by the analog input filter  2 , the analog/digital converter  4 , the programmable digital bandpass filter  6 , the digital high-pass filter  8 , whose setting is fixed, and by the frequency response correction filter  10 . In this case, the analog input filter  2  and the digital high-pass filter  8 , whose setting is fixed, can be connected via control lines  11 ,  12  to the transmission signal path of the codec circuit. If the input filter  2  is switched off via the control line  11 , the signal which is applied on the input line  1  is passed directly, without being filtered, to the input of the analog/digital converter  4 . If the digital high-pass filter  8  is switched off via the control line  12 , the output signal from the programmable digital bandpass filter  6  is passed directly to the frequency response correction filter  10 . 
     The reception signal path of the codec circuit illustrated in  FIG. 1  likewise has a frequency response correction filter  13 , whose input side is connected to a receiving line  14 . The design of the frequency response correction filter  13  is similar to that of the frequency response correction filter  10 . According to the invention, a line  15  connects a programmable digital bandpass filter  6  which is connected in the reception signal path of the codec circuit, to the frequency response connection filter  13 . The output signal, having been filtered by the programmable digital bandpass filter  6 , is supplied via a line  16  to a digital/analog converter  17 , which converts the received digital signal to an analog signal, and preferably emits this signal via a line  18  to an analog output filter  19 . In one preferred embodiment, the analog output filter  19  can be connected to the reception signal path of the codec circuit, via a control line  20 . The output signal from the output filter  19  is emitted via an output line  21  to, for example, an SLIC circuit, which is not illustrated. 
     The two programmable digital bandpass filters in the codec circuit for matching the filter characteristics of the codec circuit to the PCM signal transmitted in the transmission signal path or the reception signal path can be controlled by a signal identification device  24 , via control lines  22 ,  23 , in the preferred embodiment illustrated in  FIG. 1 . 
     The signal identification device  24  identifies the PCM signal transmitted through the codec circuit. The signal identification device  24  is connected via detection lines (which are not illustrated) to the transmission signal path and/or to the reception signal path of the codec circuit. An evaluation circuit, which is integrated in the signal identification device  24 , is used to evaluate and to identify the PCM signal in the transmission signal path or the reception signal path. The evaluation circuit identifies the type of modulation and the transmission speed such that, firstly, it is possible to determine whether the transmitted PCM signal originates from a terminal, for example a modem, or from a telephone for voice transmission. Furthermore, the evaluation circuit identifies the type of transmitting or receiving modem. For example, the evaluation circuit identifies whether the transmitting modem is a V.90 modem or a V.34 modem. The signal identification device  24  sets the optimum digital filter coefficients for the digital bandpass filters, via the control lines  22 ,  23 , to match the identified modem. In this case, the filter coefficients are preferably set automatically. 
       FIG. 2  shows the programmable digital bandpass filter, as illustrated in  FIG. 1 , according to the invention, in detail. 
     The programmable digital bandpass filter  6  has an input connection  25 , an output connection  26  and a control or setting connection  27 . The control connection  27  is connected via a control line  22  or  23  to the signal identification device  24  shown in  FIG. 1 . The input connection  25  is connected via an internal input line  28  to a programmable digital high-pass filter  29 . The programmable digital high-pass filter  29  is connected in series with a programmable digital low-pass filter  34  via a line  30 . The setting filter coefficients of the programmable digital high-pass filter  29  can be stored in an associated memory device  32 . The setting filter coefficients of the programmable digital low-pass filter  31  can be stored in an associated memory device  33 . The memory devices  32 ,  33  are preferably random access memories (RAMs). The setting filter coefficients stored there can be reprogrammed by the signal identification device  24 . To do this, the memory devices  32 ,  33  are connected to the control connection  27  via internal memory setting lines  34 ,  35 ,  36 . 
     The programmable digital low-pass filter  31  is connected on the output side via an internal output line  37  to the output connection  26  of the programmable digital bandpass filter  6 . 
     In the embodiment shown in  FIG. 2 , the programmable digital low-pass filter  31  is connected downstream from the programmable digital high-pass filter  29 . In an alternative embodiment, the programmable digital high-pass filter  29  is connected downstream from the programmable digital low-pass filter  31 . 
     In further embodiments of the programmable digital bandpass filter according to the invention, a large number of programmable digital high-pass filters  29  and a large number of programmable digital low-pass filters  31  can be connected in series. The programmable digital low-pass filters  31  may in this case at the same time be in the form of interpolation and decimation filters. 
     In one preferred embodiment of the programmable digital bandpass filter  6  according to the invention, the programmable digital filters  29 ,  31  are seventh-order filters. 
     The setting filter coefficients which can be stored in the memory devices  32 ,  33  set the filter characteristics of the digital filters  29 ,  31  via lines  38 ,  39 . 
     A bandpass filter characteristic is achieved by connecting the programmable digital high-pass filter  29  and a programmable digital low-pass filter  31  in series. The setting coefficients in this case make it possible to set the upper and the lower signal transmission cut-off frequencies of the digital bandpass filter  6  according to the invention and the gradient of the bandpass filter flanks independently of one another. However, the programmable digital bandpass filter according to the invention can be set in a flexible manner to satisfy the individual requirements of the transmission channel and the type of modem being used. 
     In this case, the lower cut-off frequency of the bandpass filter characteristic can be set by setting the setting filter coefficients of the digital high-pass filter  29 , and the upper signal transmission cut-off frequency of the bandpass filter characteristic can be set by setting the setting filter coefficients of the digital low-pass filter  31 . 
     In one preferred embodiment, the frequency response correction filters  10 ,  13 , as are shown in  FIG. 1 , are integrated in the programmable digital bandpass filter  6 , with the compensation for the ripple in the bandpass filter characteristic being carried out automatically as a function of the filter coefficient settings. 
       FIG. 3  shows the frequency response characteristic of the codec circuit shown in  FIG. 1 , in the region of the lower cut-off frequency. As can be seen from  FIG. 3 , the lower cut-off frequency occurs at about 100 to 200 Hz. 
     The filter transmission curve a shows a filter characteristic of the codec circuit when only the input filter  2  is switched on, corresponding to the switch-on control signal supplied via the control line  11 . The filter transmission curve b shows the situation when the digital high-pass filter  8 , whose setting is fixed, is also connected in the transmission signal path of the codec circuit, by means of a control signal on the control line  12 . Finally, the filter transmission curve c shows the situation in which the programmable digital high-pass filter  29  in the programmable digital bandpass filter  6  according to the invention is also connected in the transmission signal path of the codec circuit. The digital programmable bandpass filter  6  is preferably connected in the signal transmission path automatically by the signal identification device  24  on identifying a corresponding PCM signal via a separate control line. 
     As can be seen from  FIG. 3 , the lower cut-off frequency of the codec circuit is increased by the addition of the digital bandpass filter  6  according to the invention, and occurs at about 200 Hz. The lower cut-off frequency of the codec circuit can thus be set individually as a function of the transmitted PCM signal. In the example shown in  FIG. 3 , the cut-off frequency is set in the range from 50 Hz to 200 Hz. 
       FIG. 4  shows the filter transmission characteristic of the codec circuit shown in  FIG. 1  when the various high-pass filters are added, that is to say the analog input filter  2 , the digital programmable high-pass filter  29  within the digital programmable bandpass filter, and the fixed digital high-pass filter  8 . In this case, in contrast to  FIG. 3 ,  FIG. 4  also shows the filter characteristic being changed as a function of the filter coefficient. As can be seen from  FIG. 4 , not only is the lower signal transmission cut-off frequency of the codec circuit but also the gradient of the lower bandpass filter flank, as well. This allows fine adjustment of the codec circuit. 
       FIG. 5  shows the filter transmission characteristic of the digital programmable low-pass filter  31  within the programmable digital bandpass filter  6 . As can be seen from  FIG. 5 , switching the digital programmable low-pass filter  31  to different filter coefficients reduces the upper cut-off frequency. 
     As can be seen from  FIGS. 3 to 5 , the addition of the programmable digital bandpass filter  6  reduces the frequency passband, since the lower cut-off frequency is increased and the upper cut-off frequency is decreased. Conversely, the frequency passband is widened, and the data transmission rate thus increased, by changing the coefficients of the programmable digital bandpass filter  6 . 
     The filter transmission characteristic can be programmed differently in the transmission signal path and in the reception signal path of the codec circuit. For example, if a V.90 modem is used, the transmission direction can be matched to V.34 modem transmission by means of a higher upper cut-off frequency in the digital programmable high-pass filter  29  in the programmable digital bandpass filter  6  connected in the reception signal path, while the full frequency bandwidth is available in the reception direction. Use of the programmable digital bandpass filter  6  according to the invention within the codec circuit makes it possible to utilize the frequency range as far as possible, particularly for terminals and modems. The programmable digital bandpass filter  6  according to the invention furthermore offers the capability to program the cut-off frequencies independently of one another. 
     It is thus possible firstly to satisfy the predetermined specifications for voice transmission, and secondly to achieve optimum utilization of the frequency range for modem signal transmissions.