Abstract:
The invention provides a method and a communication device for cancelling echoes in the communication device, comprising: 
     receiving path means and sending path means for receiving and sending respective signals, and 
     echo compensating means for generating an echo control signal which is representative of a perceived echo, which echo compensating means are coupled between the sending and the receiving path means, and comprise an echo digital control signal generating means, and a digital-to-analog convertor coupled to the digital control signal generating means and to the receiving path means. Hardware and software implementation are easy and the dynamic range of an analog-to-digital convertor present in the receiving path is limited.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for cancelling echoes in a communication system wherein respective signals are sent through a sending path and received through a receiving path, which method comprises the steps of supplying to the receiving path an echo control signal which is derived from the sending path and is representative of an echo to be cancelled in the receiving path. 
     The present invention also relates to a communication device and to a radio base station of a cordless telephone device. 
     DESCRIPTION OF RELATED ART 
     Such a method and communication device, for example, for a radio base station of a cordless telephone device, is applied in and known from Philips Integrated Circuits Data Sheet, entitled “PCA 1070, Multistandard Programmable Analog CMOS Transmission IC”, specifying the PCA 1070 line interface features, and suitable for performing speech and line signal interface functions in electronic telephone sets. In particular the known communication device comprises an analog local-echo-cancelling means, generally also referred to as antisidetone (AST) means, for cancelling local echoes originating from its own local interface, mainly from the so-called hybrid circuit, which performs the separation and combination of respective signals in the sending and receiving paths, respectively, in order to couple a 4-wire telephone interface to the well known 2-wire a/b terminals of a telephone line. 
     In practise, the aforementioned interface is combined with a digital interface part wherein aside from analog local-echo-cancelling also digital local-echo-cancelling (LEC) is performed for cancelling echoes originating from the nearby interface itself. There is a strong tendency to cancel all kinds of echoes, irrespective of their source, to which the human ear is very sensitive and which may cause irritation and misunderstanding between parties trying to communicate with each other. In particular, echoes are more perceptible and thus more disturbing in present-day high-quality digital European cordless telephone (DECT) systems, due to the roundtrip delay of 20 msec. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an alternative method and a communication device by means of which echoes can be cancelled in an easy-to-implement way and in a way which allows flexible and programmable adjustments to a variety of national authority communication line requirements. 
     To achieve this, the present invention is characterized in that the echo-control signal is an echo digital control signal which is digital-to-analog converted and then supplied to the receiving path. 
     Although the proposed invention seems more complicated because of the inclusion of a digital-to-analog convertor (DAC) apart from the generally already present further A/D and D/A convertors in the receiving and sending paths, respectively, it has surprisingly been found that taking an echo representative control signal from the digital sending path, followed by the digital-to-analog conversion and the addition of the converted control-compensation signal to the receiving path, obviates the above disadvantages without adding substantial complexity to the above-mentioned digital software implemented signal processing and programming. 
     In order to achieve the above objective the echo compensating means comprise an echo digital control signal generating means, and a digital-to-analog converter coupled to the digital control signal generating means and to the receiving path means for compensating echoes. 
     In addition to the above advantages, the communication device according to the invention gives rise to a possible limitation of the dynamic range of an ADC which is generally also present in the receiving path of the communication device. This dynamic range is a key factor of such a convertor, and an increase thereof would add to the complexity, required chip area and current consumption in an interface IC for communication purposes. The thus achieved limited dynamic range of said ADC provides opportunities for supplying a lower supply voltage to the communication device, which supply voltage is known to limit the dynamic range at its high end in case of a remote PABX, because it is taken from the telephone line itself. At its lower end the dynamic range is limited by the lowest voltage which does not drown in noise. With the limited dynamic range of the ADC, noise requirements are thus advantageously weakened without jeopardising the overall performance of the communication device according to the invention. 
     Further embodiment of the communication device according to the invention have the advantage that the proposed inclusion or combination of echo digital control signal generating means and/or receiving filter means in the digital signal processing (DSP) means only causes a small and therefore acceptable increase of the DSP software code and execution time, as well as a minor increase—only necessitating a doubled output register—in DSP hardware. 
     In the advantageous embodiments of the communication device according to the invention outlined in claims  5  or  6  only one symmetrical digital-to-analog convertor (DAC) is used. This however does not cause a substantial increase in hardware because, for reasons of performance, a symmetrical DAC is preferred anyhow, because of its reduced output noise. Thus, in practise this does not lead to chip area increase or additional current consumption. 
     The inclusion of a dynamic noise shape (DNS) circuit as proposed in claim  8  only results in a very limited and thus acceptable increase of the chip area in the order of less than 0.05 mm 2 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The method, communication device and radio base station according to the invention will be elucidated further together with the additional advantages, while reference is made to the appended drawing, wherein similar components are referred to by the same reference numerals. In the drawing: 
     FIG. 1 shows possible arrangements of prior art communication devices, 
     FIG. 2 shows a possible embodiment of the communication device according to the invention, 
     FIG. 3 shows a possible arrangement of key impedances simulated in the communication device according to the invention, and 
     FIGS. 4A and 4B show possible and optimised connection schemes respectively for incorporating the simulated key impedances in the communication device according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a communication device  1 , such as a telephone, which is connected to a communication line  2 , such as the well known a/b telephone line. The communication device  1  usually has a low frequency data source, for example a microphone  3 , facsimile or the like, and a telephone or loudspeaker  4 . Data is thus transferred between the communication line  2  and the communication device  1 . The communication device  1  may be mobile, portable or cordless. In the case of for example a cordless communication device  1 , the device has a portable part  5  and a base station  6 , mutually communicating through antennas  7  and  8 . In general the microphone  3  and loudspeaker  4  are connected to the communication line  2  through a line interface  10 . The integrated circuit PCA 1070 mentioned earlier contains circuitry like interface  10  and in particular an analog echo cancelling means, also called analog antisidetone circuit AST, for cancelling echoes caused by a hybrid  11  included in the interface  10 . 
     FIG. 2 shows an embodiment of a part of the communication device  1 , in particular the part included in the base station  6  is exemplified. The base station  6  comprises the line interface  10  containing the hybrid  11  and several separate discrete electrical components, a usually on-chip circuit  12  connected to the interface  10 , and a time division duplex (TDD) circuit  13  connected between the circuit  12  and the antenna  7 . The portable part  5  also contains a TDD circuit  9  meant for sending signals to and receiving signals from the base station  6 . Viewed schematically, the communication device  1  has a sending path S from microphone  3  to at least a/b line  2 , and a receiving path R from at least a/b line  2  to loudspeaker  4 . This path distinction can also be made in circuit  12 . The circuit  12  contains a digital signal processing circuit, or DSP  14  for short. The DSP  14  usually comprises a digital local echo canceller (LEC)  15 , which like the analog AST mentioned above is coupled between the sending path S and the receiving path R. The DSP  14  digitally cancels or compensates echoes originating from local hybrids present in the communication device  1 . The hybrid calls for echo compensation, cancellation or antisidetone technics. These technics develop a control signal or echo compensation signal from the sending path S, which is representative of an echo experienced in the communication device  1 . Such a control signal is then inverted and added to the signal in the receiving path R in order to compensate for the echo and supply a return signal to the loudspeaker  4  which is virtually free of the echo compensated for. Local echoes originating from the communication device  1  itself and arising on a moderate timescale of approximately up to 5 msec are compensated for either in an analog way in the (AST) line interface  11  or digitally in the LEC  15 . For this purpose, transceiver  13  outputs a digital signal in the S-path to DSP  14  and receives a digital signal sent by a communication device at the other end of the line  2  through DSP  14 . DSP  14  comprises the LEC  15 , which generates a digital representation of an echo and adds it, inverted in an adder  16 , to the digital return signal in the R-path. DSP  14  further comprises a receiving filter means (RXF)  17  containing as shown in FIG. 4, a series arrangement of a highpass filter  18 , digital filter means  19  and a lowpass filter  20  for filtering out the unwanted spectral lobes of the output signal. The implementation and functioning of the receiving filter means  17  is known per se in the relevant art. The filter means output signal is then fed to a digital noise shape (DNS) circuit  21  and digital-to-analog converted in DAC means  22  so as to be provided to line interface  11 . The signal in the R-path from interface  11  is fed to a minus input and then analog-to-digital converted in ADC  23 , which in turn is connected to adder  16 . 
     Alternative echo compensation is provided for in DSP  14  by digital signal means  24  for generating a digital echo signal, which is coupled to a second DAC  25 , included in the DAC means  22 . Optionally a second digital noise shape circuit  26  is connected between the means  24  and DAC  25 . DAC  25  has an output  27  connected to a plus input of ADC  23 . The digital signal means  24  is designed and arranged to generate an echo signal to compensate for an echo originating from the communication device at the other end of the a/b line  2 . 
     Because ADC input signals are input into plus and minus inputs of the ADC  23  its dynamic range is limited. The merits thereof are such that they largely offset the disadvantages of the presence of the second DAC  25  and its optional DNS circuit  26 . However since DAC means  22  are symmetrical, the second DAC  25  is given for free, as it is implicitly present anyhow. The optional DNS  26  on the other hand costs only a very limited chip area in practise. 
     In addition, the receiving filter means  17  and the echo digital control signal generating means  24  can be combined to save memory space both for programming and data handling in the digital signal processing means  14 . Further advantages can be achieved as follows. The communication device  1  contains key impedances which are vital to match the internal impedance of the communication device  1  for each and every country with the locally present line impedance and to match an internal echo cancelling impedance with an impedance reflecting the echo properties at the specific location of the communication device  1 . These impedances (kZ SET , Z AST ), that are all variable (resistors R 1  . . . R 4 , and capacitors C 1 , C 2 ) in order to be able to adjust their values to the above mentioned requirements, are schematically shown in FIG.  3 . Given the notion of the present invention, inclusion of these key impedances by simulating them in the DSP  14  is proposed. Digital simulation provides on-chip flexibility and programmability of the key impedances as well as easy on-chip integration. Given a transfer function H(s) in terms of the Laplace operator of the circuit of FIG. 3, this transfer function can easily be transformed by bi-linear mapping of H(s) into the well known z-domain. This yields H(z) in terms of the z-operator, which is realised by adding EQ 1  after EQ to the digital filter circuit of FIG.  4 A. By properly identifying the variable coefficients in the second order digital filter function with the resistance and capacitor values, digital simulation thereof is achieved. This solution is easier to realise than building the variable programmable key impedances on-chip, for example by means of switched capacitor circuits. 
     FIG. 4B shows a combined connection scheme for the RXF  17  and the digital signal means  24 , where EQ 1  is divided out. This saves program memory space and reduces the number of instructions to be executed in DSP  14 . 
     In principle, means (not shown) can now be provided for automatically measuring local electrical subscriber line conditions and characteristics on a/b line  2  for adapting the simulated key impedances such that they automatically reflect an optimum matching to line impedance and/or provide optimum echo cancelling on the spot. These conditions are line voltage, line current, impedance behaviour as a function of frequency, echo impulse response etcetera. These conditions thus provide data for automatically calculating the optimum values for the simulated components, schematically shown in FIG.  3 . 
     In view of the foregoing it will be evident to a person skilled in the art that various modifications may be made within the spirit and the scope of the present invention as hereinafter defined by the appended claims and that the present invention is thus not limited to the examples provided.