Abstract:
In a cellular telephone system where a digital cellular telephone is connected to a regular telephone through the public switched telephone network (PSTN), a speech encoder/decoder is used with an A/μ-Law encoder/decoder causing annoying audible noise at very low levels because of the quantization characteristics of the A/μ-Law encoder/decoder. This noise is eliminated by adding a digital constant to the output of the speech coder, shifting the low level signal away from zero. The resulting DC level added to the speech signal is inaudible to the PSTN telephone user and does not degrade speech quality. Alternatively, the constant added to the output of the speech coder is confined to a small value added to the speech coder output to move the entire speech coder output during the silence period, between speech periods, above zero or below zero.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 09/127,881 abandoned filed Jul. 31, 1998 for Method And Apparatus For Speech Code Output Transformation. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention relates generally to communication systems and more particularly to a method and apparatus for improving communication between a cellular phone and a phone on the PSTN network whenever a digital speech compression algorithm is followed by an compander conversion, which is typical. 
     2. Description of Related Art 
     In the prior art, situations arise where a digital cellular phone (e.g., GSM, PCS- 1800, IS-54) is connected to a telephone on the public switched telephone network (PSTN). Such cellular systems typically employ a speech coder followed by a compander, such as a μ-Law or A-Law conversion, in order to interface to the PSTN network. Due to the “poor” quantization characteristics of A-Law and to a lesser extent of the μ-Law conversion, at very low levels (hardly audible), the output of the speech coder are transformed into an annoying audible noise after the A/μ-Law conversion at the receiving PSTN phones. The problem becomes worse as the bit-rate of the speech coding algorithm decreases and is most noticeable if a level adjustment (increase) takes place after the A/μ-Law decoding. 
     SUMMARY OF THE INVENTION 
     It has been discovered that annoying audible noise during speech intervals can be eliminated by adding a fixed number in the digital domain to the output of the speech coder. In this manner, signal samples shifting around zero are moved away from the area of “poor” quantization of the compander. The invention would typically be a part of the speech decoder algorithm, as a post operation. However, it can be used as successfully, as a stand alone block between any speech decoding algorithm and the A-Law or μ-Law conversion (compander). Adding a fixed number to the output of the speech decoder is inaudible, and does not degrade speech quality in any way. The technique eliminates the problem for any bit-rate. The effect is dramatic and will solve the noise problem for most existing standards. Alternatively, the constant added to the output of the speech coder is confined to a small value added to the speech coder output so that during the silence period between speech, when the output of the coder falls slightly below zero or slightly above zero, the constant value moves the entire speech coder output during the silence period slightly above zero or slightly below zero. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The exact nature of this invention, as well as its objects and advantages, will become readily apparent upon reference to the following detailed description when considered in conjunction with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof, and wherein: 
     FIG. 1 is a block diagram illustrating a typical interface between a digital speech coding algorithm and the PSTN network through an A/μ-Law conversion (compander). 
     FIG. 2 is a block diagram illustrating how the interface of FIG. 1 is usually simulated. 
     FIG. 3 is a block diagram illustrating the preferred embodiment of the invention. 
     FIG. 4 is a block and waveform diagram illustrating the advantage of the invention over the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventors of carrying out their invention. Various modifications, however, will remain readily apparent to those skilled in the art: 
     FIG. 1 illustrates a speech encoder/decoder  13  supplying an output signal to an A/μ-Law encoder  14  interfacing with the public switched telephone network (PSTN)  15 . At the central office, the PSTN  15  interfaces with an analog telephone line to a subscriber telephone  17  through an A/μ-Law decoder and digital to analog converter (DAC)  16 . The subscriber uses a standard PSTN telephone  17  for speech communication  18 . In this configuration, a low signal level of the output of the speech coder  13 , which occurs typically between speech intervals is transformed by the A/μ-Law conversion into an annoying audible noise at the receiving PSTN telephone  17 . 
     The typical interface to the public switch telephone network (PSTN)  15  as illustrated in FIG. 1 is usually implemented in the manner shown in FIG. 2 wherein a speech signal  12  from a cellular telephone is encoded by a speech encoder  10  into a bit stream for transmission across the transmission medium to a speech decoder  8  which converts the bit stream into an output signal,  4 . The output signal  4  is supplied to the A/μ-Law encoder/decoder  14 ,  16  which generates a signal  6  that is presented to the PSTN telephone. 
     The preferred embodiment of the present invention is illustrated in FIG. 3, as an add-on to the typical PSTN interface. A cellular signal input  12  to a speech encoder  10  supplies a bit stream to a speech decoder  8  which outputs a signal  4 . The signal  4  from the speech decoder  8  is at a low level when the input signal  12  to the speech encoder  10  is at a low level, typically when there is silence between speech. Instead of supplying the signal  4  to the A/μ-Law encoder/decoder, the present invention, by way of digital adder  21 , adds an offset  20 , which is preferably a fixed number (constant), to the signal  4  from the speech decoder  8  in the digital domain. Adding a constant to signal  4  causes the data signal to shift away from the area of “poor” quantization for the A/μ-Law converter. 
     It is important to note that adding a constant, a fixed number, for example the number  6 , to the signal stream  4  does not degrade the speech quality at the PSTN telephone  17 , while it does eliminate the annoying audible noise inherent in the prior art system of FIG.  2 . This is true for many speech coding standards, such as, ITU, ETSI, TIA, for example. By adding the constant  20  to signal  4  at the digital level through adder  21  a shifting of the signal away from 0 occurs creating shifted signal  23 . The shifted signal  23  is supplied to the A/μ-Law encoder/decoder  14 ,  16  which supplies its output signal  25  to the PSTN telephone  17 . 
     FIG. 4 illustrates how well the invention performs as compared to the prior art, such as illustrated in FIG. 2. A typical low level output signal  4  from a speech decoder which occurs, typically during periods of silence between speech, is shown as a time varying signal of very low amplitude varying around 0. In the prior art system of FIG. 2, this signal  4  is provided to an A-Law encoder/decoder or μ-Law encoder/decoder. In this example, an A-Law encoder/decoder  27  is shown because the problem is much more pronounced in this encoder/decoder. The A-Law encoder/decoder generates an output signal  6  in response. As can be seen, the output signal  6 , which started out as a low level signal  4  now has a significant higher amplitude varying around 0. This signal is perceptually annoying to the PSTN telephone user and results in degraded overall speech quality. 
     The invention of FIG. 3, takes the signal  4  from the speech decoder  8 , and adds a constant  20 , like the number 6, for example, to the signal  4  causing it to shift a constant level away from 0, as in signal  23 . The shifted signal  23  is supplied to the A-Law encoder/decoder  27  producing output signal  25 , which is shifted away from 0 by a DC offset, but without the large amplitude variation. This DC offset is inaudible to the human ear. The ear hears offset signal  25  as silence, rather than the annoying noise generated by the amplitude varying signal  6 . 
     In order to eliminate such a large DC offset signal during the silence period between speech, a second embodiment of the present invention adds the constant  20  only to values of the audio output  4  of the speech decoder  8  that fall within a certain range of digital values. To better understand how this embodiment can eliminate audible noise during the silence between speech, the cause of the audible noise is explained with reference to FIG.  4 . 
     FIG. 4 shows a low level audio output  4  that varies slightly about zero during the silence between speech. The value of zero lies within an area of “poor” quantization of a A-Law compander  27 , in which values of the audio output  4  that are equal to or slightly above zero are quantized as +8, and values that are slightly below zero are quantized as −8. As a result, the quantized output  6  of the A-Law compander  27  has an amplitude that varies between +8 and −8. This relatively large amplitude variation of the quantized output  6  produces an annoying audible noise at the PSTN telephone during the silence between speech. 
     The second embodiment of the present invention eliminates this noise by adding the constant  20  only to values of the audio output  4  that fall within a certain range of values. This can be done by choosing a range of values that include the values of the audio output  4  that are slightly below zero during the silence between speech, and adding a positive constant  20  that shifts these values to zero or above. That way, the values of the audio output  4  that are slightly below zero during the silence between speech are shifted to zero or above by the constant  20 . As a result, all of the values of the audio output  23  after the adder  21  are quantized the same by the compander  14 ,  16  during the silence between speech. This causes the quantized output  25  of the compander  14 ,  16  to have a constant amplitude during the silence between speech, thereby eliminating the audible noise caused by large amplitude variation. The constant amplitude of the quantized output  25  is perceived as silence by the human ear at the PSNT telephone, rather than an annoying audible noise. 
     In one example, the range of values is −1 or −2, and the constant  20  is a +2. The logical function for the adder  21  in this example is given by: 
     
       
           x   1 ( n )= x ( n )+2 if  x ( n )=−1 or −2, 
       
     
     
       
         otherwise  x   1 ( n )= x ( n ) 
       
     
     where x(n) is the audio output  4  of the speech decoder  8  and x 1 (n) is the audio output  23  after the adder  21 . This logical function only adds the constant  20  of a +2 for values of the audio output  4  in the range of −1 or −2. However, it is also contemplated that a +2 value could be added to all negative values of the audio output  4 . A +2 value would be added to any value within the range slightly below zero to −32,768, the maximum number of representations possible in a sixteen bit word below zero. Assuming that the values of the audio output  4  that are below zero during the silence between speech are either −1 or −2, the constant  20  shifts these values to zero or 1. As a result, the values of the audio output  23 , after the adder  21 , are quantized the same by the compander  14 ,  16  during the silence between speech. This causes the quanitized output  25  of the compander  14 ,  16  to have a constant amplitude during the silence between speech, thereby eliminating the audio noise caused by the large amplitude variation. 
     The same result can be achieved by choosing a range of values that include the values of the audio output  4  that are equal to or slightly above zero during the silence between speech, and adding a negative constant  20  that shifts these values below zero. That way, the values of the audio signal  4  that are equal to or slightly above zero during the silence between speech are shifted below zero by the constant  20 . As a result, all of the values of the audio output  23 , after the adder  21 , are quantized the same by the compander  14 ,  16  during the silence between speech. This causes the quanitized output  25  of the compander  14 ,  16  to have a constant amplitude during the silence between speech, thereby eliminating the audible noise caused by the large amplitude variation. Thus, a −2 value could be added to all the positive values of the audio output  4 . A −2 value would be added to any value within the range zero to +32,767, the maximum number of representations possible in a sixteen bit word above zero. 
     The second embodiment of the present invention can be implemented in the speech decoder  8 , as a post operation. In this case, the speech decoder  8  performs the constant addition according to the second embodiment after decoding the incoming speech signal  10  into the digital audio output  4 .