Patent Abstract:
Comfort noise is derived from a white noise signal by filtering the white noise signal in a QMF bank to produce comfort noise signal that is selectively coupled to at least one channel in a telephone. Preferably, a plurality of QMF banks are used and the magnitude of the white noise into each filter is controlled in accordance with the magnitude of the signal in a corresponding analysis sub-band in a channel. In accordance with another aspect of the invention, the signals from higher frequency analysis sub-bands are combined and control a single input to a QMF bank, thereby increasing the low frequency resolution of the comfort noise. In accordance with another aspect of the invention, the QMF banks are cascaded upwardly (the output of one bank is coupled to the low pass input of the next bank), which also enhances the low frequency resolution of the comfort noise.

Full Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a noise generator for use in telephones and other communication devices wherein it is desired to avoid complete silence during a communication. 
     As used herein, “telephone” is a generic term for a communication device that utilizes, directly or indirectly, a dial tone from a licensed service provider. As such, “telephone” includes desk telephones, cordless telephones, speaker phones (see  FIG. 1 ), hands free kits (see  FIG. 2 ), and cellular telephones (see  FIG. 3 ), among others. For the sake of simplicity, the invention is described in the context of telephones but has utility in any communication device that silences a channel temporarily. 
     Anyone who has used a speaker phone, for example, is well aware of the cut off speech and the silent periods during a conversation caused by echo canceling circuitry within the speaker phone. Such phones generally operate in what is known as half-duplex mode, which means that only one person can speak at a time. While such silent periods assure that sound from the speaker phone is not coupled directly into the microphone within the speaker phone, the quality of the call is poor. 
     Telephones of the prior art often impose a silence in an attempt to eliminate acoustic and electronic echoes. When speech is gated off by a center clipper, attenuated by a residual echo suppresser, or canceled by a noise cancellation system, the resulting output is unnaturally quiet. The silence has been interpreted by consumers as a broken connection and a party to a call might mistakenly hang up. This problem has been solved by providing so-called “comfort noise” in which a low level noise signal is applied to a line rather than silence. U.S. Pat. No. 6,122,611 (Su et al.) describes a system that not only adds noise during periods of silence but also adds a little noise during conversation to avoid changes in the apparent loudness of the speech. 
     While one might think that all noise is the same, such is not the case. An automobile produces quite a different background noise from an office or a living room full of people. Adding “white” (spectrally flat random) noise produces yet another background sound. U.S. Pat. No. 5,657,422 (Janiszewski et al.) discloses filtering the noise in a low pass filter to make it sound more natural. While better than white noise, it remains a problem to provide a comfort noise that resembles the actual noise in each individual telephone call. 
     In view of the foregoing, it is therefore an object of the invention to provide an improved generator of comfort noise. 
     Another object of the invention is to provide comfort noise that more closely matches the spectral content of actual noise during a call. 
     A further object of the invention is to provide a comfort noise that matches actual background noise as closely as possible by shaping white noise using a quadrature mirror filter bank. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are achieved in this invention in which comfort noise is derived from a white noise signal by filtering the white noise signal in a quadrature mirror filter (QMF) bank that uses a polyphase filter structure to produce a comfort noise signal that is selectively coupled to at least one channel in a telephone. Preferably, an M (M&gt;2) channel quadrature mirror filter bank with a plurality of polyphase filters is used and the magnitude of the white noise into each filter is controlled in accordance with the magnitude of the signal in a corresponding sub-band in a channel. In accordance with another aspect of the invention, the signals from higher frequency sub-bands are combined and control a single input to a QMF bank, thereby increasing the low frequency content of the comfort noise. In accordance with another aspect of the invention, the QMF banks are cascaded upwardly (the output of one bank is coupled to the low pass input of the next bank), which provides finer spectral resolution at low frequencies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a conference phone or a speaker phone; 
         FIG. 2  is a perspective view of a hands free kit; 
         FIG. 3  is a perspective view of a cellular telephone; 
         FIG. 4  is a generic block diagram of audio processing circuitry in a telephone; 
         FIG. 5  is a more detailed block diagram of audio processing circuitry in a telephone; 
         FIG. 6  is a simplified block diagram illustrating the operation of a comfort noise generator constructed in accordance with the invention; 
         FIG. 7  is a block diagram of a polyphase filter used in implementing the invention; and 
         FIG. 8  is a block diagram of a comfort noise generator constructed in accordance with a preferred embodiment of the invention. 
     
    
    
     Those of skill in the art recognize that, once an analog signal is converted to digital form, all subsequent operations can take place in one or more suitably programmed microprocessors. Reference to “signal”, for example, does not necessarily mean a hardware implementation or an analog signal. Data in memory, even a single bit, can be a signal. In other words, a block diagram herein can be interpreted as hardware, software, e.g. a flow chart, or a mixture of hardware and software. Programming a microprocessor is well within the ability of those of ordinary skill in the art, either individually or in groups. 
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention finds use in many applications where the electronics is essentially the same but the external appearance of the device may vary.  FIG. 1  illustrates a conference phone or speaker phone such as found in business offices. Telephone  10  includes microphone  11  and speaker  12  in a sculptured case. Telephone  10  may include several microphones, such as microphones  14  and  15  to improve voice reception or to provide several inputs for echo rejection or noise rejection, as disclosed in U.S. Pat. No. 5,138,651 (Sudo). Acoustic echo can occur when sound from speaker  12  is coupled to one of the microphones. Background noise can be considerable in a speaker phone because the user is typically a meter or more away from a microphone. 
       FIG. 2  illustrates what is known as a hands free kit for providing audio coupling to a cellular telephone, illustrated in  FIG. 3 . Hands free kits come in a variety of implementations but generally include powered speaker  16  attached to plug  17 , which fits an accessory outlet or a cigarette lighter socket in a vehicle. A hands free kit also includes cable  18  terminating in plug  19 . Plug  19  fits the headset socket on a cellular telephone, such as socket  21  ( FIG. 3 ) in cellular telephone  22 . Some kits use RF signals, like a cordless phone, to couple to a telephone. A hands free kit also typically includes a volume control and some control switches, e.g. for going “off hook” to answer a call. A hands free kit also typically includes a visor microphone (not shown) that plugs into the kit. Background noise in a vehicle can also be considerable but distinctly different from the background noise in a speaker phone. 
     The various forms of telephone can all benefit from the invention.  FIG. 4  is a block diagram of the major components of a cellular telephone. Typically, the blocks correspond to integrated circuits implementing the indicated function. Microphone  31 , speaker  32 , and keypad  33  are coupled to signal processing circuit  34 . Circuit  34  performs a plurality of functions and is known by several names in the art, differing by manufacturer. For example, Infineon calls circuit  34  a “single chip baseband IC.” QualComm calls circuit  34  a “mobile station modem.” The circuits from different manufacturers obviously differ in detail but, in general, the indicated functions are included. 
     A cellular telephone includes both audio frequency and radio frequency circuits. Duplexer  35  couples antenna  36  to receive processor  37 . Duplexer  35  couples antenna  36  to power amplifier  38  and isolates receive processor  37  from the power amplifier during transmission. Transmit processor  39  modulates a radio frequency signal with an audio signal from circuit  34 . In non-cellular applications, such as speakerphones, there are no radio frequency circuits and signal processor  34  may be simplified somewhat. Problems of echo cancellation and noise remain and are handled in audio processor  40 . It is audio processor  40  that is modified to include the invention. How that modification takes place is more easily understood by considering an audio processor in more detail. 
       FIG. 5  is a detailed block diagram of an audio processing circuit, including a noise reduction circuit and an echo canceling circuit, loosely based on chapter 6 of  Digital Signal Processing in Telecommunications  by Shenoi, Prentice-Hall, 1995. Sub-band filter bank  54  is not shown in the text. The following describes signal flow through the transmit channel, from microphone input  42  to line output  44 . The receive channel, from line input  46  to speaker output  48 , works in the same way. 
     Sound is converted into an electrical signal by a microphone (not shown in  FIG. 5 ) and the electrical signal is coupled to microphone input  42 . The sound may or may not include sound from a speaker (not shown in  FIG. 5 ) driven by the signal at speaker output  48 . The signal at input  42  is digitized in A/D converter  51  and coupled to summation network  52 . There is, as yet, no signal from echo canceling circuit  53  and the signal proceeds to sub-band filter block  54 , which is initially set to minimum attenuation. In sub-band filter block  54 , the transmit channel is divided by frequency into a plurality of sub-bands. In a preferred embodiment of the invention, ten sub-bands are used. As few as two sub-bands can be used. 
     The signals from at least some the sub-bands are combined and coupled through non-linear processor  55  to summation circuit  56 , where comfort noise from generator  57  can be added to the signal. Non-linear processor  55  includes, for example, a center clipper, as noted above. A center clipper fully attenuates low level signals producing the silence described above. The output signal from summation circuit  56  is converted into analog form by D/A converter  58 , amplified in amplifier  59 , and coupled to line output  44 . 
     Control circuit  60 , which includes signal inputs (not shown) from several points in the audio processing circuit, controls sub-band selection and attenuation, non-linear processing, comfort noise insertion, and echo cancellation. Echo canceller  53  reduces acoustic echo between speaker output  48  to microphone input  42 . Echo canceller  61  reduces line echo between line output  44  and line input  46 . 
     In the prior art, comfort noise is simply generated and added, as in the Su et al. patent, or white noise is filtered (in a low pass filter) as in the Janiszewski et al. patent. Unlike the prior art, the comfort noise generated in accordance with the invention mimics the power distribution of actual noise during a call, thereby producing a much more realistic background noise.  FIG. 6  illustrates the basic operation of the invention. 
     In  FIG. 6 , comfort noise generator  70  includes white noise generator  71  coupled through multiplier  72  to the high pass input of quadrature mirror filter bank  77 . White noise generator  74  is coupled through multiplier  73  to the low pass input of QMF bank  77 . The gain of each channel is controlled in accordance with the amplitude of the signals in the sub-bands defined by sub-band filter  75  and sub-band filter  76 . Filters  75  and  76  are preferably band pass filters, in which the center frequency of filter  75  is higher than the center frequency of filter  76 . By controlling gain in accordance with the amplitude, or power, in the sub-bands, one obtains a better representation of the actual noise. That is, the amplitude of each white noise signal is adjusted in accordance with the power in each sub-band. 
     White noise generators  71  and  74  are each preferably a sixteen bit white noise generator synthesizing uniformly distributed random data in the interval (−1, 1). In accordance with another aspect of the invention, a different seed (starting value) is used in each white noise generator to provide a higher degree of randomness in the channels. 
     Filter  77  uses a polyphase filter structure to implement the QMF bank.  FIG. 7  is a block diagram of a preferred embodiment of the polyphase filter structure  80  for use in the invention. 
     Filter  80  includes a low pass input coupled to summation circuit  81  and to subtractor  82 . A high pass input is also coupled to summation circuit  81  and to subtractor  82 . The input signals are added in summation circuit  81  and coupled to all pass filter  83 . The input signals are subtracted in subtractor  82  and coupled to all pass filter  84 . The output from filter  83  is up-sampled in block  85  and delayed one sample time in block  87 . The output from filter  84  is up-sampled in block  86  and added to the delayed signal in summation circuit  88 . 
     The derivation of filters  83  and  84  is described as follows. A low pass, third order elliptical filter was designed to have 1 dB ripple in the pass band, 40 dB ripple in the stop band, and a stop band frequency of 0.25 cycles per sample. These specification yielded the following low pass filter. 
                 H   0     ⁡     (   z   )       =       0.15894   +     0.40296   ⁢     z     -   1         +     0.40296   ⁢     z     -   2         +     0.15984   ⁢     z     -   3             1   -     0.30823   ⁢     z     -   1         +     0.62909   ⁢     z     -   2         -     0.19706   ⁢     z     -   3                   
The following equations are used to derive the polyphase components.
   a   0 ( z   2 )= H   0 ( z )+ H   1 ( z )  [1] and   a   1 ( z   2 )= H   0 ( z )− H   1 ( z )  [2] where   H   1 ( z )= H   0 (− z ) 
and H 1 (z) is a high pass filter. Solving these equations for a 0 (z 2 ) and a 1 (z 2 ) yields the following polyphase filters.
 
                 a   0     ⁡     (     z   2     )       =       0.15894   +     0.62715   ⁢     z     -   2         +     0.38190   ⁢     z     -   4         +     0.03132   ⁢     z     -   6             1   +     1.16320   ⁢     z     -   2         +     0.27422   ⁢     z     -   4         -     0.03883   ⁢     z     -   6                             a   1     ⁡     (     z   2     )       =       0.45195   +     0.56796   ⁢     z     -   2         +     0.17939   ⁢     z     -   4             1   +     1.16320   ⁢     z     -   2         +     0.27422   ⁢     z     -   4         -     0.03883   ⁢     z     -   6                   
Equations [1] and [2] correspond to equation 3.6.14 in P. P. Vaidyananthan,  Multirate Systems and Filter Banks , p. 87, Prentice-Hall, Upper Saddle River, N.J., 1993.  FIG. 7  implements the function represented by equations [1] and [2].
 
     Each of the filters represented by a 0 (z) and a 1 (z) are further divided into second order sections and implemented using the Direct Form I method. Direct Form I minimizes the effect of coefficient quantization noise by allowing both numerator and denominator coefficients to be multiplied and accumulated before rounding is performed. This method is more robust to quantization problems in typical fixed point implications. 
       FIG. 8  illustrates a comfort noise generator constructed in accordance with a preferred embodiment of the invention. In the embodiment of  FIG. 8 , the outputs from ten analysis sub-band filters are used for generating scaling factors for sub-band comfort noise. The sub-band filters are in existing audio processing circuitry; see  FIG. 5 . A separate set of sub-band filters is not used for the invention to reduce cost and complexity. More or fewer sub-band filters could be used instead. Obviously, if existing circuitry does not include an analysis filter bank, then one must be provided. 
     As illustrated in  FIG. 8 , there are ten sub-band filters,  90 - 99 , of progressively higher center frequency; i.e. sub-band filter  90  has the lowest center frequency and sub-band filter  99  has the highest center frequency. Although the particular frequency are not critical, the following example is representative of an effective frequency allocation. Many others could be used instead. Obviously, the range of frequencies is determined by application. In the example below, the range of frequencies is determined by the bandwidth of a telephone network. 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Analysis 
                 Analysis 
                 QMF 
                 QMF 
               
               
                   
                 Band 
                 Bandwidth (Hz) 
                 Band 
                 Bandwidth (Hz) 
               
               
                   
                   
               
             
             
               
                   
                 0 
                   102-242.5 
                 0 
                  0-250 
               
               
                   
                 1 
                 283.6-352.1 
                 1 
                 250-500 
               
               
                   
                 2 
                 370.3-456.9 
               
               
                   
                 3 
                 480.4-594.6 
               
               
                   
                 4 
                 625.3-773.1 
                 2 
                  500-1000 
               
               
                   
                 5 
                  812.9-1005.9 
               
               
                   
                 6 
                 1057.7-1309.5 
               
               
                   
                 7 
                   1377-1706.2 
                 3 
                 1000-2000 
               
               
                   
                 8 
                 1796.2-2233.9 
               
               
                   
                 9 
                 2451-3395 
                 4 
                 2000-4000 
               
               
                   
                   
               
             
          
         
       
     
     The output from sub-band filter  90  is coupled to the square root circuitry  100 . The outputs from sub-band filter  91  and sub-band filter  92  are added and coupled to the square root circuitry  101 . The outputs from sub-band filter  93 , sub-band filter  94 , and sub-band filter  95  are added and coupled to square root circuitry  102 . The outputs from sub-band filter  96 , sub-band filter  97 , and sub-band filter  98  are added and coupled to square root circuitry  103 . The output from sub-band filter  99  is coupled to square root circuitry  104 . While, in theory, one could use (n−1) polyphase filters with (n) sub-band filters, where n≧2, it is preferred to combine the outputs from several filters to reduce the number of polyphase filters and to bias comfort noise generation in favor of lower frequencies. 
     Square root circuit  100  feeds into amplifier  110 , square root circuit  101  feeds  111 , square root circuit  102  feeds amplifier  112 , square root circuit  103  feeds amplifier  113 , and square root circuit feeds amplifier  114 . The incoming signals (data) represent power or, more accurately, mean squared values. The square root circuits provide the RMS (root mean squared) value of the signal for adjusting the gain of the white noise signal. 
     The output of amplifier  110  multiplies the output of white noise generator  130  through multiplier  120 ; the output of amplifier  111  multiplies the output of white noise generator  131  through multiplier  121 ; the output of amplifier  112  multiplies the output of white noise generator  132  through multiplier  122 , the output of amplifier  113  multiplies the output of white noise generator  134  through multiplier  124 . 
     The output of multiplier  120  is coupled to the low pass input QMF bank  140 . The output of multiplier  121  is coupled to the high pass input of QMF bank  140 . The output of QMF bank  140  is coupled to the low pass input of QMF bank  141 . The output of multiplier  122  is coupled to the high pass input of QMF bank  141 . The output of QMF bank  141  is coupled to the low pass input QMF bank  142 . The output of multiplier  123  is coupled to the high pass input of QMF bank  142 . The output of QMF bank  142  is coupled to the low pass input QMF bank  143 . The output of multiplier  124  is coupled to the high pass input of QMF bank  143 . The output of QMF bank  143  is the generated comfort noise. 
     The invention thus provides an improved generator of comfort noise in which the comfort noise more closely matches the spectral content of actual noise during a call. This is achieved by shaping white noise in a M channel quadrature mirror filter bank in accordance with the amplitude of the actual noise. 
     Having thus described the invention, it is understood by those of skill in the art that various modifications can be made within the scope of the invention. For example, as noted above, other forms of filter bank architectures can be used. In analog form, the blocks shown as multipliers are programmable gain amplifiers. In software, the operation is a multiplication of the two input digital values. Fewer separate white noise generators could be used, with a consequent decrease in randomness of the signals.

Technology Classification (CPC): 6