Abstract:
A system for processing audio signals in two channels, wherein each channel includes a plurality of band pass filters for dividing the signal in each channel into a plurality of bands and a plurality of variable gain amplifiers coupled one each to a band pass filter. Control logic coupled to the variable gain amplifiers prevents the signals in adjacent bands within a channel from being attenuated simultaneously. The control logic attenuates the signals in one channel in accordance with the signal in the corresponding bands in the other channel. In some circumstances, the signal in the band in the one channel is attenuated.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a noise reduction circuit and, in particular, to a noise reduction circuit employing a plurality of channels in a band wherein the gain of a channel in controlled in accordance with the signals in certain of the other channels and wherein “noise” means unwanted sounds, such as echoes, background speech or music, and random sounds.  
           [0002]    Virtually since the invention of the telephone, techniques have been developed to improve the clarity of the sound reproduced at each station. There are a number of techniques but two are of particular interest. A first uses what is known as a comb filter; i.e. a plurality of filters wherein band pass filters alternate with band stop filters. Comb filters with complementary pass and stop bands are coupled in the two audio channels connecting the two stations of a telephone call. That is, the pass bands in one channel are the stop bands in the other channel. As a result, a signal traveling in one direction will be slightly attenuated but a signal traveling in a loop, i.e. an echo, will encounter both sets of stop bands and be highly attenuated.  
           [0003]    The use of the complementary comb filters reduces the acoustic coupling between the speaker and microphone at each station as well as inter-station echo. Echo canceling circuits, which try to recognize a delayed signal as an echo, are much more complicated than complementary comb filter circuits and the two are often used together to eliminate echoes and other noises. However, comb filters degrade the quality of speech and do not always provide a sufficient margin of acoustic stability. One reason for the degradation is that the frequency response of a room in which the microphone and speaker of a station are located is characterized by a large number of resonant peaks. The band transitions in the comb filter transfer functions are often not sharp enough to suppress the resonant peaks, because if the transitions are too sharp the quality of the transmitted audio signal is adversely affected.  
           [0004]    Complementary comb filter circuits are disclosed in U.S. Pat. No. 5,386,465 (Addeo et al.). This patent includes complementary comb filters in combination with other apparatus for processing audio signals to reduce noise. U.S. Pat. No. 4,991,167 (Petri et al.) discloses a slightly different system. Signals in the sending and receiving directions are each separated into a set of bands, each including an attenuator. The signal in a band in one direction of transmission is controlled as a function of the amplitude of the signal in the same band in the other direction of transmission.  
           [0005]    A problem with this approach is that, unlike the complementary comb filters, one can attenuate the signals in adjacent bands, thereby noticeably degrading the quality of the voice transmission. In the extreme, the telephone is reduced to “half duplex” or simplex operation, i.e. single direction at a time because sounds from the other station are inaudible. The speaker at one station must stop and the circuits must re-settle before the person at the other station can be heard.  
           [0006]    Although a complementary comb filter can attenuate an echo, current implementations of such filters cannot suppress the “tail” of the echo, i.e. an extended echo. The prior art can only eliminate an echo lasting a maximum of fifty milliseconds. It is desired to eliminate an echo lasting up to fifteen hundred milliseconds.  
           [0007]    In view of the foregoing, it is therefore an object of the invention to provide a noise reducing circuit that retains the advantages of a complementary comb filter without the signal degradation.  
           [0008]    Another object of the invention is to provide an improved technique for selectively attenuating bands in an audio communication system.  
           [0009]    A further object of the invention is to provide a circuit that can eliminate echo tails up to fifteen hundred milliseconds in length.  
           [0010]    Another object of the invention is to provide a circuit that can reduce both background noise and echo noise while maintaining full duplex communication.  
         SUMMARY OF THE INVENTION  
         [0011]    The foregoing objects are achieved in this invention in which signals in the sending and receiving channels are each separated into a set of bands, each including a variable gain amplifier. The amplitude of a signal in a band in a first channel is controlled as a function of the amplitude of the signals in the corresponding band and the two adjacent bands in a second channel. Attenuation for echoes is restricted to alternate bands. Under certain conditions, a signal in the first channel is attenuated. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0013]    [0013]FIG. 1 is a block diagram of a preferred embodiment of the invention and illustrates the response to a first sound pattern;  
         [0014]    [0014]FIG. 2 is the same block diagram illustrating the response to a second sound pattern; and  
         [0015]    [0015]FIG. 3 is the same block diagram illustrating the response to a third sound pattern. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    In FIG. 1, the upper half of the figure illustrates a first channel affecting the transmitted signal or the signal from a microphone (not shown) and the lower half of the figure illustrates a second channel affecting the received signal or the signal going to the earpiece of a telephone or to the speaker in a conferencing telephone.  
         [0017]    In addition to other processing, such as echo cancellation and weighting the inputs, the signal from a microphone is applied to input  11  and coupled to a plurality of band pass filters  12 ,  13 ,  14 ,  15 , and  16 . Although five filters are shown, performance is improved with more filters. In one embodiment of the invention, ten {fraction (1/7)} octave filters were used for each channel. The bandwidths of the filters are not identical but vary with frequency. That is, at 3,400 Hz, the bandwidth is about ⅓ or ¼ octave, whereas, at 300 Hz the bandwidth is {fraction (1/7)} octave. Either the inputs or the outputs of the filters are weighted to accommodate the fact that the human ear is more sensitive to signals at 3,000 Hz than at 300 Hz or at 10,000 Hz.  
         [0018]    The filters are preferably switched capacitor, elliptical filters, known per se in the art. Other types of filters could be used instead but maximum performance might not be obtained. One reason a switched capacitor, elliptical filter was chosen is because the filters are easily matched in integrated circuit form and a filter in discrete form would be more expensive. The sampling rate of the filters is preferably above one hundred kHz, which enables the system to detect and remove echo tails up to fifteen hundred milliseconds in length.  
         [0019]    The outputs of the filters are coupled to variable gain amplifiers  22 ,  23 ,  24 ,  25 , and  26 , respectively, and the outputs of the variable gain amplifiers are combined in summing circuit  28  for transmission from output  29 . The variable gain amplifiers are also known as programmable gain amplifiers.  
         [0020]    The receiving side of the circuit is similarly constructed in that input  31  and coupled to a plurality of {fraction (1/7)} band pass filters,  32 ,  33 ,  34 ,  35 , and  36 . As with the upper portion of the circuit, only five filters are shown for the sake of simplicity. The outputs of the filters are coupled to variable gain amplifiers  42 ,  43 ,  44 ,  45 , and  46 , respectively, and the outputs of the variable gain amplifiers are combined in summing circuit  48  for transmission from output  49 .  
         [0021]    In each set of band pass filters, the bands are adjoining, not separated or alternating as in complementary comb filters of the prior art.  
         [0022]    Transmitting channel  10  and receiving channel  30  operate independently except for control logic  60 , which controls each variable gain amplifier. When the circuit is first turned on, each variable gain amplifier is set to unity gain. At unity gain, a signal on input  11  is divided into a plurality of bands by the band pass filters and then recombined, unaffected, in summing circuit  28 . Similarly, a signal on input  31  is divided into a plurality of bands by the band pass filters and then recombined, unaffected, in summing circuit  48 .  
         [0023]    The output of each band pass filter is also coupled to a detector, such as detector  61  at the output of filter  12 . Detector  61  senses the instantaneous or short term power of the signal and provides a suitable signal to control logic  60 . Control logic  60  analyzes the information from all inputs and controls the attenuators accordingly. In particular, echoes are reduced by controlling the attenuators in one channel in accordance with the amplitude of the signal in a corresponding band in the other channel. Background noise is reduce by attenuating the signals in a channel in accordance with the amplitude of the signals in each band of that channel. Adjacent bands in a channel may not be attenuated fully, i.e. set to minimum gain/maximum attenuation. Secondly, maximum attenuation does not take place in the same band in both channels. Other combinations of events are discussed more fully below as specific examples of operation. In general, control logic  60  operates to minimize background noise and echo.  
         [0024]    In FIG. 1, some lines are distinctly heavier, indicating a first combination of input signals and the resulting control signals produced by control logic  60 . Some lines are dashed and other lines are solid to emphasize the inter-channel control by control logic  60 . For example, if the signal from band pass filter  12  and the signal from band pass filter  14  exceeded a predetermined amplitude for a short period of time, detectors  61  and  62  provide output signals to control logic  60  indicative of that fact. Control logic  60  then reduces the gain (attenuates) the signals in the corresponding bands of the other channel. Specifically, control logic  60  reduces the gain of amplifiers  42  and  44 . Similarly, control logic  60  adjusts the gain of amplifiers  22 ,  23 ,  24 ,  25 , and  26  in accordance with the outputs from the detectors in the lower channel.  
         [0025]    If, as illustrated in FIG. 2, the signal from band pass filter  12  and the signal from band pass filter  13  exceeded a predetermined amplitude for a short period of time, detectors  61  and  64  provide output signals to control logic  60  indicative of that fact. Now there is a problem because adjacent bands may not be attenuated. In accordance with the invention, the solution is to attenuate one band in one channel and the adjacent band in the other channel. Specifically, the gain of variable gain amplifier  42  is reduced and the gain of variable gain amplifier  23  is reduced.  
         [0026]    The gain of either amplifier  22  or  42  can be reduced or the gain of either amplifier  23  or  43  can be reduced but not the gain of two amplifiers in adjacent bands in the same channel. The choice can be made arbitrarily by control logic  60 , e.g. using a random number generator, or the choice can depend upon which amplitude is greater, the signal into detector  61  or into detector  64 . For best signal to noise ratio, it is preferred not to attenuate the lesser of the two signals prior to transmission. Thus, choosing by amplitude is preferred. If the amplitudes are the same or are indistinguishable, then an arbitrary choice is made.  
         [0027]    [0027]FIG. 3 illustrates a third situation in which the signals from filters  12 ,  13 , and  14  all contain a substantial amount of power. In this situation, the first rule is applied to attenuate the signals in variable gain amplifiers  42  and  44 . The second rule, attenuate the corresponding band in the other channel, is broken by reducing the gain of amplifier  23 ; i.e. by reducing the gain of the amplifier in the same channel. Unless the decision is further affected by other bands, the amplitude of the signal from filter  13  does not matter, as long as the amplitude exceeds a predetermined threshold, as determined by detector  64 .  
         [0028]    Detectors  71 ,  72 , etc. (labeled “S”) respond to signals with a long time constant, such as background noise in a room. The long term noise is factored into the amount of attenuation provided by the variable gain amplifiers to minimize noise in the system without corrupting the desired voice information. Unlike fast attack detectors  61 ,  62 , and  64 , slow attack detectors  71  and  72  affect the attenuation in the same channel as the detector, e.g. by 10 db. A signal in the corresponding band in the other channel may cause control logic  60  to increase the attenuation to 40 db, for example. For a relatively quiet room, there may be zero attenuation due to background noise. For a noisy room, the attenuation may be 10 db or even 20 db to minimize background noise. Thus, a single channel addresses background noise and two channels address echo noise.  
         [0029]    The invention thus provides a noise reducing circuit that retains the advantages of a complementary comb filter without the signal degradation and an improved technique for selectively attenuating bands in an audio communication system. The invention eliminates echo tails up to fifteen hundred milliseconds in length and can reduce both background noise and echo noise while maintaining full duplex communication. The invention can be implemented in either analog form or in digital form, although maintaining an analog voice signal is preferred because the operation of the circuit is faster.  
         [0030]    Having thus described the invention, it will be apparent to those of skill in the art that many modification s can be made with the scope of the invention. For example, logic circuit  60  can be discrete logic, a programmable logic array (PLA), or a microprocessor having appropriate A/D conversion capability for receiving the signals from the detectors. The variable gain amplifiers can be digitally controlled amplifiers, thereby saving some conversion time. The thresholds for the bands need not be the same but can correspond to one or another curve representing the sensitivity of human hearing or to the hearing of a particular individual, for example. The variable gain amplifiers can provide gain or attenuation. The gain or attenuation can be linearly related to signal strength or non-linearly related to signal strength.