Abstract:
A telephone includes a first channel having a microphone input and a line output, a second channel having a line input and a speaker output, and each channel having a plurality of band pass filters with their outputs coupled to a multiplex circuit. The multiplex circuit selects subsets of the signals in each channel in accordance with the magnitudes of the signals in each band. Thus, full duplex operation is provided in accordance with the spectral content of each channel. In one embodiment of the invention, the multiplex circuits initially allocate alternate bands within each channel and complementary bands between the channels, thereby emulating a complementary comb filter. Thereafter, bands are allocated according to spectral content.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to telephone systems and, in particular, to providing full duplex operation for more than two parties to a telephone call. As used herein, “telephone” includes cellular telephones. 
     There are two kinds of echo in a telephone system, an acoustic-echo between an earphone or a speaker and a microphone and an electrical echo occurring in the switched network for routing a call between stations. In a handset, acoustic echo is typically not much of a problem. In speaker phones, where several people huddle around a microphone and loudspeaker, acoustic feedback is much more of a problem. Hybrid circuits (two-wire to four-wire transformers) located at terminal exchanges or in remote subscriber stages of a fixed network are the principal sources of electrical echo, also known as line echo. 
     Filtering a voice signal to eliminate either or both kinds of echo is known in the art. Devices known as complementary comb filters have been used to eliminate echoes by having the signal to a speaker filtered through the pass bands of a first comb filter, thereby falling within the stop bands of a second, complementary comb filter coupled to a microphone. Matching, rather than complementary, comb filters can be used in the line out and line in channels of a telephone if one also uses a frequency shift; see U.S. Pat. No. 4,748,663 (Phillips et al.). Frequency shifting is undesirable because of the effect on the quality of the voice signal. 
     Complementary filters work well when there are only two parties to a call. For a conference call, at least one of the phones must be in half-duplex mode. Switching unobtrusively between half duplex and full duplex among the parties is difficult at best and invariably one of the parties is blocked momentarily while speaking. 
     One could provide three complementary comb filters (each containing one pass band and two stop bands) for a three party call but the cost of such a system is prohibitive. Further, the reduction in spectral power would significantly reduce the quality of the audio signal. 
     Even with well designed band pass filters, a comb filter necessarily reduces the power and spectral content of speech. For example, an amplitude peak may happen to fall within the stop band of a comb filter, substantially changing the sound characteristic of a person&#39;s voice. When fricatives fall within a stop band, intelligibility can be significantly reduced. Amplification is not a cure if the filters do not match the spectral response of an person&#39;s voice. 
     In other applications, e.g. automotive cellular telephones, certain sounds are noises characteristic of the vehicle or environment rather than the driver and it would be desirable to have a stop band match the dominant frequency of the noise. Again, comb filters of the prior art cannot remove such noise except by chance. 
     In view of the foregoing, it is therefore an object of the invention to provide an improved band pass filter system in which each band is chosen individually depending upon the spectral content of the applied signal. 
     Another object of the invention is to provide a band pass filter system in which full duplex communication is optimized for the voice characteristics of the respective speakers. 
     A further object of the invention is to provide a band pass filter system in which the pass bands can be allocated among three parties in a phone call. 
     Another object of the invention is to provide a band pass filter system that can emulate a comb filter initially and then adapt to the spectral content of each channel. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are achieved in this invention in which a telephone includes a first channel having a microphone input and a line output, a second channel having a line input and a speaker output, and each channel has a plurality of band pass filters with their outputs coupled to a multiplex circuit. The multiplex circuit selects subsets of the signals in each channel in accordance with the magnitudes of the signals in each band. Thus, full duplex operation is provided in accordance with the spectral content of each channel. In one embodiment of the invention, the multiplex circuits initially allocate alternate bands within each channel and complementary bands between the channels, thereby emulating a complementary comb filter. Thereafter, bands are allocated according to spectral content. 
    
    
     
       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 block diagram of a comb filter constructed in accordance with the prior art; 
         FIG. 2  is a chart illustrating the operation of the filters in  FIG. 1 ; 
         FIG. 3  is a block diagram of a filter system constructed in accordance with the invention; 
         FIG. 4  is a schematic of circuitry for choosing which channel receives a particular filter; 
         FIG. 5  is a block diagram of the microphone to line output channel in a telephone constructed in accordance with the invention; 
         FIG. 6  is a block diagram of the line to speaker channel in a telephone constructed in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1 , sound incident upon microphone  11  is converted into an electrical signal and coupled to telephone  12 . A portion of the circuitry within telephone  12  includes band pass filters  13 ,  14 ,  15 ,  16 , and  17 . For a bandwidth of 300–3,400 Hz, five filters are typical. More than five filters may result in too much overlap between bands. 
     Telephone  12  also includes notch filters  21 ,  22 ,  23 ,  24 , and  25 . The center frequencies of the notch filters correspond to the center frequencies of the band pass filters. Thus, a signal passing through the band pass filters, traveling along transmission line  27  and reflected back to transmission line  28  would be attenuated by the notch filters. A single telephone constructed in accordance with the invention provides approximately 10 dB of attenuation of a signal between microphone  11  and speaker  29 . 
     Telephone  30  is constructed in like manner except that the center frequencies of the filters are offset from the center frequencies of the filters in telephone  12 . Specifically, the center frequencies of notch filters  31 ,  32 ,  33 ,  34 , and  35  are between the center frequencies of the band pass filters in telephone  12 . Thus, the notch filters in telephone  30  are aligned with the dead bands between the band pass filters in telephone  12 , which further increases the effectiveness of the circuit. 
     Telephone  30  also includes band pass filters  41 ,  42 ,  43 ,  44 , and  45  having the same center frequencies as the notch filters in telephone  30 . Thus, a signal on input  47 , e.g., from a microphone (not shown), is divided among the band pass filters, summed, and transmitted over line  28  to telephone  12 . The center frequencies of the notch filters in telephone  12  correspond to the dead bands between the bands of band pass filters  41 – 45 , enhancing the operation of these filters. 
     The operation of telephones constructed in accordance with the invention is illustrated in  FIG. 2 . The center frequencies are numbered consistently with  FIG. 1 . In particular, curve  49  represents the frequency response of band pass filter  13  ( FIG. 1 ). Filters  13  and  21  have the same center frequency, thereby reducing the amount of echoes or other noises between microphone  11  and speaker  29 . 
     A problem with this construction is the need for an “A” telephone and a “B” telephone having complementary filter characteristics. In accordance with one aspect of the invention, this problem is eliminated by having a plurality of band pass filters in each channel and control circuitry for assigning the filters to each channel. 
       FIG. 3  illustrates one channel of a telephone constructed in accordance with the invention. Microphone  50  is coupled to band pass filters  51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57 ,  58 ,  59 , and  60 . The band pass filters are preferably one-third octave filters and are preferably implemented as switched capacitor filters for ease in implementing as an integrated circuit. Any form of band pass filter can be used for the invention. A sub-set of these filters is chosen to provide full duplex operation. 
       FIG. 4  is a schematic of a circuit for choosing the louder of two channels within a given band. Filter  53  is coupled to switch  61  and to peak detector  62 . When switch  61  is closed, the output signal from filter  53  is coupled to the line output (not shown) of the telephone. When switch  61  is open, the output signal from filter  53  is attenuated. Switches  61  and  67  are preferably in the form of a variable gain amplifier rather than a switching device. This not only provides greater flexibility in the circuit, it also avoids transients. 
     Filter  65 , from a second channel, has substantially the same pass band as filter  53  and is coupled to peak detector  66  and to peak detector  66 . The output from peak detector  62  is coupled to a first input of comparator  63 . The output from peak detector  66  is coupled to a second input of comparator  63 . The output from comparator  63  oppositely controls switches  61  and  67 ; that is, when switch  61  is closed, switch  67  is open and vice-versa. Thus, the louder signal in each channel prevails and is coupled to an output. 
     It is assumed that the signals in the two channels are comparable in power but differ in the spectral distribution of the power. If the signal in one channel is substantially greater than the signal in the other channel, all the filters might be assigned to the louder channel and operation would become half-duplex rather than full duplex. Additional logic (not shown) ran be used to prevent more than a certain number of adjacent filters from being set to a given channel. For example, one can provide that no more than two adjacent filters can be allocated to a given channel. 
       FIGS. 5 and 6  together illustrate a telephone constructed in accordance with a preferred embodiment of the invention.  FIG. 5  is a block diagram of a first channel, extending from microphone  71  to line output  72 , and  FIG. 6  is a block diagram of a second channel, extending from line input  73  to speaker output  74 . 
     Sound incident upon microphone  71  is converted into an electrical signal and coupled to weighting filter  76 . Weighting filter  76  reduces the amplitude of low frequency signals to provide a more even energy distribution among the bands. Filter  76  can also be used to correct for non-linearities in the frequency response of microphone  71 . The output from filter  76  is coupled to a first plurality of band pass filters, e.g. one-third octave filters. Much of the apparatus is duplicative and only one band is described. 
     Band pass filter  77  is coupled to filter  76  and to amplitude detector  78 , which, for example, includes a rectifier and a low pass filter. More complex amplitude detectors can be used instead. The output from amplitude detector  78  is coupled to sample and hold circuit  79 , which provides a stable signal for controller  81 . 
     Weighting filter  83  ( FIG. 6 ) receives signals from line input  73  and is coupled to a second plurality of band pass filters. Band pass filter  84  is coupled to filter  83  and to amplitude detector  85 . The output from amplitude detector  85  is coupled to sample and hold circuit  86 . Controller  81  receives the signals from all the sample and hold circuits and contains the logic for comparing the amplitudes of the signals in each band in each channel. The logic can be fixed or programmable. 
     In  FIG. 5 , controller  81  is coupled to the control inputs to multiplex circuit  91 . Each band pass filter, such as filter  77 , has an output coupled to a signal input of multiplex circuit  91 , which has a plurality of signal output lines coupled to summation circuit  92 . The output of summation circuit  92  is coupled to de-weighting filter  93 , which as the inverse frequency response of filter  76 . The output of de-weighting filter  93  is coupled to line output  72 . 
     In  FIG. 6 , controller  81  is coupled to the control inputs to multiplex circuit  96 . Each band pass filter, such as filter  84 , has an output coupled to a signal input of multiplex circuit  96 , which has a plurality of signal output lines coupled to summation circuit  97 . The output of summation circuit  97  is coupled to de-weighting filter  98 , which as the inverse frequency response of filter  83 . The output of de-weighting filter  83  is coupled to speaker output  74 . 
     With all the data flowing into controller  81 , there are a number of combinations of filters that could be implemented in accordance with the invention. In one embodiment, the filters having substantially the same center frequency in each channel are paired and the output from only one filter in each pair is allowed. The loudest signal in a channel is found, then the loudest signal among the remaining filters in the other channel, and so on until all filters are allocated. 
     For example, filter  101  ( FIG. 5 ) and filter  102  ( FIG. 6 ) have substantially the same center frequency. If filter  101  produces the loudest signal in channel A, i.e. the microphone channel, then the output from filter  101  is coupled to summation circuit  92  by multiplex circuit  91 . Controller  81  then looks for the loudest signal in channel B, the speaker channel, ignoring the output from filter  102 . Assuming that this is the output from filter  103 , the filter is coupled to summation circuit  97  by multiplex circuit  96  and the output from filter  104  is ignored. The process continues, ignoring previously allocated filters, until the ten bands are allocated between the two channels. 
     In a second embodiment of the invention, the loudest signal in either channel is found, then the next loudest in the same channel, and so on until five bands are allocated. As each band is allocated, the corresponding filter in the other channel is attenuated. After five bands are allocated, the remaining bands are attenuated in the channel that had the loudest signal. Thus, each channel is allocated five bands, i.e. half the number of bands in each channel. 
     In a third embodiment of the invention, the bands are allocated as in either of the first two embodiments with the additional requirement that no more than two filters in adjoining bands in a given channel are enabled simultaneously. 
     In a fourth embodiment of the invention, bands  1 ,  3 ,  5 ,  7 , and  9  in one channel are compared with the corresponding bands in the other channel. The channel having the larger signal in the majority of the bands is assigned all the odd bands and the other channel is assigned all the even bands. 
     Other combinations may be better suited to a particular application. For one-third octave or smaller filters, the invention enables one to provide full duplex operation for three parties to a call, although there will be some noticeable signal degradation due to the reduction in spectral content. The logic for implementing the embodiments is well within the capabilities of one of ordinary skill in the art, whether fixed logic or programmable logic is used. 
     The allocation process is not time consuming and can easily be repeated every fifty milliseconds or so. In accordance with another aspect of the invention, controller  81  determines whether or not sounds are repetitive or relatively continuous and, therefore, not speech. In ordinary speech, the vocal chords are not used continuously; e.g. “z” is a vocal “s”, “v” is a vocal “f”. The letters “s” and “f” do not use the vocal chords but are fricatives, produced in the front of the mouth. Thus, a relatively continuous sound is likely to be noise rather than speech. Controller  81  attenuates the output from the particular filter producing the repetitive or continuous sound and removes the band from further analysis, thereby reducing noise. This can be easily done by adding more sample and hold circuits per filter or by logic within controller  81 . 
     If additional sample and hold circuits were used, a number of sample and hold circuits are coupled to a given filter and updated at different times; e.g. at intervals of one quarter of a second. A signal having a duration greater than several sample times indicates a repetitive noise or a relatively constant sound. 
     If additional logic were used, the outputs from the sample and hold circuits are compared with a reference value to produce either a logic 1 or a logic 0, depending upon magnitude. The ten filters in each channel produce a ten bit word that is stored and compared with later obtained words, e.g. by performing a logic AND operation on the bits. Repetitive or relatively continuous noise would produce a logic 1 at the same bit location in each word, which would survive successive AND operations. 
     Other techniques can be used instead. The point is that, in accordance with the invention, relatively simple circuitry can perform sophisticated signal processing to improve the clarity and fidelity of speech in a difficult acoustic environment. 
     The invention thus provides an improved band pass filter system in which each band is chosen individually depending upon the spectral content of the applied signal. Full duplex communication is optimized for the voice characteristics of the respective speakers. A band pass filter system can emulate a comb filter initially and then adapt to the spectral content of each channel. 
     Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, although described in conjunction with a 2:1 multiplexer, other ratios can be used instead, e.g. 1:1 or 3:1. Multiplex circuit  91  includes variable gain amplifiers to attenuate, rather than switch, the outputs of some filters. Thus, a persistent sound in a band may be interpreted as noise but the output from the band pass filter is partially attenuated rather than blocked, thereby allowing some speech components to pass when they occur.