Abstract:
An audio processor for two way communication includes a signal generator for producing test signals coupled to selected test points in the audio processor. An echo canceling circuit and a voice detection circuit within the audio processor provide data representing the response of the audio processing circuit to said test signals. The test signals include a single tone signal and a sweep frequency signal. Data from the test is used for adjusting the audio processor according to the results of the tests.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to a telephone having a loudspeaker and having circuitry for echo cancellation and noise reduction and, in particular, to circuitry for calibrating such a telephone. 
         [0002]    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 (see  FIG. 1 ), speakerphones (see  FIG. 2 ), and hands-free kits (see  FIG. 3 ). For the sake of simplicity, the invention is described in the context of telephones, and in particular a hands-free kit, but can be used in any two way communication system; e.g. intercoms in banks, fast food restaurants, or intensive care units in hospitals. In other words, reference to a hands-free kit is for the sake of readability and is not meant as a limitation on use of the invention. 
         [0003]    As used herein, “loudspeaker” refers to the transducer itself and is not intended to imply any particular size, power rating, or type of enclosure. Nor is the term intended to imply any particular mechanism; e.g. electromagnetic, electrostatic, or piezoelectric. The term is simply used to distinguish from a “speaker,” a person who may be speaking. 
         [0004]    There are many sources of noise in a telephone system. Some noise is acoustic in origin while other noise is electronic, from the telephone network, for example. As used herein, “noise” refers to any unwanted sound, whether the unwanted sound is periodic, purely random, or somewhere in-between. As such, noise includes background music, voices of people other than the desired speaker, tire noise, wind noise, and so on. As thus broadly defined, noise could include an echo of the speaker&#39;s voice. However, echo cancellation is treated separately in a telephone. 
         [0005]    In addition to noise, the electrical and physical characteristics of the system can affect or “color” the sound of a person&#39;s voice. It has long been known in the art to provide some sort of calibration, for headsets, e.g. U.S. Pat. No. 4,788,708 (Hendrix), and for speakerphones, e.g. U.S. Pat. No. 4,887,288 (Erving). The Hendrix patent discloses a stand alone unit for testing headsets. The test stimuli and responses are all generated externally. This means that the device under test and test station must be physically near each other, which is not always convenient. Also, the test conditions are usually different from operating conditions. 
         [0006]    The Erving patent describes a speakerphone with a self-calibration circuit included in the speakerphone. The speakerphone disclosed in the Erving patent is voice activated; i.e. the speakerphone does not operate in full duplex. 
         [0007]    In view of the foregoing, it is therefore an object of the invention to provide a method and apparatus for testing or calibrating a hands-free kit. 
         [0008]    Another object of the invention is to provide a method and apparatus for testing or calibrating hands-free kit by way of a computer. 
         [0009]    A further object of the invention is to test or calibrate a hands-free kit by generating stimuli and sensing responses with the device under test itself. 
         [0010]    Another object of the invention is to provide a method and apparatus for tuning the mechanical elements of a hands-free kit. 
         [0011]    A further object is to provide a method and apparatus for testing a hands-free kit remotely, such as by wireless interface. 
       SUMMARY OF THE INVENTION 
       [0012]    The foregoing objects are achieved in this invention in which an audio processor for two way communication includes a signal generator for producing test signals coupled to selected test points in the audio processor. An echo canceling circuit and a voice detection circuit within the audio processor provide data representing the response of the audio processing circuit to said test signals. The test signals can include a single tone signal, multi-tone signals, a sweep frequency signal, white noise and/or recorded voice signals. Data from the tests is used for adjusting the audio processor according to the results of the tests. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: 
           [0014]      FIG. 1  is a perspective view of a desk telephone; 
           [0015]      FIG. 2  is a perspective view of a conference phone or a speakerphone; 
           [0016]      FIG. 3  is a perspective view of a hands-free kit; 
           [0017]      FIG. 4  is a perspective view of a cellular telephone (“cellphone”); 
           [0018]      FIG. 5  is a block diagram of the major components of a cellular telephone; 
           [0019]      FIG. 6  is a detailed block diagram of an audio processing circuit constructed in accordance with a preferred embodiment of the invention; 
           [0020]      FIG. 7  illustrates the effect known as echo return loss; 
           [0021]      FIG. 8  is a block diagram of a calibration system constructed in accordance with a preferred embodiment of the invention; 
           [0022]      FIG. 9  is a block diagram of a calibration system constructed in accordance with an alternative embodiment of the invention; 
           [0023]      FIG. 10  is a flowchart of a process for calibrating a hands-free kit in accordance with the invention; and 
           [0024]      FIG. 11  is a screen shot of the second step illustrated in  FIG. 10 . 
       
    
    
       [0025]    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 can be interpreted as hardware, software, e.g. a flow chart or an algorithm, 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 
       [0026]    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 desk telephone including base  10 , keypad  11 , display  13  and handset  14 . As illustrated in  FIG. 1 , the telephone has speakerphone capability including loudspeaker  15  and microphone  16 . 
         [0027]      FIG. 2  illustrates a conference phone or speakerphone such as found in business offices. Telephone  30  includes microphone  31  and loudspeaker  32  in a sculptured case. Telephone  30  may include several microphones, such as microphones  34  and  35  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). 
         [0028]      FIG. 3  illustrates what is known as a hands-free kit for providing audio coupling to a cellular hands-free kit, illustrated in  FIG. 4 . Hands-free kits come in a variety of implementations but generally include powered loudspeaker  36  attached to plug  37 , which fits an accessory outlet or a cigarette lighter socket in a vehicle. A hands-free kit also includes cable  38  terminating in plug  39 . Plug  39  fits the headset socket on a cellular telephone, such as socket  41  ( FIG. 4 ) in cellular telephone  42 . 
         [0029]    In a sense, a hands-free kit is a special kind of speakerphone and comments relating to one should not be interpreted as excluding the other unless referring to a unique characteristic. A hands-free kit typically includes a volume control and some control switches, e.g. for going “off hook” to answer a call. A hands-free kit may include a visor microphone (not shown) that plugs into the kit. Some hands-free kits use “wireless” or RF signals, such as the “BlueTooth®” interface, to couple to a telephone. 
         [0030]      FIG. 5  is a block diagram of the major components of a cellular telephone. Typically, the blocks correspond to integrated circuits implementing the indicated function. Microphone  61 , speaker  62 , and keypad  63  are coupled to signal processing circuit  64 . Circuit  64  performs a plurality of functions and is known by several names in the art, differing by manufacturer. For example, Infineon calls circuit  64  a “single chip baseband IC.” QualComm calls circuit  64  a “mobile station modem.” The circuits from different manufacturers obviously differ in detail but, in general, the indicated functions are included. 
         [0031]    A cellular telephone includes both audio frequency and radio frequency circuits. Duplexer  65  couples antenna  66  to receive processor  67 . Duplexer  65  couples antenna  66  to power amplifier  68  and isolates receive processor  67  from the power amplifier during transmission. Transmit processor  69  modulates a radio frequency signal with an audio signal from circuit  64 . In non-cellular applications, such as speakerphones, there are no radio frequency circuits and signal processor  64  may be simplified somewhat. Problems of echo cancellation and noise remain and are handled in audio processor  70 . 
         [0032]      FIG. 6  is a block diagram of audio processing circuit  71 , constructed in accordance with a preferred embodiment of the invention. The following describes signal flow through the transmit channel, from  MIC  input  72  to  LINE OUT    74 . The receive channel, from  LINE IN    76  to  SPKR  output  78 , works in the same way. 
         [0033]    A new voice signal entering input  72  may or may not be accompanied by a signal from output  78 . The signals from input  72  are digitized in A/D converter  81  and coupled to summation network  82 . There is, as yet, no signal from echo canceling circuit  83  and the data proceeds to non-linear processor  84 , which is initially set to minimum attenuation. 
         [0034]    The output from non-linear processor  84  is converted back to analog form by D/A converter  87 , amplified in amplifier  88 , and coupled to output  74 . Data from the two VAD circuits is supplied to control  90 , which uses the data for controlling echo elimination and other functions. Circuit  83  reduces acoustic echo and circuit  91  reduces line echo. The operation of these last two circuits is known per se in the art. 
         [0035]    In accordance with the invention, audio processing circuit  71  includes signal generator  85  that is selectively coupled to one or more test points within audio processing circuit  71 . The test points are represented by a “+” sign within a circle in  FIG. 6  and signal generator  85  is shown coupled to test point  86 . Signal generator  85  is controlled by control circuit  90 . 
         [0036]    In accordance with another aspect of the invention, signal generator  85  can be the same circuit used for generating DTMF (dual tone multi-frequency) signals or a separate circuit, such as a white noise generator. Signal generator  85  generates tones for testing, including a sweep frequency signal for some tests. A sliding tone is obtained by sequentially changing data in a register to produce progressively increasing, or decreasing, pitch. The change is incremental, not continuous, but the incremental change is sufficiently small not to matter. Similarly, a sine wave is approximated in a digital circuit by incremental changes in amplitude. An internal linear feedback shift register can be used as a psuedo-random, white noise generator. 
         [0037]    In accordance with another aspect of the invention, the audio processor is used to test the mechanical and acoustical aspects of a hands-free kit. This enables a system designer to optimize the performance of the enclosure and transducers in the enclosure, typically permitting a higher degree of full duplex operation. 
         [0038]    The mechanical and acoustical aspects include loudspeaker response and distortion, microphone response, echo to near end speech ratio (ENR), acoustic echo return loss (ERL), and line interface characteristics (if applicable). These tests are important and desired because the tests can identify mechanical or acoustic problems that may prevent the system from achieving acceptable performance. The tests are also used to optimize voice quality and obtain maximum loudness without significant distortion. The process will enable a customer to achieve a higher level of system performance by helping him to improve the design of his enclosure and the selection of his transducers. 
         [0039]    As illustrated in  FIG. 7 , Echo Return Loss (ERL) is the amount of coupling, represented by arrow  93 , from loudspeaker  94  to microphone  95  in speakerphone  96 . The coupling can be acoustical, mechanical or electrical. An ERL that is flat across the frequency range of interest is preferred. Spikes in the ERL can result from case rattles, the frequency response of the loudspeaker, or other non-linearities. ERL can be improved by separating the microphone and loudspeaker as much as possible, aiming the loudspeaker away from microphone, reducing mechanical coupling, e.g. by soft mounting the loudspeaker and microphone, sealing all air paths from the microphone to the loudspeaker, and by electronically compensating for transducer response. 
         [0040]    Echo to Near end speech Ratio (ENR) is the ratio of Echo Power to Near End Speech Power as measured at the microphone and represented by arrow  98 . Improved ENR will directly improve performance during double talk (both parties speaking). The ENR measurement is independent of microphone sensitivity or preamp gain. Maximum loudness should be used for an accurate measurement of ENR. Low values of ENR provide satisfactory full duplex performance. If the system does not have a low ENR, ERL should be reduced as described above. Specific values are system dependent and easily determined empirically. Measuring ENR is useful, for example, in diagnosing problems with the plastics, electronics, transducers, or the enclosure of a hands-free kit. 
         [0041]    A repeatable stimulus is desirable. The stimulus can be a sine wave (tone), recorded speech, or white noise. Preferably, one plays a “.wav” file with a range of speech or noise on a computer. Recorded tones could be used also. The stimulus is inherently repeatable because it is recorded, or computer generated. To obtain a relatively standardized voice signal, automated voice menus are recorded as “.wav” files. 
         [0042]    One embodiment of the hardware for calibrating is illustrated in  FIG. 8 . Computer  101  is coupled to hands-free kit  105  by way of adapter  103 . Hands-free kit  105  includes the audio processing circuit illustrated in  FIG. 6 . In one embodiment of the invention, adapter  103  includes a USB (Universal Serial Bus) connection to computer  101  and a two wire serial bus connection to hands-free kit  105 . Hands-free kit  105  and cellphone  102  communicate in the usual manner, as indicated by the (analog) lines interconnecting “ MIC ” and “ HEADSET.”   
         [0043]    The connection through adapter  103  is a control interface for sending commands to hands-free kit  105 . Computer  101  and hands-free kit can also be coupled by bidirectional data bus  106  for injecting test signals. This bidirectional data bus can be wired or wireless, such as “Bluetooth” ®. 
         [0044]    Another embodiment of the hardware for calibrating is illustrated in  FIG. 9 . Computer  101  is coupled to speakerphone  109  by way of adapter  103 . A wireless interface can be used instead of adapter  103 . Speakerphone  109  includes the audio processing circuit illustrated in  FIG. 6 . 
         [0045]    A process for calibrating a speakerphone or hands-free kit is illustrated in  FIG. 10 . In the audio processing circuitry, there are a plurality of registers for storing variable data and default data when the system is first turned on. The data can represent magnitude, thresholds, amplifier gain, filter coefficients, and the like. The hands-free kit being calibrated is able to monitor and to measure the signals applied to it or produced by it. 
         [0046]    Referring to  FIG. 10 , as a first step, the interface is calibrated by adjusting the gains of the inputs and outputs of the audio processor. For example, one measures line output to make sure that the maximum output signal matches the rating for maximum input&#39;signal of the device attached to line output. 
         [0047]    The next step is to measure  ERL , as indicated at block  110 . A “screen shot” of a computer display is illustrated in  FIG. 11 . As indicated in  FIG. 11 , the test is conducted with the loudspeaker operated at full volume (level “F”). This enables one to measure the echo path and to adjust the microphone input to avoid clipping. The room should be quiet while the test takes place. A sweep frequency stimulus is applied to the speaker while monitoring the amplitude of the signal at the MIC input (e.g. with VAD  73 ,  FIG. 6 ). The response is presented graphically on a screen. The user is then advised whether or not to adjust the near side adapt threshold (an internal system parameter). Another step would similarly test for clipping at the MIC input. If clipping is detected, one should reduce case vibration, reduce microphone gain, or reduce loudspeaker gain, preferably in the order listed. 
         [0048]    Referring to block  113  in  FIG. 10 , line ERL is measured. This is the same test as in block  110 , only for the line input instead of the MIC input and using the line output instead of the speaker output. Depending upon the capabilities of the particular device under test, additional tests can be performed using a wireless or wired link to the device. Such tests include adjusting the threshold for switching between half duplex and full duplex mode, adjusting noise cancellation, and adjusting the frequency response of the transmit channel and the receive channel. 
         [0049]    The following data is given by way of example, not as a limit on signal levels, which depend upon the particular system being tested. A digital full-scale sine wave produces a signal level of −4 dB. Any signal above this level will typically be distorted due to saturation. For speech, a peak of −12 dB is a useful the full-scale limit. 
         [0050]    The invention thus provides a method and apparatus for testing or calibrating a hands-free kit by way of a computer. The hands-free kit itself generates stimuli and senses responses. The testing enables one to tune the mechanical elements of a hands-free kit. The testing can be conducted remotely, such as by wireless interface. 
         [0051]    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, the mention of USB and BlueTooth® interfaces is not intended to be exhaustive of the manner in which signals can be coupled to the system under test. Simulating actual conditions as closely as possible is desired. For example, if a transmission line simulator is available, it can be included to simulate various lengths of line between a telephone and a switching station. Computer  101  can be a programmed personal computer or a test apparatus dedicated to calibrating hands-free kits or speakerphones. Signal generator  85  can have plural outputs individually coupled to test points by amplifiers whose gain is adjustable or can be coupled by a multiplex circuit to the test points. Either way, the coupling is selective.