Patent Publication Number: US-6708024-B1

Title: Method and apparatus for generating comfort noise

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to communications, and, more particularly, to a method and apparatus for generating comfort noise in a communications device, such as a cordless telephone. 
     2. Description of the Related Art 
     The telecommunications industry has undergone explosive growth over the past several years. A significant contribution to this growth has been the high demand for radio communication services, such as cordless telephone service, for example. Cordless telephones provide a greater flexibility to users than traditional landline phones by allowing them to move freely, not being tethered to the landline telephone system. 
     A typical cordless telephone system includes a handset unit and a base unit. The base unit is coupled to a telephone line and includes an antenna, a transmitter, and a receiver for communicating via radio frequencies with the handset unit. A local power line generally supplies the power for the base unit. The handset unit includes a speaker and a microphone, and also an antenna, a transmitter and a receiver for likewise communications with the base unit. Typically, the handset unit is powered by at least one battery. This battery is usually charged by the local power line when the handset unit is placed inside a cradle of the base unit. 
     The base and handset units generally communicate through transmission of digital signals. Typically, analog speech signals are digitized and coded before transmission. Speech signals are digitized because digitized signals are less susceptible to channel noise since they may be regenerated, as well as amplified, along the way, thereby reducing the possibility of being corrupted by the transmission system. On the receiving end, digitized signals are decoded and converted back to its analog form. A CODEC (CODing and DECoding device) commonly performs the coding/decoding functions, and sometimes analog-to-digital (A/D) and digital-to-analog (D/A) conversions. Since the base and handset units transmit, as well as receive signals, each unit typically includes a CODEC. 
     To achieve a greater bandwidth, cordless telephone systems employ voice compression algorithms. One popular voice compression algorithm is Adaptive Differential Pulse Code Modulation (ADPCM). The ADPCM scheme takes advantage of a high sample-to-sample correlation that exists in speech waveforms to reduce a transmission bit rate, while preserving an overall signal quality. In the ADPCM scheme, an analog voice signal is converted into digital representation and compressed into a lower bit stream through an encoding process for transmission. 
     Transmitted digitized, compressed signals, however, may not reach the intended destination error free. For example, a transmission from the base unit of the cordless telephone to the handset unit may include an error or errors such that quality of voice is jeopardized. Additionally, the transmission errors may introduce noise that result in undesirable sound, thereby causing discomfort to a listener on the receiving end. 
     The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a method is provided. The method includes receiving a signal, scaling the signal to a preselected value, indicating whether an error occurred during transmission of the signal, and providing the scaled signal as an output signal in response to an indication that the error occurred during transmission. 
     In another aspect of the present invention, an apparatus is provided. The apparatus includes a scaler for receiving a signal and being capable of scaling the signal to a preselected value. The apparatus includes an indicator capable of indicating that an error occurred during transmission of the signal, wherein the scaled signal is provided as an output signal in response to an indication that the error occurred during transmission. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
     FIG. 1 is a simplified block diagram of a communications system in accordance with the present invention; 
     FIG. 2 is a simplified block diagram of one embodiment of the communications system of FIG. 1; 
     FIG. 3 depicts a stylized diagram of a remote unit of the communications system of FIG. 2; 
       5 FIG. 4 illustrates a stylized block diagram of an encoder and decoder that may be employed in the remote unit of FIG. 2; and 
     FIG. 5 illustrates one embodiment of a method in accordance with the present invention that may be implemented in the communications systems of FIGS. 1 and 2. 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     Referring now to the figures, and in particular to FIG. 1, a block diagram of a communications system  100  in accordance with the present invention is illustrated. FIG. 1 includes a first telecommunications device  110  capable of communicating with a second telecommunications device  120  over a connection  130 . The connection  130  may be a wire-line connection or a wire-less connection, depending on the application. In one embodiment, the communications system  100  may include communication between any two telephones or communications within a telephone system, such as between a handset and base station of a cordless telephone system. In an alternative embodiment, the communications system  100  may include communication between any telecommunications devices  110 ,  120  capable of performing substantially an equivalent function of a telephone, which may include, but not limited to, transmitting and/or receiving voice and data signals. Examples of the telecommunications devices  110 ,  120  include any telephone employing a digital signal processor or any data processing system (DPS) utilizing a modem to perform telephony, a television phone, a wireless local loop, a DPS working in conjunction with a telephone, Internet Protocol (IP) telephony, and the like. IP telephony is a general term for the technologies that use the Internet Protocol&#39;s packet-switched connections to exchange voice, fax, and other forms of information that have traditionally been carried over the dedicated circuit-switched connections of the public switched telephone network (PSTN). One example of IP telephony is an Internet Phone, a software program that runs on a DPS and simulates a conventional phone, allowing an end user to speak through a microphone and hear through the DPS speakers. The calls travel over the Internet as packets of data on shared lines, avoiding the tolls of the PSTN. 
     Turning now to FIG. 2, a stylized block diagram of one embodiment of the communications system  100  of FIG. 1 is shown in accordance with the present invention. In the illustrated embodiment, the communications system  100  is a cordless telephone system  140 . Accordingly, the first telecommunications device  110  is a base unit  150  of the cordless telephone system  140 , and the second telecommunications device  155  is a remote unit  155  of the cordless telephone system  140 . The base and remote units  150 ,  155  each include an antenna  160  for communication over a wireless connection  165 . In the illustrated embodiment, the connection  130  (see FIG. 1) is a wireless connection  165 . The base unit  150  is coupled to an external line  170  via a telephone line interface  175  that is affixed to a fixed structure  180 . The fixed structure  180 , for example, may be a wall. The external line  170  may be a public switched telephone network (PSTN) line or a private branch exchange (PBX) line. The base unit  150  is coupled to the external line  170  to provide telephonic services to the remote unit  155 . In accordance with one embodiment, the remote unit  155  includes conventional components (i.e., microphone, speaker, dial keypad, etc.) inherent to cordless phones. Such components are well known to those of ordinary skill in the art and are not discussed herein to avoid unnecessarily obscuring the present invention. 
     The base unit  150  includes a CODEC  185 , and the remote unit  155  includes a CODEC  190  for performing requisite coding and decoding functions. Since the CODECs  185 ,  190  generally perform similar functions, in certain applications the two CODECs  185 ,  190  may be substantially similar. 
     As can be seen in FIG. 3, the disclosed embodiment of the instant invention is described herein with respect to the remote unit  155 . However, it should be appreciated that the instant invention may also be applicable to the base unit  150 . FIG. 3 illustrates a stylized block diagram of one embodiment of the remote unit  155  in accordance with the present invention. The remote unit  155  is capable of establishing a radio communication link with the base unit  150 . In the interest of clarity and to avoid obscuring the invention, only that portion of the remote unit  155  that is helpful in understanding the invention is illustrated. More specifically, FIG. 3 illustrates a receive unit  210  of the remote unit  155  that may be utilized for receiving signals from the base unit  150 . Those skilled in the art will appreciate that the remote unit  155  may also include a transmitting unit (not shown), as well as other logic for implementing other telephonic features such as a caller identification system, for example. Additionally, although the remote unit  155  illustrated in FIG. 3 employs a time division duplex (TDD) architecture, it is envisioned that the remote unit  155  may also employ a frequency division duplex (FDD) architecture without departing from the spirit of the instant invention. 
     The receive unit  210  receives a transmitted radio signal from the antenna  160 , and passes the signal through a first impedance matching filter  212 . The radio signal may comprise a plurality of signals, at least one of which may be carrying a synchronization signal transmitted by the base unit  150 . The first impedance matching filter  212  matches the impedance of the antenna  160  with the impedance of the rest of the receive unit  210 , thereby reducing the signal reflection from the remaining portion of the receive unit  210 . An output signal from the first impedance matching circuit  212  is passed through a first bandpass filter  215 , which filters out the unwanted frequencies from the radio signal. The radio signal is then passed through a first amplifier  220 , and subsequently through a second impedance matching filter  225 . The second impedance matching filter  225  matches the output impedance of the first amplifier  220  to the impedance of the rest of the receiving unit  210 . Although not so limited, in the illustrated embodiment, the first and second impedance matching filters  212 ,  225  have a real 50-ohm impedance. Furthermore, in the illustrated embodiment, the center frequency of the first bandpass filter  215  is 900 MHz, and its band-width is approximately 2 MHz. Those skilled in the art will appreciate that the impedance of the impedance matching filters  212 ,  225 , as well as the center frequency and bandwidth of the first bandpass filter  215 , may vary, depending on the application in which they are employed. 
     The voice signal is then provided from the second impedance matching filter  225  to a second amplifier  230  and then to a mixer  240  (or downconverter). The mixer  240  mixes the incoming signal with a signal generated by a local oscillator  245  and provides an intermediate frequency (IF) signal. The intermediate frequency signal is substantially equal to the difference between the radio frequency signal and the oscillator frequency generated by the local oscillator  245 . The IF signal from the mixer  240  is then provided to a third amplifier  250  and to a second bandpass filter  255 . The output from the second bandpass filter  255  is amplified by a fourth amplifier  260 , passed through a third bandpass filter  265 , amplified by a first limiting amplifier  270 , passed through a fourth bandpass filter  275 , and then amplified by a second limited amplifier  280 . In accordance with one embodiment of the present invention, the second, third, and fourth bandpass filters  255 ,  265 ,  275  are ceramic filters that have a center frequency of approximately 10.7 MHz and a bandwidth that is capable of allowing a channel through. 
     The output signal from the second limited amplifier  280  is provided to a demodulator  284 , which outputs a voltage signal that is proportional to the frequency of the input signal. The demodulator  284  employs a discriminator  286  that allows the demodulator  284  to demodulate a wide bandwidth. The output signal from the demodulator  284  is passed through a low pass filter  288 , which substantially removes unwanted noise from the voltage signal provided by the demodulator  284 . An output of the low pass filter  288  is provided to a comparator  290 , which compares the input signal against a threshold and provides a substantially square output that is then delivered to a controller  292  of the remote unit  155 . 
     The controller  292  may, in one embodiment, control a variety of functions of the remote unit  155 . For example, in the instant embodiment, the controller  292  includes a CODEC  190 , GMSK generator  294 , battery monitor  296  for monitoring usage of a battery  298 , keypad interface  300 , and analog-to-digital converter  302  and digit-to-analog converter  304  for converting analog signals to digital signals, and vice-versa. The CODEC  190 , GMSK generator  294 , battery monitor  296 , keypad interface  300 , and analog-to-digital converter  302  and digit-to-analog converter  304  are well known to those of ordinary skill in the art and are therefore not discussed in detail herein. The term “controller,” as utilized herein, refers to control logic capable of providing one or more desirable functions for the remote unit  155 . Accordingly, in one embodiment the controller  292  may provide fewer functions than the illustrated functions in FIG. 3, and in other embodiments it may provide additional functions not expressly illustrated in FIG. 3, such as a caller identification system (not shown), for example. 
     Turning now to FIG. 4, one embodiment of the CODEC  190  is shown in accordance with the present invention that may be employed by the remote unit  155 . Specifically, the CODEC  190  comprises an ADPCM encoder  305  and decoder  310 , wherein the decoder  310  is imbedded in the encoder  305 . The ADPCM scheme is not described in detail herein, as it is well-known to those skilled in the art. Additionally, it will be appreciated that the instant invention is not limited the ADPCM scheme, but rather may be applicable to other compression schemes as well. 
     In the interest of clarity and to avoid obscuring the invention, only that portion of the CODEC  190  that is helpful in understanding the invention is illustrated. The encoder  305  receives a log-PCM input signal, S(k), and transcodes it to an ADPCM signal, I(k). Generally, a parity check may be performed on the I(k) signal, wherein parity bits associated with the I(k) signal are also transmitted along with I(k) signal. The input signal S(k) is provided to a first input terminal of a signal adder  312 , while an estimate signal, Se(k), of the input signal S(k) is provided to a second terminal of the signal adder  312 , which subtracts the Se(k) signal from the S(k) signal and provides a difference signal, d(k) to an adaptive quantizer  315 . The adaptive quantizer  315  adaptively quantizes the difference signal, d(k). In one embodiment, the difference signal, d(k), may be adaptively quantized by taking the log (base  2 ) of the difference signal, d(k), then normalizing the d(k) signal by the quantization scale factor, y(k), and coding the result, I(k). The quantization scale factor y(k) is generated by an adaptation speed and scale factor estimator  320 . The normalization provides the adaptation to the quantization and is based on past coded samples. In one embodiment, the adaptation is controlled bimodally, and comprises a fast adaptation factor for signals with large amplitude fluctuations (e.g., speech) and a slow adaptation factor for signals which vary more slowly (i.e., data). The adaptation speed and scale factor estimator  320 , based on a speed-control factor, weighs the fast and slow adaptation factors to form a single quantization scale factor. 
     The decoder  310  receives the ADPCM signal, I(k), and transcodes it to a log-PCM signal, Se(k). The decoder  310  includes an inverse adaptive quantizer  325  that uses the I(k) signal to reconstruct a quantized version of the difference signal, Dq(k). The inverse adaptive quantizer  325  uses the same adaptive quantization characteristics as the adaptive quantizer of the encoder  305 . The quantized difference signal, Dq(k), is input to an adaptive predictor  330 , which then computes a signal estimate, Se(k). The Se(k) signal is provided to the signal adder  312 , which then subtracts the Se(k) signal from the next input signal, S(k), to complete the feedback loop. Although not so limited, in the illustrated embodiment, the adaptive predictor  330  makes use of both an all-pole filter (not shown) and an all-zero filter (not shown). The all-pole filter is a second-order filter with constrained adaptive coefficient values designed to match the slowly varying aspects of the speech signal. Since the predictor  330  is particularly sensitive to errors, the predictor  330  makes use of a sixth-order all-zero filter to offer signal stability even with transmission errors. 
     In accordance with the present invention, the decoder  310  includes a comfort noise generator  335 . The comfort noise generator  335  includes a scaler  340 , a noise power estimator  345 , and a multiplexer  350  controlled by a indicator  355 . The CODEC  190  employs a method of FIG. 5 to provide a suitable level of noise during communication between the base unit and remote unit, making the connection appear more alive. The method of FIG. 5 begins at block  405 , where the quantized difference signal, Dq(k), is received. The quantized difference signal, Dq(k), may comprise a plurality of samples. 
     At block  410 , the scaler  340  scales the Dq(k) signal by a scaling constant. The noise power estimator  345  provides the scaling constant to the scaler  340 , after estimating the noise power based on the difference signal, Dq(k). The noise power estimator  345  in one embodiment estimates the instantaneous power as follows: 
      power( k )=0.85*power( k −1)+0.95 * Dq ( k )* Dq ( k ).  (1) 
     where power(k−1) is the instantaneous power value of a previous sample. 
     The scaling constant may be computed once the value of power(k) is determined using the following equation: 
     
       
         scaling_constant=sqrt(0.0001*1/power( k ))  (2) 
       
     
     The scaler  340  generates the scaling constant such that the samples of the Dq(k) signals are below approximately −30 dB, thereby producing comfort level noise. Because the noise level in the quantized Dq(k) signal may vary substantially from one sample to another, the scaler  340 , in conjunction with the instantaneous power value generated by the noise power estimator  345  based on a recursive algorithm, scales the Dq(k) sample to a comfort noise level. In one embodiment, the scaling constant may be obtained from a table, rather than computing equation (2), which requires a division operation. A table having pre-calculated values for given values of power(k) may be utilized to obtain a value for the scaling constant. 
     It should be appreciated that the constants utilized in equation (1), such as 0.85 and 0.95, may vary from one application to another, depending on the specific requirements. Likewise, constant in equation (2), namely 0.0001, may vary, depending on implementation requirements. Equations (1) and (2) may be one of any variety of equations that generate a scaling constant that scales the samples of the quantized difference signal, Dq(k), to a comfort noise level. For the purposes of this invention, a comfort noise level is any level that may not cause substantial discomfort to a user. 
     At block  420 , the indicator  355  indicates whether an error occurred in the received signal during transmission. The indicator  355  in one embodiment may derive its signal from an existing error indicator of the remote unit  155 . In the illustrated embodiment, the indicator.  355  is a parity check logic that identifies any errors in the transmission based on the parity bits that accompany the I(k) signal. The indicator analyzes the parity bits transmitted with the I(k) signal to identify erroneous transmissions. A Telecommunication devices  110 ,  120  (see FIG. 1) typically employ error-indicating logic (not shown) that identifies erroneous transmissions, and, accordingly, the signal from such logic may be utilized for the same purpose as that served by the indicator  355 . 
     At block  430 , the mutliplexer  350  provides the scaled signal from the scaler  340  in response to an indication that the error occurred during transmission. If the indicator  355  indicates no transmission error, then the estimate signal, Se(k) from the adaptive predictor coefficient estimator  330  is provided from the multiplexer  350 . 
     The present invention provides a suitable level of noise for a conversation over the connection  165  without a separate signal generator. That is, no separate generator is required to produce a signal that provides an acceptable level of noise to the connection  165 . Instead, the instant invention scales the received quantized difference signal, Dq(k), to provide the a suitable level of noise to the connection  165 . 
     It is noted that the present invention is not limited to telephony, and, instead, may also be applicable to wireless LAN, wireless telemetry, and any other wireless technology employing ADPCM compression scheme or any other compression schemes. The comfort noise generator  335  (see FIG. 4) may be implemented in hardware, software, or any combination thereof. Additionally, the steps of the method of FIG. 5 may be implemented within a digital signal processor (not shown). 
     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.