Abstract:
A method, apparatus, and system for reducing audio signal noise in a communication system includes, biasing ( 214 )an audio signal noise level beyond a transmission device ( 200 ) operating range to condition an audio signal intended to be received by a receiving device ( 400 ) having an operating range including the audio signal noise level.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to the field of audio communications and, more specifically, to the reduction of noise in an audio signal.  
         BACKGROUND OF THE INVENTION  
         [0002]    Audio communication systems having transmitting and receiving devices are commonly used in applications such as cordless telephones, cellular telephones, corded telephones, and many other two-way communication devices.  
           [0003]    Conventional cordless telephone devices utilize a stationary base unit and a portable handset unit. The base unit is physically and electrically connected to both a source of electrical power and to the telephone company central office. The portable handset unit is wireless and in general communicates with the base unit via a radio frequency link to in turn communicate with an outside party via the telephone company hardwire system. Voice signals from a far party (i.e., an outside party) are received from the hardwire telephone lines by the base unit and transmitted over the RF link to the handset to complete the two-way conversation. Additionally, when a near party (i.e., the party using the handset) speaks into the microphone, a small amount of his own voice is present in the earpiece as a result of a proportion of the speech signal received by a microphone in the portable handset unit being fed back to the output signal produced by the base unit and supplied to the earpiece of the portable handset unit. This is commonly referred to as sidetone. This is a desirable feature in a telephone because it is interactive and gives the user a feeling that the telephone is “alive” and therefore functioning properly. However, the implementation of sidetone in telecommunication systems can introduce noise or static into the output signal produced by the base unit and supplied to the earpiece of the portable handset unit. A major component of this noise in systems implementing sidetone is derived from the audio acoustic noise surrounding the microphone on the portable handset unit. The audio noise is picked up by the microphone on the portable handset unit and transmitted to the base unit. The noise is then amplified by the base unit and transmitted to the far end of the telephone line through the telephone company central office. Meanwhile this noise is transmitted through the audio path (sidetone) to the earpiece of the portable handset unit. The result is a static voice quality signal in the earpiece of the portable handset unit and on the side of the far end talker as well.  
         SUMMARY OF THE INVENTION  
         [0004]    The invention comprises a method, apparatus, and system for reducing audio signal noise in communication systems.  
           [0005]    A method according to one embodiment of the invention for reducing audio signal noise in a communication system includes, determining the level of the audio signal noise, and biasing the audio signal noise level beyond a transmission device operating range to condition the audio signal intended to be received by a receiving device having an operating range including the audio signal noise level.  
           [0006]    In another embodiment of the present invention, a transmission device for transmitting audio signals includes a pre-amplifier, for biasing an audio signal noise level of the audio signals beyond a compander operating range, and the compander for compressing the dynamic amplitude range of the audio signals.  
           [0007]    In another embodiment of the present invention, a communication system includes at least one pre-amplifier, for biasing an audio signal noise level of audio signals beyond an operating range of at least one transmission device, the at least one transmission device for transmitting the audio signals, the at least one transmission device including a first compander for compressing the dynamic amplitude range of the audio signals, and at least one receiving device for receiving the audio signals from the at least one transmission device, the at least one receiving device including a second compander for expanding the compressed audio signals back to their full dynamic range. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0009]    [0009]FIG. 1 depicts a block diagram of an embodiment of a cordless telephone system;  
         [0010]    [0010]FIG. 2 depicts an embodiment of a transmitting circuit, suitable for use in the portable handset unit of the cordless telephone system of FIG. 1;  
         [0011]    [0011]FIG. 3 graphically depicts a response curve of a typical 2:1 compressor suitable for use in the transmitting circuit of the portable handset unit of the cordless telephone system of FIG. 1;  
         [0012]    [0012]FIG. 4 depicts an embodiment of a receiving circuit suitable for use in the base unit of the cordless telephone system of FIG. 1;  
         [0013]    [0013]FIG. 5 graphically depicts a response curve of a typical 1:2 expander suitable for use in the receiving circuit of the base unit of the cordless telephone system of FIG. 1; and  
         [0014]    [0014]FIG. 6 graphically depicts a response curve of a typical 2:1 compressor suitable for use in the transmitting circuit of the portable handset unit of the cordless telephone system of FIG. 1, operating on an audio signal conditioned in accordance with the present invention. 
     
    
       [0015]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    The present invention will be described within the context of a cordless telephone system and associated devices. However, it will be appreciated by those skilled in the art that the subject invention may be advantageously employed in any system having transmitting and receiving devices wherein a compander is implemented. Thus, it is contemplated by the inventors, that the subject invention has broad applicability beyond the cordless telephone system described herein. The invention may be used to provide a means for reducing noise in an audio signal transmitted from a transmission device and supplied to a receiving device.  
         [0017]    [0017]FIG. 1 depicts a block diagram of an embodiment of a cordless telephone system  100 . The cordless telephone system  100  includes a base unit  120  and a portable handset unit  110 . The base unit  120  is physically and electrically connected to both a source of electrical power and to a telephone company hardwire system. The portable handset unit  110  is wireless and in general communicates with the base unit  120  via a radio frequency link to in turn communicate with a caller connected to the other end of the telephone company hardwire system (herein referred to as a far end talker). Voice signals from the far end talker are received from the hard-wire telephone lines by the base unit  120  and transmitted over the RF link to the portable handset unit  110 . Voice signals from the microphone  150  of the portable handset unit  110  are transmitted to the base unit  120 . A portion of the voice signals from the microphone  150  of the portable handset unit  110  is routed, by the base unit  120 , to the hard-wire telephone line. The other portion of the voice signal is sent back to the earpiece  160  of the portable handset unit  110  through a sidetone circuit. This completes the two-way conversation. The portable handset unit  110  of the cordless telephone system  100  includes a compander  130 . The compander  130  of the portable handset unit  110  includes a compressor  131  and an expander  132  as a part of a transmitting circuit. The base unit  120  of the cordless telephone system  100  also includes a compander  140 . The compander of the base unit  120  includes a compressor  141  and an expander  142  as a part of a receiving circuit.  
         [0018]    [0018]FIG. 2 depicts an embodiment of a transmitting circuit  200 , suitable for use in the portable handset unit  110  of the cordless telephone system  100  of FIG. 1. The transmitting circuit  200  of the portable handset unit  110  includes a microphone  150 , an amplifier  214 , a compander  130  including a compressor  131  and an expander  132 , an audio amplifier  215 , a modulation block  216  (illustratively a frequency inversion scrambler), and an output audio amplifier  218 .  
         [0019]    An audio signal originates in the microphone  150  in the transmitting circuit  200  of the portable handset unit  110 . The audio signal then propagates through the compressor  131  of the transmitting circuit  200 . The microphone amplifier  214  adjusts the level of audio signal to be processed by the compressor  131 . The compressor  131  compresses the dynamic amplitude range of the audio signal from the microphone  150  in preparation for transmission across a narrow-band transmission link. The signal then propagates through the audio amplifier  215  to adjust the audio signal to a standard level. Subsequently, the signal is processed through a frequency inversion scrambler  216  that inverts the frequency spectrum. Frequency inversion is optional but useful in cordless telephone systems to add privacy of transmitted signals. The inverted frequency spectrum is buffered through an amplifier  218  for providing the output signal TX OUT. The output signal TX OUT is FM modulated and transmitted by radio link to a receiving circuit in the base unit  120  of the cordless telephone system  100  of FIG. 1.  
         [0020]    In cordless telephone systems, the audio signal from a microphone on a portable handset unit includes not only the voice of the party speaking into the microphone (herein referred to as a near end talker), but also any ambient sound picked up by a microphone within the dynamic range of a compander of a transmitting circuit in a portable handset unit. This extraneous ambient sound noise is referred to as audio signal noise. If audio signal noise is transmitted by a transmitting circuit of a portable handset unit to a receiving circuit in a base unit of a cordless telephone system, the result is a degradation in the quality of an audio signal not only in an earpiece of a portable handset unit through a sidetone circuit of the base unit, but also in the far end telephone line.  
         [0021]    [0021]FIG. 3 graphically depicts a response curve of a typical 2:1 compressor suitable for use in the transmitting circuit  200  of the portable handset unit  110  of the cordless telephone system  100  of FIG. 1. FIG. 3 illustrates how audio signal noise in an audio signal is compressed by the compressor  131  of the compander  130  of the transmitting circuit  200  of the portable handset unit  110 . The dynamic range of the compressor  131  in FIG. 3 is −80 dBV to 0 dBV. The normal sound level of an audio signal received by the microphone  150  of the portable handset unit  110  is typically 94 dB SPL (approximately −40 dBV). Additionally, the inventors have determined that the audio signal noise component of an audio signal of a microphone of a portable handset unit as in the cordless telephone system  100  of FIG. 1, is 64 dB SPL (approximately −70 dBV). It will be appreciated by those skilled in the art that the audio signal noise level of cordless telephone systems can be readily measured by the microphone of a portable handset unit or by any other means known to those skilled in the art. Thus, it is evident that both the normal sound level (−40 dBv) and the audio signal noise level (−70 dBv) are included within the dynamic range of the compressor  131  in the portable handset unit  110 . As such, the −40 dbV normal sound signal and the −70 dBV audio signal noise signal are compressed to −20 dBV and −35 DBV, respectively, and are transmitted to an expander in a receiving circuit of the base unit  120  through RF link.  
         [0022]    [0022]FIG. 4 depicts an embodiment of a receiving circuit  400 , suitable for use in the base unit  120  of the cordless telephone system  100  of FIG. 1. The compressed and FM modulated signal from the transmitting circuit  200  of the portable handset unit  110  is applied as RF In to the receiving circuit  400  of the base unit  120 . The receiving circuit  400  of the base unit  120  includes a demodulation block  410  (illustratively an FM demodulator), a compander  140  including a compressor  141  and an expander  142 , and an output audio amplifier  414 .  
         [0023]    The receiving circuit  400  in the base unit  120  receives the compressed and FM modulated signal from the transmitting circuit  200  of the portable handset unit  110  and propagates the signal through the demodulation block  410 . As discussed above, the audio signal from the microphone  150  of the portable handset unit  120  includes not only the voice of the near end talker, but also the audio signal noise picked up by the microphone  150  and compressed by the compressor  131  of the transmitting circuit  200 . Through the FM demodulator  410 , the signal is re-inverted back to its original frequency spectrum. The FM demodulator  410  is optional but useful in cordless telephones where, in combination with the frequency inversion scrambler  216  of the transmitting circuit  200  of the portable handset unit  110  provides privacy of the conversation between conversing parties. The output signal of the FM demodulator  410  is applied to the expander  142  of the receiving circuit  400  of the base unit  120 . The expander  142  decompresses the narrow-band compressed signal RF In of the transmitting circuit  200  of the portable handset unit  110  to return it to its full dynamic range for a far end talker to hear the received voice signal. The output signal of the expander  142  is then amplified by the output amplifier  414  to provide the voice signal from the near end talker to the far end talker. The signal is subsequently transmitted over conventional telephone lines to the far end talker.  
         [0024]    [0024]FIG. 5 graphically depicts a response curve of a typical 1:2 expander suitable for use in the receiving circuit  400  of the base unit  120  of the cordless telephone system  100  of FIG. 1. FIG. 5 illustrates how the compressed signals transmitted from the portable handset unit  110  are expanded at the proportion of 1:2. As depicted, the input dynamic range of the 1:2 expander  142  of FIG. 4 is −40 dB to 0 dB. As such, normally the compressed −20 dBV normal sound signal and the compressed −35 dBV audio signal noise discussed above, are transmitted into the expander  142  of the base unit  120  through RF link from the portable handset unit  110  and expanded back to −40 dBV and −70 dBV by the expander  142  in the base unit  120 . This results in a degradation of the quality of an audio signal not only in the earpiece  160  of the portable handset unit  110  through a sidetone circuit of the base unit  110 , but also in the far end telephone line.  
         [0025]    In accordance with the present invention, the audio signal from the microphone  150  of the portable handset unit  110  is biased by an amplifier such that the dynamic range of the compressor  131  does not include the level of audio signal noise picked up by the microphone  150  and as such is not compressed by the compressor  131  in the transmitting circuit  200  of the portable handset unit  110  and is not transmitted to a receiving circuit of the base unit  120 . Briefly stated, the gain of the amplifier  214  of the transmitting circuit  200  of the portable handset unit  110  is adapted to attenuate by 20 dB; that is −40 dBV normal sound signal and −70 dBV audio signal noise become −60 dBV and −90 dBV respectively, while maintaining the dynamic range of the compressor  131  at −80 dBV to 0 dBV. Thus the −90 dBV audio signal noise is out of the operating range of the compressor  131  of the transmitting circuit  200  of the portable handset unit  110 .  
         [0026]    [0026]FIG. 6 graphically depicts a response curve of a typical 2:1 compressor suitable for use in the transmitting circuit  200  of the portable handset unit  110  of the cordless telephone system  100  of FIG. 1, operating on an audio signal conditioned in accordance with the present invention. The gain of the amplifier  214  of the transmitting circuit  200  of the portable handset unit  110  is adapted to produce attenuations in the normal sound signal and the audio signal noise of −20 dBv. The normal sound signal and audio signal noise become −60 dBV and −90 dBV, respectively. Since the dynamic range of the compressor  131  is still −80 dBV to 0 dBV, the compressor  131  would not work effectively at the level of the audio signal noise, −90 dBv. That is, the audio signal noise would not be compressed at the proportion of 2:1, it would be between 2:1 and 1:1, depending on the specification and performance of the compressor  131 . As such, the output of the compressor  131  for the normal sound signal and the audio signal noise are −30 dBV and −50 dBV, respectively.  
         [0027]    After compressing, the normal sound signal, −30 dBv, and the audio signal noise, −50 dBv, are compensated +10 dB by the audio amplifier  215  of the transmitting circuit  200  of the portable handset unit  110  for standard modulation. As such, the −30 dbV normal sound signal and the −50 dBV audio signal noise are compensated to −20 dBV and −40 DBV, respectively. The compressed and compensated signals are then modulated by the modulation block  216 , and transmitted to an expander in the base unit  120  through RF link. As mentioned above and with reference to FIG. 4, the input dynamic range of a typical expander in a receiving circuit of the base unit  120  has an input dynamic range of −40 dBV to 0 dBV and 1:2 proportion. As such, the signal comprising a range of −20 dBV to −40 dBV transmitted from the transmitting circuit  200  of the portable handset unit  110  is expanded to −40 dBV and −80 dBV. Comparing to the −40 dBV signal and −70 dBV audio signal noise discussed above with reference to FIG. 3, there is a 10 dB audio signal noise attenuation in the signal transmitted to the base unit  120 . The attenuation of the audio signal noise results in better audio signal quality in the receiver of the far end of the telephone line.  
         [0028]    By the same principle, the expander  132  in the portable handset unit will also experience the same 10 dB audio signal noise attenuation in the signal transmitted from transmitting circuit  200  of the portable handset unit, resulting in better audio quality in the earpiece  160  of the portable handset unit  110  via the sidetone circuit of the base unit  120 . Additionally, by biasing the compressor  141  of the base unit  120  in a substantially similar manner as described above for the compressor  131  of the portable handset unit  1   10 , better audio quality in the earpiece  160  of the portable handset unit  110  can be achieved.  
         [0029]    It will be appreciated by those skilled in the art that a cordless telephone system or the like can be advantageously adapted to configure the compressor of either a portable handset unit or a base unit, or both, or in any combination, and still be within the bounds of the present invention.  
         [0030]    While the forgoing is directed to some embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the invention is to be determined according to the claims, which follow.