Abstract:
A wireless microphone having a split-band audio frequency companding system is disclosed. The companding system includes a compression circuit in which one amplification element is utilized in connection with a number of frequency bands. Each frequency band has a rectifier and filter element associated therewith. High-pass filter elements are utilized in the higher frequency bands of the compression and expander circuits to reduce the transfer of low-frequency signals to the rectifier elements that process the low-frequency signals, thereby reducing undesirable modulations of a variable resistance element associated therewith.

Description:
FIELD OF THE INVENTION 
   This invention generally relates to wireless microphone systems and, more particularly, to a wireless microphone system having a split-band audio frequency companding system that provides improved noise reduction and sound quality with reduced cost. 
   BACKGROUND OF THE INVENTION 
   Companding circuits that include a compressor circuit and an expander circuit may be used to increase the useable dynamic range of a signal that passes through a modulated space. In audio applications, this is done by first compressing the dynamic range of the information signal prior to modulation, and then expanding the dynamic range of the information signal after demodulation. Referring to  FIGS. 1A and 1B , general, schematic representations of a typical compression circuit  10  and a typical expansion circuit  12 , respectively, are shown. 
   In the circuit shown in  FIG. 1A , a rectifier and filter element  14  is used to detect the amplitude of an input signal at terminal  16 . The amplitude information is utilized to control the resistance of a variable resistance element  18  that is provided in the feedback loop of an operational amplifier  20 . This circuitry is arranged so that relatively higher amplitude information generally reduces the resistance of the variable resistance element  18 , while relatively lower amplitude information generally increases the resistance of the variable resistance element  20 . This has the effect, for example, of reducing the gain of amplifier  20  for higher level signals at terminal  16 , and of increasing the gain of amplifier  20  for lower level signals at terminal  16 . Signal processing is continuously done across the frequency spectrum for signals of low amplitudes to signals of high amplitudes. 
     FIG. 1B  is a general, schematic diagram of a typical expander circuit  12  that is used to expand the dynamic range of the signal at terminal  22 . Circuit  12  includes generally the same circuit components that form the compression circuit shown in  FIG. 1A , such components including a variable resistance element  24 , a rectifier and filter element  26 , an operational amplifier  28 , and a feedback resistor  30 . However, the circuit components shown in  FIG. 1B  are rearranged as shown so that the gain of the operational amplifier  28  increases as the amplitude of the signal at terminal  22  increases, and so that the gain of the operational amplifier  28  decreases as the amplitude of the signal at terminal  22  decreases. 
   The problems associated with such compandor circuitry are largely due to the time constant of the integrating filter of the rectifier that forms a portion of the rectifier and filter components  14  and  26  (FIGS.  1 A and  1 B). If the time constant is made relatively large, then amplitude modulation of the higher frequency components of the noise by lower frequency components of the signal can be heard by a user. This is commonly referred to as “breathing,” which is undesirable, especially in high-end audio applications. 
   Signals with a quick rise time often are distorted by typical compandor circuits because, for example, the compressor circuit portion of the compandor circuit may not be able to react fast enough to keep the signal within the linear range of the modulated space. If the time constant is short, “breathing” goes away, but lower frequency signals become distorted due to rectifier ripple. Both of the above-referenced problems are particularly evident in wide band audio implementations such as, for example, wireless microphone applications. 
   Distortion problems can be reduced by establishing separate attack and release time constants for the rectifiers that are used in the compression and expansion circuits. However, there is still a serious compromise in performance that must be made for wide band signals present in high-end audio applications. 
   Various specific compander circuits are known. See, for example, U.S. Pat. No. 4,353,035 that discloses a circuit for compression or expansion of an electrical signal. This patent states that a two-band compander pre-emphasis is carried out during compression and de-emphasis during expansion in the lower frequency range. This patent states that noise suppression is improved by the pre-emphasis and de-emphasis operations. The content of U.S. Pat. No. 4,353,035 is incorporated by reference into this application as if fully set forth herein. 
   In another application, U.S. Pat. No. 4,449,106 discloses a noise reducing apparatus that includes a circuit that processes signals in a plurality of frequency bands. This patent states that the mid and high frequencies are intensified when the signal level is low. This patent also states that the signal level versus noise level ratio in the mid and high frequency ranges is increased with respect to the noise introduced in the transmission path. The content of U.S. Pat. No. 4,449,106 is incorporated by reference into this application as if fully set forth herein. 
   In yet another application, U.S. Pat. No. 5,832,097 discloses a multi-channel synchronous companding system for hearing aids. This patent states that an input signal is directed through a 2:1 compressor, and then is directed through a band splitting filter to divide the signal into a desired number of frequency bands. This patent further states that the divided signals are then passed through expander/compressors to provide selected expansion/compression of each frequency band as a function of the user&#39;s hearing impairment. The content of U.S. Pat. No. 5,832,097 is incorporated by reference into this application as if fully set forth herein. 
   The above-described circuits are suitable for their intended purposes. However, such circuits may not be suitable for a number of applications such as, for example, high end wireless microphone applications. In such applications, a premium is placed on the quality of the audio reproduction. Also, a premium is placed on the ability to manufacture such microphones with reduced cost that allows the manufacturer&#39;s profit margin to be maximized. Furthermore, battery life is a concern in the transmitter portion of typical wireless microphones because the presence of a greater number of active elements increases current drain and correspondingly decreases battery life. 
   SUMMARY OF THE INVENTION 
   It is desirable to provide a wireless microphone having a multi-band audio companding system. One aspect of the present invention is that the companding system includes a compression circuit in which one amplification element is utilized in connection with a number of frequency bands, each frequency band having a rectifier and filter element associated therewith. A further aspect of the present invention is that high-pass filter elements are utilized in the higher frequency bands of the compression and expander circuits to reduce the transfer of low-frequency signals to the rectifier elements that process the high-frequency signals, thereby reducing undesirable modulations of a variable resistance element associated therewith. 
   Providing a wireless microphone with such a companding system has a number of distinct advantages. First, the time constants of the integrating filter of the rectifier utilized in the compression and expansion circuits are individually set with respect to each frequency band that is processed, thereby allowing improved audio reproduction. Second, breathing problems are reduced. Third, problems associated with rectifier ripple are minimized. Fourth, the material costs associated with manufacturing such a wireless microphone are significantly reduced because a smaller number of circuit components are utilized. 

   
     Other features and advantages of the invention will become apparent from the description that follows. 
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a general, schematic representation of a prior art compression circuit; 
       FIG. 1B  is a general, schematic representation of a prior art expansion circuit; 
       FIG. 2  is a general, schematic representation of a wireless microphone system that incorporates a split-band audio frequency companding system that provides improved noise reduction and sound quality; 
       FIG. 3A  is a general, schematic representation of the transmitter portion  34  of the wireless microphone system  32  shown in  FIG. 2 ; 
       FIG. 3B  is a general, schematic representation of the receiver portion  36  of the wireless microphone system  32  shown in  FIG. 2 ; 
       FIG. 4A  is a general, schematic representation of the compressor circuit  52  shown in  FIG. 3A ; 
       FIG. 4B  is a general, schematic representation of the expander circuit  66  shown in  FIG. 3B ; 
       FIG. 5  is a detailed schematic diagram of a specific application of the compressor circuit  66  shown in  FIG. 4A ; 
       FIG. 6  is a detailed schematic diagram of a specific application of the expander circuit shown in  FIG. 4B ; and 
       FIG. 7  is a graph that illustrates the audio performance of the companding circuitry illustrated in  FIGS. 5 and 6  that is utilized in the wireless microphone system shown in FIG.  2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   While the present invention is susceptible of embodiment in various forms, there is shown in the drawings a number of presently preferred embodiments that are discussed in greater detail hereafter. It should be understood that the present disclosure is to be considered as an exemplification of the present invention, and is not intended to limit the invention to the specific embodiments illustrated. 
     FIG. 2  is a general, schematic diagram of a wireless microphone system  32  that incorporates a split-band audio frequency companding system that provides improved noise reduction and sound quality. System  32  includes a transmitter portion  34  and a receiver portion  36 . Transmitter portion  34 , includes a microphone  38 , a transmitter circuit  40 , and an antenna  42  that may be located at a first location when the system  32  is in use. In an exemplary application of the present invention, microphone  36  may be a directional microphone such as, for example, the digital and analog microphone disclosed in U.S. Pat. No. 6,084,973. The content of U.S. Pat. No. 6,084,973 is incorporated by reference into this application as if fully set forth herein. 
   The receiver portion  36  of the wireless microphone system  32  includes an antenna  44  and a receiver circuit  46 . The receiver portion  36  produces an audio level output signal at terminal  48 . 
     FIG. 3A  is a general, schematic representation of the transmitter portion  34  of the wireless microphone system  32  shown in FIG.  2 . Transmitter portion  34  includes an audio frequency amplifier  50  that amplifies the signal provided to it from microphone  38 . A unique compressor circuit  52  compresses the signal provided to it at terminal  52   a  from amplifier  50 , and provides a compressed signal at terminal  52   b  as discussed in greater detail hereafter. Radio frequency oscillator  54 , modulator  56 , and radio frequency amplifier  58  are utilized to provide a modulated and amplified signal to antenna  42  that is broadcast therefrom. It should be understood that any suitable means of modulation such as, for example, amplitude or frequency modulation techniques may be utilized in connection with the present invention. 
     FIG. 3B  is a general, schematic representation of the receiver portion  36  of the wireless microphone system  32  shown in FIG.  2 . Receiver portion  36  includes a radio frequency amplifier  60  that provides an amplified signal to the tuning and amplification circuit  62  that is operatively coupled to a local oscillator  64 . An expander  66  is electrically coupled to the tuning and demodulation circuit  62  via terminal  62   a  to allow an audio output signal to be provided at terminal  48  via terminal  62   b  and audio frequency amplifier  68  as discussed in greater detail hereafter. 
     FIG. 4A  is a general, schematic representation of the compressor circuit  52  shown in  FIG. 2A , while  FIG. 4B  is a general, schematic representation of the expander circuit  66  shown in FIG.  2 B. In the embodiment of the present invention illustrated in  FIGS. 4A and 4B , two frequency bands are utilized to allow low and high frequency signals to be companded separately in two distinct frequency bands. However, it should be understood that any number of frequency bands can be utilized in accordance with the principles of the present invention disclosed in this application. 
   Referring to  FIG. 4A , a low-frequency rectifier  70  and a high-frequency rectifier  72  are utilized for compression purposes. Each rectifier  70  and  72  is provided with separate attack and release time constants as discussed in greater detail hereafter. Two variable resistance elements  74  and  76  are provided in the feedback loop of operational amplifier  78 . A low-pass filter  80  and a high-pass filter  82  are electrically coupled to the inputs of the variable resistance elements  74  and  76 , respectively. A high-pass filter  84  is electrically coupled to the high-frequency rectifier  72  as shown in FIG.  4 A. 
   Compression circuit  52  includes an attack capacitor  86  and a release capacitor  88  that are operatively electrically coupled to the low-frequency rectifier  70 . Capacitors  86  and  88  are optimized for frequencies below the crossover point. An attack capacitor  90  and a release capacitor  92  are operatively electrically coupled to the high frequency rectifier  72  as shown in FIG.  4 A. Capacitors  90  and  92  are optimized for frequencies above the cross-over point. 
   In operation, the low-pass filter  80  causes the low-frequency adjustable resistance element  74  to control the gain of operational amplifier  78  at frequencies below the crossover point. Similarly, the high-pass filter  82  causes the high-frequency adjustable resistance element  76  to control the gain of the operational amplifier  78  at frequencies above the crossover point. High-pass filter  84  reduces the transfer of low-frequency components into the high-frequency rectifier  72  that would create harmonics that would modulate the high frequency adjustable resistance element  76 . Modulation of component  76  in this manner is especially undesirable in high-end audio applications. 
   Referring to  FIG. 4B , the expander circuit  66  includes a low-frequency rectifier  94  and a high-frequency rectifier  96 . Rectifiers  94  and  96  include attack capacitors  98  and  100 , as well as release capacitors  102  and  104 , respectively, that provide separate attack and release time constants for each rectifier. The attack and release capacitors  98  and  102  on low-frequency rectifier  94  are optimized for frequencies below the crossover point, and are equal to the timing capacitors on low-frequency rectifier  70  (FIG.  4 A). Likewise, the attack and release times for high-frequency rectifier  96  are optimized for frequencies above the crossover point, and are equal to the timing capacitors on high-frequency rectifier  72 . 
   Expander circuit  66  also includes two high-pass filters  106  and  108 , a low-pass filter  110 , a low-frequency variable resistance element  112 , a high-frequency variable resistance element  114 , and two operational amplifiers  116  and  118 . Appropriate resistors  120  and  122  are provided in a feedback loop of the operational amplifiers  116  and  118  as shown. 
   In operation, high-pass filter  106  causes the high-frequency variable resistance element  114  to control the gain of operational amplifier  116  above the crossover point. Similarly, the gain of operational amplifier  116  is controlled for low frequencies by the low-frequency variable resistance element  112 . However, the output of operational amplifier  118  is filtered by low-pass filter  110 . Placing low-pass filter  110  after the operational amplifier  118  is advantageous because, for example, a reduction in noise and distortion over placing it before the operational amplifier  118  is obtained. 
   High-pass filter  108  reduces the transfer of low frequency signal components from entering high-frequency rectifier  96 . This provides a number of distinct advantages such as, for example, a reduction in the production of harmonics that would modulate the high-frequency variable resistance element  114  in an undesirable manner. The high and low expanded components are summed via resistors  124  and  126  to create a single output signal at terminal  62   b.    
   The crossover frequency between the two frequency bands utilized in the compressor circuit  52  and the expander circuit  66  is determined by the time constants of the following circuit components: high-pass filter  82 , high-pass filter  84 , high-pass filter  106 , high-pass filter  108 , low-pass filter  80 , and low-pass filter  110 . In the illustrated embodiment of the invention, the time constants of these filter elements are all generally equal to each other. 
     FIGS. 5 and 6  are detailed schematic diagrams of a specific implementation of the compression circuit  52  and the expander circuit  66  shown in FIG.  2 . In the embodiment of the invention illustrated in  FIGS. 5 and 6 , four commercially available programmable analog compander circuits  128 ,  130 ,  132 , and  134  are utilized. A circuit that is preferred for this application is available from Phillips Semiconductors as programmable analog compander circuit model no. SA572. Use of such circuit components provides a number of distinct advantages such as, for example, allowing for separate attack and release timing capacitors. 
   Referring to  FIG. 5 , R 3  and C 4  correspond to the low-pass filter  80  shown in FIG.  4 A. C 12  and a 6.8K resistor that is internal to the circuit  130  form the high-pass filter element  82  shown in  FIG. 4   a . The high pass-filter element  84  ( FIG. 4A ) is formed from C 18  and R 12 . C 6  and C 3  correspond to the attack and release capacitors  86  and  88  (FIG.  4 A), while C 14  and C 16  correspond to attack and release capacitors  90  and  92  (FIG.  4 A). The low-frequency rectifier  70  and the low-frequency variable resistance element  74  ( FIG. 4A ) are internal to compandor circuit  128 , while high-frequency rectifier  72  and the high-frequency variable resistance element  76  ( FIG. 4A ) are internal to the compander circuit  130 . 
   Referring to  FIG. 6 , R 12  and C 23  form the low-pass filter  110  (FIG.  4 B). C 8  and a 6.8K resistor that is internal to the compandor circuit  132  form the high-pass filter  106  (FIG.  4 B). The high pass filter  108  shown in  FIG. 4B  is formed from C 15  and R 7  shown in FIG.  5 . C 13  and C 10  correspond to attack capacitor  100  and release capacitor  104  (FIG.  4 B), while C 24  and C 21  correspond to attack capacitor  98  and release capacitor  102 . Low-frequency rectifier  94  and low-frequency variable resistance element  112  are internal to compander circuit  134 . High-frequency rectifier  96  and high-frequency variable resistance element  114  are internal to compander circuit  132 . Amplifier  136  is a buffer amplifier that is used to drive the expander circuit, while amplifier  138  a summing amplifier. 
     FIG. 7  shows the excellent frequency and amplitude linearity of the companding circuitry shown in  FIGS. 4A and 4B  across the full audio band. Linearity in this manner is quite desirable for high-end audio applications such as wireless microphone applications. 
   The illustrated embodiments of the present invention concern wireless microphone applications. However, it should be understood that the unique split-band companding circuitry disclosed herein may be utilized in other applications such as, for example, wireless musical instruments such as electric guitars, electric bases and the like. 
   From the foregoing it will be observed that numerous modifications and variations can be effectuated without departing from the true spirit and scope of the novel concepts of the present invention. It is to be understood that no limitation with respect to the specific embodiments illustrated is intended or should be inferred. The disclosure is intended to cover by the appended claims all such modifications as fall within the scope of the claims when the claims are properly interpreted.