Abstract:
A compact and lightweight wireless microphone and transmitter. The transmitter is connected to the microphone using a standard connector. The transmitter includes a low-frequency amplifier for amplifying a received voice signal and a surface acoustic wave oscillator providing a high-frequency oscillation signal. A modulator combines the amplified voice signal and the high frequency oscillation signal into a modulated signal. The modulated signal is amplified using a high-frequency amplifier to create a high-frequency modulated signal. The high-frequency modulated signal is output as radio waves through an antenna. A power supply is included for powering the transmitter. To improve sound quality, the transmitter additionally includes a compander circuit for improving signal-to-noise ratio and increasing a dynamic range through a logarithmic compression algorithm. Also, a pre-emphasis circuit which reduces noise by providing higher gain in the high frequency amplification than in the low frequency amplification.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a wireless microphone apparatus for performing frequency modulation (FM) of a voice signal for transmission as radio waves, and particularly to a transmitter device for FM wireless microphone with a band of about 800 MHz.  
           [0003]    2. Description of the Related Art  
           [0004]    Wireless microphone systems used in a concert halls, live stages or wayside recording are roughly classified into two types: (1) hand-held microphone transmitter systems and (2) belt pack microphone transmitter systems. Ultimately, the respective transmitter systems should be compact, lightweight and offer long term operation (about ten hours). However, it is difficult to achieve both size and weight reductions along with long-term operation because the transmitter systems are battery operated.  
           [0005]    A hand-held transmitter apparatus typically has an integral structure of a microphone element, a window screen and a body case for receiving a battery. In addition, it is becoming desirable to be able to change certain components within a microphones in order to adapt it to differences in voice quantity, voice quality or preference of tone characteristics of a singer or a speaker.  
           [0006]    Accordingly, it is desired to create a wireless microphone where the microphone is severable from the transmitter. However, this is complicated by the number of different circuits required to improve sound quality such as reducing signal-to-noise ratio or increasing dynamic range. Because of the large number of circuits, the body of the transmitter must be formed in a large size which is inconvenient and potentially interferes with the operation of the transmitter. Thus, conventional apparatus and circuits result in a microphone which is not optimal.  
           [0007]    For example, in a conventional transmitter for a wireless microphone, a high-frequency oscillating part generally requires multiplying an output from a quartz oscillator to obtain a desired frequency. Such multiplication is done using a Phase Lock Loop (PLL) circuit and a conventional quartz multiplication method.  
           [0008]    Such a conventional quartz multiplication method is illustrated in FIG. 7. The method involves multiplying a fundamental frequency of 23 MHz derived from a quartz crystal by 36 in order to obtain a desired frequency of 828 MHz related to a frequency band of 800 MHz. Such a multiplication method requires a circuit which consumes a significant physical mounting area. In part, the large surface area is due to intermediate filters required to reduce unnecessary radiation. In addition, in the case of an overtone oscillation type of the quartz oscillator, it is difficult if not impossible to perform deep modulation and oscillation with frequencies at or above 800 MHz. In part, this is due to the possibility of breaking the quartz oscillator which must be made very thin.  
           [0009]    Alternatively, such conventional methods allow for frequencies that are easily changed. Such changes can be made by modifying circuit configuration. However, as illustrated in FIG. 8, such a circuit is complex. In addition to complexity, the circuit consumes significant power and thus requires larger or additional batteries. Thus, such conventional methods are not suited for miniature applications.  
           [0010]    Accordingly, circuits and/or methods capable of miniaturization are desired. Further, it is desirable to provide a wireless microphone which provides for connection of a transmitter to a microphone through a standard connector, such as a canon connector (i.e. XLR).  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention provides a compact and lightweight wireless microphone for business. In addition, the present invention provides a transmitter for the microphone with a size compatible with a canon connector. In view of the aforementioned problems, it is an object of the present invention to provide a transmitter for a wireless microphone. The transmitter comprises a connector means for making connection to a microphone and inputting a voice signal from the microphone, a low-frequency amplification means for amplifying the voice signal, a surface acoustic wave (SAW) oscillation means for outputting a high-frequency oscillation signal, a modulation means for performing frequency modulation of the high-frequency oscillation signal by the amplified voice signal, a high-frequency amplification means for amplifying a high-frequency modulation signal, an antenna means for radiating a high-frequency output signal amplified by radio waves, and a power source for supplying electric power of a predetermined voltage to each of the aforementioned means.  
           [0012]    To significantly improve sound quality required for business applications, the present transmitter device further comprises a compander circuit for improving a signal-to-noise ratio and for increasing a dynamic range by compressing the voice signal using logarithmic compression. In an embodiment, the compression is done using ½ logarithm compression. Beyond compression, a pre-emphasis circuit is included for reducing noise by providing a degree of amplification of high pass frequencies larger than the degree of amplification for other frequencies.  
           [0013]    In some embodiments, the present invention comprises a SAW oscillation means which outputs a high-frequency oscillation signal with a band of about 800 MHz directly without multiplication.  
           [0014]    The modulation means includes an oscillation element, a modulation and oscillation circuit, and a buffer amplification circuit. The voice signal is superimposed on a bias voltage at a variable capacitance diode at the modulation means. The voice signal is applied to the oscillation element and modulation is performed by changing a load of the SAW oscillation means. The SAW oscillation means is inserted into a terminal of the oscillation element.  
           [0015]    The modulated signal is amplified using a high frequency amplification means. The amplified modulated signal has a power in an 800 MHz band of about +10 dBm. Additionally, an embodiment of the present invention provides for a band-pass filter circuit for eliminating harmonics of the high-frequency oscillation signal and for matching the impedance of the antenna means.  
           [0016]    In part, the antenna means comprises the body case of the transmitter. The body case is about ¼ the wavelength of the high-frequency oscillation signal. In an embodiment, the body case is about 7.5 cm long with a diameter of 2.3 cm. In an embodiment, the power source means comprises a battery and a DC-DC converter for increasing an output voltage of 1.5 V DC of said battery to 3.0 V DC. In some embodiments, the converted voltage is stabilized to 2.7 V DC by a series regulator. Each the circuit means in the device is designed to operate at the same voltage. In an embodiment, the battery used is a single AAA dry battery offering 10 hours or longer of continuous operation. In part because of the battery, a weight of the present device including the metal receiving case and the battery is about 55 g.  
           [0017]    In some embodiments, the SAW resonator is capable of directly oscillating at or about a frequency band of 800 MHz. To avoid frequency changes of the SAW resonator due to changes in ambient temperature, an embodiment includes a plurality of transistors connected in series to increase negative resistance and thus provide sufficient oscillation margin.  
           [0018]    Moreover, in some embodiments, a high-frequency amplifier performs matching for an antenna and a band-pass filter reduces unnecessary radiation. Such a band pass filter is simple because the SAW resonator directly provides a frequency at or about 800 MHz.  
           [0019]    The antenna means provides good radiation efficiency. In an embodiment, such antenna means are formed from a microphone body while a transmitter body forms a ground. Accordingly, a structure in which the antenna is not exposed outside of the structure is achieved. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 shows a schematic block diagram illustrating an embodiment of a circuit configuration in a transmitter device for a wireless microphone according to the present invention;  
         [0021]    [0021]FIG. 2 shows a block diagram of a more detailed circuit configuration of an embodiment of the transmitter device for a wireless microphone according to the invention;  
         [0022]    [0022]FIG. 3 shows an embodiment of a detailed circuit diagram of the transmitter device for a wireless microphone according to the invention;  
         [0023]    [0023]FIG. 4 shows an embodiment of a component assembly construction of the transmitter for a wireless microphone according to the invention;  
         [0024]    [0024]FIG. 5 shows an embodiment of a combination of the transmitter for a wireless microphone and a microphone fitted to the transmitter according to the invention;  
         [0025]    [0025]FIG. 6 shows an external view of an embodiment of a wireless microphone apparatus according to the invention;  
         [0026]    [0026]FIG. 7 shows a schematic circuit for a transmitter providing a quartz multiplication method of a conventional art; and  
         [0027]    [0027]FIG. 8 shows a schematic circuit configuration of a transmitter for a wireless microphone according to a PLL method of a conventional art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    Details on a wireless microphone apparatus and a transmitter device for the apparatus according to the present invention will be described later with reference to the accompanying drawings.  
         [0029]    [0029]FIG. 1 shows a schematic block diagram of an embodiment of a circuit configuration  100  according to the present invention. Referring to FIG. 1, circuit configuration  100  includes a modulation and oscillation circuit  110 . In modulation and oscillation circuit  110 , an AF signal from a modulation signal input terminal  112  along with a bias voltage is applied to a variable capacitance diode  114  (e.g., diode D 2  which is a KV1832E). In addition, a load of a SAW resonator  118  with a band of 800 MHz is inserted into a base circuit of an oscillation element  116  (e.g., Q 2  which is a FH102). In operation, the load is varied to perform FM modulation. In order to increase a negative resistance and stabilize oscillation in the SAW resonator  118 , two transistors (e.g., FH102) are provided in series. In addition, modulation and oscillation circuit  110  includes a buffer amplification circuit  120  formed of transistors and included on the same package.  
         [0030]    Circuit configuration  100  further includes a high-frequency amplification circuit  130 . In high-frequency amplification circuit  130 , about 0 dBm of electric power of a band of 800 MHz outputted from buffer amplification circuit  120  is amplified to +10 dBm of transmission power at the antenna end  132  by a high-frequency amplification element  134  (e.g., Q 3  which is a 2SC5231).  
         [0031]    Circuit configuration  100  further includes a band-pass filter  150  which is formed of two stages of π type low-pass filters. The π type low-pass filters act to attenuate any harmonics greater than two times 800 MHz. Band-pass filter  150  matches the impedance of an antenna circuit (not shown). In an embodiment, the antenna circuit includes a microphone body (not shown) and a body case of a transmitter (not shown). The impedance match does not include matching the attenuation of a harmonic noise component.  
         [0032]    [0032]FIG. 2 shows a block diagram of a more detailed circuit configuration  200  of the transmitter according to the invention. Referring to FIG. 2, more detailed circuit configuration  200  includes a microphone input terminal  40  which comprises an industry-standard XLR connector for connecting the microphone to the transmitter. As a result of this, different transmitters can be attached to the microphone. The different transmitters can accommodate tone characteristics of different users, such as a singer or a speaker.  
         [0033]    Input amplifier  41  provides for low-frequency amplification of a voice signal received from microphone input terminal  40 . The amplification is performed to a predetermined level. The amplified voice signal is logarithmically compressed by a compander  42 . In an embodiment, the compression is performed to ½ logarithm, however, it should be recognized that other compression ratios can be used. As a result of the amplification and compression, the dynamic range and signal-to-noise ratio characteristics of the wireless microphone are significantly improved.  
         [0034]    A frequency of the logarithmically compressed voice signal is input to a pre-emphasis circuit  43  to improve sound quality. Sound quality is augmented for a high frequency region. Such augmentation raises a high frequency band of a voice signal to a higher level than other voice regions. Also, any hiss noise is reduced. Such a logarithmically compressed and pre-emphasized voice signal is subsequently de-emphasized and expanded when received in a receiver unit.  
         [0035]    The pre-emphasized voice signal is processed through a low-pass audio filter  44  to reduce a high-pass noise. The signal emerging from low-pass audio filter  44  is modulated in block  45 . The modulation creates an FM signal using SAW resonator oscillation  46  at a frequency band of 800 MHz. The signal is amplified using a high-frequency amplifier  47  and is transmitted through antenna  80 .  
         [0036]    Also illustrated in FIG. 2 is power source  210 . Power source  210  includes a AAA dry battery  49  which provides power at 1.5V DC. The power level from battery  49  is monitored by a voltage reduction check  52 . The voltage reduction check monitors decreases in the level of output voltage of battery  49 . Further, the voltage is indicated and visually ensured by a light emitting diode. Voltage from battery  49  is increased to 2.7 V DC by a DC-DC converter  50 . Also, switching noise generated by DC-DC converter  50  is removed by a series regulator  51  and it is constructed so as not to lose the sound quality. Power from power source  210  is supplied to each circuit within the embodiment.  
         [0037]    [0037]FIG. 3 shows an embodiment of a detailed circuit diagram of the transmitter device for a wireless microphone according to the invention. In FIG. 3, individual circuit configuration blocks of the transmitter described in relation to FIG. 2 are described in more detail in relation to FIG. 3.  
         [0038]    Block  40 . Microphone Terminal Input Circuit  
         [0039]    Microphone Terminal Input Circuit  40  includes four terminals that are connected to a canon connector with  3  pins and a field ground with a connector shape. An inductor L 100  and a Resistor R 49  act in combination to isolate the microphone body from the transmitter. Particularly, the inductor and resistor combination provide high-frequency isolation. High-frequency power supplied from block  49  is introduced to connectors MIC-HOT and INPUT-GND for microphone connection.  
         [0040]    Block  41 . Input Amplifier Circuit  
         [0041]    Input Amplifier Circuit  41  is a low noise amplifier with low-voltage operation. Amplifier circuit  41  is formed of an FET and a PNP transistor, and amplification is performed to the optimum input necessary for a compander. A variable resistor VR 4  is preset. Changing variable resistor VR 4  provides gain adjustment in respective microphone units. Thus, an optimal adjustment level is achievable.  
         [0042]    Block  42 . Compander Circuit  
         [0043]    Compander Circuit  42  compresses the voice signal amplified by the input amplifier. The voice signal is compressed to ½ using a logarithmic compression. Additionally, a network of resistors and capacitors is connected to an input terminal of the compander. The network corrects middle and high pass regions of the voice signal to improve the signal-to-noise ratio in the whole system.  
         [0044]    Block  43 . Pre-emphasis Circuit  
         [0045]    Pre-emphasis Circuit  43  emphasizes high pass regions of the voice signal to reduce a triangular noise peculiar to FM in a comprehensive manner. In an embodiment, Pre-emphasis Circuit  43  includes a 50 μsec delay connected to an input terminal of OPAMP U 4 . Resistors R 35  and R 9  along with capacitor C 2  defines the 50 μsec delay covering the high pass region.  
         [0046]    Block  44 . Audio Filter Circuit  
         [0047]    Audio Filter Circuit  44  is a low-pass filter with a cut-off frequency of 20 KHz comprising a diode D 2  for limiting inductor T 1 , capacitor C 50 , capacitor C 51  and capacitor C 49 . Such a circuit reduces adjacent channel leakage power and improves the signal to noise ratio.  
         [0048]    Block  45 . Modulation Circuit  
         [0049]    Modulation Circuit  45  optimizes the voice signal. Such optimization is performed and a reverse voltage is applied to the variable capacitance diode D 2 . Modulation circuit  45  includes variable resistors VR 3  and VR 2 . VR 3  is a pre-set resistor for adjusting modulation degree and VR 2  is a pre-set resistor for controlling a DC current to a variable capacitance diode and setting a center frequency.  
         [0050]    Block  46 . Oscillation and Buffer Amplification Circuit  
         [0051]    Oscillation and Buffer Amplification Circuit  46  includes a dual transistor connected in series to optimally use circuit current. Oscillation and Buffer Amplification Circuit  46  is operable to prevent oscillation failures in the SAW resonator by providing a negative resistance which is variable. The buffer amplification circuit is an emitter grounded amplification circuit that minimizes reflection from the antenna and the high-frequency amplification circuit.  
         [0052]    Block  47 . High-frequency Amplification Circuit  
         [0053]    High-frequency Amplification Circuit  47  amplifies a high-frequency signal with about 0 dBm at a frequency of 800 MHz provided from the buffer amplification circuit. Circuit  47  amplifies the received signal and outputs a high frequency signal with about +10 dBm. The amplified signal is provided to the antenna.  
         [0054]    Block  48 . Band-pass Filter Circuit  
         [0055]    Band-pass Filter Circuit  48  suppresses second or higher harmonic signals generated during high frequency amplification. Further, Band-pass Filter Circuit  48  matches the impedance of the antenna and a secondary π type filter for passing only a fundamental component of an 800 MHz band.  
         [0056]    Block  49 . Battery  
         [0057]    Battery  49  is a AAA alkaline battery for supplying energy to all the transmitter circuits.  
         [0058]    Block  50 . DC-DC Converter Circuit  
         [0059]    DC-DC Converter Circuit  50  increases the 1.5 V received from battery  49  to about 3 V. Current at the input side of about 60 mA is decreased to about 25 mA at the output side. A voltage of operation completion is 0.9 V.  
         [0060]    Block  51 . Power Stabilization and Noise Elimination Circuit  
         [0061]    Power Stabilization and Noise Elimination Circuit  51  is a series regulator for eliminating noise created during operation of the voltage step-up DC-DC converter circuit  50 . The voltage output from Circuit  51  is 2.7 V DC.  
         [0062]    Block  52 . Voltage Reduction Check Circuit  
         [0063]    Voltage Reduction Check Circuit  52  monitors voltage output from battery  49 . In addition to monitoring the voltage, Voltage Reduction Check Circuit  52  includes an alarm indicating a voltage drop. In an embodiment, the alarm comprises an LED which does not emit light when the voltage drops below 1.1 V.  
         [0064]    Referring now to FIG. 4, an embodiment of a component assembly  400  construction of the transmitter for a wireless microphone according to the invention is illustrated. Component assembly  400  includes a canon connector  11  with XLR specifications. Connector  11  provides both mechanical and electrical connections with a microphone  31  (not shown) and a transmitter  21 . Connector  11  also includes a lock mechanism  12 . Component assembly  400  also includes a ring-shaped elastic member  13  comprised of rubber material. Elastic member  13  is interposed between connector  11  and transmitter  21 . Elastic member  13  advantageously eliminates mechanical rattle at the interface of connector  11  and transmitter  21 . By eliminating the rattle, the mutual electrical connection between cannon connector  11  and transmitter  21  is more secure.  
         [0065]    A molded plastic module  14  is contained in a tubular body case  17 . Module  14  is adapted for receiving a dry battery  15  in one side and a circuit board  16  in the opposite side. In an embodiment, circuit board  16  contains the circuitry described with reference to FIGS. 1 through 3. Circuit board  16  is protected by a plastic cover  19 . A seal  23  which in some embodiments has printed letters thereon is pasted on the plastic cover. Tubular body  17  which covers plastic module  14  is made of metal. The bottom of tubular body  17  is blocked with a base plate  18 .  
         [0066]    A length of tubular body  17  is about ¼ of the wavelength of a high-frequency oscillation signal used in the embodiment. In an embodiment, the length is on the order of 75 to 90 mm. The embodiment includes an antenna structure with good radiation efficiency in which microphone body  31  is formed as an antenna and transmitter body  21  is formed as a ground.  
         [0067]    Referring now to FIG. 5, a combination of transmitter  21  and a microphone  31  is illustrated according to the invention. As illustrated, canon connector  11  connects microphone  31  to transmitter  21 . Thus, microphone  31  can easily be mounted in and removed from transmitter  21  in a snap manner. Transmitter  21  provides substantially the same shape as a wired microphone.  
         [0068]    [0068]FIG. 6 illustrates an external view of a wireless microphone according to the present invention. Microphone  31  and transmitter  21  are connected through connector  11  (not shown) and form an integral outward appearance which is both compact and lightweight while maintaining functions necessary for the wireless microphone. Microphone  31  is particularly useful for business.  
         [0069]    By improving the conventional compander part, high-frequency part, power source part, and body, transmitter  21  can be manufactured as a micro-miniature package. Such a micro-miniature package results in a compact and light weight wireless microphone and a transmitter device for the microphone with a size compatible with a canon connector (XLR).  
         [0070]    From the foregoing discussion, a number of advantages of the present invention are evident. For example, a SAW resonator capable of directly oscillating at a frequency of 800 MHz is provided. A second advantage is that all internal component circuits including the SAW resonator modulation circuit and a pre-emphasis circuit operate at a common voltage of 2.7 V DC. Because of this, circuit current consumption is minimized. Third, a high-frequency amplification part matches the antenna and the SAW resonator. This allows for directly oscillating at a frequency in the 800 MHz band. Additionally, further unnecessary radiation is reduced by using a band-pass filter. Fourth, the antenna provided by the body case of the transmitter has a size with about ¼ the wavelength of the SAW resonator output signal. Thus, a length of 7.5 cm and the diameter of 2.3 cm are achieved. This size provides the wireless microphone with good portability as well as limited weight and size.  
         [0071]    In addition, the present invention provides for a wireless microphone operable with a single AAA dry battery. On the single battery, the microphone can provide 10 hours or longer of continuous operation. One embodiment of the present invention has a weight, including the metal receiving case and the battery, of about 55 g.