Abstract:
A transmitter for modulating an externally supplied audio signal according to a predetermined modulation system and for transmitting an obtained modulation signal includes a modulator for modulating the audio signal according to the modulation system; a balance convertor for converting the modulation signal in an unbalanced mode generated by the modulator into a balanced mode; and an antenna having a predetermined length formed by a leakage transmission line, the antenna emitting the modulation signal converted into the balanced mode.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present invention contains subject matter related to Japanese Patent Application JP 2004-211785 filed in the Japanese Patent Office on Jul. 20, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to transmitters, and more particularly, to an on-vehicle portable frequency-modulation transmitter, that is, a so-called FM transmitter.  
         [0004]     2. Description of the Related Art  
         [0005]     FM transmitters have been available as accessories for vehicles on which playback apparatuses, such as cassette tape players, compact disk (CD) players, or minidisk (MD) players, are not mounted.  
         [0006]     For example, as shown in  FIG. 8 , an FM transmitter  1  includes a stereo mini plug  4  provided at a leading end of a cord  3  extending from a casing  2 . The FM transmitter  1  is used such that the stereo mini plug  4  is inserted into a headphone jack of a portable audio apparatus, such as a cassette tape recorder, a CD player, or an MD player. Playback sound acquired from the portable audio apparatus is FM-modulated by a circuit board accommodated within the casing  2  and transmitted at a predetermined frequency via an antenna  5 . (See, for example, the home page of TDK Marketing Corporation, searched on Jul. 18, 2004, the Internet &lt;URL: http://www.tmac.tdk.co.jp/nyn/nyn00502.htm&gt;.)  
         [0007]     Thus, even in a vehicle on which a cassette tape recorder or the like is not mounted, a user of the FM transmitter  1  is able to listen to playback sound from a cassette tape or the like played back by a portable audio apparatus via a car stereo by receiving FM radio waves via a car radio antenna of the vehicle.  
         [0008]     In addition to the FM transmitter  1  shown in  FIG. 8  in which a battery is used as a power source, for example, an FM transmitter  10  shown in  FIG. 9  in which a cigar lighter socket of a vehicle supplies power is available. (See, for example, the home page of Audio-Technica Corporation, searched on Jul. 18, 2004, the Internet &lt;URL: http://www.audio-technica.co.jp/products/caracc/at-fmt7dc.html&gt;.)  
         [0009]     The FM transmitter  10  includes a cigar lighter plug  12  provided at a leading end of a casing  11 . By inserting the cigar lighter plug  12  into a cigar lighter socket (not shown) of the vehicle, power is supplied from the cigar lighter socket to the FM transmitter  10 .  
         [0010]     In addition, the FM transmitter  10  further includes a stereo mini plug  14  and a direct current (DC) power plug  15  provided at a leading end of a cable  13  extending from the casing  11 . By inserting the stereo mini plug  14  into a headphone jack (not shown) of a portable audio apparatus  16  and by inserting the DC power plug  15  into a DC power terminal (not shown) of the portable audio apparatus  16 , audio signals of playback sound from the portable audio apparatus  16  are input to the FM transmitter  10  while driving power is supplied to the portable audio apparatus  16 .  
         [0011]     In the FM transmitter  10 , the audio signals input from the portable audio apparatus  16  are FM-modulated by an FM modulation circuit accommodated within the casing  11 , and the FM-modulated audio signals are transmitted at a predetermined frequency via an antenna (not shown) accommodated within the casing  11 .  
       SUMMARY OF THE INVENTION  
       [0012]     Since the FM transmitter  1  shown in  FIG. 8  uses a battery as a power source, the FM transmitter  1  achieves high portability. Thus, the FM transmitter  1  can be used in a desired position in a vehicle. However, the FM transmitter  1  is likely to be affected by the environment. In addition, depending on the position in a vehicle, FM radio waves transmitted from the FM transmitter  1  may be blocked by the body of the vehicle. Thus, FM radio waves may not be received at an antenna of the vehicle with a radio field intensity that is adequate from a practical point of view.  
         [0013]     The FM transmitter  10  shown in  FIG. 9  can be used in only a limited position in a vehicle. In addition, depending on the structure of the vehicle or depending on the position of an antenna of the vehicle, FM radio waves transmitted from the FM transmitter  10  may not be received at the antenna with a radio field intensity that is adequate from a practical point of view.  
         [0014]     It is desirable to provide a transmitter that is less likely to be affected by the environment and that achieves stable transmission of radio waves.  
         [0015]     According to an embodiment of the present invention, a transmitter includes modulation means for modulating an audio signal according to a predetermined modulation system, balance conversion means for converting an unbalanced modulation signal from an unbalanced mode to a balanced mode, and an antenna having a predetermined length formed by a leakage transmission line, the antenna emitting the modulation signal converted into the balanced mode.  
         [0016]     As a result, the transmitter ensures a transmission quality adequate from a practical point of view even in a space having a large environmental change. In addition, adopting the balance conversion means causes the transmitter to be much less likely to be affected by the use environment and enables stable transmission of radio waves.  
         [0017]     According to an embodiment of the present invention, a transmitter for modulating an externally supplied audio signal according to a predetermined modulation system and for transmitting an obtained modulation signal includes modulation means for modulating the audio signal according to the modulation system, balance conversion means for converting the modulation signal in an unbalanced mode generated by the modulation means into a balanced mode, and an antenna having a predetermined length formed by a leakage transmission line, the antenna emitting the modulation signal converted into the balanced mode. Thus, even in a space having a large environmental change, a transmission quality adequate from a practical point of view can be ensured. In addition, transmitting the modulation signal in the balanced mode causes the transmitter to be much less likely to be affected by the use environment and enables stable transmission of radio waves. Thus, a transmitter that is less likely to be affected by the environment and that enables stable transmission of radio waves can be achieved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a perspective view showing the external structure of an FM transmitter according to an embodiment of the present invention;  
         [0019]      FIG. 2  is a front view showing the external structure of the FM transmitter according to the embodiment;  
         [0020]      FIG. 3  is a schematic diagram showing an operating state of the FM transmitter according to the embodiment;  
         [0021]      FIGS. 4A and 4B  are schematic diagrams showing operating states of the FM transmitter according to the embodiment;  
         [0022]      FIG. 5  is a block diagram showing the internal structure of the FM transmitter according to the embodiment;  
         [0023]      FIG. 6A  to  6 C show the structure of a balun;  
         [0024]      FIG. 6D  is an equivalent circuit diagram of the balun;  
         [0025]      FIG. 7  shows the relationship between a set transmit frequency and control data;  
         [0026]      FIG. 8  is a perspective view of the external structure of a known FM transmitter; and  
         [0027]      FIG. 9  is a perspective view of the external structure of a known FM transmitter. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     Embodiments of the present invention will be described with reference to the drawings.  
         [0029]      FIGS. 1 and 2  show the external structure of an FM transmitter according to an embodiment of the present invention. An FM transmitter  20  includes a cigar lighter plug  22  provided at a leading end of an L-shaped bent casing  21 . By inserting the cigar lighter plug  22  into a cigar lighter socket  23  inside a vehicle, as shown in  FIG. 3 , power is supplied from the cigar lighter socket  23  to the FM transmitter  20 .  
         [0030]     A power lamp  24  is provided on a top face of the casing  21 . A user is able to determine whether or not power is being supplied from the cigar lighter socket  23  to the FM transmitter  20  by seeing whether or not the power lamp  24  is turned on.  
         [0031]     A rotatable frequency selection dial  25  is provided at a rear end of the downward-bent casing  21 . Transmit frequencies that can be set are written on a front face of the frequency selection dial  25  with predetermined angular spaces therebetween. A window  26  is provided at the rear end of the casing  21  so as to correspond to the frequency selection dial  25 .  
         [0032]     In the FM transmitter  20 , a desired transmit frequency is selected from among the transmit frequencies written on the frequency selection dial  25 . By adjusting the rotation position of the frequency selection dial  25 , the set transmit frequency can be seen through the window  26 . Thus, the frequency of FM radio waves transmitted from the FM transmitter  20  can be set to the desired transmit frequency.  
         [0033]     A cord  27  and an antenna cable  29  extend from a back face of the casing  21  and are retained by a bush  30  provided for exit protection. One end of each of the cord  27  and the antenna cable  29  is connected to a circuit board (not shown) accommodated within the casing  21 .  
         [0034]     A headphone mini plug  28  is provided at a leading end of the cord  27 . Thus, by inserting the headphone mini plug  28  into a headphone jack (not shown) of a portable audio apparatus  31 , such as a cassette tape recorder, as shown in  FIG. 3 , audio signals of playback sound output from the portable audio apparatus  31  are input to the FM transmitter  20 .  
         [0035]     The antenna cable  29  is formed by covering a twisted-pair cable with a flexible insulator. A leading end of the twisted-pair cable is terminated with, for example, 100 Ω, and the twisted-pair cable has a characteristic impedance of, for example, 100 Ω. Thus, leakage radio waves can be generated along the antenna cable  29 .  
         [0036]     Taking into consideration the portability of the FM transmitter  20  and the length of a typical vehicle, the length of the antenna cable  29  is set to about three meters. If an antenna  32  (shown in  FIG. 4B ) of a vehicle is located at a front portion of the vehicle, the antenna cable  29  is mounted along the interior trim of the vehicle as shown in  FIG. 4A . If the antenna  32  of the vehicle is located at a rear portion of the vehicle, the antenna cable  29  is mounted along the interior trim of the vehicle as shown in  FIG. 4B . Thus, leakage radio waves from the antenna cable  29  are received at the antenna  32  of the vehicle with a radio field intensity that is adequate from a practical point of view.  
         [0037]     Thus, a user of the FM transmitter  20  is able to listen to sound in a good condition based on FM radio waves transmitted from the FM transmitter  20  via a car stereo, irrespective of the type of vehicle in which the FM transmitter  20  is used.  
         [0038]     The FM transmitter  20  includes an input level switch  33  provided on the back face of the casing  21 , as shown in  FIG. 2 . In the FM transmitter  20 , the input level switch  33  is normally set to “high”. However, when the volume of playback sound output from a car stereo is small even if the volume of the connected portable audio apparatus  31  (see  FIG. 3 ) is turned up, the input level switch  33  is set to “low”. Thus, the volume of the playback sound can be increased.  
         [0039]      FIG. 5  shows the circuit structure of the circuit board (not shown) accommodated within the casing  21  of the FM transmitter  20 .  
         [0040]     Referring to  FIG. 5 , in the FM transmitter  20 , a power supply voltage VCC of 12 V or 24 V supplied from a vehicle-mounted battery via a cigar lighter jack is input to a regulator circuit  41  via a noise filter  40 . The regulator circuit  41  transforms the input power supply voltage VCC into 5 V, and the transformed power supply voltage is supplied as a driving voltage VCC′ to each circuit block.  
         [0041]     In addition, in the FM transmitter  20 , audio signals S 1 L and S 1 R for left and right channels supplied from the portable audio apparatus  31  connected via the headphone mini plug  28  are input to an automatic level control amplifying circuit  43  via a noise filter circuit  42 .  
         [0042]     A level adjusting circuit  61  including an attenuator  60  and the input level switch  33  is provided between the noise filter circuit  42  and the automatic level control amplifying circuit  43 . When the input level switch  33  is set to “high” or “low”, the attenuator  60  and the ground are connected or disconnected, respectively.  
         [0043]     Thus, when the input level switch  33  is set to “high”, the audio signals S 1 L and S 1 R are attenuated and input to the automatic level control amplifying circuit  43 . In contrast, when the input level switch  33  is set to “low”, the audio signals S 1 L and S 1 R are input to the automatic level control amplifying circuit  43  without being attenuated.  
         [0044]     The automatic level control amplifying circuit  43  performs amplification and amplitude limiting processing on the supplied audio signals S 1 L and S 1 R for the left and right channels to produce amplitude-limited audio signals S 2 L and S 2 R. The automatic level control amplification circuit  43  transmits the obtained amplitude-limited audio signals S 2 L and S 2 R to a preemphasis circuit  44 .  
         [0045]     The preemphasis circuit  44  performs 75 microsecond preemphasis for emphasizing high-bandwidth components including many noise components, that is, preemphasis for emphasizing high-bandwidth components of 2.2 kHz or more, on the amplitude-limited audio signals S 2 L and S 2 R to produce preemphasis audio signals S 3 L and S 3 R. Then, the preemphasis circuit  44  supplies the obtained preemphasis audio signals S 3 L and S 3 R to a multiplexer circuit  45 .  
         [0046]     A 7.6-MHz clock CLK 1  is generated by an oscillator circuit  47  using a quartz crystal  46  as an oscillator. The clock CLK 1  is ¼ frequency-divided by a first frequency divider  48  to produce a 1.9-MHz clock CLK 2 . The clock CLK 2  is 1/50 frequency-divided by a second frequency divider  49  to produce a 38-kHz clock CLK 3 . The clock CLK 3  is supplied to the multiplexer circuit  45 .  
         [0047]     The multiplexer circuit  45  generates a sum signal S 4 A in an AM-FM system of FM stereo broadcasting by adding the supplied preemphasis audio signals S 3 L and S 3 R for the left and right channels. The multiplexer circuit  45  also generates a difference signal S 4 B in the AM-FM system by subtracting the preemphasis audio signal S 3 R for the right channel from the preemphasis audio signal S 3 L for the left channel and by multiplying the 38-kHz clock CLK 3  by the subtraction result. Then, the multiplexer circuit  45  transmits the sum signal S 4 A and the difference signal S 4 B to an adding circuit  50 .  
         [0048]     The 38-kHz clock CLK 3  output from the second frequency divider  49  is ½ frequency-divided by a third frequency divider  51  to produce a pilot signal S 5  of a 19-kHz clock. The pilot signal S 5  is supplied to the adding circuit  50 . The pilot signal S 5  is used as a reference signal when a receiving side generates audio signals for the left and right channels in accordance with the sum signal S 4 A and the difference signal S 4 B. Thus, the adding circuit  50  adds the sum signal S 4 A, the difference signal S 4 B, and the pilot signal S 5 , and obtains a stereo composite signal S 6 . The adding circuit  50  transmits the stereo composite signal S 6  to a voltage-controlled oscillator circuit  52 .  
         [0049]     A carrier frequency determination unit including a diode, a trimmer capacitor, and a coil is provided as an input stage of the voltage-controlled oscillator circuit  52 . The voltage-controlled oscillator circuit  52  FM-modulates the supplied stereo composite signal S 6  to generate an FM modulation signal S 7  at a carrier frequency corresponding to the capacitance of the diode, the trimmer capacitor, and the coil of the carrier frequency determination unit. The voltage-controlled oscillator circuit  52  transmits the generated FM modulation signal S 7  to a band pass filter circuit  54  via a radio frequency (RF) amplifying circuit  53 .  
         [0050]     The band pass filter circuit  54  extracts only signal components in a range between 76 MHz and 90 MHz, which is a transmission range of the FM transmitter  20 , from the supplied FM modulation signal S 7 , and transmits the extracted signal components as an FM modulation signal S 8  to a balun  55 .  
         [0051]     The balun  55  is formed by winding three leads  61  around a core  60  made of, for example, a Ni—Zn ferrite, as shown in  FIGS. 6A  to  6 C. The balun  55  has an equivalent circuit structure in which first to third coils L 1  to L 3  are connected in series with each other, as shown in  FIG. 6D . The balun  55  converts the unbalanced FM modulation signal S 8  from an unbalanced mode to a balanced mode, and obtains a balanced FM modulation signal S 9 . The balun  55  supplies the balanced FM modulation signal S 9  to a twisted-pair cable  29 A contained in the antenna cable  29 . Accordingly, the FM modulation signal S 9  is leaked out and emitted from the antenna cable  29 .  
         [0052]     In contrast, the FM modulation signal S 7  output from the RF amplifier  53  is also supplied to a program counter circuit  56 . Here, the program counter circuit  56  receives from a switching circuit  57  control signals D 0  to D 3  for four channels corresponding to the transmit frequency of the FM transmitter  20  set by operating the frequency selection dial  25  (see  FIG. 1 ).  
         [0053]     The control signals D 0  to D 3  for respective channels rise to logic “1” (that is, “H”) or fall to logic “0” (that is, “L”) in a pattern corresponding to the transmit frequency set by the user, as shown in  FIG. 7 . This pattern changes depending on the set transmit frequency. For example, when the transmit frequency is set to 76.8 MHz, the logic levels of the control signals D 0  to D 3  for first to fourth channels are set to “0”, “1”, “1”, and “1”, respectively. When the transmit frequency is set to 77.0 MHz, the logic levels of the control signals D 0  to D 3  for the first to fourth channels are set to “1”, “0”, “1”, and “1”, respectively.  
         [0054]     The program counter circuit  56  generates a comparison signal S 10  of approximately 100 kHz. In accordance with the set transmit frequency, which is recognized from the pattern of the logic levels of the control signals D 0  to D 3  supplied from the switching circuit  57 , the comparison signal S 10  is generated by multiplying the FM modulation signal S 7  supplied from the RF amplifier  53  by the reciprocal of the transmit frequency, in other words, the reciprocal of the value indicated in the column “value of X” in  FIG. 7 , that is, 1/X. The program counter circuit  56  transmits the generated comparison signal S 10  to a phase-locked loop (PLL)  59 .  
         [0055]     The clock CLK 2  output from the first frequency divider  48  is 1/19 frequency-divided by a fourth frequency divider  58  to produce a clock CLK 4  of approximately 100 kHz. The clock CLK 4  is supplied to the PLL circuit  59 . Accordingly, the PLL circuit  59  compares the phase of the comparison signal S 10  received from the program counter circuit  56  and the phase of the clock CLK 4  received from the fourth frequency divider  58 . Then, the PLL circuit  59  transmits a PLL signal S 11  for synchronizing the phases to the voltage-controlled oscillator circuit  52 . The voltage-controlled oscillator circuit  52  changes the capacitance of the diode of the carrier frequency determination unit in accordance with the PLL signal S 1 .  
         [0056]     Thus, the voltage-controlled oscillator circuit  52  FM-modulates the stereo composite signal S 6 , which is received from the adding circuit  50 , to a signal of a carrier frequency corresponding to the capacitance of the diode of the carrier frequency determination unit, that is, to a signal of the transmit frequency set by operating the frequency selection dial  25  (see  FIG. 1 ).  
         [0057]     Accordingly, in the FM transmitter  20 , the audio signals S 1 L and S 1 R input from the portable audio apparatus  31  (see  FIG. 3 ) via the headphone mini plug  28  can be FM-modulated at a transmit frequency set by the user and emitted from the antenna cable  29  (see  FIG. 1 ).  
         [0058]     With the foregoing structure, in the FM transmitter  20 , the FM modulation signal S 8 , which is generated based on the audio signals S 1 L and S 1 R (see  FIG. 5 ) supplied from the portable audio apparatus  31  (see  FIG. 3 ) connected via the headphone mini plug  28 , is transmitted as a leakage radio wave via the antenna cable  29 .  
         [0059]     Thus, in the FM transmitter  20 , by mounting the antenna cable  29  along the interior trim of a vehicle to a position near the antenna  32  of the vehicle, a transmission quality adequate from a practical point of view can be ensured even in a space in which a large environmental change occurs, such as a cabin of the vehicle. Thus, when the field intensity of a reception point is constant, the FM transmitter  20  achieves the same effect with a smaller transmission output compared with a case where the FM transmitter  1  shown in  FIG. 8  or the FM transmitter  10  shown in  FIG. 9  is used.  
         [0060]     In addition, in the FM transmitter  20 , since the antenna cable  29  is formed by the twisted-pair cable  29 A, the antenna cable  29  has a high flexibility. Thus, compared with a case where another leakage transmission line is used as an antenna cable having a predetermined length, the storage capability and portability of the antenna cable  29  can be dramatically improved. Furthermore, in a case where the antenna cable  29  is mounted along the interior trim of a vehicle, by improving the flexibility of mounting the antenna cable  29 , such a mounting operation can be performed easily.  
         [0061]     In addition, in the FM transmitter  20 , the FM modulation signal S 8 , which is generated based on the audio signals S 1 L and S 1 R supplied from the portable audio apparatus  31 , is converted from the unbalanced mode to the balanced mode by the balun  55  and transmitted via the antenna cable  29 . Thus, the FM transmitter  20  is much less likely to be affected by the use environment of a vehicle or the like, and stable transmission of FM radio waves can be achieved.  
         [0062]     With the foregoing structure, after the FM modulation signal S 8 , which is generated based on the audio signals S 1 L and S 1 R supplied from the portable audio apparatus  31  connected via the headphone mini plug  28 , is converted from the unbalanced mode to the balanced mode by the balun  55 , the converted FM modulation signal S 8  is transmitted as a leakage radio wave via the antenna cable  29  formed by the twisted-pair cable  29 A. Thus, the leakage radio wave from the antenna cable  29  can be received in a good condition at the antenna  32  without being interrupted by the body of a vehicle. Thus, an FM transmitter that is less likely to be affected by the environment and that achieves stable transmission of radio waves can be realized.  
         [0063]     Although the present invention is applied to an on-vehicle FM transmitter in the foregoing embodiment, the present invention is not limited to this. The present invention is also applied to various transmitters, such as portable FM transmitters that are not mounted in vehicles, fixed FM transmitters that are used at home, and transmitters in which an externally supplied audio signal is modulated according to a predetermined modulation system other than FM modulation and transmitted.  
         [0064]     In addition, although the twisted-pair cable  29 A (see  FIG. 5 ) whose one end is terminated with a characteristic impedance is used as a leakage transmission line used for an antenna in the foregoing embodiment, the present invention is not limited to this. For example, a balanced feeder wire whose one end is terminated with a characteristic impedance or a leakage co-axial cable whose one end is terminated with a characteristic impedance may be used. In addition, various other types of leakage transmission lines may be used as leakage transmission lines used for antennas.  
         [0065]     In addition, although the balun  55  has the structure shown in  FIG. 6  in the foregoing embodiment, the present invention is not limited to this. Various other structures are also possible.  
         [0066]     In addition, although the antenna cable  29  has a length of, for example, three meters in the foregoing embodiment, the present invention is not limited to this. When the length of the antenna cable  29  is set so that an antenna of a vehicle receives radio waves, the antenna cable  29  may have any length as long as various conditions, such as the transmission power of the FM transmitter  20 , the position of the antenna of the vehicle, and the length of body of the vehicle, are taken into consideration.  
         [0067]     In addition, although the present invention is applied to the FM transmitter  20  of a type in which the power is supplied from a cigar lighter socket of a vehicle in the foregoing embodiment, the present invention is not limited to this. The present invention is widely applicable to other types of FM transmitter, such as an FM transmitter using a battery as a power source.  
         [0068]     The present invention is widely applicable to various other types of transmitters as well as on-vehicle FM transmitters.  
         [0069]     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.