Patent Publication Number: US-2006008090-A1

Title: Separation adjusting circuit

Description:
TECHNICAL FIELD  
      The present invention relates to a separation adjustment circuit for increasing the separation degree between a right signal and a left signal in a stereo receiver.  
     BACKGROUND ART  
       FIG. 1  shows a conventional stereo receiver.  
      As shown in  FIG. 1 , a stereo receiver  70  comprises an antenna  71 , a front end part  72  for implementing a synchronization processing and a frequency conversion processing, etc., an F.M. detection part  73  outputting the composite signal including an L (left signal) +R (right signal) component signal, an L−R component signal and a pilot signal (signal for determining a stereo signal), a stereo demodulation circuit  74  demodulating the composite signal into a stereo right signal and a stereo left signal and a separation adjustment circuit  75  that is provided at the former stage of the stereo demodulation circuit  74  and adjusts the intensity ratio between an L+R component signal and an L−R component signal in order to increase the separation degree (separation) between stereo signals.  
      The separation adjustment circuit  75  comprises a capacitor  76  for cutting a direct current component, a capacitor  77 , a variable resistance  78 , a resistance  79 , a buffer amplifier  80  and a resistance  81 . This circuit adjusts the intensity of the L+R component signal by changing a resistance value of the variable resistance  78 . Meanwhile, the capacitors  76  and  77 , the variable resistance  78  and the resistance  79  are external components that are not configured on an IC chip.  
      The intensity of the L+R component signal that is outputted from the F.M. detection part  73  receives the influence of the variable resistance  78  and the resistances  79  and  81 . The intensity of the L−R component signal, however, hardly receives the influence of the variable resistance  78  and it receives only the influences of the resistances  79  and  81 .  
      That is, the following equations are satisfied.  
      Intensity of L+R component≈resistance value of resistance  81 /(resistance value of variable resistance  78 +resistance value of resistance  79 )  
      Intensity of L−R component≈resistance value of resistance  81 /resistance value of resistance  79   
      A frequency band of the L−R component signal is obtained by centering the frequency of 38 kHz. In the case where the L−R component signal passes through the variable resistance  78 , the impedance of the capacitor  77  is small and this signal passes through the capacitor  77  without receiving much influence of the variable resistance  78 . On the other hand, in the case of the L+R component, the impedance of the capacitor  77  is large and the intensity (signal level) changes according to the resistance value of the variable resistance  78 .  
      By changing the resistance value of the variable resistance  78  in this way, the intensity of the L+R component signal can be changed. Therefore, the intensity ratio between the L+R component signal and the L−R component signal can be adequately adjusted so that it becomes possible to increase the separation degree between stereo signals.  
      In the conventional stereo receiver  70 , however, there is a problem such that many components such as the capacitor  77 , the variable resistance  78 , etc. are mounted on an IC chip and the cost for mounting the external components increases.  
      Furthermore, there is another problem such that the mounting area of a printed board increases as the number of external components increases when the stereo receiver  70  is mounted on a printed board.  
      In addition, it is conceivable that the variable resistance  78  includes, for example, a trimmer resistance, etc. In the production line or the adjustment line of the stereo receiver  70 , the resistance value is adjusted by adjusting the trimmer resistance using a driver. In this way, the adjustment of the resistance value of the variable resistance  78  is carried out by an adjustment operator so that the adjustment cannot work well. Therefore, it sometimes takes a long time to implement this adjustment.  
      Thereupon, the object of the present invention is to offer a separation adjustment circuit where a few external components are mounted and the intensity ratio between composite signals can be easily adjusted, in consideration of the above-mentioned problems.  
     DISCLOSURE OF INVENTION  
      In the present invention for solving the above-mentioned problems, the following configuration is adopted.  
      That is, a separation adjustment circuit for adjusting an intensity ratio between a sum signal and a difference signal in a stereo composite signal and for increasing a separation degree between a stereo right signal and a stereo left signal, comprises a sum signal retrieving unit retrieving a sum signal from the stereo composite signal; a difference signal retrieving unit retrieving a difference signal from the stereo composite signal; a mixing unit mixing the sum signal and the difference signal, thereby obtaining a stereo right signal and a stereo left signal; a first adjustment unit adjusting a current amount that flows in the sum signal retrieving unit or the difference signal retrieving unit and adjusting an intensity of the sum signal or an intensity of the difference signal; and a generation unit generating a control signal for controlling an adjustment operation of the first adjustment unit.  
      The sum signal is a signal that shows an L+R component signal obtained by adding the stereo right signal and the stereo left signal. The difference signal is a signal showing an L−R component signal that is the difference between the stereo right signal and the stereo left signal.  
      The sum signal retrieving unit is, for example, a current mirror circuit for retrieving a sum signal from the stereo composite signal that is inputted into a differential amplifier. Similarly, the difference signal retrieving unit is, for example, a current mirror circuit for retrieving a difference signal from the stereo composite signal.  
      The mixing unit is, for example, a mixer circuit for mixing the sum signal and the difference signal, thereby obtaining a stereo right signal and a stereo left signal. A stereo demodulation function is implemented by the sum signal retrieving unit, the difference signal retrieving unit and the mixing unit.  
      The first adjustment unit is, for example, a transistor on the output side for configuring the current mirror circuit. An intensity of the sum signal or the difference signal is adjusted by adjusting the current amount of a transistor on the side of this output so that the separation degree between a stereo right signal and a stereo left signal can be increased. In this way, the number of external components for adjusting an intensity of the sum signal or the difference signal, which are required for the conventional stereo receiver can be decreased. Therefore, it becomes possible to decrease the mounting area of a printed board.  
      Since the operation of the first adjustment unit is controlled based on the control signal generated by the generation unit, the intensity of the sum signal or the difference signal can be easily adjusted and the separation degree between a stereo right signal and a stereo left signal can be increased, without a manual operation.  
      In the separation adjustment circuit, the first adjustment unit comprises a plurality of transistors and a selection unit selecting the transistors based on the control signal. This first adjustment unit may adjust an intensity of the sum signal or the difference signal based on the total current amount of the transistors that are selected by the selection unit.  
      The selection unit is, for example, a switch and this switch is connected to each transistor. By adjusting a current amount that flows in the current mirror circuit by controlling an ON operation or OFF operation of the switch on the basis of the control signal, the switch can adjust the intensity ratio between a sum signal and a difference signal. Therefore, it becomes possible to increase the separation degree between a stereo right signal and a stereo left signal.  
      The separation adjustment circuit comprises a resistance that is connected to an output stage of the separation adjustment circuit and a second adjustment unit being connected to the resistance in parallel and adjusting a current amount flowing in the resistance. The second adjustment unit may adjust a current amount based on a current amount adjusted by the first adjustment unit.  
      Thus, the DC bias at an output of the separation adjustment circuit can be also adjusted in the first adjustment unit according to the intensity adjustment of the sum signal or the difference signal. Therefore, it becomes possible to set this DC bias to a predetermined DC bias that does not distort an output signal.  
      Additionally, the separation adjustment circuit is configured in such a way that the control signal is generated based on the separation degree between a stereo right signal and a stereo left signal that are outputted from the separation adjustment circuit.  
      Thus, it becomes possible to increase the reliability of the control signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention is further clarified if both the detailed explanation that is described later and the attachment drawings are referred to.  
       FIG. 1  shows a conventional stereo receiver;  
       FIG. 2  shows a stereo receiver that is a preferred embodiment of the present invention;  
       FIG. 3  shows the circuit configuration of a separation adjustment circuit;  
       FIG. 4  shows the concrete examples of a current mirror circuit of a broken-line part C of  FIG. 3 ;  
       FIG. 5  shows the configuration of the separation adjustment circuits of another preferred embodiment; and  
       FIG. 6  shows the configuration of the separation adjustment circuits of still another preferred embodiment. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      The following is the detailed explanation of the present invention in reference to the drawings.  
       FIG. 2  shows the stereo receiver that is a preferred embodiment of the present invention.  
      As shown in  FIG. 2 , a stereo receiver  10  comprises an antenna  11 , a front end part  12  for implementing a synchronization processing and a frequency conversion processing, etc., an F.M. detection part  13  outputting the composite signal including an L (leftsignal)+R(right signal) component signal, an L−R component signal and a pilot signal (signal for determining a stereo signal) and a separation adjustment circuit  14  having both a stereo demodulation function of demodulating the composite signal into a stereo right signal and a stereo left signal and a separation adjustment function of adjusting the intensity ratio between the L+R component signal and the L−R component signal in order to increase the separation degree (separation) between the stereo signals.  
      At the former stage of the separation adjustment circuit  14 , a resistance  15  for supplying a reference voltage in order to adjust the DC bias of a reception signal, a buffer amplifier  16  and a capacitor  17  for cutting a direct current component as an external component, are provided.  
      Next, the separation adjustment circuit  14  is explained in detail.  
       FIG. 3  shows the circuit configuration of the separation adjustment circuit  14 .  
      At first, a composite signal and a DC reference voltage are inputted into a differential amplifier that includes a P channel MOS transistor  20 . Then, the composite signal that is inputted into the differential amplifier is distributed to a broken line frame A and a broken line frame B by the current mirror circuit (sum signal retrieving unit) that includes N channel MOS transistors  21  and  22  and another current mirror circuit (difference signal retrieving unit) that includes transistors  21  and  23 . A signal that is distributed to the broken line part A expresses an L+R component signal while a signal that is distributed to the broken line part B shows an L−R component signal. The L−R component signal is inputted into the mixer circuit (mixing unit) including a transistor  24  and then a 38 kHz component signal is removed. And, in the mixer circuit, an L−R component signal obtained by removing the 38 kHz component signal and an L+R component signal are mixed (sum or difference computation) so that a stereo right signal (R signal) and a stereo left signal (L signal) are obtained.  
      Then, in the separation adjustment circuit  14  of the preferred embodiment, the current amount of a current that flows in a current mirror circuit including the transistors  21  and  23 , that is, the transistor  23  (first adjustment unit) of the broken line part B is adjusted by a control signal generated by the control circuit  25  (generation unit). By changing the current value that flows in the transistor  23  in this way, the intensity of the L−R component signal is adjusted so that the intensity ratio between the L+R component signal and the L−R component signal can be adjusted. As a result, it becomes possible to increase the separation degree between the L signal and the R signal.  
      The control signal that is generated by the control circuit  25  is the digital signal of an optional bit number and this signal controls the ON operation or OFF operation of a current amount adjustment switch that is described later.  
       FIG. 4  shows the concrete example of a current mirror circuit of the broken line part C of  FIG. 3 . Meanwhile, it is assumed that a current mirror circuit including the transistors  21  and  23  outside the broken line part C has also the same configuration. Furthermore, the same control signal is supplied to two transistors  23   0 from the control circuit  25  and they function in the same way.  
      A current mirror circuit  30  as show in  FIG. 4A  includes a plurality of transistors  23  ( 23 - 1 ,  23 - 2 , . . . ,  23 -n) and a switch  31  (selection unit) that is connected to a drain of each transistor  23 . The ON operation or the OFF operation of each switch  31  is controlled based on a control signal that is outputted from the control circuit  25 . Meanwhile, the switch  31  includes a semiconductor switching element.  
      According to the selected switch  31 , the current value of the current that flows in the current mirror circuit  30  changes and the intensity of the L−R component signal is adjusted. In other words, as the number of the switches  31  that become ON increases, the current amount that flows in a contact point D increases. Accordingly, the L−R component signal with a strong intensity is inputted into the mixer circuit. When the number of the switches  31  is small, on the contrary, the current amount that flows in the contact point D decreases and the L−R component signal with a weak intensity is inputted into the mixer circuit. By adjusting the intensity of the L−R component signal based on the number of the switches  31  that are ON in this way, the intensity ratio between the L+R component signal and the L−R component signal can be adjusted. Therefore, it becomes possible to increase the separation degree between an L signal and an R signal.  
      A current mirror circuit  32  of another example as shown in  FIG. 4B  includes a plurality of transistors  23  ( 23 - 1 ,  23 - 2 , . . . ,  23 -n) and the switch  31  that is connected to a gate of each transistor  23 . The ON operation or the OFF operation of each switch  31  is controlled based on a control signal that is generated by the control circuit  25 . Since the current value of a current that flows in the current mirror circuit  32  changes according to the number of the selected switches  31  in the same way as in  FIG. 4A . Therefore, the intensity of the L−R component signal is adjusted in accordance with the change.  
      The control signal that is generated by the control circuit  25  is generated based on the separation degree between the stereo right signal and the stereo left signal that are outputted. That is, for example, in the case where a separation degree between the stereo right signal and the stereo left signal is low and an intensity of the L−R component signal is higher than the intensity of the L+R component signal, the control circuit  25  generates a control signal for decreasing the number of the switches  31  that are ON. Furthermore, the number of the switches  31  at this time is adjusted in such a way that the intensity of the L+R component signal becomes the same as the intensity of the L−R component signal. In this way, the current amount that flows in a mixer circuit is decreased and the intensity of the L+R component signal becomes the same as the intensity of the L−R component signal. Therefore, it becomes possible to increase the separation degree between the stereo right signal and the stereo left signal.  
      Furthermore, it is possible to change the size of each transistor  23  of the current mirror circuits  30  or  32  and to optionally select the transistor  23  using the switch  31 . By changing each size of the transistor  23  of the current mirror circuit  30  or  32  in this way, it becomes possible to set to an optional value, the current amount of the current that flows in the current mirror circuit of the broken-line part C, according to the combination of the selected transistors  23 .  
      The following is the explanation of the configuration of the separation adjustment circuit in another preferred embodiment. The separation adjustment circuit  14  is configured to change the current amount of an L−R component for separation adjustment. By changing the current amount of this L−R component, the DC bias component of the L signal and the R signal that are output signals sometimes change. For example, in the case where the current amount of the L−R component signal is adjusted to be increased, the DC bias of the L signal and the R signal that are outputted from the separation adjustment circuit  14  increases. Accordingly, distortion sometimes occurs on the L signal and the R signal.  
       FIG. 5  shows a separation adjustment circuit for not generating distortion on the outputted L signal and R signal. In  FIG. 5 , the same constituent elements as those shown in  FIG. 3  indicate the same articles so that the explanations are omitted.  
      In a separation adjustment circuit  40  shown in  FIG. 5 , a current mirror circuit that includes a transistor  41  is connected to an output part of the separation adjustment circuit  14  of  FIG. 3  and both a resistance  42  and a fixed current source  43  (second adjustment unit) that is connected to the resistance  42  in parallel are connected to the output stage of this current mirror circuit. In addition, the other terminal of the resistance  42  and that of the current source  43  are connected to ground.  
      The current amount of the fixed current source  43  is changed by the control signal of the control circuit  25  based on the intensity of an L−R component signal and the changed current amount is adjusted to a predetermined DC bias in such a way that the output signal does not generate distortion.  
      By changing the current amount of the fixed current source  43  based on the intensity of the L−R component signal in this way, the current amount of a current that flows in the resistance  42  can be changed. Therefore, for example, even if the current amount is adjusted to increase the intensity of the L−R component signal, the output can be suppressed not to exceed the fixed DC bias and the distortion of an output signal can be avoided.  
      In the changing method of a current amount of the fixed current source  43 , it is possible that the current that flows in the resistance  42  is adjusted by connecting a fixed current source on a drain side of the transistor  23  and by selecting the transistor  23  using the switch  31 , in a current mirror circuit which includes the transistor  21 , a plurality of the transistors  23  and the switch  31  that is connected to each transistor  23  as shown in  FIG. 4 . In other words, a method of changing a current amount by connecting the fixed current source  43  to the current mirror circuit which includes a plurality of transistors and by selecting the number of the transistors on the output side using the switch, etc. is conceivable.  
      Meanwhile, the resistance  42  and the fixed current source  43  provided at the output stage of the separation adjustment circuit  40  as shown in  FIG. 5m aybe configured not to be connected to the current mirror circuit.  
       FIG. 6  shows a separation adjustment circuit configured in such a way that it adjusts the current amount of an output stage without being connected to the current mirror circuit.  
      As shown in  FIG. 6 , the resistance  42  and the fixed current source  43  that is connected to the resistance  42  in parallel are connected to the output stage of the separation adjustment circuit  50 . The other terminal of the resistance  42  and that of the fixed current source  43  are connected to a power source (VDD). The current value that flows in the fixed current source  43  is changed on the basis of the control signal that is outputted from the control circuit  25  in the same way as in  FIG. 5 .  
      Even if the separation adjustment circuit is configured to adjust the current amount that flows in the resistance  42  at the output stage without being connected to the current mirror circuit, the DC bias of an output signal can be controlled at a predetermined value.  
      Furthermore, a variable resistance may be provided instead of the transistor  23  of the separation adjustment circuit  14  of  FIG. 3 .  
      Both the stereo demodulation function of implementing the sum or difference computation between the L−R component signal from which the 38 kHz component signal is removed and the L+R component signal and the separation adjustment function of adjusting the intensity ratio between the L−R component signal and the L+R component signal can be realized by adjusting the bias current (current amount of the current mirror circuit) of a mixer circuit having the stereo demodulation function.  
      Since the number of the external components such as a capacitor  77  that is indispensable for a conventional stereo receiver  70  can be decreased, it becomes possible to decrease the mounting area of a printed board.  
      In addition, it becomes possible to increase the separation degree between a right signal and a left signal without manual operations, which is realized by an electric control based on the control signal that is generated by the generation unit.  
      Furthermore, a variable resistance  78 , etc. that are required at the conventional stereo receiver  70  in  FIG. 1  can be omitted using the separation adjustment circuit of the preferred embodiment of the present invention so that a reception signal does not receive the influence of the impedance based on the resistance. Therefore, it becomes possible to decrease the capacity of a capacitor  76  for cutting the DC component of the reception signal. In this way, the capacitor  76  for cutting a DC component (capacitor  17  in the preferred embodiment of the present invention) having a small capacity, in other words, the capacitor having a small size can be mounted. Accordingly, it becomes possible to further decrease the mounting area of a printed board.  
      Additionally, the above-mentioned separation adjustment circuit is configured to adjust the current amount of the L−R component but this circuit may be also configured to adjust the current amount of the L+R component.  
      According to the separation adjustment circuit of the present invention, the current amount that flows in the retrieving unit used when a sum signal or a difference signal is retrieved from a stereo composite signal is adjusted based on the control signal generated by the generation unit so that these configurations can be realized on an IC chip, etc. Therefore, the number of the external components for the separation adjustment that are required in a conventional stereo receiver can be decreased and accordingly the mounting area of a printed board can be also decreased.  
      Furthermore, since the intensity ratio between a sum signal and a difference signal are adjusted on the basis of the control signal that is generated by the generation unit, it becomes possible to increase the separation degree between a right signal and a left signal without manual operations.