Patent Publication Number: US-2009232316-A1

Title: Multi-channel blend system for calibrating separation ratio between channel output signals and method thereof

Description:
BACKGROUND  
     The present invention relates to a multi-channel blend scheme, and more particularly to a multi-channel blend system and method for calibrating a transfer curve of multiple channel output signals. 
     Multi-channel blend system generally adjusts the output signal strength when the input signal strength is smaller than a particular threshold value. For example, a stereo blend system utilizes a stereo blend scheme to adjust the separation ratio between signal strengths of left and right channel output signals while switching between a stereo mode and a mono mode, such as adjusting its output from stereo mode to mono mode in response to the decreasing input. 
       FIG. 1  is a diagram illustrating an ideal transfer curve CV of the above-mentioned stereo blend scheme. Values on the horizontal axis represent different signal strengths of a received (FM/AM) audio signal, and values on the vertical axis represent magnitudes of left and right channel output signals respectively. The magnitudes of the left and right channel output signals are almost identical when the signal strength of the received audio signal is smaller than the predetermined threshold value V 1 . Within the range between predetermined threshold values V 1  and V 2 , when the signal strength of a received audio signal becomes larger, the magnitude of the left channel output signal is increased while the magnitude of the right channel output signal is decreased; When the signal strength of the received audio signal becomes smaller, the magnitude of the left channel output signal is decreased while the magnitude of the right channel output signal is increased. 
     More specifically, in the stereo mode, if the signal strength of the received audio signal is initially at the predetermined threshold value V 2  and becomes smaller, the stereo blend system gradually adjusts the separation ratio between the magnitudes of the left and right channel output signals, so as to be capable of switching from the stereo mode to the mono mode when the signal strength of the received audio signal is at the predetermined threshold value V 1 . When the signal strength of the received audio signal becomes smaller than the predetermined threshold value V 1 , the stereo blend system enters the mono mode and outputs the left and right channel output signals with almost identical magnitudes. 
     However, the actual transfer curve of the real world is usually not like the ideal transfer curve CV illustrated above because the signal strength of the received audio signal is not estimated correctly and some fabrication process variation or mismatch arises. A detailed explanation of this actual transfer curve is described.  FIG. 2-FIG .  4  are diagrams illustrating actual transfer curves corresponding to different situations. 
     As shown in  FIG. 2 , starting splitting points of signal strength of the actual transfer curve CV 1 /CV 2  are different from that of the ideal transfer curve CV. As shown in  FIG. 3 , although starting splitting points of signal strength of the actual transfer curves CV 1 ′ and CV 2 ′ are identical to those of the ideal transfer curve CV, the slopes of these curves CV 1 ′ and CV 2 ′ from the starting splitting points of signal strength are not equal to that of the ideal transfer curve CV. As shown in  FIG. 4 , actual transfer curves CV 1 ″ and CV 2 ″ are different from the ideal transfer curve CV in both their slopes and starting splitting points of signal strength. 
       FIG. 5  is a diagram of a conventional stereo blend system  500 . The stereo blend system  500  includes an anti-aliasing filter  505  and a decoding circuit  510 . The decoding circuit  510  includes a gain amplifying module  515 , a mixer  520 , a separating module  525 , a plurality of amplifiers assumed to have an identical gain AV 2 , and a plurality of low-pass filters (LPF)  530  and  535 , where the gain amplifying module  515  has an amplifier  5151  with a gain value AV 1  and a gain controller  5153 . In general, an input signal V in  passing through the anti-aliasing filter  505  comprises an L+R signal component positioned in a lower band and an L−R signal component positioned in a higher band. The lower band is usually regarded to a frequency range 200 Hz-15 KHz and the higher band is regarded to a frequency range nearby 38 KHz. Thus, through the mixer  520 , the separating module  525 , and the amplifiers having the gain AV 2 , the left and right channel output signals can be separated from the input signal V in  and then outputted to the LPFs  530  and  535  respectively. The left and right channel output signals can be illustrated by the following equations (bypassing the effect of mixer  520 ): 
     
       
         
           
             
               
                 
                   LOUT 
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                       V 
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                   ROUT 
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                       AV 
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                     S 
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                         LOUT 
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     where the parameter SEP is a separation ratio between the left and right channel output signals LOUT and ROUT. 
     According to Equation (3), the separation ratio SEP is determined by the gain value AV 1 , which is provided by the gain amplifying module  515 . In general, the method of controlling the gain value AV 1  is to utilize the gain controller  5153  to output a control voltage to adjust the gain value AV 1  according to a receiver signal strength indicator (RSSI) V RSSI . The value of RSSI V RSSI  is usually proportional to the signal strength of the received audio signal V in . If the value of the RSSI V RSSI  becomes larger, it means that the separation ratio SEP should also be adjusted to become larger, and thus an adjusted control voltage provided from the gain controller  5153  increases the gain value AV 1  in order to effectively increase the separation ratio SEP. However, the value of the RSSI V RSSI  may not correctly correspond to the signal strength of the received audio signal (e.g. the signal strength of the received audio signal is not correctly estimated), and some fabrication process variation or mismatch may arise in the amplifier  5151  and the gain controller  5153 . Both of these will cause the actual transfer curve of the stereo blend system  500  become substantially different from the ideal transfer curve CV shown in  FIG. 1 . 
     SUMMARY  
     Therefore one of the objectives of the present invention is to provide a multi-channel blend system and method for calibrating a transfer curve between the multiple channel output signals, to solve the above-mentioned problems. 
     According to an embodiment of the present invention, a multi-channel blend system is disclosed. The multi-channel blend system comprises a decoding circuit and a calibration circuit. The decoding circuit is utilized for receiving an input signal to generate a first channel output signal and a second channel output signal, and the decoding circuit has a gain amplifying module, which is used for providing a gain value utilized for determining a separation ratio between the first channel output signal and the second channel output signal according to a calibration signal. The calibration circuit is utilized for providing a predetermined test signal serving as the input signal so as to generate the calibration signal according to at least one of the first and second channel output signals generated from the predetermined test signal. 
     According to the embodiment of the present invention, a multi-channel blend method is disclosed. The multi-channel blend method comprises the following steps of: receiving an input signal to generate a first channel output signal and a second channel output signal; providing a gain value utilized for determining a separation ratio between the first channel output signal and the second channel output signal according to a calibration signal; and providing a predetermined test signal serving as the input signal so as to generate the calibration signal according to at least one of the first and second channel output signals generated from the predetermined test signal. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a diagram illustrating an ideal transfer curve of a stereo blend scheme. 
         FIG. 2  is a diagram illustrating a first type of an actual transfer curve of the stereo blend scheme. 
         FIG. 3  is a diagram illustrating a second type of the actual transfer curve of the stereo blend scheme. 
         FIG. 4  is a diagram illustrating a third type of the actual transfer curve of the stereo blend scheme. 
         FIG. 5  is a diagram of a conventional stereo blend system. 
         FIG. 6  is a diagram of a multi-channel blend system according to an embodiment of the present invention. 
         FIG. 7  is a timing diagram showing a procedure for determining a target offset calibration parameter. 
         FIG. 8  is a timing diagram showing a procedure for determining a target gain calibration parameter. 
         FIG. 9  is a diagram illustrating detailed implementations of the comparison module, the gain controller, and the test signal generator shown in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION  
     Certain terms are used throughout the description and following claims to refer to particular components. As one skilled in the art will appreciate, electronic equipment manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following description and in the claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is coupled to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
       FIG. 6  is a diagram of a multi-channel blend system  600  according to an embodiment of the present invention. The multi-channel blend system  600  includes an anti-aliasing filter  605 , a decoding circuit  610 , and a calibration circuit  640 . In this embodiment, the multi-channel blend system  600  is a stereo blend system; however, this is not a limitation of the multi-channel blend system of the present invention. The operation and function of the anti-aliasing filter  605  is similar to that of the anti-aliasing filter  505  as stated above. Except for a gain amplifying module  615  and switch units  6101  and  6103 , operation and function of other elements within the decoding circuit  610  are also similar to those of corresponding elements having the same names within the decoding circuit  510  shown in  FIG. 5 , and thus further description of them is not detailed for brevity. 
     The gain amplifying module  615  provides a gain value AV 1  utilized for determining a separation ratio between a first channel output signal LOUT (i.e. a left channel output signal) and a second channel output signal ROUT (i.e. a right channel output signal) according to a calibration signal S cal , a reference signal S ref , and an indication signal S ind . The calibration circuit  640  generates a predetermined test signal S test  for inputting into the decoding circuit  610  instead of an original input signal during calibration, and determines the calibration signal S cal  according to at least one of the channel output signals LOUT and ROUT generated from the predetermined test signal S test . 
     In this embodiment, the calibration circuit  640  includes a test signal generator  6405 , an indication signal generating module  6410 , a comparison module  6415 , a decision module  6420 , and switch units  6425  and  6430 . The test signal generator  6405  is utilized for generating the predetermined test signal S test , and the indication signal generating module  6410  is used for generating the indication signal S ind  into the gain amplifying module  615 . The indication signal S ind  (i.e. an RSSI value) is indicative of a preset separation ratio between the first and second channel output signals LOUT and ROUT. For example, the value of the indication signal S ind  being 20 dBuV emf means that the preset separation ratio equals 3.52 dB, and the value of the indication signal S ind  being 30 dBuV emf means that the preset separation ratio equals 19.1 dB. Each preset separation ratio corresponds to a specific gain value AV 1  that should be provided by the gain amplifying module  615 . For instance, when the preset separation ratio equals 3.52 dB, the gain value AV 1  is almost equal to 0.2; when the preset separation ratio equals 19.1 dB, the gain value AV 1  is almost equal to 0.8. In addition, the comparison module  6415  compares channel signal magnitudes from the first channel output signal LOUT to output a comparison result; of course, instead of comparing the channel signal magnitudes from the first channel output signal LOUT, calibrating the gain value AV 1  to adjust the separation ratio can also be achieved by comparing channel signal magnitudes from the second channel output signal ROUT, that still falls within the scope of the present invention. Then, the decision module  6420  is utilized for determining the calibration signal S cal  according to the comparison result and the specific gain value, which corresponds to the value of the indication signal S ind  generated from the indication signal generating module  6410  and inputted into the decoding circuit  610  for generating the channel output signals. 
     The predetermined test signal S test  is a DC voltage level and the calibration signal S cal  comprises at least an offset calibration parameter SB off  and a gain calibration parameter SB gain . When adjusting a control voltage V c  for controlling the gain value AV 1  of the amplifier  6151 , the switch unit  6101  is turned off while the switch units  6103 ,  6425 ,  6430 , and  64105  are turned on. The multi-channel blend system  600  receives the predetermined test signal S test  as its input signal and outputs the channel output signals LOUT and ROUT according to the gain value AV 1 . In this embodiment, the control voltage V c  is determined by the indication signal S ind , the reference signal S ref , and the calibration signal S cal  having the offset calibration parameter SB off  and the gain calibration parameter SB gain . This can be simply illustrated by the following equation: 
         V   c   =SB   off   +SB   gain ×( S   ref   −S   ind )   Equation (4) 
     wherein it is assumed that the control voltage V c  and the indication signal S ind  are inversely proportional. However, this is not intended to be a limitation of the present invention. The indication signal S ind  is generated by the indication signal generating module  6410 , which further includes an indication signal generator  64110  and a signal strength indicator  64115 . The indication signal generator  64110  generates an intermediate frequency (IF) signal where the IF signal may be a square wave signal. The signal strength indicator  64115  then outputs a signal strength value according to the IF signal as the indication signal S ind  where the signal strength indicator  64115  is practically an RSSI indicator and usually implemented by a rectifier. 
     During the calibration of the offset point (i.e. the starting splitting point) in the actual transfer curve, initially the gain value AV 1  is assigned to a first initial value (e.g. the first initial value is set as zero in this embodiment), and the voltage level of predetermined test signal S test  (serving as the input signal V in  during the calibration) is set to VDC. The decoding circuit  610  generates the channel output signals LOUT according to the gain value AV 1  of the first initial value, that is, the channel signal magnitude of the channel output signal LOUT equals VDC*AV 2  (due to AV 1 =0) according to Equation (1). 
     Next, the decision module  6420  selects a candidate offset calibration parameter as the offset calibration parameter SB off , and the gain amplifying module  615  generates a first control voltage V c1  so as to provide the gain value AV 1  of a first adjust value according to Equation (4) while the gain calibration parameter SB gain  is not considered. The decoding circuit  610  generates the channel output signals LOUT according to the gain value AV 1  of the first adjust value, that is, the channel signal magnitude of the channel output signal LOUT equals VDC*(1+AV 1 )*AV 2  according to Equation (1). 
     Then, the comparison module  6415  compares channel signal magnitudes of the channel output signal LOUT corresponding to the first initial value and the first adjusted value to generate a first comparison result, e.g. (1+AV 1 ). When an actual gain value derived from the first comparison result is greater than the specific gain value, the decision module  6420  decreases the candidate offset calibration parameter. Otherwise, when the actual gain value derived from the first comparison result is smaller than the specific gain value, the decision module  6420  increases the candidate offset calibration parameter. 
     For example, it can be calculated that the preset separation ratio ideally equals 3.52 dB when the signal strength of the received audio signal corresponds to 20 dBuV emf (the indication signal S ind  corresponds to 20 dBuV emf under this condition). According to Equation (3), if the preset separation ratio equals 3.52 dB, the specific gain value AV 1  should approach 0.2, and therefore the required first comparison value is set as 1.2 since the first initial value of the gain value AV 1  equals zero. The decision module  6420 , e.g. a digital SAR, utilizes a digital successive-approximation algorithm to either increase or decrease the candidate offset calibration parameter SB_OFF and finally determines a target offset calibration parameter SB off  to make the gain value AV 1  approach 0.2. Further description is illustrated in  FIG. 7 . 
       FIG. 7  is a timing diagram showing a procedure for determining the target offset calibration parameter SB off . The candidate offset calibration parameter SB_OFF is represented by a digital code having 5 bits, and is adjusted by one bit per three time slots. The gain value AV 1  is initially assigned as the first initial value (i.e. zero), and the channel signal magnitude of the channel output signal LOUT at time slot T 0  according to the first initial value is equal to S 0 . Next, as mentioned above, the decision module  6420  determines the gain value AV 1  of the first adjusted value according to the candidate offset calibration parameter SB_OFF (i.e. a code ‘10000’ shown in  FIG. 7 ), and the channel signal magnitude of the channel output signal LOUT at time slot T 1  becomes S 1  according to the gain value AV 1  of the first adjusted value. The comparison module  6415  compares the channel signal magnitudes S 0  and S 1  to output the first comparison result of time slot T 2 . In this example, the actual gain value derived from the first comparison result of time slot T 2  is greater than the specific gain value (e.g. 0.2) and the candidate offset calibration parameter SB_OFF is therefore decreased to become another code ‘01000’ during time slots T 3 -T 5 . During the time slots T 3 -T 5 , since the actual gain value derived from the first comparison result of time slot T 5  (e.g. a ratio of a channel signal magnitude S 2  corresponding to the code ‘01000’ to the channel signal magnitude S 0 ) is still greater than the specific gain value (e.g. 0.2), the candidate offset calibration parameter SB_OFF is decreased again to become another code ‘00100’. The above-mentioned procedure is repeatedly performed and ended when the actual gain value derived from the first comparison result is equal to the specific gain value (e.g. 0.2). 
     In the above-described example, the target offset calibration parameter is finally determined as a code ‘00111’, as shown in  FIG. 7 ; the channel signal magnitude of the channel output signal LOUT at time slot T 14  is slightly higher than a channel signal magnitude V 1 ′ which corresponds to the preset separation ratio 3.52 dB. At time slot T 14 , the gain value AV 1  almost equaling 0.2 is achieved and the separation ratio is adjusted precisely. Therefore, the offset point (i.e. the starting splitting point) in the actual transfer curve of the multi-channel blend system  600  is to be calibrated. 
     For calibrating the slope of the actual transfer curve to approximate the slope of the ideal transfer curve CV shown in  FIG. 1 , initially the gain value AV 1  is assigned a second initial value (the second initial value is set as zero in this embodiment), and the voltage level of predetermined test signal S test  (serving as the input signal V in  during the calibration) is set to VDC. The decoding circuit  610  generates the channel output signals according to the gain value AV 1  of the second initial value, that is, the channel signal magnitude of the channel output signal LOUT equals VDC*AV 2  (due to AV 1 =0) according to Equation (1). 
     Next, the decision module  6420  selects a candidate gain calibration parameter as the gain calibration parameter SB gain , and the gain amplifying module  615  generates a second control voltage V c2  to provide the gain value AV 1  of a second adjusted value according to Equation (4) while the offset calibration parameter SB off  is not considered. The decoding circuit  610  generates the channel output signals LOUT according to the gain value AV 1  of the second adjust value, that is, the channel signal magnitude of the channel output signal LOUT equals VDC*(1+AV 1 )*AV 2  according to Equation (1). Then, the comparison module  6415  compares channel signal magnitudes of the channel output signal LOUT corresponding to the second initial value and the second adjusted value to generate a second comparison result, e.g. (1+AV 1 ). When an actual gain value derived from the second comparison result is greater than the specific gain value, the decision module  6420  decreases the candidate gain calibration parameter. Otherwise, the actual gain value derived from the second comparison result is smaller than the specific gain value, the decision module  6420  increases the candidate gain calibration parameter. 
     For example, it can be calculated that the preset separation ratio ideally equals 19.1 dB when the signal strength of the received audio signal corresponds to 30 dBuV emf (the indication signal S ind  corresponds to 30 dBuV emf under this condition). According to Equation (3), if the preset separation ratio equals 19.1 dB, the specific gain value AV 1  should approach 0.8, and therefore the required second comparison value is set as 1.8 since the second initial value of the gain value AV 1  equals zero. The decision module  6420 , e.g. a digital SAR, utilizes the digital successive-approximation algorithm to either increase or decrease the candidate gain calibration parameter SB_GAIN and finally determines a target gain calibration parameter SB gain  to make the gain value AV 1  approach 0.8. Further description is illustrated in  FIG. 8 . 
       FIG. 8  is a timing diagram showing a procedure for determining the target gain calibration parameter SB gain . As shown in  FIG. 8 , the candidate gain calibration parameter SB_GAIN is represented by a digital code having 3 bits, and is adjusted by one bit per three time slots. The gain value AV 1  is initially assigned as the second initial value (e.g. zero), and the channel signal magnitude of the channel output signal LOUT at time slot T 0  according to the second initial value is equal to S 0 ′. Next, the decision module  6420  determines the gain value AV 1  of the second adjusted value according to the candidate gain calibration parameter SB_GAIN (i.e. a code ‘100’ shown in  FIG. 8 ), and the channel signal magnitude of the channel output signal LOUT at time slot T 1  becomes S 1 ′ according to the gain value AV 1  of the second adjusted value. The comparison module  6415  compares the channel signal magnitudes S 0 ′ and S 1 ′ to output the second comparison result of time slot T 2 . In this example, the actual gain value derived from the second comparison result of time slot T 2  is greater than the specific gain value (e.g. 0.8) and the candidate gain calibration parameter SB_GAIN is therefore decreased to become another code ‘010’ during time slots T 3 -T 5 . During the time slots T 3 -T 5 , since the actual gain value derived from the second comparison result of time slot T 5  is smaller than the specific gain value (e.g. 0.8), the candidate gain calibration parameter SB_GAIN is then increased to become a code ‘011’. The above-mentioned procedure is repeatedly performed and ended when the actual gain value derived from the second comparison result is equal to the specific gain value (e.g. 0.8). 
     In the above-described example, the target gain calibration parameter is found and equals the value of the code ‘011’, as shown in  FIG. 8 ; the channel signal magnitude of the channel output signal LOUT at time slot T 8  is slightly higher than a channel signal magnitude V 2 ′ which corresponds to the preset separation ratio 19.1 dB. At time slot T 8 , the gain value AV 1  almost equaling 0.8 is achieved and the separation ratio is adjusted precisely. Accordingly, the slope of the actual transfer curve of the multi-channel blend system  600  is also to be calibrated. 
     More particularly, implementations of the comparison module  6415 , the gain controller  6153 , and the test signal generator  6405  are illustrated in  FIG. 9 . As shown in  FIG. 9 , the comparison module  6415  compares the channel signal magnitudes of the channel output signal LOUT at different time slots by using a switched capacitor technique, and the gain controller  6153  uses a difference amplifier to generate the control voltage for controlling the gain value AV 1 . For brevity, the indication signal generating module  6410  is not shown in  FIG. 9  and further description of the comparison module  6415 , gain controller  6153 , and the test signal generator  6405  is also not detailed here. Furthermore, the above-mentioned first and second initial values can be modified according to design requirements. Of course, the above-mentioned step of calibrating the slope of an actual transfer curve can be performed before the step of calibrating the offset point arising in the actual transfer curve; calibrating only the slope of the actual transfer curve or the offset point arising in the actual transfer curve also helps to reduce the problems due to the actual transfer curve. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.