Abstract:
A signal distortion compensating apparatus and method in a digital TV translator are provided that extract a reference signal from a modulator output of a digital TV translator, and conduct a pre-correction based on the extracted reference signal to compensate for a signal distortion involved in the digital TV translator. The compensating apparatus can include a modulator that modulates an input signal while conducting a distortion pre-correction controlled by a control signal applied thereto on the input signal. The modulator outputs baseband signals divided from the input signal. A signal processing unit processes the signals outputted from the modulator, and transmits the resultant signal to subscribers. An auto correction unit compares the signal, which is outputted in a distorted state from the signal processing unit, with a reference signal derived from the baseband signals output by the modulator, and generates the control signal based on the result of the comparison.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a signal distortion compensating apparatus and method, and more particularly to a signal distortion compensating apparatus and method in a digital TV translator. 
     2. Background of the Related Art 
     A variety of linearizing techniques have been proposed to compensate for non-linear behaviors of radio frequency (RF) high-power amplifiers (HPAs) to allow those RFHPAs to meet a standard associated with digital communication systems. Representative related art linearizing methods include a pre-distortion scheme, a feed-forward scheme, and a look-up table (LUT) scheme using a vector signal analyzer. 
     First, a linearizing method based on the related art pre-distortion scheme will be described. In accordance with this linearizing method, a pre-distorter is arranged upstream from the main amplifier in an amplifying system. The pre-distorter has characteristics opposite to those of the main amplifier. Thus, the upstream pre-distorter compensates for a signal distortion occurring in a particular direction in terms of gain and phase characteristics when an input signal passes through the main amplifier, thereby maintaining the output signal from the amplifying system to be the same as the input signal in terms of characteristics. The related art pre-distortion linearizing method makes it possible to obtain outputs improved in non-linearity. 
     In accordance with a linearizing method based on the related art feed-forward scheme, a delay line and an error amplifier are used. When an output signal from a main amplifier amplifying an input signal contains distorted intermodulation components, a 180° phase adjustment is conducted for those distorted intermodulation components by the delay line and the error amplifier to offset phase distortions of the intermodulation components. As a result, only the signal components similar to the components of the original input signal are outputted. 
     However, both of the above described related art pre-distortion and feed-forward linearizing methods can be implemented only with hardware configurations. Further, they are limited and problenmatic in that only the linear distortion can be compensated. 
     For this reason, a related art linearizing method based on a look-up table scheme using a vector signal analyzer has been mainly used. FIG. 1 is a block diagram schematically illustrating a related art signal distortion compensating apparatus applied to digital TV translators. 
     Referring to FIG. 1, the related art signal distortion compensating apparatus includes a transmitting unit  100  and a linear/non-linear correction receiving unit  200 . The transmitting unit  100  includes a modulator  110  consisting of a vestigial sideband (VSB) processor  111 , a linear filter  112 , and a non-linear pre-corrector  113 . The transmitting unit  100  also includes an intermediate frequency (IF) modulator  120 , an up-converter  130 , an intermediate power amplifier (IPA)  140 , an HPA  150 , a directional coupler  160 , and an antenna  170 . The linear/non-linear correction receiving unit  200  includes a vector signal analyzer  210  consisting of a down-converter  211  and a demodulator  212 , and a computer  220 . 
     FIG. 2 is a block diagram schematically illustrating an internal software configuration of the computer  220  for producing a reference signal associated with a distorted signal in the related art signal distortion compensating apparatus shown in FIG.  1 . Referring to FIG. 2, the internal software configuration of the computer  220  includes a termination slice unit  221  a linear filter unit  222 , a non-linear pre-corrector  223 , a root raised cosine (RRC) filter unit  224 , a scaling unit  225 , a pilot signal recovering unit  228 , a complex division unit  226 , and a look-up table (LUT) storing memory  227 . The related art signal distortion compensating apparatus will now be described in additional detail, in conjunction with FIGS. 1 and 2. 
     Input data is first applied to the VSB processor  111 , which in turn conducts a channel coding for the input signal, thereby producing symbols. The produced symbols are applied to the linear filter  112 , which in turn produces an in-phase (I) signal and a quadrature (Q) signal, based on the symbols. 
     The I and Q signals from the linear filter  112  are transmitted to the IF modulator  120 , and then to the up-converter  130 . The I and Q signals are subjected to a frequency up-conversion while passing through the up-converter  130 . The resultant signals pass through the IPA  140  and HPA  150 . The resultant output signal from the HPA  150  is transmitted as a TV signal for general subscribers over the antenna  170  after passing through the directional coupler  160 . 
     However, the TV signal, which is transmitted to general subscribers from the antenna  170 , may involve a distortion because the VSB signal outputted from the baseband modulator  110  may be distorted because of non-linear factors of temperature, degradation, and noise while passing through the IF modulator  120 , IPA  140 , and HPA  150 . To correct such a signal distortion, distortion components are extracted from the output signal of the HPA  150  through the directional coupler  160 . Based on the extracted distorted signal, a reference signal is also produced in accordance with a termination slice scheme. The reference signal is then compared with the distorted signal. Based on the result of the comparison, the signal distortion of the transmitting unit  100  is measured. 
     That is, the extracted distorted signal from the directional coupler  160  is applied to the down-converter  211  of the vector signal analyzer  210  that down-converts the up-converted frequency signal into an IF signal of 44 MHz. The resultant signal outputted from the clown-converter  211  passes through the demodulator  212 , which in turn extracts I and Q digital data. 
     Based on the extracted I and Q digital signals from the demodulator  212 , a reference signal is produced using the internal software of the computer  220  to correct a distortion in the transmission signal outputted from the transmitting unit. The software producing the reference signal consists of a routine for producing a reference signal for distortion correction based on a distorted signal, which is illustrated in FIG.  2 . 
     Referring to FIG. 2, a source reference signal is produced in the termination slice unit  221  based on the distorted signal. The source reference signal is then sequentially processed by the linear filter unit  222  and the non-linear pre-corrector  223 , so that it has the same signal as that of the output from the modulator shown in FIG.  1 . 
     Since the distorted signal has been subjected to a root raised cosine filtering process, a pilot removal process, and a scaling process while passing through the vector signal analyzer, the reference signal should also be subjected to those processes so that it can be compared with the distorted signal. To this end, those processes are implemented in a software fashion by the RRC unit  224  and the scaling unit  225 . 
     The resultant reference signal is then applied to the complex division unit  226  at which the distorted signal emerging from the pilot signal recovering unit  228  is also received. Based on the distorted signal and the reference signal extracted using the distorted signal, the complex division unit  226  produces an LUT coefficient for error calculation and non-lineaarity correction. 
     The produced LUT coefficient is temporarily stored in the LUT storing memory  227 , and then inputted to the linear filter  112  and the non-linear pre-corrector  113  via a standard serial cable such as an RS-232C. The linear distortion may be corrected using an adaptive complex equalizer included in the vector signal analyzer. 
     As described above, the related art signal distortion compensating apparatus and method applied to digital TV translators have various problems. First, it is necessary to use additional devices such as the vector signal analyzer and computer, which result in a great increase in costs. Second, the signal distortion compensation method based on the look-up table scheme using the vector signal analyzer requires a lengthened period of time for coefficient calculation because it uses software processing. Third, the signal distortion compensation method based on the look-up table scheme using the vector signal analyzer involves a complicated production of the reference signal in that the RF signal outputted after passing through the IF modulator, IPA, and HPA is down-converted again. Fourth, the vector signal analyzer involves an inconvenience in transportation at least in the case when it is to be used for an unmanned translator installed at an alpine region because it is heavy. 
     The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter. 
     An object of the present invention is to provide an apparatus and method of using same that solves one or more problems caused by the limitations and disadvantages of the related art. 
     Another object of the present invention is to provide a signal distortion compensating apparatus and method in a digital TV translator that extracts a reference signal from a modulator output of a digital TV translator, and conducts a pre-correction based on the extracted reference signal to compensate for a signal distortion involved in the digital TV translator. 
     Another object of the present invention is to provide a signal distortion compensating apparatus and method in a digital TV translator that compensates for distorted signals using a software control technique without using additional hardware. 
     Another object of the present invention is to provide a signal distortion compensating apparatus and method that removes or reduces non-linearity resulting from non-linear devices such as high power amplifiers having non-linear characteristics, or other factors such as degradation or temperature variation. 
     In order to achieve at least the above-described objects of the present invention in a whole or in parts, there is provided a signal distortion compensating apparatus in a digital TV translator that includes a modulator that modulates an input signal, wherein the modulator outputs baseband signals divided from the input signal, a signal processor that processes the baseband signals from the modulator, and transmits a resultant signal to subscribers, and an auto correction unit that compares the resultant signal outputted by the signal processor in a distorted state with a reference signal derived from the baseband signals of the modulator, and generates a control signal based on the comparison, wherein the modulator receives the control signal and conducts a distortion pre-correction for the input signal. 
     To further achieve the above-described objects of the present invention in a whole or in parts, there is provided a signal distortion compensating apparatus in a digital TV translator that includes a vestigial sideband (VSB) modulator that modulates an input signal, a signal processor that processes an output signal from the VSB modulator and transmits the processed signal to subscribers, a VSB receiver that receives the processed signal output by the signal processor and generates a VSB demodulation signal based on the processed signal, and a controller that receives the VSB demodulation signal from the VSB receiver, demodulates the VSB demodulation signal, calculates a distortion pre- correction value based on the demodulated signal, and controls the VSB modulator based on the calculated distortion pre-correction value. 
     To further achieve the above-described objects of the present invention in a xv hole or in parts, there is provided a signal distortion compensating method in a digital TV translator that includes modulating an input signal, dividing baseband signals from the modulated signal, and outputting the divided baseband signals, processing the divided baseband signals to transmit resultant signals to subscribers, and comparing one of the resultant signals, which is distorted during the processing step with an associated one of the divided baseband signals as a reference signal and generating a distortion correction signal, wherein the modulating step further performs pre-distortion correcting responsive to the distortion correction signal. 
     To further achieve the above-described objects of the present invention in a whole or in parts, there is provided a signal distortion compensating method in a digital TV translator that includes modulating an input signal to output modulated signals, processing the modulated signals and transmitting a resultant signal to subscribers, processing a signal distorted from each of the modulated signals during an execution of the step to extract distortion components from the distorted signal, executing a vestigial sideband demodulation for the distortion components extracted from the distorted signal, and calculating a correction value for the distorted signal based on the demodulated distortion components. 
     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein: 
     FIG. 1 is a block diagram schematically illustrating a related art signal distortion compensating apparatus applied to digital TV translators; 
     FIG. 2 is a block diagram schematically illustrating the internal software configuration of a computer for producing a reference signal associated with a distorted signal in the related art signal distortion compensating apparatus shown in FIG. 1; 
     FIG. 3 is a block diagram schematically illustrating a first preferred embodiment of a signal distortion compensating apparatus in a digital TV translator in accordance with the present invention; 
     FIG. 4 is a diagram illustrating a correction for a non-linear distortion of VSB signals; 
     FIG. 5 is a diagram illustrating a correction for a linear distortion of VSB signals; and 
     FIG. 6 is a block diagram schematically illustrating a second preferred embodiment of a signal distortion compensating apparatus in a digital TV translator in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 3 is a block diagram schematically illustrating a first preferred embodiment of a signal distortion compensating apparatus in a digital TV translator in accordance with the present invention. As shown in FIG. 3, the digital TV translator according to the first preferred embodiment includes a transmitting unit or transmitter  300  and an automatic correction unit  400 . 
     The transmitting unit  300  includes a modulator  310  preferably having a VSB processor  311 , a linear filter  312 , and a non-linear pre-corrector  313 . The transmitting unit  300  also includes a signal processing unit  320  preferably having an IF modulator  321 , an up-converter  322 , an IPA  323  and an HPA  324 . The transmitting unit  300  further includes a directional coupler  330  and an antenna  340 . The automatic correction unit  400  includes a down-converter  410 , an I and Q signal demodulator  420 , an analog/digital (A/D) converter  430 , a data acquisition and time synchronization unit  440 , a complex division unit  450 , an LUT storing unit  480 , an adaptive complex equalizer  460  and a control unit  470 . 
     Operations of the first preferred embodiment of the signal distortion compensating apparatus in the digital TV translator will now be described in conjunction with FIG.  3 . Input data, which in this case is a digital signal converted to have a Moving Picture Expert Group 2 (MPEG2) format, is first applied to the VSB processor  311  that conducts a channel coding for the input signal to produce symbols. The produced symbols are processed by the linear filter  312  that produces an I signal and a Q signal based on those symbols. The I and Q signals from the linear filter  312  are transmitted to the IF modulator  321 , and then to the up-converter  322 . The I and Q signals are subjected to a frequency up-conversion while passing through the up-converter  322 . 
     The up-converted signals pass through the IPA  323  and HPA  324 . The resultant output signal from the HPA  324  is transmitted as a TV signal for general subscribers over the antenna  330  after passing through the directional coupler  330 . 
     However, the VSB signal outputted from the modulator  310  may be distorted because of non-linear factors of temperature, degradation, and noise while passing through the IF modulator  321 , the up-converter  322 , the IPA  323 , and the HPA  324 . Such a distortion of the VSB signal may be a non-linear distortion or a linear distortion. The following description is made in conjunction with the compensation of the VSB signal preferably for both the non-linear distortion and the linear distortion. 
     FIG. 4 is a diagram illustrating a correction for the non-linear distortion of the VSB signal. As shown in FIG. 4, it can be determined that the non-linear distortion of the VSB signal is generated while having a function relation with amplitude and phase. 
     Such a non-linearly distorted signal can preferably be corrected using a signal distortion compensating method in which the distorted signal is compared with a reference signal to calculate the distortion thereof. The reference signal can preferably be derived from the output of the modulator  310  included in the transmitting unit  300 . 
     Further, the distorted signal outputted from the HPA  324  and fed back via the directional coupler  330  is applied to the down-converter  410  that down-converts the applied RF signal into an IF signal preferably having a frequency band of 44 MHz. The resultant signal outputted from the down-converter  410  passes through the I and Q demodulator  420 , which in turn extracts I and Q signals. The A/D converter  430  receives the I and Q signals, and converts them to have the same form as the output signal from the modulator  310  of the transmitting unit  300 . 
     To synchronize in terms of time the distorted signal and the reference signal with each other, both the distorted signal and the reference signal are applied to the data acquisition and time synchronization unit  440 . Using a cross-correlation process, the data acquisition and time synchronization unit  440  preferably synchronizes the distorted signal with the reference signal in terms of time. 
     The synchronization between the reference signal and the distorted signal can be obtained by calculating a correlation of the distorted signal with respect to the reference signal. The data acquisition and time synchronization unit  440  then shifts the distorted signal in a direction exhibiting a higher correlation with respect to the reference signal, and derives a synchronization value at a point where the distorted signal exhibits a highest correlation with respect to the reference signal. 
     Finally, an LUT coefficient for error calculation and non-linear distortion correction is preferably produced in the complex division unit  450  using the synchronized reference and distorted signals. The produced LUT coefficient is temporarily stored in the LUT storing unit  480 . 
     Productions of LUT coefficients can be carried out as follows. First, the reference and distorted signals are normalized in magnitude. Then an error of the reference signal from the distorted signal is calculated. The calculation is achieved by conducting a complex division for the normalized reference and distorted signals because those normalized reference and distorted signals have a complex form of I and Q signals as shown in equation 1 as follows.                    V   ref       V   dis       =         V   P       V   D       =     α   +     j                 β                
          α   =         (       VP   1     *     VD   1       )     +     (       VP   Q     *     VD   Q       )             (     VD   1     )     2     +       (     VD   Q     )     2                
          β   =         (       VP   1     *     VD   1       )     -     (       VP   Q     *     VD   Q       )             (     VD   1     )     2     +       (     VD   Q     )     2                   (   1   )                                
     In Equation 1, “VP 1 ” represents an I signal of the reference signal, “VP Q ” represents a Q signal of the reference signal, “VD 1 ” represents an I signal of the distorted signal, and “VD Q ” represents a Q signal of the reference signal. As shown in Equation 1, the gain and phase values in the non-linear pre-corrector  313  included in the modulator  310  are initially 0 and 1, respectively. Accordingly the initial output signal from the modulator  310  may be expressed as Equation 2 as follows: 
     
       
         ( I+jO )*( I+jQ )= I+jQ   (2) 
       
     
     The initial output signal from the modulator  310  is varied after being processed by a non-linear correction algorithm. The control unit  470  performs a control for sending an output from the complex division unit  450  to the non-linear pre-corrector  313  of the modulator  310  included in the transmitting unit  300  as a coefficient for the correction of the non-linearly distorted signal. Thus, the non-linear characteristics of the transmitting unit  300  are pre-corrected. 
     The auto correction unit  400  can be implemented using very simple hardware and software configurations. However, the present invention is not intended to be so limited. For example, where the auto correction unit  400  is implemented in the form of a digital signal processor (DSP), even the simple hardware configuration can be eliminated. 
     FIG. 5 is a diagram illustrating a correction for the linear distortion of the VSB signal. As shown in FIG. 5, it can be determined that a linear distortion is generated at a bandwidth of 6 MHz of the VSB signal while having a function relation with frequency. Due to such a linear distortion, the VSB signal exhibits a non-flat channel form. 
     To compensate for such a linear distortion, the output signal from the data acquisition and time synchronization unit  400  is processed by a zero-forcing, least means square (LMS) process or algorithm or the like while passing through the adaptive complex equalizer  460  under the control of the control unit  470 . As a result, a tap coefficient for the correction of the linear distortion is produced by the adaptive complex equalizer  460 . 
     The LMS process is preferably an adaptive process using a data channel including actual user information, and a channel for transmitting a known reference signal (training signal) to both the transmitting and receiving stages. The LMS process stably updates the filter coefficient in that it uses the reference signal. In accordance with the LMS process, a convergence of the filter coefficient to a global minimum value is ensured because the evaluation function is convex. 
     FIG. 6 is a block diagram schematically illustrating a second preferred embodiment of a signal distortion compensating apparatus in a digital TV translator in accordance with the present invention. As shown in FIG. 6, the signal distortion compensating apparatus according to the second preferred embodiment includes a signal processing unit  620  with a VSB modulator  610  that amplifies a digital TV signal received by the translator via a receiving antenna included in the translator, an up-converter  621  that converts an output signal from the VSB modulator  610  into a corresponding signal of an RF band, an IPA  622  and a HPA  623 . The apparatus also includes a directional coupler  630 , a VSB receiver or receiving unit  670 , and a computer-based control unit  680 . 
     The VSB receiving unit or receiver  670  includes a down-converter  671 , an I and Q channel separator  672 , an A/D converter  673 , a synchronous signal detector  674 , and an RF controller  675 . The computer-based control unit includes a VSB demodulator  681 , and a linear/non-linear characteristic correction algorithm processor  682 . 
     Operations of the second preferred embodiment of the apparatus according to the present invention will now be described. As shown in FIG. 6, a digital TV signal, which is received by the translator via a receiving antenna included in the translator, is amplified by the VSB modulator  610 . The amplified signal is converted into a corresponding signal of an RF band by the up-converter  621  and amplified again while passing through the IPA  622  and the HPA  623 . The resultant signal is then applied to the directional coupler  630  and fed back to the VSB receiver  670 . 
     The digital TV signal is distorted while passing through the up-converter  621 , the IPA  622 , the HPA  623 , and the directional coupler  630 . The distorted signal, which is received by the VSB receiver  670 , is converted from the form of an RF signal of preferably 470 MHz to 806 MHz to the form of an IF signal of 44 MHz. 
     The resultant 44 MHz signal is separated into I and Q-channel baseband signals by the I and Q channel separator  672 . The I and Q signals are then subjected to an A/D conversion by the A/D converter  673 . 
     A synchronous signal for the A/D converter is supplied from the synchronous signal detector  674 , which is preferably configured using a digital TV VSB transmitting/receiving chip. The I and Q channel separator  672  and the down-converter  671  are preferably controlled by the RF controller  675 . 
     The I and Q signals from the VSB receiver  670  are transmitted to the computer-based control unit  680  via  104 -bus lines, respectively, in order to allow the VSB demodulation thereof to be processed preferably using application software running on the computer. That is, the I and Q signals are subjected to a VSB demodulation while passing through the VSB demodulator  681  of the computer-based control unit  680 , and then applied to the linear/non-linear characteristic correction algorithm processor  682 , which in turn analyzes the characteristics of the applied signals. 
     In the case of a compensation for the characteristics of a linear input signal, the coefficients, which are for I and Q data, of a digital filter internally included in the VSB demodulator  681  are corrected based on the characteristics of the linear input signal. The corrected coefficients are fed back to the VSB modulator  610  via a serial cable such as an RS-232C. 
     In the case of a compensation for the characteristics of a non-linear input signal, the gain and phase values (α and β) in the digital filter of the VSB demodulator  681  are corrected based on the characteristics of the non-linear input signal using Equation 1 described above. The corrected values are fed back to the VSB modulator  610  via the RS-232C serial cable. 
     The application software used for the correction of linear/non-linear characteristics is preferably the same as those in the related art case of FIG.  2 . Accordingly, no detailed description will be made for the software. 
     Using the above mentioned operations, corrections for linear and non-linear characteristics may be conducted in a sequential fashion at the point of time desired by the user. These corrections for linear and non-linear characteristics may also be carried out in an automatic fashion. 
     As described above, preferred embodiments of an apparatus for signal distortion compensation and methods in a digital TV translator have various advantages. In accordance with the preferred embodiments, it is possible to correct linear and non-linear characteristics without using additional hardware such as an expensive vector signal analyzer. It is also possible to automatically execute the correction software for linear/nonlinear characteristics. Further, since a reference signal is extracted from a modulator output, there is an advantage in that superior characteristics are obtained, as compared to the related art in which a reference signal is extracted from a distorted signal. 
     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.