Abstract:
A differential phase detection device receives first through fourth detection signals from a photodetector and detects a differential phase signal therefrom, the differential phase detection device includes a slicer slicing and digitizing each of the detection signals with respect to a reference level. A synthesizer synthesizes the digitized detection signals and generates therefrom synthesis signals. A phase difference detector compares phases of the synthesis signals and outputs a first phase difference signal and a second phase difference signal. A matrix circuit processes the first and second phase difference signals to output the differential phase signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Korean Application No. 2000-82052, filed Dec. 26, 2000, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a differential phase detection device having an improved structure and a tracking error signal detection apparatus using the same. 
   2. Description of the Related Art 
   Generally, as shown in  FIGS. 1 and 2 , a tracking error signal detection apparatus using a differential phase detection (DPD) device includes a four-division photodetector  10  for receiving light reflected from a recording medium, and differential phase detection devices  20  and  30  for receiving four detection signals a, b, c, and d output from the four-division photodetector  10 , generating a plurality of phase difference signals P 1 , P 2 , P 3 , P 4 , P 5 , and P 6 , and performing an operation on the plurality of phase difference signals to detect a tracking error signal TES′ and TES″. 
   The four-division photodetector  10  is divided into two parts, one part in a direction (R′ direction) corresponding to a radial direction of the recording medium and another part in a direction (T′ direction) corresponding to a tangential direction of the recording medium. First through fourth division plates A, B, C, and D of the four-division photodetector  10  are arranged counterclockwise and generate the first through fourth detection signals a, b, c and d, respectively. 
   Referring to  FIG. 1 , the differential phase detection device  20 , as an example of conventional one, includes four capacitors  21 , four equalizers  22 , four slicers  23 , four phase shifters  24 , two phase difference detectors  25  and  27  and a matrix circuit  29 . The capacitors  21  perform AC coupling on the respective first through fourth detection signals a, b, c, and d, thereby removing DC components. Each equalizer  22  amplifies the high-frequency component of a signal input via the corresponding capacitor  21 . Each slicer  23  digitizes the signal amplified by the corresponding equalizer  22 . Each phase shifter  24  shifts the phase of the digitized signal to control an offset or a balance of a final output. The phase difference detector  25  detects a phase difference between the digitized signals corresponding to the first and second detection signals a and b, which are input from corresponding two phase shifters  24 , and outputs first and second phase difference signals p 1  and p 2 . The other phase difference detector  27  detects a phase difference between the digitized signals corresponding to the third and fourth detection signals c and d, which are input from corresponding two phase shifters  24 , and outputs third and fourth phase difference signals p 3  and p 4 . The matrix circuit receives the first through fourth phase difference signals p 1 , p 2 , p 3 , and p 4  and performs an operation on the signals, thereby outputting the tracking error signal TES′. The tracking error signal TES′ is a differential signal between a sum p 1 +p 3  of the first and third phase difference signals p 1  and p 3  and a sum p 2 +p 4  of the second and fourth phase difference signals p 2  and p 4 . Because the conventional differential phase detection device  20  includes the two phase difference detectors  25  and  27  to detect the phase difference signals, the entire volume of the differential phase detection device  20  is large, and an output signal can be degraded due to the difference between the two phase difference detectors  25  and  27  in a gain characteristic. 
   Referring to  FIG. 2 , the differential phase detection device  30 , as another example of a conventional device, has a structure that overcomes the problems of the differential phase detection device  20  of  FIG. 1 . The differential phase detection device  30  includes four capacitors  31 , four delay units  32 , two equalizers  33   a  and  33   b , two slicers  34   a  and  34   b , two phase shifters  35   a  and  35   b , a phase difference detector  37  and a matrix circuit  39 . In  FIGS. 1 and 2 , the same reference numerals denote same elements having the same functions. 
   The capacitors  31  perform AC coupling on respective first through fourth detection signals a, b, c and d received from the four-division photodetector  10 , thereby removing DC components. The delay units  32  time-delay the first through fourth detection signals a, b, c, and d received from the respective capacitors  31 . The delay units  32  are provided for compensating for an offset of a final output, that is, a tracking error signal TES″, when a shift of an objective lens of an optical pickup occurs, or when a depth of a pit recorded on an optical disc deviates from a specified value. The delay units  32  relatively delay the first and second detection signals a and b output from the preceding first and second division plates A and B, respectively, in a T′ direction or the third and fourth detection signals c and d output from the succeeding third and fourth division plates C and D, respectively. The delay units  32  appropriately delay the first detection signal a and/or the third detection signal c such that a delay value applied to the first detection signal a becomes positive or negative with respect to the third detection signal c. Likely, the delay units  32  appropriately delay the second detection signal b and/or the fourth detection signal d such that a delay value applied to the second detection signal b becomes positive or negative with respect to the fourth detection signal d. 
   The first and third detection signals a and c output from the delay units  32  are summed and equalized by the equalizer  33   a . The second and fourth detection signals b and d output from the delay units  32  are summed and equalized by the equalizer  33   b . The slicers  34   a  and  34   b  digitize the amplified sum signals from the equalizers  33   a  and  33   b , respectively. The phase shifters  35   a  and  35   b  shift phases of the respective digitized sum signals to control an offset or a balance of a final output. The phase difference detector  37  detects a phase difference between sum signals received from the respective phase shifters  35   a  and  35   b  and outputs two phase difference signals p 5  and p 6 . The matrix circuit  39  performs a differential operation on the two phase difference signals p 5  and p 6  received from the phase difference detector  37  to output the tracking error signal TES″. 
   Because the differential phase detection device  30  of  FIG. 2  includes one phase difference detector  37 , a problem of a gain error between the two phase difference detectors  25  and  27  occurring in the device of  FIG. 1  does not occur. However, in order to realize a structure using the one phase difference detector  37 , the differential phase detection device  30  of  FIG. 2  employs the four delay units  32  to compensate for the phase difference between the first and third detection signals a and c and the phase difference between the second and fourth detection signals b and d. Accordingly, the total volume of the differential phase detection device  30  is large due to a large volume of the block of the delays  32 , and a large amount of power is required to operate the delays  32 . In addition, because the delay values of the delay units  32  are related to the frequency of a reproduced signal, a unit for selecting a delay value appropriate for a multiple speed at which data will be reproduced from an optical disc is required. Consequently, the differential phase detection device  30  of  FIG. 2  has a larger volume than the differential phase detection device  20  of  FIG. 1  and consumes a larger amount of power. 
   SUMMARY OF THE INVENTION 
   Various objects and advantages of the invention will be set forth in part in the description that follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
   To solve the above problems, the present invention to provides a differential phase detection device having an improved structure for employing a single phase difference detector without using a delay unit and a tracking error signal detection apparatus using the same. 
   Accordingly, to achieve the above and other objects of the invention, there is provided a differential phase detection device detecting a differential phase signal from first through fourth signals. The differential phase detection device includes a slicer slicing and digitizing each of the first through fourth signals with respect to a reference level; a first synthesizer synthesizing the first digitized signal and the third digitized signal to generate a first synthesis signal; a second synthesizer synthesizing the second digitized signal and the fourth digitized signal to generate a second synthesis signal; a phase difference detector comparing a phase of the first synthesis signal with a phase of the second synthesis signal to generate a first phase difference signal and a second phase difference signal; and a matrix circuit determining a difference between the first and second phase difference signals received from the phase difference detector to output the differential phase signal. 
   Here, the first and second synthesizers perform a synthesis method including one of time averaging, an AND operation, or an OR operation on the first and third detection signals and the second and fourth detection signals to generate the first and second synthesis signals, respectively. 
   The differential phase detection device further includes an alternating current (AC) coupler removing direct current (DC) components from the first through fourth signals. An equalizer amplifies the first through fourth signals or the first through fourth signals from the AC coupler, wherein the equalizer is provided between the AC coupler and the slicer. 
   The differential phase detection device further includes a phase shifter shifting a phase of an input signal between the slicer and the first and second synthesizers or between the first and second synthesizers and the phase difference detector. 
   There is also provided a tracking error signal detection apparatus including a photodetector including first through fourth division plates which are disposed counterclockwise or clockwise along directions corresponding to radial and tangential directions of the recording medium, wherein the first and third division plates are positioned in one diagonal direction and the second and fourth division plates are position in another diagonal direction and the first through fourth division plates receive light reflected from a recording medium to generate first through fourth detection signals, respectively; and a differential phase detection device detecting a tracking error signal from the first through fourth detection signals output from the photodetector, the differential phase detection device including: a slicer slicing and digitizing each of the first through fourth detection signals with respect to a reference level, a first synthesizer synthesizing the first digitized signal and the third digitized signal to generate a first synthesis signal, a second synthesizer synthesizing the second signal and the fourth signal to generate a second synthesis signal, a phase difference detector comparing a phase of the first synthesis signal with a phase of the second synthesis signal to generate a first phase difference signal and a second phase difference signal, and a matrix circuit processing the first or the second phase difference signals received from the phase difference detector to output a tracking error signal. 
   These together with other objects and advantages, which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part thereof, wherein like numerals refer to like parts throughout. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above object and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a schematic block diagram illustrating a tracking error signal detection apparatus using a conventional differential phase detection device; 
       FIG. 2  is a schematic block diagram illustrating a tracking error signal detection apparatus using another conventional differential phase detection device; 
       FIG. 3  is a schematic block diagram illustrating a tracking error signal detection apparatus using a differential phase detection device according to an embodiment of the present invention; 
       FIG. 4  is a graph illustrating a principle of synthesis of signals performed by time averaging, an AND operation, and an OR operation in each synthesizer of the differential phase detection device of  FIG. 3 ; 
       FIG. 5  is a schematic block diagram illustrating a tracking error signal detection apparatus using a differential phase detection device according to an alternative embodiment of the present invention; 
       FIG. 6A  is a graph illustrating a tracking error signal detected by the conventional differential phase detection device of  FIG. 2  with respect to a DVD-ROM disc; 
       FIG. 6B  is a graph illustrating a tracking error signal detected by the conventional differential phase detection device of  FIG. 2  with respect to a DVD-RW disc; 
       FIG. 7A  is a graph illustrating a tracking error signal detected by the differential phase detection device according to the present invention with respect to a DVD-ROM disc, when each of the first and second synthesizers of the differential phase detection device is realized as an AND gate; 
       FIG. 7B  is a graph illustrating a tracking error signal detected by the differential phase detection device according to the present invention with respect to a DVD-RW disc, when each of the first and second synthesizers of the differential phase detection device is realized as an AND gate; 
       FIG. 8A  is a graph illustrating the tracking error signals detected by the differential phase detection device according to the present invention with respect to a DVD-ROM disc, when each of the first and second synthesizers of the differential phase detection device is realized as an OR gate; 
       FIG. 8B  is a graph illustrating the tracking error signal detected by the differential phase detection device according to the present invention with respect to a DVD-RW disc, when each of the first and second synthesizers of the differential phase detection device is realized as an OR gate; 
       FIG. 9  is a graph illustrating an offset of the tracking error signal detected by the tracking error signal detection apparatus according to the present invention in an on-track state with respect to a radial tilt; 
       FIG. 10  is a graph illustrating an offset of the tracking error signal detected by the tracking error signal detection apparatus according to the present invention in the on-track state with respect to a tangential tilt; 
       FIG. 11  is a graph illustrating an offset of the tracking error signal detected by the tracking error signal detection apparatus according to the present invention in the on-track state with respect to a defocus; 
       FIG. 12  is a graph illustrating an offset of the tracking error signal detected by the tracking error signal detection apparatus according to the present invention in the on-track state with respect to a detrack; 
       FIG. 13  is a graph illustrating an offset of the tracking error signal detected by the tracking error signal detection apparatus according to the present invention in the on-track state with respect to a shift of an objective lens; 
       FIG. 14  is a graph illustrating an offset of the tracking error signal detected by the tracking error signal detection apparatus according to the present invention in the on-track state with respect to a change in the thickness of an optical disc; 
       FIG. 15  is a graph illustrating an offset of the tracking error signal detected by the tracking error signal detection apparatus according to the present invention in the on-track state with respect to a change in a depth of a pit recorded on the optical disc; and 
       FIG. 16  is a graph illustrating an offset of the tracking error signal detected by the tracking error signal detection apparatus according to the present invention in the on-track state with respect to a shift of the objective lens plus a change in a depth of a pit recorded on the optical disc. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3  is a schematic block diagram illustrating an embodiment of a tracking error signal detection apparatus using a differential phase detection device according to the present invention. Referring to  FIG. 3 , the tracking error signal detection apparatus includes a photodetector  50  for receiving light reflected from a recording medium and a differential phase detection device  70  for detecting a differential phase from detection signals a, b, c and d output from the photodetector  50 . 
   The photodetector  50  includes first through fourth division plates A, B, C and D in R′ and T′ directions counterclockwise (or clockwise). The first and third division plates A and C are positioned in one diagonal line, and the second and fourth division plates B and D are positioned in another diagonal line. The first through fourth detection signals a, b, c and d output from the first through fourth division plates A, B, C and D, respectively, are input to the differential phase detection device  70 . 
   The differential phase detection device  70  includes a slicer unit  75  having four slicers, first and second synthesizers  77  and  79 , a phase difference detector  85  and a matrix circuit  87 . The differential phase detection device  70  may also include an AC coupling unit  71  and/or an equalizer unit  73  prior to the slicer unit  75 . 
   The AC coupling unit  71  includes capacitors to perform AC coupling on the first through fourth detection signals a, b, c and d to remove DC components. After passing through the AC coupling unit  71 , the first through fourth detection signals a, b, c and d only have high frequency components. The equalizer unit  73  amplifies the high frequency components of the first through fourth detection signals a, b, c and d from the AC coupling unit  71 . The slicer unit  75  slices and digitizes each of the first through fourth detection signals a, b, c and d from the equalizer unit  73  with respect to a predetermined reference level, thereby generating first through fourth digitized signals d 1 , d 2 , d 3 , and d 4 . 
   Each of the first and second synthesizers  77  and  79  is a logical matrix and includes a unit for time-averaging an input signal, an AND gate, or an OR gate. The first synthesizer  77  performs time averaging, an AND operation or an OR operation on the first and third digitized signals d 1  and d 3  from the slicer unit  75 , thereby generating a first synthesis signal sa. Like the first synthesizer  77 , the second synthesizer  79  performs time averaging, an AND operation or an OR operation on the second and fourth digitized signals d 2  and d 4  from the slicer unit  75 , thereby generating a second synthesis signal sb. 
   As shown in  FIG. 4 , when the first synthesizer  77  performs time-averaging on the first and third digitized signals d 1  and d 3 , a first synthesis signal sa AVERAGE  is generated. When the first synthesizer  77  performs an AND operation on the first and third digitized signals d 1  and d 3 , a first synthesis signal sa AND  is generated. When the first synthesizer  77  performs an OR operation on the first and third digitized signals d 1  and d 3 , a first synthesis signal sa OR  is generated. The second synthesizer  79  also generates the second synthesis signal sb by synthesizing the second and fourth digitized signals d 2  and d 4  similarly to the first synthesizer  77 . Because each of the first and second synthesis signals sa and sb generated by the first and second synthesizers  77  and  79 , respectively, changes a little depending on a synthesizing method used, the first and second synthesizers  77  and  79  may be provided such that the first and second synthesizers  77  and  79  can generate the first and second synthesis signals sa and sb using the same synthesis method. 
   The phase difference detector  85  detects a phase difference between the first and second synthesis signals sa and sb output from the first and second synthesizers  77  and  79 , thereby outputting first and second phase difference signals pa and pb. It is assumed that the first synthesis signal sa is input to a positive (+) input terminal of the phase difference detector  85 , and the second synthesis signal sb is input to a negative (−) input terminal of the phase difference detector  85 . The first phase difference signal pa corresponds to a phase difference between the first and second synthesis signals sa and sb when the phase of the first synthesis signal sa input to the (+) input terminal leads the phase of the second synthesis signal sb. The second phase difference signal pb corresponds to the phase difference between the first and second synthesis signals sa and sb when the phase of the second synthesis signal sb input to the (−) input terminal leads the phase of the first synthesis signal sa. 
   When the differential phase detection device  70  is used in the tracking error signal detection apparatus, the matrix circuit  87  performs a differential operation and integration on the first and second phase difference signals pa and pb output from the phase difference detector  85 . A result signal from the matrix circuit  87  by performing the differential operation and integration on the first and second phase difference signals pa and pb is a tracking error signal TES. The matrix circuit  87  of the differential phase detection device  70  is appropriately modified depending on a system or apparatus employing the differential phase detection device  70 . For example, when the differential phase detection device  70  is employed by a seek direction detection system; the matrix circuit  87  has a structure of summing and integrating the two input phase difference signals pa and pb. 
   The differential phase detection device  70  according to the present invention, for instance, may include a phase shifter  81  for shifting a phase of an input signal to control an offset or balance of a final output. In this instance, two phase shifters  81  may be installed such that one phase shifter is disposed between the first synthesizer  77  and one input terminal of the phase difference detector  85 , and the other phase shifter is disposed between the second synthesizer  79  and the other input terminal of the phase difference detector  85 . Alternatively, the phase shifter  81  may be disposed between the slicer unit  75  and the input terminals of the first and second synthesizers  77  and  79 , as shown in  FIG. 5 . When the phase shifter  81  is disposed between the slicer unit  75  and the input terminals of the first and second synthesizers  77  and  79 , as shown in  FIG. 5 , different time delays may be applied to each individual first through fourth digitized signals d 1 , d 2 , d 3 , and d 4 , so that an offset and balance of an output signal can be controlled more effectively. 
   The following description concerns characteristics of a signal detected by a tracking error signal detection apparatus employing the differential phase detection device  70  according to the present invention.  FIGS. 6A and 6B  are graphs illustrating tracking error signals TES″ detected by the conventional differential phase detection device  30  of  FIG. 2  with respect to a DVD-ROM disc and a DVD-RW disc, respectively.  FIGS. 7A and 7B  are graphs illustrating tracking error signals TES detected by the differential phase detection device  70  according to the present invention with respect to a DVD-ROM disc and a DVD-RW disc, respectively, when each of the first and second synthesizers  77  and  79  is realized as an AND gate.  FIGS. 8A and 8B  are graphs illustrating tracking error signals TES detected by the differential phase detection device  70  according to the present invention with respect to a DVD-ROM disc and a DVD-RW disc, respectively, when each of the first and second synthesizers  77  and  79  is realized as an OR gate. 
   As shown in  FIGS. 6A ,  7 A, and  8 A, for a DVD-ROM disc, the differential phase detection device  70  according to the present invention can detect a tracking error signal which is not inferior to that detected by the conventional differential phase detection device  70  employing the delay unit  32 . As shown in  FIGS. 6B ,  7 B, and  8 B, for a DVD-RW disc, the differential phase detection device  70  according to the present invention can also detect a tracking error signal which is not inferior to that detected by the conventional differential phase detection device  70  employing the delay unit  32 . In addition, it can be appreciated by comparing  FIG. 7A  and  FIG. 8A  and comparing  FIG. 7B  and  FIG. 8B , tracking error signals including similar qualities are detected when each of the first and second synthesizers  77  and  79  is realized as an AND gate and when each of the first and second synthesizers  77  and  79  is realized as an OR gate. 
     FIGS. 9 through 15  are graphs illustrating offsets of the tracking error signal TES detected by the tracking error signal detection apparatus according to the present invention in an on-track state with respect to a radial tilt, a tangential tilt, a defocus, a detrack, a shift of an objective lens, a change in thickness of an optical disc, a change in depth of a pit recorded on the optical disc, respectively. Here, the optical disc is a DVD-ROM. 
   As shown in  FIGS. 9 through 15 , a tracking error signal detection apparatus, according to the present invention, can detect a tracking error signal TES which is rarely affected by the radial tilt, the tangential tilt, the defocus, the shift of the objective lens, the change in the thickness of the optical disc, and the change in the depth of the pit, but reacts greatly to the detrack. Referring to  FIG. 9 , a tracking error signal TES detected by the tracking error signal detection apparatus, according to the present invention, is slightly influenced by the radial tilt, but this influence can be ignored because an offset value with respect to the radial tilt is within a tilt margin of the system. 
   Even when a change in the depth of the pit recorded on an optical disc and the shift of an objective lens occur simultaneously, as shown in  FIG. 16 , the tracking error signal detected by the tracking error signal detection apparatus, according to the present invention, does not have an offset.  FIG. 16  is a graph illustrating an offset depending on a change in the depth of the pit in a state in which the objective lens is shifted by 200 μm. As seen from the above description, the differential phase detection device  70  according to the present invention includes fewer components than the conventional differential phase detection device  70  and responds to and compensates for offsets similarly to the conventional differential phase detection device  70 . 
   It has been described that the differential phase detection device  70  according to the present invention receives and performs an operation on the four detection signals a, b, c and d output from the four-division photodetector  50  and outputs the tracking error signal TES according to a differential phase detection method. Specifically, the differential phase detection device  70 , according to the present invention, may be modified and thereby applied to a variety of systems for detecting a differential phase. Here, a divided structure of the photodetector  50  varies with a type (e.g., radial tilt, tangential tilt or defocus) of differential phase signal to be detected. 
   As described above, the differential phase detection device, according to the present invention, includes a pair of synthesizers for synthesizing two digitized signals so that a delay unit conventionally used may be omitted while only a single phase difference detector is used. Consequently, the number of components is less than the number of components used by conventional tracking error signal detection apparatuses thereby reducing the size of the differential phase detection device, according to the present invention. In addition, a tracking error signal detection apparatus employing a differential phase detection device, according to the present invention, can detect a tracking error signal that is relatively less influenced by a radial tilt, a tangential tilt, a defocus, a change in thickness of an optical disc, a change in depth of a pit recorded on an optical disc and/or a shift of an objective lens, compared to the conventional differential phase detection device. 
   While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.