Abstract:
An optical storage apparatus, a preamplifier and a method of generating a tracking error signal. The optical storage apparatus includes a pickup head, a preamplifier, and a servo control. The pickup head generates radio frequency (RF) signals. The preamplifier includes a signal adjusting apparatus, a phase detector, a charge pump, and a circuit. The signal adjusting apparatus has input terminals for receiving the RF signals. At least two of the input terminals are substantially short-circuited when the optical storage apparatus is in a calibration mode. The phase detector generates control signals based on the phase differences between the radio frequency signals. The circuit generates current control signal based on the tracking error signal when the optical storage apparatus is in the calibration mode. The current magnitude of at least one of the current sources in the charge pump is determined according to the current control signal.

Description:
This application claims the benefit of Taiwan application Serial No. 95112899, filed Apr. 11, 2006, the subject matter of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates in general to an optical storage apparatus, and more particularly to a preamplifier for an optical storage apparatus. 
   2. Description of the Related Art 
   An optical storage system, such as a conventional CD system or DVD system, or any other more advanced optical disc system, usually includes several functional blocks, as shown in  FIG. 1 . The blocks include, for example, a pickup head  108  for receiving a reflected beam from an optical disc  110 , a preamplifier (preamp)  102  for receiving a number of radio frequency (RF) signals generated by the pickup head  108 , a servo control  104  for receiving a control signal, such as a tracking error signal TE, generated by the preamplifier  102  and according thereto controlling the various motors of the optical disc system, and a video processor  106  for performing an image decoding process (e.g., the processing according to the MPEG4 specification), according to the information read from the optical disc, to generate a playable video signal. 
   When the optical drive is reading the disc, the laser beam has to be precisely impinged on a track path of the disc without offset in order to make the optical pickup head read data from the disc or write the data to the disc smoothly. The above-mentioned operation has to be performed based on the tracking error signal TE, or the value of any other signal having the same or similar function, so as to obtain a real-time tracking state, and then adjust the operation of the optical pickup head with the servo control. 
   Generally speaking, the tracking error signal TE is generated based on phase differences between RF signals corresponding to various areas, such as areas A, B, C, and D in  FIG. 1 , on the optical pickup head  108 . That is, the value of the tracking error signal TE represents the phase deviations between the RF signals generated by the optical pickup head receiving the reflected beam. It is of great significance for the tracking error signal TE to faithfully reflect such phase deviations, in order to precisely perform the servo control and smoothly read the data on the optical disc. 
   However, non-ideal factors such as circuit mismatch caused by process variation or the path delay mismatch caused by the unsymmetrical circuit layout, often result in the value of the tracking error signal TE being not able to correctly reflect the actual phase deviations or differences in the RF signal, and consequently impose bad influences on the servo control result. 
   SUMMARY OF THE INVENTION 
   The invention is directed to an optical storage apparatus, a preamplifier, and a method of generating a tracking error signal, wherein the precision of the tracking error signal can be enhanced through calibration, which eliminates the phase shift caused in the tracking error signal by circuit mismatch. 
   According to a first aspect of the present invention, an optical storage apparatus is provided. The optical storage apparatus includes a pickup head, a preamplifier, and a servo control. The pickup head receives a beam reflected from an optical disc to generate a number of radio frequency (RF) signals. The preamplifier is coupled to the pickup head. The preamplifier includes a signal adjusting apparatus, a phase detector, a charge pump, and a circuit. The signal adjusting apparatus has a number of input terminals for receiving the RF signals. At least two of the input terminals of the signal adjusting apparatus are substantially short-circuited when the optical storage apparatus is in a calibration mode. The phase detector is coupled to the signal adjusting apparatus. The phase detector generates a number of control signals based on the phase differences between the radio frequency signals. The charge pump is coupled to the phase detector. The charge pump includes a number of current sources and a number of switches coupled to the current sources. The switches are controlled by the control signals to generate a tracking error signal. The circuit is coupled to the charge pump. The circuit generates at least one current control signal based on the tracking error signal when the optical storage apparatus is in the calibration mode. The current magnitude of at least one of the current sources is determined according to the current control signal. The servo control is coupled to the preamplifier and receiving the tracking error signal. 
   According to a second aspect of the present invention, a method of generating a tracking error signal in an optical storage apparatus is provided. The method includes the steps of: receiving a first radio frequency signal at a first input terminal, receiving a second radio frequency signal at a second input terminal, receiving a third radio frequency signal at a third input terminal, and receiving a fourth radio frequency signal at a fourth input terminal; detecting a first phase difference between the first radio frequency signal and the second radio frequency signal, and detecting a second phase difference between the third radio frequency signal and the fourth radio frequency signal; short-circuiting the first input terminal with the second input terminal and short-circuiting the third input terminal with the fourth input terminal when the optical storage apparatus is in a calibration mode, and generating the tracking error signal at an output terminal of a charge pump based on the first phase difference and the second phase difference; adjusting the current magnitude of at least one current sources in the charge pump according to the tracking error signal at the output terminal when the optical storage apparatus is in the calibration mode; and generating the tracking error signal by using the adjusted current sources in the charge pump when the optical storage apparatus is not in the calibration mode. 
   According to a third aspect of the present invention, a preamplifier to be disposed in an optical storage apparatus is provided. The preamplifier includes a first input terminal, a second input terminal, a third input terminal, a fourth input terminal, a first switch, a second switch, a first equalizer, a second equalizer, a third equalizer, a fourth equalizer, a phase detector, a charge pump, and a circuit. The first switch is coupled to and between the first input terminal and the second input terminal. The second switch is coupled to and between the third input terminal and the fourth input terminal. The first equalizer is coupled to the first input terminal. The second equalizer is coupled to the second input terminal. The third equalizer is coupled to the third input terminal. The fourth equalizer is coupled to the fourth input terminal. The phase detector is coupled to the first equalizer, the second equalizer, the third equalizer and the fourth equalizer. The charge pump is coupled to the phase detector. The charge pump includes a number of current sources and a number of switches coupled to the current sources. The circuit is coupled to the charge pump. The circuit generates at least one current control signal based on a tracking error signal generated by the charge pump when the first switch and the second switch are turned on, and the current control signal is outputted to the charge pump to determine the current magnitude of at least one of the current sources. 
   The invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  (Prior Art) is a block diagram showing an optical storage system. 
       FIG. 2  is a block diagram showing a differential phase detector in an optical storage system according to a preferred embodiment of the invention. 
       FIG. 3  is a circuit diagram showing a charge pump according to a preferred embodiment of the invention. 
       FIG. 4  is a circuit diagram showing a variable current source. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  is a block diagram showing a differential phase detector in an optical storage system  200  according to a preferred embodiment of the invention. Referring to  FIG. 2 , the differential phase detector includes a signal adjusting apparatus Q, a phase detector PD 1 , and a charge pump K 2 , and is disposed in a preamplifier of an optical storage apparatus, for detecting phase differences among RF signals A, B, C, and D transmitted from a pickup head and thus generating a tracking error signal TE required by a servo control. 
   The signal adjusting apparatus Q includes high-pass filters HPF 1  to HPF 4 , gain control amplifiers GCA 1  to GCA 4 , equalizers EQ 1  to EQ 4 , and input terminals Ia, Ib, Ic, and Id. 
   The high-pass filter HPF 1  receives a signal A through an input terminal Ia and after high-pass filtering generates a signal SA 1 . A gain control amplifier GCA 1  receives the signal SA 1  and adjusts the level of the signal SA 1  to generate a signal SA 2 . An equalizer EQ 1  improves quality of the signal SA 2  and then generates a signal SA 3 . Similarly, an adjusting path composed of HPF 2 , GCA 2 , and EQ 2 , an adjusting path composed of HPF 3 , GCA 3 , and EQ 3 , and an adjusting path composed of HPF 4 , GCA 4 , and EQ 4  adjust the signals B, C, and D to generate signals SB 3 , SC 3 , and SD 3 , respectively. 
   It is to be appreciated that the functions, operating principles, and implementations of the high-pass filter, the gain control amplifier, and the equalizer in the signal adjusting apparatus Q are well know in the art associated with the optical storage technology, so details thereof will be herein omitted. People of ordinary skill in the art may also understand that the embodiment signal adjusting apparatus Q of  FIG. 2  serves only as an example of the present invention, and other additions, omissions, and modifications to each of the elements are intended to fall within the protected scope of the invention. 
   The phase detector PD 1  detects a phase difference between the signal A and the signal B based on the signal SA 3  and the signal SB 3 , and thus outputs control signals UPab and DNab serving as detected results. The phase detector PD 1  detects a phase difference between a signal C and a signal D based on the signal SC 3  and the signal SD 3 , and thus outputs control signals UPcd and DNcd serving as the detected results. The charge pump K 2  generates and outputs a signal Vout 2  based on control signals UPab, DNab, Upcd, and DNcd. It is to be appreciated that the implementation of the phase detector PD 1  is well known in the art, so detailed descriptions thereof will be herein omitted. 
     FIG. 3  is a circuit diagram showing the charge pump K 2  according to a preferred embodiment of the invention. Referring to  FIG. 3 , the charge pump K 2  includes current sources I 1  to I 4  and switches SW 1  to SW 4 , wherein a capacitor C 41  and a resistor R 41  represent an equivalent RC effect of a later stage circuitry. In this embodiment, the current sources I 1  to I 4  are respectively coupled to an output node Vout 2  through the switches SW 1  to SW 4  and charge/discharge the capacitor C 41  when the corresponding switch turns on, so as to generate the tracking error signal TE. On/off states of the switches SW 1 , SW 2 , SW 3 , and SW 4  are respectively controlled by the control signals Upab, DNab, Upcd, and DNcd generated by the previous stage phase detector PD 1 . According to such a mechanism, the preamplifier can generate the corresponding tracking error signal TE based on detected results of the phase differences between the RF signals A, B, C, and D. 
   In order to eliminate the inferior influence on the detected results of the phase differences between the RF signals caused by the unsymmetrical condition, such as circuit mismatch or geometrical mismatch of circuit layout, in this embodiment a switch apparatus SWAB is coupled to and between the input terminals a 1  and b 1 , which respectively receive the RF signals A and B, of the preamplifier, and a switch apparatus SWCD is coupled to and between the input terminals c 1  and d 1 , which respectively receive the RF signals C and D, of the preamplifier. The switch apparatuses SWAB and SWCD may be implemented by MOS transistors or other frequently used switch apparatuses. 
   When the optical drive is initialized, has not yet engaged in read/write operations, or at any user-specified or periodical time for calibration, a control circuit (not shown) in the optical drive generates a mode signal which turns on the switch SWAB and short-circuits the input terminals Ia and Ib, so that no phase difference exists therebetween. The control circuit also turns on the switch SWCD and short-circuits the input terminals Ia and Ib, so that no phase difference exists therebetween. By so configuring, the phase difference between the signals A and B represented by the control signals UPab and DNab now reflects the influence caused by the circuit mismatch on the phase difference; the phase difference between the signals C and D represented by the control signals UPcd and DNcd reflects the influence caused by the circuit mismatch on the phase difference; and the value of the tracking error signal TE on the output terminal Vout 2  also reflects such an influence. 
   Consequently, the value of the tracking error signal obtained in the calibration mode can be used to adjust the configuration of the charge pump, in order to eliminate the influence from mismatch causing phase shift. In this embodiment, the optical storage system  200  further includes an analog-to-digital converter ADC for acquiring the value of the tracking error signal, converting the value of the tracking error signal into a digital value, and then transmitting the digital value to a digital signal processor DSP. The digital signal processor DSP generates corresponding control signals C 1  and C 2  based on the digital value, so as to adjust the configuration of the charge pump. In this embodiment, the values of the control signals C 1  and C 2  generated by the digital signal processor DSP can be used to adjust the value of the signal on the output terminal Vout 2  to a nominal value, which is the desired value of the output terminal Vout 2  in the calibration mode when no inferior influence such as the circuit mismatch exists. 
   In this embodiment, the current sources I 1  and I 3  are variable current sources, which serve to achieve the object of controlling the value of the current for charging/discharging the capacitor C 41 . The control signals C 1  and C 2  respectively control the current magnitudes of the current sources I 1  and I 3 .  FIG. 4  is a circuit diagram showing a variable current source. As shown in  FIG. 4  as an example, the current source I 1  includes sub-current sources I 11  to I 1 N and sub-switches SW 11  to SW 1 N. The sub-current sources I 11  to I 1 N are respectively coupled to the sub-switches SW 11  to SW 1 N. The control signal C 1  is a digital signal having N bits for respectively controlling the sub-switches SW 11  to SW 1 N. When the number of turned-on sub-switches among the sub-switches SW 11  to SW 1 N is greater, the output current becomes larger. The current source I 3  may also have a similar configuration. 
   The control signals C 1  and C 2  may be set as the same signal. When entering the calibration mode, the values of the control signals C 1  and C 2  are adjusted according to the value of the signal Vout 2  so that the value of the signal Vout 2  is the nominal value, and the values of the control signals C 1  and C 2  are fixed. Thereafter, when the disc is actually written/read, the values of the control signals C 1  and C 2  are continuously fixed so that the currents of the current sources I 1  and I 3  are held constant to eliminate the phase difference contributed by the circuitry. 
   The control signals C 1  and C 2  may also be set as different signals. The control signal C 1  is adjusted to calibrate the phase difference between the signals A and B caused by circuit mismatch, and the control signal C 2  is adjusted based on the phase difference between the signals C and D caused by circuit mismatch. When the control signal C 1  is being adjusted, the circuit path for processing the signals C and D may be set as open-circuit, or the circuit path of the signals C and D may be kept in a non-operational state. At this time, the charge pump K 2  is only fed with the inputs of the control signals UPab and DNab so that the value of the control signal C 1  may be adjusted, and the influence caused by the circuitry on the phases of the signals A and B may be eliminated. When the signals C and D is being calibrated, the circuit path for processing the signals A and B may be set as open-circuit, or the circuit path of the signals A and B may be kept in a non-operational state. At this time, the charge pump K 2  is only fed with the inputs of the control signals UPcd and DNcd so that the value of the control signal C 2  may be adjusted, and the influence caused by the circuitry on the phases of the signals C and D may be eliminated. 
   In this embodiment, the current sources I 1  and I 3  are variable current sources. In the practical circuit design, however, the current source I 2  may be a variable current source controlled by the control signal to adjust the value of the current, or the current sources I 1  and I 2  may both be variable current sources for respectively receiving different control signals so that the phase shift occurred when the differential phase detector is processing the signals A and B may be adjusted. 
   As for the current sources I 3  and I 4 , the current source I 4  may be a variable current source controlled by the control signal to adjust the value of the current. Also, the current sources I 3  and I 4  may both be variable current sources for respectively receiving different control signals to adjust the phase shift occurred when the differential phase detector is processing the signals C and D. 
   If the current sources I 2  and I 4  are variable current sources, the current sources I 2  and I 4  may receive the same control signal so that the currents thereof are adjusted. The current sources I 2  and I 4  may alternatively receive different control signals so that the currents thereof may be respectively adjusted in a manner similar to that performed when the current sources I 1  and I 3  are variable current sources. Other circuit combinations may be easily derived according to the descriptions mentioned hereinabove, and detailed descriptions thereof will be omitted. 
   The differential phase detector and the charge pump thereof according to the embodiment of the invention may eliminate the time delay caused by and inherent in the circuitry based on the fine adjustment at initialization. Thus, the precision of the tracking error signal may be enhanced, and the data read/write error caused by the incapability of precise tracking operation when the disc is being read/written can be avoided. 
   While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.