Abstract:
An apparatus has a plurality of rotary heads for reproducing digital data from a recording medium, and delays a reference signal phase-locked with the rotary heads on the basis of the detection output of predetermined reference data in the reproduced digital data, thereby generating a signal indicating the reproducing start timing of the digital data by the plurality of rotary heads.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a digital data reproducing apparatus and, more particularly, to an apparatus for reproducing digital data from a recording medium using a rotary head. 
   2. Related Background Art 
   As a conventional apparatus of this type, a VTR for recording/reproducing an image signal and an audio signal on/from a magnetic tape using a rotary head is known. 
   In such a VTR, the recording/reproducing start position of the head is controlled with high precision using a head switch pulse (to be referred to as an SWP hereinafter) which indicates the positional relationship of the head with respect to the tape. 
   The SWP is normally generated using a PG signal indicating the rotation phase of a cylinder. In this case, the phase relationship between the head and the PG signal is determined by the mounting precision of a motor of the cylinder which mounts the head, and the cylinder, and varies in units of apparatuses. For this reason, since the phase difference between the PG signal and the SWP varies in units of apparatuses, an appropriate SWP is obtained by adjusting the phase difference between the PG signal and the SWP in units of apparatuses. 
   However, in the above-mentioned conventional apparatus, the phase adjustment of the SWP must be performed in each apparatus, resulting in a very cumbersome operations. 
   The phase adjustment of the SWP is normally performed by a specialist upon delivery from a factory. Thus, it is impossible to perform the phase adjustment when the phase of the SWP deviates from that upon delivery due to the poor mounting precision of the motor and the cylinder, deterioration of the motor performance, and the like after delivery of the apparatus to a user. 
   Furthermore, in a domestic-use VTR, the recording start positions of data in tracks on tapes do not always precisely match among cassettes, and the phase of the SWP may be proper in a given cassette but may be improper in another cassette. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to solve the above-mentioned problems. 
   It is another object of the present invention to precisely generate a recording/reproducing start reference signal of a head upon recording/reproducing of a signal. 
   In order to achieve the above objects, according to one aspect of the present invention, there is provided a digital data reproducing apparatus comprising (a) reproducing means for reproducing digital data from a recording medium using rotary head means, (b) detection means for detecting predetermined reference data in the reproduced digital data, and (c) generation means for generating a reference signal indicating a reproducing start timing of the digital data by the rotary head means in correspondence with an output from the detection means. 
   It is still another object of the present invention to precisely reproduce digital data even from recording media having different data recording start positions. 
   The above and other objects and features of the present invention will become apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing the arrangement of a digital VTR according to an embodiment of the present invention; 
       FIG. 2  is a view showing the recording format in the embodiment of the present invention; 
       FIG. 3  is a block diagram showing the arrangement of an SWP generation circuit shown in  FIG. 1 ; 
       FIG. 4  is a timing chart showing the states of signals in the respective units of the circuit shown in  FIG. 3 ; and 
       FIG. 5  is a block diagram showing another arrangement of the SWP generation circuit shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiment of the present invention will be described in detail hereinafter with reference to the accompanying drawings. 
   In this embodiment, the present invention is applied to a digital VTR.  FIG. 1  is a block diagram showing the arrangement of a reproducing system of the digital VTR. 
   Referring to  FIG. 1 , digital signals reproduced from a magnetic tape  1  by magnetic heads  3  and  4  are selectively output to a switch pulse (SWP) generation circuit  8  via a switch  5 , and the digital signal is then output to a reproduced signal processing circuit  6 . The information amount of a reproduced digital signal is compressed using techniques such as DCT, quantization, and the like upon recording. Thus, the reproduced signal processing circuit  6  expands the information amount by performing processing opposite to that upon recording for the reproduced digital signal to convert the digital signal into an image signal with an original format, and outputs the image signal. 
   A PG generation circuit  7  generates a PG signal in synchronism with the rotation of a cylinder  2  on which the heads  3  and  4  are arranged, and outputs the PG signal to the SWP generation circuit  8 . The SWP generation circuit  8  generates an SWP on the basis of the PG signal, as will be described later, and outputs the SWP to the switch  5  and the reproduced signal processing circuit  6 . In the reproduced signal processing circuit, the SWP is used for the purpose of, e.g., a start signal of processing such as error correction in units of tracks. The switch  5  is switched in response to the SWP, and alternately outputs the reproduced signals from the heads  3  and  4 . 
   The format of data handled in this embodiment will be explained below.  FIG. 2  shows the format of data handled in this embodiment. 
     FIG. 2  shows the format of one sync. block. The sync. block includes sync. data  201  which is located at the beginning of the sync. block and serves as reference data upon detection of data in each block, ID data  202  which indicates the position, on a track, of the sync. block, the attribute of effective data included in the sync. block, and the like, parity data  203  used for error correction/detection of the ID data, effective data  204  consisting of image data, audio data, and the like, and parity data  205  used for error correction of the effective data. In this embodiment, a plurality of such sync. blocks recorded on a single track are reproduced. 
   The SWP generation circuit  8  will be described below with reference to  FIG. 3 . 
     FIG. 3  is a block diagram showing the arrangement of the SWP generation circuit. Referring to  FIG. 3 , a reproduced signal output from the switch  5  is input via a terminal  101 , and is supplied to a serial-to-parallel (S-P) conversion circuit  102  and a sync. detection circuit  103 . 
   The S-P conversion circuit  101  converts the serially input reproduced signal into parallel data in units of a predetermined number of data, and outputs the parallel data to a latch circuit  104 . The latch circuit  104  outputs the signal output from the S-P conversion circuit  102  in response to clocks SCLK (to be described later). 
   On the other hand, the sync. detection circuit  103  generates a sync. detection signal SDET by detecting sync. data in the reproduced signal, and also generates clocks SCLK in units of bits of the reproduced signal. 
   The signal SDET output from the sync. detection circuit  103  is supplied to a symbol counter  106  and a block counter  107 . 
   The symbol counter  106  counts the number of data in one block. The counter  106  is reset in response to the signal SDET, and then performs a count-up operation in response to clocks SCLK. When the count value of the counter  106  has reached a value corresponding to data for one block, the counter  106  generates a block enable signal BEN to the block counter  107 . The symbol counter  106  resets itself in response to the signal BEN. 
   The block counter  107  is reset in response to edge pulses SWPE indicating the leading and trailing edges of the SWP output from an edge generation circuit  118 . The block counter  107  performs two different operations depending on whether or not it receives the signal SDET. 
   When the sync. detection circuit  103  detects sync. data in each sync. block, and outputs a signal SDET, the signal SDET is input to a load input terminal L of the block counter  107 . Upon reception of the signal SDET at the input terminal L, the block counter  107  extracts the ID data following the sync. data in each sync. block in reproduced data output from the latch circuit  104  using the clocks SCLK. The ID data of each block includes a block serial number (block number) in one track, and the block number is loaded. 
   Therefore, the output from the counter  107  is written for each sync. block depending on the contents of the ID data when the counter  107  receives the signal SDET. 
   A case will be explained below wherein sync. data in the reproduced signal cannot be detected due to, e.g., dropout of the reproduced signal. 
   In this case, after the block number in the ID data is loaded immediately before the dropout, the signals BEN are counted in synchronism with the clocks SCLK. More specifically, the signal BEN is output when the counter  106  has counted data for one block, and by counting the signals BEN, the block number of currently reproduced data can be confirmed even when ID data is not input. 
   Counters  108 A and  108 B are respectively reset in response to the pulses SWPE. The counter  108 A counts the clocks SCLK, and outputs a signal BEN′ and resets itself when it has counted a value corresponding to data for one block as in the symbol counter  106 . The counter  108 B counts signals BEN′. The roles of the counters  108 A and  108 B will be described later. 
   The outputs from the symbol counter  106  and the block counter  107  are respectively output to comparators  110  and  111 , and are compared with the outputs from registers  112  and  113 , as will be described later. 
   The PG signal generated by the PG generation circuit  7  is input from a terminal  116  as a rectangular wave frequency-divided to ½, and is supplied to an MPU (Micro Processing Unit)  114  and a variable delay circuit  117 . 
   Although the PG signal supplied from the terminal  116  is synchronized with the rotation of the cylinder, it cannot be directly used as the SWP since the relationship between the reproduced signal and the head position is not determined. Therefore, in this embodiment, the SWP is generated by delaying the PG signal using the variable delay circuit  117 . 
   The MPU  114  outputs and stores the data position on an appropriate track designated by an operation unit (not shown) as a symbol position S and a block position B in the registers  112  and  113 . Note that the symbol position S and the block position B can be arbitrarily set. 
   The comparators  110  and  111  respectively compare the outputs from the counters  106  and  107  with the outputs from the registers  112  and  113 . When comparators  110  and  111  find coincidences between the outputs, they output signals indicating the coincidences to an AND gate  115 . The AND gate  115  logically ANDs the outputs from the comparators, and outputs the AND to the MPU  114 . 
   Upon reception of the signal from the AND gate  115 , the MPU  114  fetches the count values of the counters  108 A and  108 B, and sets the delay time of the variable delay circuit  117  using these count values. The output from the variable delay circuit  117  is output as the SWP. 
   Note that a memory address counter  109  generates and outputs a write address of a memory in the subsequent signal processing circuit using the count values of the symbol counter  106  and the block counter  107 . 
   The detailed operation of the circuit shown in  FIG. 3  will be described below with reference to the timing chart shown in  FIG. 4 . 
   Assume that the delay time of the variable delay circuit  117  is temporarily set to be a period for two sync. blocks, and is adjusted using an actual reproduced signal. Also, assume that the number of sync. blocks to be recorded in one track is  10  for the sake of simplicity. 
   A waveform A in  FIG. 4  indicates the state of the signal SDET which is free from any dropout. A waveform B indicates the state of the PG signal supplied from the terminal  116 . If the PG signal rises at time t 1 , the SWP indicated by a waveform C rises at time t 2  after an elapse of a period for two sync. blocks from the PG signal. 
   When the SWP rises, the edge generation circuit  118  generates pulses, as indicated by a waveform D, and the block counter  107  and the counters  108 A and  108 B are reset. Therefore, the counter  107  and the counters  108 A and  108 B are reset at the generation timings t 5  and t 8  of the pulses SWPE. 
   In this embodiment, assuming that “2” is set in the register  112  and “2” is also set in the register  113 , the output from the AND gate  115  is as indicated by a waveform I. The MPU  114  fetches the count values of the counters  108 A and  108 B in response to the signal output from the AND gate  115 , and compares the outputs from the counters with the values set in the registers  112  and  113 . 
   In this embodiment, the delay time of the variable delay circuit  117  is set on the basis of the time difference, T, between the output time of the signal from the AND gate  115  and the PG signal. 
   More specifically, when the output is obtained from the AND gate  115  at time t 3 , the count values of the counter  106  and the counters  108 A and  108 B respectively indicate 2 sync. blocks+2 symbols, and these values are equal to those set in the registers  112  and  113 . For this reason, the delay time is not changed. 
   A case will be described below wherein the PG signal is input at a timing earlier by one sync. block than the above-mentioned case, i.e., the PG signal is input at the timing of time t 7  earlier by one sync. block than time t 10 . 
   As shown in  FIG. 4 , if the PG signal rises at time t 7 , a pulse SWPE is generated at time t 8 , and the counter  107  and the counters  108 A and  108 B are reset. After the counters are reset, they start counting operations. In this case, since the reproducing heads are switched at an earlier timing than in the above-mentioned case, a reproduced signal for the shortened period cannot be supplied to the terminal  101 . Therefore, the state of the counter  107  is as indicated by time t 8  of a waveform F in  FIG. 4 , and a count value “0” continues for a period of two sync. blocks after the counter is reset. For this reason, the output from the AND gate  115  is generated at time t 9 . 
   The counter  108 A performs substantially the same operation as that of the symbol counter  106 , except that it is reset in response to the pulse SWPE. Therefore, the signal BEN′ is generated at the same period to have a predetermined phase difference from the signal BEN. In the case of this embodiment, the signals BEN and BEN′ have no phase difference therebetween. Since the counter  108 B counts the signals BEN′ output from the counter  108 A after it is reset, it continues the counting operation after it is reset at time t 8 , as indicated by a waveform H in  FIG. 4 . Therefore, when the output from the AND gate  115  is supplied to the MPU  114 , the counter  108 B outputs a count value indicating block  3 , and the counter  108 A outputs a count value indicating data  2 . 
   The MPU  114  sets the delay time of the variable delay circuit  117  on the basis of the comparison results of the count values of the counters  108 A and  108 B fetched at time t 9  and the values set in the registers  112  and  113 . More specifically, in this case, the combination of the count values of the counters  108 A and  108 B upon reception of the signal from the AND gate  115  indicates 3 blocks+2 symbols, and this value is delayed by one sync. block from the combination of the values set in the registers  112  and  113 . Therefore, the delay time can be prolonged by a period for one sync. block, i.e., ΔT as compared to the above-mentioned case. 
   As described above, in this embodiment, the position of currently reproduced data is confirmed on the basis of sync. data and ID data in the reproduced signal, and the delay time of the SWP with respect to the PG signal is determined with reference to predetermined data in the reproduced data, thus automatically attaining phase control of the SWP with high precision. 
   Therefore, a normal user can perform the phase control of the SWP in correspondence with apparatuses and tapes using a normally recorded tape without help of a specialist, thus always obtaining a high-quality reproduced signal. 
   In addition, the deviation amount of the phase of the SWP with respect to the PG signal can be detected by a simple arrangement, i.e., by comparing a value indicating a reference sync. block (symbol) with the count value of the counter upon actual detection of the sync. block (symbol) after generation of the SWP. 
   In the above embodiment, the SWP is generated by controlling the delay time of the variable delay circuit  117  by the MPU  114 . Alternatively, the MPU itself may generate the SWP by delaying the PG signal on the basis of the outputs from the AND gate  115  and the counters  108 A and  108 B.  FIG. 5  shows the arrangement in this case. 
   According to the arrangement shown in  FIG. 5 , since the variable delay circuit can be omitted, the circuit scale can be reduced. 
   In the above embodiment, the counters  108 A and  108 B are arranged, and their count values are fetched in response to the output from the AND gate  115 . Alternatively, an internal counter of the MPU may be used to perform the counting operation. 
   Furthermore, in the above embodiment, appropriate symbol and block positions are stored in the registers, and are compared with the count values of the symbol and block counters, thereby setting the delay time. Alternatively, only an appropriate block position may be stored in the register, and the delay time may be set in correspondence with the comparison result with the count value of the block counter. 
   In this case, although the control becomes rougher than the case wherein the symbol position is also set, the register for storing the symbol position and the comparator can be omitted, and the circuit scale can be further reduced. 
   In the above embodiment, the phase difference between the PG signal and the SWP is detected by utilizing the count values of the counters  108 A and  108 B at the output timing of the signal from the AND gate  115 . Alternatively, a counter which is reset in response to the edge of the PG signal and counts clocks SCLK may be arranged, and the count value of this counter may be fetched in response to the output from the AND gate  115 , thereby directly detecting the delay time T. 
   As can be seen from the above description, since a signal indicating the reproducing start timing of a plurality of heads is generated upon detection of a predetermined reference signal in input digital data, a reference signal can be generated with high precision by a very simple arrangement. 
   Since the number of blocks in a reproduced signal is counted, and a reference signal indicating the reproducing start timing is generated upon reproduction of a predetermined block, a reference signal can also be generated with high precision, and deterioration of quality of the reproduced signal can be prevented.