1. Field of the Invention
The present invention relates to a digital signal reproducing apparatus for reproducing digital signals from signals read from a recording medium on which the digital signals are recorded, and more particularly, to a digital signal reproducing apparatus in accordance with a sampled servo system.
2. Description of Background Information
On an optical disk called a DRAW (Direct Read After Write) disk, servo bytes are recorded as shown in FIG. 1.
Each sector on an optical disk comprises 43 servo blocks, each of which is composed of two servo bytes followed by 16 data bytes. A servo byte consists of two wobbled pits Wp1, Wp2 and a clock pit Cp, wherein the wobbled pits Wp1, Wp2 are located on the left and right sides of the track center, respectively.
As an information detecting point of an pickup (a light spot for detecting information) is moving on the track center, decreased light amounts are equal at the locations of the left and right wobbled pits Wp1, Wp2. However, if the light spot deviates to the left or right from the track center, the decreased light amounts at the locations of the two wobbled pits Wp1, Wp2 change corresponding to the direction and amount of the deviation of the light spot. Thus, a tracking error signal may be generated from the difference between the decreased light amounts at the two positions (difference in level between RF signals). This tracking error signal is held for a subsequent data byte block.
The distance D between the wobbled pit Wp2 and the clock pit Cp is set longer than a maximum distance between two adjacent pits in a data byte. This distance D may therefore be detected as a synchronizing signal. A variety of timing signals are generated on the basis of the detected synchronizing signal. Also, a clock is generated corresponding to a signal indicating that the clock pit CP has been detected.
A mirror portion in the distance D is used as a focus area in which a focus error signal is detected and held for a subsequent data byte block.
When a DRAW disk of, for example, five inches, on which servo bytes as described above have been recorded thereon, is rotated at 1800 rpm, edges generated in the RF signal by the presence of clock pits have a repeating frequency of 41.28 KHz.
There is known a digital signal reproducing apparatus which reads address data and so on recorded subsequent to the servo bytes on a DRAW disk as mentioned above, and utilizes the read data to reproduce digital signals recorded on data information sections of the DRAW disk.
Such reproduction should be performed in consideration of a delay time of the apparatus. However, it is rather difficult to set the delay time since delays caused by a differential circuit and an edge detecting circuit of the reproduction apparatus must be taken into account for deriving an accurate delay time. Further, if changes in temperature, power supply voltage and so on result in fluctuating the delay amount, a clock for reproduction cannot be generated with a high accuracy.