an embodiment of the present invention will be explained with reference to the drawings . fig1 shows the configuration of an optical disc device 10 according to the embodiment of the present invention , and fig2 shows the configuration of an optical disc 20 used in the optical disc device 10 . as shown in fig2 , an optical disc based on , e . g ., the hd dvd standard is used as the optical disc 20 . a data area 30 is formed on the outer peripheral side of the optical disc 20 , and a bca 40 is formed on the inner peripheral side . pits ( not shown ) for recording data such as video data are formed in the data area 30 . sync patterns as synchronization signals are also recorded in the pits . grooves , i . e ., wobbles ( not shown ) are also formed in the data area 30 to wobble in fixed cycles . each wobble mainly serves as a guide groove . a barcode - like pattern ( not shown ) is formed in the bca 40 to radiate in the radial direction of the optical disc 20 . for example , unique identification information assigned to the optical disc 20 and copy protection information are recorded in the bca 40 . note that the optical disc device 10 is based on the hd dvd standard and thus configured to play back data recorded in the bca 40 without fail at the time of playback . this configuration makes it possible to , e . g ., prevent unauthorized copying . at the time of playback , the optical disc 20 is attached to a rotating shaft 50 , and the optical disc device 10 rotates the optical disc 20 together with the rotating shaft 50 , using a spindle motor 60 . at this time , the optical disc device 10 moves an optical pickup head 70 to a desired position and irradiates , e . g ., pits with laser light output from the optical pickup head 70 . the optical disc device 10 receives light reflected from the pits using the optical pickup head 70 and performs predetermined signal processing for obtained electrical signals ( rf signals ), thereby generating video signals . at this time , a signal processing circuit 80 performs the predetermined signal processing for the electrical signals output from the optical pickup head 70 . the rotational speed of the spindle motor 60 is controlled on the basis of the processing result . as described above , the optical disc device 10 is based on the hd dvd standard and thus configured to play back data recorded in the bca 40 without fail at the time of playback , in order to , e . g ., prevent unauthorized copying . however , the hd dvd standard prescribes a rotational speed for the spindle motor 60 with which data can be played back from the bca 40 . even if the optical disc 20 is rotated at a rotational speed different from the prescribed rotational speed , data cannot be played back from the bca 40 . accordingly , the optical disc device 10 needs to rotate the optical disc 20 at the predetermined rotational speed prescribed by the hd dvd standard to play back data from the bca 40 , unlike the case of the data area 30 . in this embodiment , when the optical disc device 10 plays back data from the bca 40 , it first moves the optical pickup head 70 to the data area 30 . the optical disc device 10 controls the rotational speed of the spindle motor 60 such that the rotational speed of the spindle motor 60 becomes almost equal to the rotational speed which is prescribed by the hd dvd standard and with which data can be played back from the bca 40 , by playing back data from the data area 30 . after that , the optical disc device 10 moves the optical pickup head 70 to the bca 40 . at this time , the rotational speed of the spindle motor 60 may change slightly . in this case , the optical disc device 10 plays back the data from the bca 40 and detects , from the played - back data , a sync pattern ( a reference signal serving as a reference for calculating the rotational speed of the spindle motor 60 ) which is a unique bit pattern . with this operation , the optical disc device 10 controls the rotational speed of the spindle motor 60 such that the rotational speed of the spindle motor 60 becomes the rotational speed prescribed by the hd dvd standard . as described above , the optical disc can be stably rotated at the predetermined rotational speed by controlling the rotational speed of the spindle motor 60 with high accuracy while playing back the data from the bca 40 . accordingly , data readout accuracy can be increased . fig3 shows a rotational speed control procedure rt 10 according to this embodiment when playing back data from the bca 40 . in fig3 , at the time of playback , the rotational speed control procedure rt 10 starts . a controller 110 of the signal processing circuit 80 rotates the spindle motor 60 at the predetermined rotational speed by controlling the operation of a rotational speed controller 120 and maintains the rotational speed . after that , in step sp 10 , the controller 110 moves the optical pickup head 70 to the data area 30 . in this case , the optical disc device 10 irradiates , e . g ., pits with laser light emitted from the optical pickup head 70 and receives light reflected from the pits using the optical pickup head 70 . the optical disc device 10 supplies obtained electrical signals to the signal processing circuit 80 . the signal processing circuit 80 inputs the electrical signals to a pit / sync detector 90 and a bca / sync detector 100 . in step sp 20 , the controller 110 turns a switch sw to the side of the pit / sync detector 90 and causes the rotational speed controller 120 to stop maintaining the rotational speed of the spindle motor 60 . in this case , when the pit / sync detector 90 detects sync patterns recorded in the pits of the data area 30 from electrical signals output from the optical pickup head 70 , it notifies the rotational speed controller 120 that the sync patterns are detected . in step sp 30 , the rotational speed controller 120 calculates the rotational speed of the spindle motor 60 on the basis of time intervals at which the sync patterns are detected and controls the rotational speed of the spindle motor 60 based on the calculation result . with this operation , the rotational speed of the spindle motor 60 is controlled to be almost equal to the rotational speed which is prescribed by the hd dvd standard and with which data can be played back from the bca 40 . in step sp 40 , the controller 110 controls the operation of the rotational speed controller 120 , thereby maintaining the rotational speed of the spindle motor 60 . after that , the flow shifts to step sp 50 , and the controller 110 moves the optical pickup head 70 to the bca 40 . at this time , the rotational speed controller 120 tries to maintain the rotational speed of the spindle motor 60 . however , since the rotational speed of the spindle motor 60 is not controlled by calculating the actual rotational speed of the spindle motor 60 , the rotational speed of the spindle motor 60 may change slightly . in step sp 60 , the controller 110 turns the switch sw to the side of the bca / sync detector 100 and causes the rotational speed controller 120 to stop maintaining the rotational speed of the spindle motor 60 . in this case , when the bca / sync detector 100 detects sync patterns recorded in the bca 40 from electrical signals output from the optical pickup head 70 , it notifies the rotational speed controller 120 that the sync patterns are detected . in step sp 70 , the rotational speed controller 120 calculates the rotational speed of the spindle motor 60 on the basis of time intervals at which the sync patterns are detected and controls the rotational speed of the spindle motor 60 based on the calculation result . with this operation , the rotational speed of the spindle motor 60 is controlled with high accuracy to be almost equal to the rotational speed which is prescribed by the hd dvd standard and with which data can be played back from the bca 40 . as described above , according to this embodiment , the optical disc can be stably rotated at the fixed rotational speed when playing back the data from the bca 40 . accordingly , the data readout accuracy can be increased . this embodiment eliminates the need to provide an fg signal generator for the spindle motor 60 and makes it possible to correspondingly simplify the configuration of the optical disc device 10 . this embodiment also eliminates the need to provide an fg signal detector for detecting an fg signal in the signal processing circuit 80 and makes it possible to correspondingly simplify the configuration of the signal processing circuit 80 . this eliminates the need to provide an input terminal for inputting , to the signal processing circuit 80 , an fg signal output from an fg signal detector . note that the above - described embodiment is merely an example and not intended to limit the present invention . for example , it is also possible to control the rotational speed of the spindle motor 60 by detecting anything other than a sync pattern as a reference signal for calculating the rotational speed of the spindle motor 60 when playing back the data from the bca 40 . examples of the reference signal include a resync pattern which is another type of synchronization pattern , a preamble indicating the start of data , and a postamble indicating the end of data . it is also possible to control the rotational speed of the spindle motor 60 by detecting not sync patterns obtained from pits but wobble signals obtained from wobbles when playing back data from the data area 30 . as the optical disc 20 , an optical disc based on a dvd standard may be used instead of an optical disc based on the hd dvd standard . when the controller 110 plays back data from the bca 40 , it may move the optical pickup head 70 to the bca 40 without playing back data from the data area 30 . after that , the controller 110 may try to detect sync patterns recorded in the bca 40 while changing the rotational speed of the spindle motor 60 . the rotational speed of the spindle motor 60 may be controlled only if the detection of the sync patterns is successful . in this case , the controller 110 turns the switch sw to the side of the bca / sync detector 100 in advance . fig4 shows a rotational speed control procedure rt 20 according to another embodiment when playing back data from a bca 40 without playing back data from a data area 30 . in fig4 , at the time of playback , the rotational speed control procedure rt 20 starts . in step sp 100 , a controller 110 of a signal processing circuit 80 rotates a spindle motor 60 at a predetermined rotational speed by controlling the operation of a rotational speed controller 120 . at the same time , the controller 110 moves an optical pickup head 70 to the bca 40 . in step sp 110 , the rotational speed controller 120 determines whether it is notified by a bca / sync detector 100 that sync patterns are detected . if an affirmative result is obtained in step sp 110 , the flow shifts to step sp 120 , and the rotational speed controller 120 controls the rotational speed of the spindle motor 60 . after that , the flow shifts to step sp 130 , and the procedure rt 20 ends . on the other hand , if a negative result is obtained in step sp 110 , the flow shifts to step sp 140 , and the rotational speed controller 120 determines whether a current rotational speed is equal to a predetermined upper limit . if a negative result is obtained in step sp 140 , the flow shifts to step sp 150 . in step sp 150 , the rotational speed controller 120 increases the rotational speed of the spindle motor 60 by a predetermined value , and the flow returns to step sp 110 to repeat the above - described operation . attempts are made to detect a sync pattern while stepwise increasing the rotational speed of the spindle motor 60 . on the other hand , if an affirmative result is obtained in step sp 140 , the flow shifts to step sp 160 . in step sp 160 , the rotational speed controller 120 determines whether it is notified by the bca / sync detector 100 that sync patterns are detected . if an affirmative result is obtained in step sp 160 , the flow shifts to step sp 120 , and the rotational speed controller 120 controls the rotational speed of the spindle motor 60 . after that , the flow shifts to step sp 130 , and the procedure rt 20 ends . on the other hand , if a negative result is obtained in step sp 160 , the flow shifts to step sp 170 . in step sp 170 , the rotational speed controller 120 determines whether the current rotational speed is equal to a predetermined lower limit . if a negative result is obtained in step sp 170 , the flow shifts to step sp 180 . in step sp 180 , the rotational speed controller 120 reduces the rotational speed of the spindle motor 60 by a predetermined value , and the flow shifts to step sp 160 to repeat the above - described operation . attempts are made to detect a sync pattern while stepwise reducing the rotational speed . on the other hand , if an affirmative result is obtained in step sp 170 , the flow returns to step sp 110 , and the rotational speed controller 120 repeats the above - described operation . attempts are made again to detect a sync pattern while stepwise increasing the rotational speed of the spindle motor 60 .