| 摘要 |
PURPOSE:To make tracking possible without a pregroup, by generating a signal for the radius of a light scattering material on a basis of a detected Doppler shift quantity and comparing this signal with a control signal designating the radius position to lead a head to the designated position. CONSTITUTION:An output signal 221 from a photodetector 319 is separated into a read signal 332 and a Doppler signal 333 by a BPF341. A control signal 338 for track selection is converted to an analog voltage signal, and a signal having a frequency corresponding to this voltage is generated by a voltage control oscillator 36. A signal of the sum or the difference between frequencies of this signal and a signal from a quartz oscillator is transmitted as a reference signal 337 of a frequency corresponding to a desired track. The Doppler signal 333 and the reference signal 337 are sent to a comparator 35 to output an error signal 334. The error signal 334 is sent to a driving mechanism 322 to position an optical head 32 onto the desired track. Thus, tracking of the optical head is realized without a pregroup. |
