OPTICAL READING DEVICE COMPRISING A FOCUSSING DETECTION DEVICE

摘要

The invention relates to the exact focussing of a light beam on a track carried by an optically readable information support. The object of the invention is to provide in an optical reading device a dichotomic focussing error detection device of which the two states correspond to the absence and to the presence of the reading signal. In the absence of the reading signal, the lens of the optical reading device is moved at a uniform speed between two limit positions by means of a triangular control signal, the position control lopp of the lens being open. In the presence of the reading signal, the lens is no longer controlled by the triangular control signal, but instead by an error signal emanating from the position control loop which is then closed.