Eduardo E Lopez

44995695, Feb 12, 1985

Sep 7, 1982

6/415501

Eduardo A. Lopez De Romana - Torrance CA

Discovision Associates - Costa Mesa CA

G11B 700

369 45

A method and apparatus for monitoring the focus condition of a writing light beam impinging the writing surface of a recording medium. The present invention operates on the recognition of the fact that light from a laser is very slightly divergent as it emerges from the laser. Accordingly, the reflected light from the medium is convergent as it passes in the reverse direction and it forms a real image of the source plane wave. The image is compared to a reference image indicative of proper focus, and the focus is adjusted until the image matches the reference image.

44659776, Aug 14, 1984

Jun 4, 1982

6/385171

Eduardo A. Lopez de Romana - Torrance CA

Discovision Associates - Costa Mesa CA

H03K 519

328120

A drop-out detector for detecting the absence of a pulse in either of first and second pulse trains, especially for use with the spindle servo error generator of a videodisc mastering machine. Prior attempts at providing a drop-out detector function for the spindle servo error generator rely on the use of RC based timing circuits to create time windows within which to look for the presence or absence of a reference or tach pulse, as the case may be. Such arrangements provide satisfactory performance in connection with constant angular velocity type discs, since reference and tach frequencies remain constant throughout the mastering process. However, problems arise in utilizing such RC based circuits in connection with constant linear velocity type discs. The present invention overcomes these problems by operating on a pulse width modulation technique for controlling the spindle motor speed. In such an arrangement, rather than to phase compare two AC signals, one from the reference oscillator and one from the tachometer, a three-stage shift register is provided whose output is pulse width modulated by the difference in time relationship between two pulse train inputs, one pulse train representing a reference standard and the other representing the tachometer output.