Dynamic information storage or retrieval – Information location or remote operator actuated control – Selective addressing of storage medium
Patent
1987-02-05
1989-07-04
Shoop, Jr., William M.
Dynamic information storage or retrieval
Information location or remote operator actuated control
Selective addressing of storage medium
369 43, 369 44, G11B 1722
Patent
active
048456985
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a method as in the preamble to the major claim.
The method is especially concerned with preventing the scanning system from moving in undesired directions during the radial scanning of compact disks with equipment that has a simple servo system without a direction-recognition circuit.
Rapid searching for a desired point on the disk by determining the difference tracks between the target and the instantaneous position and establishing them as a reference is state of the art. When the targets are relatively remote, carrying out the search with the fine-drive mechanism alone is insufficient. The procedure consists in principle of switching the coarse-drive mechanism over to rapid search, meaning that the coarse-drive mechanism is accelerated and maintained at high speed, with the fine-drive mechanism control circuit simultaneously being switched off. While the coarse-drive mechanism continues to move (entraining the fine-drive mechanism with it), the radially transversed tracks are detected and counted. As soon as the previously established reference value (the target track) is attained the fine-drive mechanism control circuit is switched on again and the coarse-drive mechanism switched from search mode to control. Search systems that operate on this principle usually have a direction-recognition circuit that determines the direction of the fine- and coarse-drive mechanism in relation to the disk that is being scanned. This device prevents tracks that have been crossed in the wrong direction from being incorrectly identified. Motions of this type, which are incorrect in relation to the prescribed direction, occur in rapidly operating search systems only during the starting and landing phase. This is due first of all to the disk that is being scanned having radial eccentricities that result from disk positioning and retention and from manufacturing tolerances in the disk. During the starting and landing phases, when the coarse-drive mechanism is moving relatively slowly, a motion on the part of the track that derives from the instantaneous direction of a radial disk eccentricity sometimes catches up with or overtakes the motion of the scanner. The resulting direction of motion in relation to the disk will be opposite the prescribed direction. This will be evident from FIG. 1. With search systems that have no direction-recognition circuit no difference can be detected, and tracks that are crossed in the wrong direction are incorrectly evaluated as correct. The result is, however, that an incorrect target is arrived at and corrections must be made. This can lead to essentially longer search times. The second cause of the problem that is to be addressed is a brief incorrect direction of movement during the coarse-drive mechanism starting phase. At the instant of start-up, when, that is, the coarse-drive mechanism is switched on with the fine-drive mechanisms control circuit discontinued, an incorrectly opposed motion can also occur, depending on the instantaneous preliminary diversion of the fine-drive mechanism. Since the coarse-drive mechanism carries out practically no motion during this initial instant, this can lead to traversing a large numer of tracks in the wrong direction. An extreme eccentricity at this point will increase any errors that occur and increase the time needed to find the desired point on the track. FIG. 2 illustrates an example. When the instantaneous fine-drive mechanism operating point is not at the zero point of its range of motion, the error can, as FIG. 2 also shows, be increased even more in the corresponding skip direction.
European OS No. 0 090 379 discloses a search system of this type.
Search systems with direction detectors or direction-recognition circuits are at the present time restricted for reasons of cost to top-of-the-line CD players. In simple search systems with no direction-recognition circuit the aforesaid conditions lead on the one hand to considerably longer searching times. Search systems with a very lhigh searching speed or rapid and brief b
REFERENCES:
patent: 4598394 (1986-07-01), Nonaka
patent: 4623994 (1986-11-01), Nabeshima et al.
Deutsche Thomson-Brandt GmbH
Fogiel Max
Shoop Jr. William M.
Young Brian
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