1 2
pick-up relative to the disc, the tracking error signal
OPTICAL DISC TRACKING SYSTEM WITH tends to accompany a steady drift which will bring
SWITCHING OF TRACKING ERROR SIGNALS AT about a continuing shift in tracking. Accordingly, in the
BOUNDARY BETWEEN TRACK GUIDE AND event that the pick-up has inclined relative to the disc,
TRACK ADDRESS 5 the pattern of diffraction occurring at the guide groove
region and that at the address information region differ
BACKGROUND OF THE INVENTION from each other and> as a result thereof( the amount of
1. (Field of Technology) tracking shift necessarily deviates to such an extent as to The present invention relates to a tracking system for result in a disturbed tracking at the boundary. FIGS.
an optical memory device of a type wherein any of the 10 12(a) to 12(c) and FIGS. 13(a) to 13(c) are schematic
information recording, reproduction and erasing can be representations illustrative of the change in tracking
carried out by radiating a laser beam to a memory me- error signal occurring during the tracking at the bound
dium. ary according to these two methods, respectively.
2. (Description of the Prior Art) In these figures, the servo region is considered to be Optical memory devices have recently drawn the 15 sufficiently lower than the pit reproducing frequency
attention of people as a high density, high capacity and> therefore, an output of the detector during the
memory device. The reason that an optical memory tracking at the address information region is shown as
device is of high density and high capacity is because an a e yalue FIG 12 ^ where {he difference
the size of each bit which represents a unit of informa- ifl mnt of beam reflected fa taken M a tracki efror
twnstoragecanbereducedtoadiameterofaboutl^m. 20 &. 13 applies where the difference in
This, in turn, however, imposes some limitations on the ° . . , . . c , „ .. „ ... ,
. . j • »/ -t- H ■ j f output from detectors for detecting two split beam
optical memory device. More specifically, m order for v . , . ,. ° . * .
information to be recorded on, or reproduced from, a 'W*nts ,s taken as the tracking error signal.
predetermined location, the light beam is required to be SUMMARY OF THE INVENTION
accurately positioned. 25 ,. , . . , . ,
Because of the foregoing, when using a disc capable Accordingly, the present invention has been devised
of recording information simultaneously with erasure of t0 substantially eliminate the above described problems
previously recorded information, it is a general practice and nas for lts essential object to provide a tracking
for the disc substrate to be permanently provided with system wherein a relatively easy signal processing
beam guide tracks or address information. 30 means is employed to enable tracking to be performed
The guide tracks generally have a shape as shown in in a stabilized manner, even to discontinuous guide
FIG. 10 of the accompanying drawings and are in the grooves, with no need to take special measures for any
form of grooves of a depth generally equal to the wave- disc substrate.
... ... » **** DBSCRtPTION OF THE DRAWNGS
reproduction and erasing is earned out while the light This and other objects and features of the present
beam scans the tracks of the disc guided along these invention will become clear from the following descrip
guide grooves. tion taken in conjunction with preferred embodiments
As a means for sensing a tracking signal from the thereof with reference to the accompanying drawings,
guide grooves, two methods are well-known; a Twin 40 jn which:
Spot method (a three-beam method) such as generally piG. 1 is a schematic diagram showing an optical
used in association with VD (video disc) and CD (com- head for a magneto-optic disc according to one embodi
pact disc) and a push-pull method such as generally ment of the present invention;
used in association with an optically writeable disc. The FIG 2 is a diagram sh0wing an electric circuit formTwin Spot method and the push-pull method are illus- 45 ■ a detecting means for detecting a tracking error trated respectively m FIGS. 12 and 13 of the accompa- signal
nyj^ drawin|s- FIG. 3 is a circuit block diagram showing the circuit
The Twin Spot method has an advantage in that a of {he detectin„ means.
stable tracking performance can be achieved even pIG 4 { b). ^ showi waveforms of si als
though a pick up ,s inclined relative to the optical disc 50 \ ^ *
substrate. However, it has a problem in that, when a JU,-* = • ii i_- i
* i,- u _ u J u * -j FIG. 5 is a schematic diagram showing an optical
tracking beam scans a boundary between a guide , , , . , ,. ^ ®. . ^ ,..e f
groove region G and an address information region A head. f°r an °Ptlcal dlsc to which a tracking system is
constituted by a plurality of pits as shown in FIG. 12(a), aPPhed accordlng t0 a second embodiment of the pres
the tracking tends to be disturbed because of the differ- 55 en* invention;
ence between a diffraction efficiency on the leading FIG- 618 a schematic diagram showing a relationship
beam Bi and that on the trailing beam B2. It is to be between the respective positions of laser beam spots
noted that reference character R used in FIG. 12(b) incident on the optical disc, the arrangement of detect
represents recorded bits. m§ elements of a photo-detector and the respective
On the other hand, although the push-pull method is 60 positions of the laser beam spots reflected toward the
generally free from the above mentioned problem in- detecting elements;
herent in the Twin Spot method because of the tracking KG- 7 is a circuit block diagram showing a circuit performed by a single beam B4 as shown in FIG. 13(a), used to effect a processing necessary to produce a feedit has a problem in that, because the position of the light back control signal;
beam which has been reflected towards a detector D 65 FIGS. 8(a,b) are charts showing waveforms of sig
shown in FIG. 13(6) tends to become displaced relative nals;
thereto in the event of a shift in position of a lens as a FIG. 9 is a diagram similar to FIG. 7 showing a modiresult of tracking or in the event of inclination of the fied form of the circuit;
