The invention relates to an index pulse generator for optical information recording and reproduction systems, and more particularly, to an improvement of an index pulse generator which is used in an optical information recording and reproduction system such as is used in combination with an optical disc, optical magnetic disc or the like. 
In optical information recording and reproduction systems, an index pulse is utilized in detecting the number of revolutions of the disc, detecting the presence or absence of a disc loaded, deriving a timing signal for the recording and reproduction of information, or deriving a track jump signal for a disc having spiral track information. 
An index pulse generator of this kind has been constructed as illustrated in FIG.  1.  Specifically, a disc 1 on which optical information is recorded or from which such information is reproduced is detachably mounted on an output shaft of a drive motor 3 by means of a clamp member 2.  As shown in FIG.  2, the bottom surface of the disc 1 is partly formed with an index mark as may be formed by a light reflector 4 of aluminium or the like, in combination with a photoreflector 5 which is located so as to be aligned with the path of movement of such mark.  The photoreflector 5 directs a light beam toward the light reflector 4, which then reflects the light beam, which is collected by the photoreflector to be converted into an electrical signal.  A current signal which is outputted from the photoreflector 5, passes through a resistor R1, across which a voltage signal V1 is developed for application to a positive (i.e. non-inverting) input of a comparator 6.  A reference voltage Vref of a given value from a source, not shown, is applied to the negative (i.e. inverting) input of the comparator 6.  In this manner, the comparator 6 develops an index pulse V2 in response to the comparison. 
When the disc 1 is rotated by the motor 3, the photoreflector 5 scans the disc along a path indicated by dotted lines A shown in FIG.  2.  It will be understood that the light reflector 4, which may be formed of aluminium, presents an optical reflectivity of an increased magnitude while the remainder of the bottom surface of the disc 1 exhibits an optical reflectivity of a reduced magnitude.  Accordingly, the photoreflector 5 outputs a signal V1 which is subject to a periodic variation as indicated by a solid line curve in FIG.  3(a).  In other words, the signal V1 has an increased value only when the photoreflector 5 receives reflected light from the light reflector 4.  It will be recognized that the signal V1 has a d.c.  component which is attributable to dark current or the like. 
If the slicing level of the reference voltage Vref is established as indicated by broken lines in FIG.  3(a), a comparison of the output voltage V1 against the reference voltage Vref in the comparator 6 results in an output pulse voltage V2 as shown in FIG.  3(b).  In the prior art practice, the pulse voltage V2 is utilized as an index pulse in controlling data write/read operation. 
However, it is generally difficult to establish a value of the reference voltage Vref, as indicated in FIG.  3(a).  It will be understood that the amount of light reflected varies when the disc 1 undergoes undulation as when the disc 1 is changed, when the non-uniformity of the clamp member 2 results in a change in the distance between the disc 1 and the photoreflector 5, and when the reflectivity of the light reflector 4 varies with time.  Accordingly, the waveform of the voltage signal V1 changes, and may not be crossed by the reference voltage Vref as illustrated in FIG.  4(a).  Accordingly, the width of the index pulse may change, or such pulse may be completely removed as indicated in FIG.  4(b), thus preventing a proper control of data write/read operation.