1.  Field of the Invention
The present invention relates to a recording and/or reproducing apparatus for a disc-shaped recording medium.  More the present invention relates to a recording and/or reproducing apparatus for reproducing information signals having differing reproduction principles which are recorded on a disc-shaped recording medium. 
2.  Background of the Invention
Systems using optical discs and magneto-optical discs as a recording medium where the user can reproduce or record data such as music data are well known.  For example, optical disc reproducing apparatuses such as CD (Compact Disc) players are dedicated to reproduce only, and recording and/or reproducing apparatuses which use magneto-optical discs can both reproduce and record data. 
While data is reproduced from the optical disc by this kind of recording and/or reproducing apparatus, a light beam is radiated onto the signal recording surface by an optical head and data is then read out by detecting the light beam which is reflected by the recording surface.  In order that the light beam be radiated in an appropriate manner, a tracking servo and a focusing servo control the position of an objective lens within the optical head and a sled servo feeds the optical head in a radial direction of the disc. 
Also, the operating modes, for example, the tracking operation modes and the spindle servo modes at the time of recording and reproducing are different for read only type optical discs such as CDs, where the information is physically formed as convex or concave-shaped pits, and magneto-optical discs, where magneto-optical areas are provided in which the user can record information. 
It therefore follows that with recording and/or reproducing apparatuses for optical discs which have both regions in which the information is recorded using pits, hereinafter referred to as pit regions, and recordable regions which are capable of being recorded in, it is necessary to switch over between the tracking operation mode and the spindle servo mode. 
When the optical disc is a magneto-optical disc on which the user can, for example, record music etc.,  a pit region 80a is provided at the innermost side of the disc, as shown in FIG.  1 (a).  At this pit region, TOC information (referred to hereinafter as a "P-TOC") for managing the disc attributes and recorded data etc.  is recorded using pits.  This is followed by a recordable region 80b, in which management information (referred to hereinafter as a "U-TOC") to be used in re-writing by the user is recorded along with usual data such as music etc. 
A wobbled pregroove is formed at the recordable region 80b.  The address information for this pregroove is then formed by wobbling using, for example, an FM modulated signal.  By using the pregroove in this way, address information, which in this case is absolute position information, can be deduced even for recordable regions where there is no data recorded.  Naturally, when data is being recorded no address exists for the data, so the recording operation can be controlled using address information obtained from the pregroove. 
A hybrid-type optical disc is one in which the recording region includes a pit region, and a recordable region is also provided, as is shown in FIG.  1(b).  This hybrid type optical disc has data such as a P-TOC and/or music data recorded in the pit region as data exclusively for reproducing, and is also capable of having data such as a U-TOC and music data recorded in the recordable region.  This means that it is possible to record data such as music etc.  as read only data which cannot be overwritten together with data which can be overwritten on the same optical disc. 
However, with regards to discs on which the pit region and the recordable region exist together, the principles behind recording and reproducing back from and to a pit region and from and to a recordable region are different, and their data read out methods are not compatible.  It follows that when reading data from these respective regions, a system controller for the recording and/or reproducing apparatus has to change over between the tracking operation mode and the spindle servo mode in order to correspond with the region which is to be scanned by the optical head. 
For example, with the optical disc in FIG.  1(a), when the optical disc is loaded into the apparatus, the tracking operation mode is first put into the mode for scanning the pit region and the TOC information is read out.  Next, when data such as music is actually being recorded or reproduced, the optical head is fed towards the outside of the disc, and the tracking operation mode is put into the mode for scanning the recordable region (hereinafter referred to as "MO model") and the data is read out.  The recording/reproducing of data etc.  is then achieved by having the optical head access a prescribed position using the address data which was read out. 
When reading out usual data using the optical head 82, that is to say, when reading out data from the MO region 80b, the output signal for a photodetector PD is supplied to an RF amplifier 83 (see FIG.  2) where it undergoes arithmetic processing and amplification processing.  This photodetector PD is constructed from a four division detector which has a pair of side spot detectors and a pair of RF signal detectors at its center. 
At the RF amplifier 83, items such as the reproducing data, focus error signal and tracking error signal are generated by performing arithmetic processing on the output from the photodetector PD.  These signals and their corresponding circuit systems are, however, omitted from this explanation. 
A push-pull signal for extracting the address modulation information which detects the pregroove, for example the information from the four-partition detector in the arithmetic amplifier 83a of the RF amplifier 83, is used as the signal for operating the spindle servo system. 
An address decoder 84 is a circuit for demodulating information from the aforementioned pregroove and then extracting the address information from this demodulated information.  The signal for the spindle servo is also extracted by the address decoder 84. 
For example, at the address decoder 84, the groove information is put into binary after being passed through a band pass filter of a prescribed central frequency and biphase data is deduced using PLL demodulation.  Also, a clock signal of a prescribed frequency is reproduced by putting this biphase data through PLL processing and this bit clock is also used as rotational speed information for the spindle. 
The signal for use with the spindle servo extracted from the address decoder 84 is supplied to a signal processing unit 85.  The signal processing unit 85 then generates a spindle error signal ES.sub. AD necessary for a servo circuit 86, which operates the spindle servo, to control the number of rotations of the spindle motor 81. 
Also, while the optical head 82 reads the pit region 80a, the RF signal obtained at the arithmetic amplifier 83b in the RF amplifier 83 is sent to the signal processing unit 85.  A spindle error signal ES.sub. EFM is then generated from the bit clock which is being reproduced and this is then outputted.  For example, a synchronization signal is extracted from the EFM signal for the data which is generated to decode the RF signal.  This is injected into the PLL circuit and a reproduction bit clock is obtained from this PLL circuit.  This reproduction bit clock is then compared with the standard system clock and an error signal ES.sub. EFM is obtained. 
By then comparing the level of the inputted spindle error signals ES.sub. AD or ES.sub. EFM with a prescribed standard value, the servo circuit 86 can determine whether or not the current rotational speed of the spindle motor 81 is faster or slower than the regulation speed.  If the rotational speed of the spindle motor is not the regulation speed, the servo circuit 86 generates a motor control signal SPO for correcting this and supplies this to the motor driver 87. 
The rotational speed of the spindle motor 81 can be accelerated or decelerated with regards to the forward direction depending on whether the current applied from the motor driver 87 is positive on negative, so that a normal operating speed can be maintained. 
For example, if the spindle motor 81 is rotating slowly, a forward current is applied based on the motor control signal so as to provide acceleration.  If the rotational speed is fast, a current is applied in the reverse direction based on the motor control signal SPO so as to provide deceleration. 
Unfortunately, the recording and/or reproducing apparatus for magneto-optical discs which have both pit regions and recordable regions cannot distinguish between a pit region and a recordable region while the optical head is actually scanning these regions and may lapse into a state where it cannot read out the data. 
For example, when reading out data from the recordable region 80b of the optical disc 80, if the apparatus is affected by interference such as impacts or vibration, or if a large error occurs in the data which is read out for whatever reason, the optical head 82 may scan into the pit region 80a. 
As the tracking operation modes for the pit region 50a and the recordable region 80b differ, it is no longer possible to carry out an appropriate tracking operation if, for example, the pit region 80a is entered into while the apparatus is still in the mode for a recordable region. 
If the tracking operation cannot be carried out as usual, the data cannot be read.  The recording and/or reproducing apparatus will therefore not be able to return to normal operation, with the reason why being unclear. 
This is the same if the optical head 82 goes from the pit region 80a into the recordable region 80b as a result of interference etc. 
Also, with recording and/or reproducing apparatus equipped with the kind of spindle servo apparatus shown in FIG.  2, there is a problem where the spindle motor may rotate quickly or rotate in a reverse direction, so as to go into servo run away. 
For example, if the apparatus is subjected to interference such as an impact or vibration etc.  or a large error occurs in the data being read out for whatever reason, the light beam scanning process being carried out by the optical head 82 may go into the pit region 80a while reading data from the recordable region 80b of the disc 80. 
The spindle servo methods for this kind of pit region 80a and recordable region 80b are different.  For this reason, if the pit region 80a is entered while still in the reading mode for the recordable region 80b, the signal supplied to the signal processing unit will be completely inappropriate.  As a result of this, the signal for the spindle error signal ES will no longer function as a CLV (Constant Linear Velocity) control signal.  The result of a completely inappropriate signal applied for the functioning of the spindle servo loop may result in reverse rotation if a decelerating current is continuously applied to the spindle motor 81 in the reverse direction.  Alternatively, rotation of an extremely high speed may occur in the forward direction for the spindle motor. 
If this kind of servo run away occurs, it is, of course, not possible to recover the spindle motor from this run away state using servo control and it is therefore not possible to return the spindle motor to normal operation. 
In particular, once the spindle motor 81 starts to rotate in the reverse direction, even if the optical head 82 could return to the recordable region 80b, the data read out would not be the same data.  The servo information could therefore not be extracted accurately and appropriate rotation conditions for the spindle motor 81 could not be returned to.  In other words, the spindle motor 81 could not be brought out of a state of reverse rotation and recording/reproduction would not be possible. 
This is the same if the optical head 82 goes from the pit region 80a into the recordable region 80b. 
When the spindle servo has stopped functioning in this way, it is possible to consider turning off the motor power supply so that the spindle motor 81 will come to a halt as a way of resolving temporary spindle runaway.  However, it will not be possible to return the spindle motor to normal operation quickly using this method. 
This kind of problem occurs not only when shifting between a recordable region and a pit region, but also, for example, when making an access within the same region (it follows that this problem also occurs in reproducing apparatus with discs which only have pit regions). 
This is to say that both the tracking servo and the sled servo have to be turned off during the access of the optical head 82, but the aforementioned signal for generating a spindle error signal ES required to do this cannot be read.  Because of this, it is extremely difficult to maintain the rotational speed of the spindle motor within a range which can be reined in by the spindle servo. 
Also, when the optical head 82 goes to the innermost position or the outermost position on the disc, so that a reading operation from a signal recording part of the disc known as a "Miller surface" is to be carried out, the signal for generating the spindle error signal ES cannot be obtained.  This means that the spindle servo cannot be applied in a suitable manner.