WO2006006089A1

WO2006006089A1 - Drive comprising means for determining the tracking polarity of a record carrier

Info

Publication number: WO2006006089A1
Application number: PCT/IB2005/052070
Authority: WO
Inventors: Bart Van Rompaey; Cornelis M. Schep
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-07-07
Filing date: 2005-06-23
Publication date: 2006-01-19

Abstract

The present invention relates to a drive and a corresponding method for accessing an optical record carrier (20) which can have different tracking polarities. For enabling the determination of the tracking polarity of the record carrier a drive is proposed according to the invention comprising: means (15) for detecting the push pull signal, means (14) for detecting the track cross signal and means (13) for determining the tracking polarity of the record carrier by use of said push pull signal and said track cross signal.

Description

Drive comprising means for determining the tracking polarity of a record carrier

The present invention relates to a drive for accessing an optical record carrier which can have different tracking polarities. Further, the present invention relates to a corresponding method as well as to a computer program for implementing said method on a computer.

Generally it is possible to provide optical record carriers having different tracking polarities. For instance, as will be the case with BD (Blu-ray Disc) record carriers, manufacturers will be given the possibility to produce ROM discs with in-pit or on-pit tracks and recordable discs with in-groove or on-groove tracks. This was not the case with CD or DVD record carriers since there only on-groove or on-pit tracks are used so that the tracking polarity is always the same. A drive, for instance in a player, a recording apparatus or a PC, has to know what type of tracks are present so that the tracking system of the drive can correctly follow the tracks. If the drive was reading the data from the record carrier assuming the wrong type of tracking polarity, no valid data would be read.

It is thus an object of the present invention to provide a drive and a corresponding method which allow to correctly recognize the tracking polarity of an optical record carrier.

The object is achieved according to the present invention by a drive as claimed in claim 1.

A corresponding method is defined in claim 5. A computer program comprising computer program means are causing a computer to carry out the steps of the method when said program is run on a computer as defined in claim 6.

The invention is based on the idea to use, in addition to the push pull signal, also the track cross signal which is generally used for track counting. More particularly, the track cross signal is the low-pass filtered sinusoidal sum signal (of the two detector halves) in the HF read channel, when the focus of the optical beam crosses the tracks. It shows a minimum when the spot of the optical beam is on the tracks. The push pull signal is the low pass filtered sinusoidal difference signal (of the two detector halves) in the PP read channel, when the focus of the optical beam crosses the tracks. The push pull signal crosses two times the zero line, one time when the spot is on the tracks and another time when the spot is in between the tracks. By use of these two signals it can not only be determined when the spot is on track, but furthermore it can be determined which kind of tracking polarity the actual optical record carrier has.

In a preferred embodiment the tracking polarity is determined by finding the minimum of the track cross signal and by determining the algebraic sign of the gradient of the push pull signal at the corresponding radial position, which algebraic sign of the gradient then determines the tracking polarity. In other words it is determined which zero-crossing of the push pull signal correspond to a minimum of the track cross signal, and the tracking polarity derives from the gradient of the push pull signal in that zero-crossing.

More particularly, in a further embodiment, the algebraic sign of the gradient of the push pull signal at a radial position where the track cross signal shows a minimum is used for determining that the record carrier is an on-pit or an on-group type record carrier if said gradient has a positive value and for determining that the record carrier is an in-pit or an in-groove type record carrier if the gradient has a negative value. However, since it is mainly a question of how the push pull signal has been defined, the determination can also work in the other way round.

The present invention thus provides a simple but effective method for determining the tracking polarity of an optical record carrier, which method can be used by a drive each time a new record carrier is inserted into the drive.

The invention will now be explained in more detail with reference to the drawings in which

Fig. 1 shows a block diagram of a drive according to the present invention, Fig. 2 illustrates the generation of the push pull signal, Fig. 3 illustrates the partition of the detector for generation of push pull signal,

Fig. 4 shows the relationship between the PP and the TCS signal, Fig. 5 shows part of a track cross signal and a push pull signal for an on-pit type record carrier and Fig. 6 shows a part of a track cross signal and a push pull signal for an in-pit type record carrier.

Fig. 1 shows in a simple block diagram a drive 10 according to the present invention for accessing an optical record carrier 20, such as, for instance, a BD disc. For the drive only the elements which are relevant for understanding the present invention are shown. It comprises a reading unit 11 for reading data from the record carrier 20 and a writing unit 12 for writing data to the record carrier 20. Further, a polarity determination unit 13 is provided for determining for each new record carrier that is inserted into the drive the tracking polarity. The drive has to know what type of tracks are present on the record carrier 20, i.e. what kind of tracking polarity it has, so that the tracking system (not shown) of the drive 10 can correctly follow the tracks. If the drive was reading or writing data assuming the wrong type of tracking polarity, no valid data would be read or written, respectively. Since according to the present invention the determination of the tracking polarity is based on the use of the track cross signal (TCS) and the push pull signal (PP), a corresponding unit 14 for determining the track cross signal and a corresponding unit 15 for determining the push pull signal are provided. The generation of these two signal and the determination of the tracking polarity therefrom will be explained below in more detail. First, the nature of the push pull signal and the track cross signal shall be explained.

The PP signal is actually an error signal, it is a measure for being off-track: the larger its value the larger the distance the optical spot is away from the nominal tracking direction along the spiral of an optical disc. This error signal can then be used as a servo signal to control the radial position of the optical spot which is scanning the optical disc along the spiral. When the servo loop is closed the optical spot should be on the nominal tracking direction and the PP error signal is zero.

The generation of the PP signal is based on the diffraction phenomena of the spot while scanning the spiral. Fig. 2 shows a schematic diagram of the principle. Fig. 2A shows three parallel tracks of the spiral. The spot is tracking along the center track. It should be noted that the figure shows pits and lands of a ROM disc, but the principle can also be used for R (recordable) and RW (rewritable) systems where instead of pits and lands a groove is present to enable tracking on an empty disc, p is the track pitch and r indicates the radius on the optical disc where the spot is located. Fig. 2B shows a photodiode detector which has four separate quadrants each producing their own signals. The circle indicates the image of the readout spot on the detector. It contains the data information and is also used to generate the PP signal. The 'half circles' above and below the center circle are caused by diffraction on the grating structure which is on the disc in the radial direction (period is the track pitch p).

Diffraction causes side lobes of order -1 and +1 which partly overlap with the center spot thus generating the half circles. Depending on the position of the spot (r=0, r=p/4, r=p/2, ....) the phase of the reflected light in the side lobes changes, causing interference with the zero-th order, central spot. This interference is indicated by the gray- value of the overlapping area between the center spot and a side lobe. The dark color indicates destructive interference, the lighter one constructive.

When the spot is on the nominal tracking direction or right between two tracks (r=0 and r = p/2) both side lobs have the same phase and the interference is the same above and below. When the spot is halfway off the track (r= p/4 or r= 3p/4) then the phase is different above and below.

The generation of a PP signal (also called tracking error signal; TES) is now easy based on the four signals Al, A2, A3, A4 of the four quadrants of the photo detector as shown in Fig. 3:

PP = (A1+A2-A3-A4)/(A1+A2+A3+A4).

It should be noted that the PP signal also has an algebraic sign or polarity. With this it is meant that the PP signal can also be 180 degrees phase shifted. Whether the PP signal is positive or negative when the spots deviates to one particular side of the nominal tracking direction depends on the sign of the PP signal.

The sign or polarity of the PP signal depends on whether the disc is in-groove or on-groove for BD-R or in-pit or on-pit for BD-ROM. The polarity of the PP signal detected on the grooves of a BD rewritable disc is said to be positive. All other polarities are then determined through this definition. For CD and DVD systems there are only on-groove or on-pit type record carriers existing; the determination of the tracking polarity of the record carrier is thus not required since it is always the same. However, for the BD system and probably also for future optical storage systems this is not the case. For BD-ROM in-pit and on-pit type record carriers are allowed, and for BD-R in-groove and on-groove type record carriers will be allowed as well. The difference thereof shall be briefly explained in the following.

The groove corresponds to the area that has been exposed by the mastering spot. In general the groove can be as well a depression as an elevation on the disc. If the groove is nearer to the entrance surface of the optical read-out beam than the land, the recording method is called on-groove recording. If the groove is farther from the entrance surface than the land, the recording method is called in-groove recording. A similar definition is valid for a ROM disc, which does not have a groove, but only pits that have been exposed to the mastering spot. If the pit surface is nearer to the entrance surface than the land, the recording method is called on-pit recording. If the pit-surface is farther from the entrance surface than the land, the recording method is called in-pit recording.

With reference to Fig. 4 the definition of the PP signal and the track cross signal (TCS) shall be explained. The amplitudes of all signals are linearly related to currents through the photo detector, and therefore linearly related to the optical power falling on the detector. The push pull signal PP is shown in the lower part of Fig. 4. It is the low-pass filtered sinusoidal difference signal (Ii -I₂) in the PP read channel, when the focus of the optical beam crosses the tracks. Therein it holds: Il = A1+A2, and 12 = A3+A4. The PP signal is usually used by the drive for radial tracking.

The track cross signal TCS is shown in the upper part of Fig. 4. It is the low- pass filtered sinusoidal sum signal (I1+I₂) in the HF read channel, when the focus of the optical beam crosses the tracks. The TCS signal is usually used by the drive for track jumping. The value of the TCS signal is defined as:

TCS = ((Ii+I₂)_max - (I_!+I₂)_min)/(Il+l2)maχ.

As can be seen from Fig. 4, the PP signal crosses two times the zero line. One time when the spot is on the tracks and another time when the spot is in between the tracks (corresponding to the position r=0 and r=p/2 in Fig. 2). Of course it is desired to be able to set the working point of the servo mechanism around the zero crossing that corresponds with the on-track position also indicated in Fig. 4.

When a disc is inserted in a drive, the drive will need to know the polarity of the PP signal, i.e. the tracking polarity of the disc, in order to read back the data. The drive needs this information because then it knows which zero crossing is the one corresponding to on-track. According to the present invention, the TCS signal is determined and used to indicate the zero crossing in the PP signal that corresponds with the on-track situation. While the TCS has always been used for track counting, it has been found that for it can also be used to determine the tracking polarity as will be necessary for BD discs. As a further illustration Figs. 5 and 6 show (in the case of on-pit shown in Fig.

5 and on-pit shown in Fig. 6) the TCS signal (Figs. 5A, 6A) and the PP signal (Figs. 5B, 6B). The minimum in the TCS signal is the on track position. As can be seen from Figs. 5B and 6B the algebraic gradient (i.e. the he polarity) of the PP signal at the radial position, where the TCS signal has a minimum (i.e. at the on track position), is opposite for in-pit and on-pit disc. For an on-pit disc the gradient of the PP signal has a positive value, while for an in-pit disc the gradient of the PP signal has a negative value. The same holds for an in- groove and an on-groove disc.

In principle the information whether a disc is on-type or in-type can also be mentioned in the Disc Information which is embossed in the lead-in zone of the optical disc. The only problem is that in order to read out the Disc Information one needs to know the tracking polarity (or equivalently, one needs to know whether the disc is on-type or in-type.) In alternative or in addition the drive may attempt to read data assuming a tracking polarity, and to maintain or change the tracking polarity according to whether valid data are read or not. However this trial-and-error approach would imply an increase of start-up time in average. So, another method is desirable to determine this type, which is made possible by the present invention. The present invention thus provides a simple but effective and easy to implement method for determining the tracking polarity of a record carrier by use of the push pull signal and the track cross signal.

Claims

CLAIMS:

1. Drive for accessing an optical record carrier (20) which can have different tracking polarities comprising: means (15) for detecting a push pull signal, means (14) for detecting a track cross signal and - means (13) for determining a tracking polarity of the record carrier by use of said push pull signal and said track cross signal.

2. Drive as claimed in claim 1, wherein said means (13) for determining the tracking polarity are adapted for finding the minimum of the track cross signal and for determining the tracking polarity from the algebraic sign of the gradient of the push pull signal at the corresponding radial position.

3. Drive as claimed in claim 2, wherein said means (13) for determining the tracking polarity are adapted for determining that the record carrier is an on-pit or an on- groove type record carrier if said gradient has a positive value and for determining that the record carrier is an in-pit or in-groove type record carrier if the gradient has a negative value.

4. Drive as claimed in claim 2, wherein said means (13) for determining the tracking polarity are adapted for determining that the record carrier is an on-pit or an on- groove type record carrier if said gradient has a negative value and for determining that the record carrier is an in-pit or in-groove type record carrier if the gradient has a positive value.

5. Method for accessing an optical record carrier (20) which can have different tracking polarities comprising the steps of: - detecting a push pull signal, detecting a track cross signal and determining a tracking polarity of the record carrier by use of said push pull signal and said track cross signal.

6. Computer program comprising computer program means for causing a computer to carry out the steps of the method as claimed in claim 5 when said program is run on a computer.