1.  Technical Field
The present invention relates generally to retrieving information from an optical disk.  More specifically, the invention relates to a new method for generating a tracking error signal for any ablative, phase pit or phase change type of media such as compact disk (CD) or digital video disk (DVD).  Still more specifically, the invention generates a tracking error signal by using a differential amplitude reading from a quad photodetector, or multiple pixel detector. 
2.  Description of Related Art
The use of optical disks for storing information has become popular in recent years and a number of products are commercially available, including compact audio disks, and digital video disks.  Originally, these devices were read only memory devices, such as the music and personal computer CD players, but more recently, recordable compact disks have emerged in the marketplace.  Despite the many different formats and options available in optical disk technology, there are considerable pressures to have standards which enable optical disk players to work with all data formats, and all manufacturers' products widely interchangable. 
With reference now to the figures and in particular with reference to FIG.  1 is a block diagram schematic which illustrates the components and connections of a known optical disk player.  This configuration is well known in the prior art and is included here to motivate and frame the present invention.  An optical disk system 100 includes semiconductor diode laser 102 that is typically used to generate laser beam 104.  Miniature optics 106 collimates and steers laser beam 104.  Generally, a movable, controllable lens 108 focuses laser beam 104 to a focused spot, also referred to as beam spot, and position on a spinning optical storage disk 110, which is connected to a variable speed motor 112.  Light reflected from optical storage disk 110 is routed to both detector 114 and a position sensitive detector, such as a quad photodetector 116.  Detector 114 and quad photodetector 116 may be one single unit instead of the two units shown in FIG.  1 Electronic data for the data channel is routed through read channel 118 into a servo controller/decoder 120.  The electronic signals from quad photodetector are routed through preamplifier 122, and a servo preprocessor 124, into servo controller/decoder 120.  These electronics generate an error signal which, through a power amplifier 126 and servo control motor 128 adjust the position of lens 108 in the vertical and horizontal directions in order to keep laser beam 104 aligned in focus and on track. 
Thus, these optical disk memory devices operate by bouncing light emitted from a semiconductor diode laser off an inhomogeneous reflective surface of a spinning optical disk.  The reflected light is then routed to a detector, which outputs an electronic signal that is processed to recover the stored digital data.  The laser beam is nominally stationary, however, the laser assembly is slid along the radius of the disk being read.  Furthermore, fine adjustments of the beam focal spot and position are made based on a feedback signal that drives an electromechanical armature.  The data is encoded on the disk in a variety of ways, including ablative, phase pit or phase change type of coding. 
In general, the optical disk rotates, often at high speeds, and this gives rise to a time signal, or time vector of digital data.  A single speed compact disk rotates at several hundred revolutions per minute (rpm).  Because it is desirable to provide a constant time spacing between data readings, a constant linear velocity is maintained as the data is read from the disk.  Thus the precise rotational speed depends on the portion of the disk being read.  For example, typical rotational speeds for a single speed compact disk are 200 rpm at the outermost track and 450 rpm at the innermost track.  Faster microprocessors, as well as an overall improvement in control technology, have enabled disk drives that operate at multiples of the original single speed CD player.  Thus, a 6.times. ( six times single speed) CD player rotates at speeds up to 2700 rpm when reading the innermost track.  Advantages of higher speeds include shorter data acquisition time, and error checking for misread data points.  At these faster operating speeds, optical beam alignment tolerances, and their error correction become more challenging.  Further, it is highly desirable for any processing protocols, and error correction methods, to be general across all data formats. 
In early optical disk readers, tracking was accomplished by advantageously coded spot patterns on the disk.  This scheme is not widely used because it is an overly specific format, and valuable disk area is used for tracking instead of data storage. 
One existing method for generating a tracking error signal for audio compact disks is the three spot tracking technique.  Here, a tracking error signal generator includes three light receptors for receiving three optical signals reflected from a recorded medium and for generating respective three electrical output signals.  The three light beams impinge upon the recorded medium along a line having a predetermined angle with respect to a recorded track.  Thus, the outer two spots sandwich the inner, data reading spot, and line up with the inside and outside edges of the data track.  The difference in power between the outer beams is compared to generate the tracking error signal.  The tracking error signal is set to the difference between the first and third output signals.  To be widely useful, it is desirable that digital video disk players be able to play compact disks.  However, because there is a difference between the track pitch of the CD and that of the DVD, the three spot tracking technique is no longer suitable for generating error tracking signals which are independent of data format. 
Differential phase tracking error overcomes the limitation of data format.  To generate a differential phase tracking error signal which can be used to align the laser beam onto the data track of an optical disk, the quad photodetector 116 of FIG.  1 is used in the following manner.  If the pit is left of the beam spot, the beam spot appears on quadrant A of quad photodetector 116 first and then on quadrant D. In this situation, the pit leaves quadrant A and then leaves quadrant D. The modulation of the output from these elements in quad photodetector 116 will have (quadrant A+quadrant C) ahead of (quadrant B+quadrant D) in the time domain.  Measuring this phase difference provides a differential phase tracking error signal that may be used to drive servo control motor 128 in FIG.  1 to realign the laser beam onto the optical disk data track.  A limitation of differential phase tracking error signal generation is that it is dependent on the specific data pattern, and hence proves to be more effective with certain formats than with others. 
Therefore, it would be advantageous to have a method and apparatus for generating a tracking error signal that is robust across different data formats and is independent of data pattern.