US Patent No. 7719781 - Prepared by Attorney David Tran for Toshiba and filed by Schwegman (SLWK)
Brief description: The magnified portion of FIG. 2 illustrates one example servo pattern 200. The servo pattern shown is a null-burst pattern. The null-burst servo pattern 200 includes an A burst 210, a B burst 220, a C burst 230 and a D burst 240. The phase of the A burst is 180 degrees out of phase with the B burst. The A burst and the B burst are adjacent one another, and the border between them is on the centerline of a track. The phase of the C burst is 180 degrees out of phase with the D burst. The C burst and the D burst are adjacent one another and the border between them is on the edge of a track. When a read head is passing over the center of a track, the A burst and the B burst will be null or zero because the adjacent servo patterns will cancel. When the read head is off center, the signal will have a varying amplitude and phase. The phase can be detected through a demodulation scheme. The amplitude can be detected through peak detection. The amplitude can also be determined using a demodulation scheme. Given the phase and the amplitude, the location of the read head from the center track of the disk can be determined. Similarly, the same demodulation can be done with respect to the C burst and the D burst. The C burst and the D burst information can be used as further information regarding the position of the read head with respect to the center of the track or with respect to the border between a first track and a second track. The information from the C burst and the D burst can be used to confirm the position of the read head or may, in some instances, provide information necessary to determine if the read head is on one side (above in FIG. 2) or on the other side (below in FIG. 2) the centerline of the track. The magnified portion of FIG. 2 illustrates one example null pattern having an A burst 210, a B burst 220, a C burst 230, and a D burst 240 written in phase relationship with respect to the preamble 202. It should be noted that in FIG. 2, the signals forming the A burst 210, the B burst 220, the C burst 230, and the D burst 240 written in phase relationship with respect to the preamble 202 are shown in a simplified manner for the sake of illustration. The disk drive 100 not only includes many mechanical features and a disk with a servo pattern thereon, but also includes various electronics for reading signals from the disk 120 and writing information representing data to the disk 120. FIG. 3 is a schematic diagram of a disk drive 100 that more fully details some of example electronic portions of the disk drive 100, according to an example embodiment. Referring to FIG. 3, the disk drive device 302 is shown as including a head disk assembly (HDA) 306, a hard disk controller (HDC) 308, a read/write channel 313, a microprocessor 310, a motor driver 322 and a buffer 324. The read/write channel 313 is shown as including a read/write path 312 and a servo demodulator 304. The read/write path 312, which can be used to read and write user data and servo data, may include front end circuitry useful for servo demodulation. The read/write path 312 may also be used for writing servo information in self-servowriting. It should be noted that the disk drive 100 also includes other components, which are not shown because they are not necessary to explain the example embodiments.