Modified Frequency Modulation

Abstract: A disk drive that can read from and write to both standard floppy disks and high capacity floppy disks. The present invention provides a single slot disk drive that can accept both standard floppy disks and high capacity floppy disks. After a floppy disk is inserted, physical cues on the floppy disk are used to inform the disk drive as to the nature (standard or high capacity) of the inserted disk. The disk drive incorporates two interfaces, a standard floppy disk controller and an IDE interface for high capacity disks. The disk drive is configured so that both interfaces can communicate with a host computer at all times, regardless of what type of disk is inserted in the disk drive. This allows the host computer to operate as if it had two separate disk drives available.

Abstract: In magnetic recording devices, such as floppy disk drives, operational difficulties in the faithful reproduction of recorded data include peak shifting. The invention overcomes these difficulties by introducing a read back compensation circuit comprising a frequency equalizer which favorably amplifies a certain harmonic of the original read signal. In a recording device using a modified frequency modulation encoding scheme (MFM), for instance, this harmonic would be the second harmonic. In order to minimize the effect of circuit paramater variations the frequency equalizer may be split between two circuits, specifically between the read filter network and the read differentiating network. First switching means connected to the filter network and second switching means connected to the differentiating network are arranged as to change the damping factor and the frequency of the filter and differentiating network. The switching means are controlled in dependence either of the speed of the recording medium or of a track signal indicating the position of the read head.

Abstract: A method and apparatus for recording data on a rotating magnetic disk at plural density rates on different areas of the disk is disclosed. The method is particularly useful in magnetic disk recording systems in which the header or control fields of the recorded information are recorded in "single" density FM recording and the data fields of the recorded information are in "double" density modified frequency modulation (MFM) code. The technique involves changing the conventional MFM coding rules to prevent certain data patterns from appearing to the data recording system read circuitry as part of the control fields. In particular, the data pattern 011110 is encoded according to the invention as a data pattern which consists of 000000 with synchronization bits between the first and second, third and fourth, and fifth and sixth data bits. This encoding modification prevents any misinterpretation of the data field as part of a control field. The data recording circuitry used to decode the MFM information is modified from the conventional decoding circuitry so that adjacent "zero" data bits having no synchronization bit between them are decoded as adjacent "one" bits. The original data stream is thereby recovered.

Abstract: A modulation method and apparatus is provided which can coexist with conventional Manchester (Ethernet) coding schemes and which can be transmitted and received over twisted pair cabling. The method according to the invention is a three-level D.C.-free code without bipolar violations wherein intersymbol interference is sufficiently controllable that the received signal can be detected as a two-level code. The code has a different power spectrum than Manchester coding for the same baud rate. The modulation method is a modified frequency modulation (Miller) code comprising three levels wherein each of the transitions is of a limited duration and the transition returns to zero d.c. after a predetermined pulse duration. The modulation method may be termed pulsed modified frequency modulation or pulsed Miller encoding. The signal so modulated is intended to be transmitted over twisted pair cabling of limited bandwidth such that the cabling serves as a low-pass filter. The pulsed modified frequency modulation encoding method according to the invention produces a fundamental at a frequency which is about half of the fundamental of a Manchester coding scheme at the same bit rate.