Frequency Modulation (FM) Frequency  modulation  (FM)  is  the  simplest  method of encoding data to include enough timing pulses so that the controller and disk drive remain synchronized. Using FM, each data bit is split into two clock periods. A logic ONE is encoded as two pulses or flux reversals. A logic ZERO is encoded as a pulse followed by no pulse. Therefore the byte 11000101 would be encoded on the disk as PPPPPNPNPNPPPNPP (P = pulse, N = no pulse). FM is an effective method for encoding data, but it wastes a lot of space on the disk. To maximize data storage on the disk, a method is needed that reduces the number of pulses yet does not allow too many no pulse time periods. Modified Frequency Modulation (MFM) Modified  frequency  modulation  (MFM)  refines data encoding to reduce the number of pulses written on the disk. Using MFM, a logic ONE is always encoded as no pulse followed by a pulse. A logic ZERO, when preceded by a logic ONE, is encoded as two no pulses. A logic ZERO, when preceded by another logic ZERO, is encoded as a pulse followed by no pulse. Using MFM, the byte 11000101 would be encoded NPNPNNPNPNNPNNNP for a total of six pulses or flux reversals on the disk. Compare this with the 12 pulses required to store the same data using FM. MFM  is  currently  used  with  all  floppy  drives,  most large disk memory sets, and many fixed disk systems. Run Length Limited (RLL) The run length limited encoding schemes take data encoding  to  a  new  level.  Usually  the  RLL  specification will be followed by two numbers such as 1,7 or 2,7. These numbers represent the minimum and maximum run of 0 bits between two 1s. The most common RLL scheme is RLL 2, 7. RLL 2,7 is a complex encoding scheme that groups bits together and uses a table to encode the data in these groups. For   example, 1100   is   encoded   as NNNNPNNN,  1101  is  NNPNNPNN,  and  111  is NNNPNN. RLL increases the density and transfer rate of data by 50 percent. A 20M MFM drive can store 30M if formatted as an RLL drive. Whether a drive is MFM or RLL depends on the controller and not the drive. FIXED DISK CONTROLLERS The disk controller determines what encoding scheme will be used and interfaces the disk with the computer. You can change the disk controller to make a 20M drive into a 30M drive by changing from an MFM controller to an RLL controller. RLL encoding requires that the drive work harder; therefore, be sure your drive can handle the demands of a new controller. Of particular concern is the type of head actuator and the magnetic medium of the drive. Stepper  motor  head  actuators  are  slower  and  the problems they can encounter with temperature can cause the drive to be very unreliable if formatted as an RLL drive. Iron oxide medium has a lower signal-to- noise ratio than the thin film medium. The noise picked up by the heads can be interpreted as data and result in read errors. FIXED DISK INTERLEAVE FACTOR The interleave factor is a method of numbering the sectors on a fixed disk to provide the optimal transfer of data between the controller and the computer. When a fixed disk is formatted, sector numbers are written on each  track.  Interleaving  refers  to  the  relationship between the physical sectors on a track and the logical sectors on a track. Each sector on a fixed disk in a personal computer has 512 bytes per sector. Most files are larger than 512 bytes; therefore, it is assumed that if you want to retrieve the data at cylinder 225, sector 1, you will next need the data in sector 2. Since the fixed disk spins at 60 revolutions per second, the heads read data at 512 bytes per sector, 17 sectors per track or a data rate of over 500 kilobytes per second. With no interleave factor, the head reads the data from sector 1 and sends it to the controller. While the controller assembles the data to send it to the computer, sector 2 is under the head but the controller is not ready to accept the data. So  the  disk  must  make  another revolution to retrieve the data from sector 2. To avoid this problem, the disk is interleaved. This means the logical sector numbers do not necessarily follow the physical  sectors. Figure 10-19 illustrates the sector numbering of a disk with a 3:1 interleave. Physically the sectors are numbered 1, 7, 13, 2, 8, 14, 3, 9, 15, 4, 10, 16...12, and back to 1. With a 3:1 interleave, the head reads logical sector 1 and sends the data to the controller. While  the  controller  processes  the  data,  the  next physical sector and part of the following sector pass by 10-26