1. DATA ENCODING METHODS

2. When the data is received from a serial digital circuit, it is in a format know as NRZ or Non- Return to Zero. This is most common format of data output by a parallel to serial shift register. A very easy solution to this problems as we have would be to record a clock signal with the data signal for synchronization purposes.

3. As writing a clock bit with each data bit would require double space for the data storage, another alternative method, which is normally used in the disk storage method, is to encode the data that is to be written to the disk. This encoding is normally done using some data encoding technique from which the clock synchronization information can b separated, without any loss of the data information.

4. Three most common encoding methods are known as FM encoding method MFM encoding method RLL encoding method

5. FM Encoding Scheme FM- Frequency Modulation, Also known as “single density recording” FM was the original data encoding scheme used for storing data on the magnetic recording surface. This method was popular in the 1970s and is no longer used today.

6. FM Encoding Scheme In this method a clock signal is put with every data signal on the recording surface. This clock signal is used for synchronizing the read operation, as there will always be a clock signal, whether the data signal is thee or not.

7. FM Encoding Scheme Eg:- store 1011 on magnetic recording media using FM recording technique: 1 bit- two pulses (one clock pulse and one data pulse PP ) 0 bit- one pulse and one no pulse PN PP PN PP PP

8. FM Encoding Scheme FM method of data recording also known as (0,1)Run Length Limited recording. the reason is, in series or a running length the minimum number of 0s is zero. i.e. you can have two pulses next to each other without any 0 or no pulse in btw and the maximum number of 0s is one

9. More data can be stored on the same surface, if the number of pulses required to storage density can be minimized. When minimizing the pulse, one should be careful that the number of no pulse together should not be very long, otherwise the disk controller may go out of synchronization with the data.

10. MFM- Modified Frequency modulation The MFM method of data storage, by reducing the number of pulses, is able to store more data without any data and synchronization loss. MFM Encoding Scheme

11. In MFM recording the 0s and 1s are encoded as given below:- :- 1 is always stored as a no pulse, and a pulse(NP) :- 0, when preceded by another 0, is stored as a pulse, and no pulse (PN) :- 0, when preceded by a 1,is stored as two no pulse (NN) MFM Encoding Scheme

12. Eg:- store 1001 using MFM storage method NP NN PN NP ( we can see, this required only three pulses in MFM same data store in FM method PP PN PN PP. Total six pulses required ) MFM Encoding Scheme

13. Advantage:- (MFM over FM) to record data at a higher density. MFM technique allow twice as many bits to be stored in a given length of disk space as FM. Disadvantage:- the circuitry required to encode and decode the data is more complex. MFM Encoding Scheme

14. RLL Encoding Scheme RLL- Run Length Limited. The RLL encoding is most common encoding scheme used in the hard disk storage. This store 50 percent more information than MFM and 3times as much information than FM. RLL Encoding Scheme RLL- Run Length Limited. The RLL encoding is most common encoding scheme used in the hard disk storage. This store 50 percent more information than MFM and 3times as much information than FM.