Frequency modulation was the original data encoding scheme used for storing data on magnetic recording surface. It is also known as the single density recording method.
In frequency modulation, a clock pulse is written at the beginning of each bit cell. The data pulse is written at the center of each cell. If data is 1, the data pulse is present. If the data is 0, then there is no data pulse.
In Frequency modulation, there are two flux changes per bit cell when 1´s are recorded in all bit cells. It is used in single density floppy disk drives. Frequency modulation encoding in no longer used.
Frequency modulation method of data recording is also known as (0, 1) Run length limited (RLL) recording. For example, suppose you want to store the binary digit 1011 on magnetic media using frequency modulation recording technique. The data recording a 1 bit is stored as two pulses(one clock pulse and one data pulse) and a 0 bit is stored as one pulse and one gap or no pulse(only 1 clock pulse and no data pulse).
- A binary digit 1 is stored as two pulses (PP).
- A binary digit 0 is stored as one pulse and one “No pulse” (PN).