Editing Modified frequency modulation (section)

==Frequency modulation==
[[Frequency modulation encoding]] (FM) was the first widely used system to perform this operation on disk drives. The [[drive controller]] includes an accurate clock running at half the selected data rate of the disk media. When data is written to the disk, the clock signal is interleaved with the data. On reading, the clock signals are used as short-term triggers to time the presence or lack of a following signal that represents the data bits.<ref>{{cite journal
 |url=https://ieeexplore.ieee.org/document/1449179
 |title=Delay Modulation
 |journal=Proceedings of the IEEE | volume= 57 | issue= 7
 |date=July 1969
 |publisher=IEEE
 |doi=10.1109/PROC.1969.7249
 |access-date=
 |last1=Hecht
 |first1=M.
 |last2=Guida
 |first2=A.
 |pages=1314–1316
 |url-access=subscription
 }}</ref>

The upside to the FM approach is that it is extremely easy to implement the writing circuitry, and the clock recovery on reading is also relatively simple. The downside is that it uses up half of the disk surface for the clock signal, thus halving the total amount of data the disk can store. This led to the development of new forms of encoding that were more efficient.