Digital frequency modulation

Time course of a digital bit data sequence encoded as FM and below that as MFM

The digital frequency modulation , abbreviated FM , also called delay code and after John Milton Miller also called Miller code , is a digital modulation or line coding and, in contrast to frequency modulation, does not provide an analog and continuous modulated signal, but a time-discrete digital bit sequence. It is used, among other things, for magnetic data carriers such as floppy disks as a line code for data recording.

In the technical literature, the separation between modulation and line coding is not clear in this border area and so this method is referred to as both a form of modulation and a type of line coding.

Digital frequency modulation (FM) is related to modified frequency modulation (MFM) and is its forerunner. In contrast to MFM, however, FM has higher frequency components in the spectrum and therefore requires greater bandwidths on magnetic storage media. In contrast to the Manchester coding , which is also used, the frequency spectrum of FM is reduced, but it is not free of equal components. The absence of DC components in the signal, which is necessary for magnetic recording, must therefore be guaranteed by additional line coding.

With the FM, there is a signal change after every user data bit, regardless of whether it is logical 1 or logical 0 . With a logical 1 bit, there is also a signal change in the middle of the bit, as shown in the adjacent figure above. Therefore, this code can also be regarded as a frequency modulation: the logical- 0 bits is a low frequency f ₀  = 1 / T assigned, and the logical- 1 bits is a high frequency f ₁  = 2 / T assigned. T represents the period of a bit. No other frequency assignments are provided.

Individual evidence

1. ^ John M. Miller , IEEE History Center biography, accessed on August 12, 2011