First, we see Return to Zero (RZ) recording, where a one is represented by a pulse of magnetization in one direction, and a zero is represented by no magnetization.Then, we see Bipolar Return to Zero (RZ(B)) recording, in which one bits are recorded as pulses of magnetization in one direction, and zero bits as pulses of magnetization in the other direction.
And then we see Carrier-Suppressed Return to Zero (CSRZ) modulation, where pulses represent ones, and the absence of a pulse represents a zero, but successive pulses alternate in direction, thereby avoiding the presence of a DC component in the signal. More bandwidth-efficient forms of modulation which avoid a DC component that we will see later have been used for magnetic recording, but this form of modulation has found applications in fiber-optic communications.
Next, we see Return to Bias (RB) recording. Here, pulses are recorded in one direction for a one bit, but no pulse is recorded for zero.
Then, we see Non-Return to Zero (NRZ) recording. Here, a signal is recorded one way for one, and the opposite way for zero, without space between bits.
Below that, we see Non-Return to Zero Inverting (NRZI) recording, in which a transition from one direction of magnetization to the other indicates a one, and no change indicates a zero. This was a popular method of recording on classic magnetic tapes, and this principle also forms a step within more advanced methods we will see later. (This modulation method is also referred to as NRZ-M, to distinguish it from a complement, NRZ-S, in which a zero bit is indicated by a change in polarity, and a one bit is indicated by the absence of such a change.)
Next, there is Phase Encoding (PE), also known as Biphase Level or Manchester II + 180°. This method was first used when the density of reel-to-reel computer tapes was increased to 1600 bpi. Tapes for use with this method were also labelled 3200 fci, since with this method (like RZ and RB, but unlike NRZ and NRZI) the tape had to be capable of twice as much bandwidth, in the forms of changes of the direction of magnetization, as the raw data itself called for. As is the case with some of the other modulation methods noted here, it was also known by some other names. For example, LINCtape and DECtape used this modulation method, as illustrated by waveform charts in their maintenance manuals, but the customer-level documentation referred to the modulation method used simply as Manchester code, which name properly applies to the next variant of phase encoding to be shown.
Then, we see Biphase Mark (BM), also known as Manchester code and as Frequency Modulation. This variant of phase encoding has a transition at every boundary between bit times, but indicates a one by a polarity transition within the bit, and a zero by no transition within the bit. (Using a transition within the bit to indicate a zero is also possible; this is known as Biphase Space, or Manchester I + 180°.)
After that, we see Differential Manchester (DM) encoding; this version of phase encoding has a polarity transition within each bit, and alternates the direction of this transition when the bits stay the same, and keeps the direction of the transitions constant when the bits differ. Note that this format, unlike the preceding ones shown here, is ambiguous, except for a convention regarding initial conditions, about which bits are ones and which bits are zeroes.
Finally, we see Modified Frequency Modulation (MFM), also known as Miller Code. This combines the low bandwidth requirement of NRZI with the self-clocking characteristic of PE. One can think of it as a modification of NRZI recording, in which a flux change is inserted between two consecutive zero bits. Note that although flux changes can occur either in the middle of a bit, or on the boundary between bits, they still cannot be closer together than the width of a bit. Essentially, room is made for clocking information, without increasing the bandwidth from NRZI, by allowing flux changes at odd half-bit positions. This method is used with floppy disks, and was used with early computer hard drives as well. This method can also be ambiguous, since both a string of consecutive ones and a string of consecutive zeroes lead to a square wave of the same frequency, differing only in phase; the most common way to deal with such an ambiguity is simply to begin a block with a known, fixed sequence of synchronization characters.