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Baseband Pulse and Digital Signaling Chap. 3
Manchester Signaling. Each binary 1 is represented by a positive half-bit period
pulse followed by a negative half-bit period pulse. Similarly, a binary 0 is represented by a
negative half-bit period pulse followed by a positive half-bit period pulse. This type of signaling
is also called split-phase encoding.
Later in this book, we will often use shortened notations. Unipolar NRZ will be
denoted simply by unipolar, polar NRZ by polar, and bipolar RZ by bipolar. In this regard,
unfortunately, the term bipolar has two different conflicting definitions. The meaning is
usually made clear by the context in which it is used: (1) In the space communication
industry, polar NRZ is sometimes called bipolar NRZ, or simply bipolar (this meaning will
not be used in this book); and (2) in the telephone industry, the term bipolar denotes
pseudoternary signaling (this is the meaning we use in this book), as in the T1 bipolar RZ
signaling described in Sec. 3–9.
The line codes shown in Fig. 3–15 are also known by other names [Deffeback and
Frost, 1971; Sklar, 2001]. For example, polar NRZ is also called NRZ-L, where L denotes the
normal logical level assignment. Bipolar RZ is also called RZ-AMI, where AMI denotes
alternate mark (binary 1) inversion. Bipolar NRZ is called NRZ-M, where M denotes inversion
on mark. Negative logic bipolar NRZ is called NRZ-S, where S denotes inversion on
space. Manchester NRZ is called Bi-w-L,
for biphase with normal logic level.
Other line codes, too numerous to list here, can also be found in the literature [Bylanski
and Ingram, 1976; Bic, Duponteil, and Imbeaux, 1991]. For example, the bipolar
(pseudoternary) type may be extended into several subclasses as briefly discussed following
Eq. (3–45).
Each of the line codes shown in Fig. 3–15 has advantages and disadvantages. For
example, the unipolar NRZ line code has the advantage of using circuits that require only one
power supply (e.g., a single 5-V power supply for TTL circuits), but it has the disadvantage of
requiring channels that are DC coupled (i.e., with frequency response down to f 0), because
the waveform has a nonzero DC value. The polar NRZ line code does not require a
DC-coupled channel, provided that the data toggles between binary 1’s and 0’s often and that
equal numbers of binary 1’s and 0’s are sent. However, the circuitry that produces the polar
NRZ signal requires a negative-voltage power supply, as well as a positive-voltage power
supply. The Manchester NRZ line code has the advantage of always having a 0-DC value,
regardless of the data sequence, but it has twice the bandwidth of the unipolar NRZ or polar
NRZ code because the pulses are half the width. (See Fig. 3–15.)
The following are some of the desirable properties of a line code:
• Self-synchronization. There is enough timing information built into the code so that bit
synchronizers can be designed to extract the timing or clock signal. A long series of
binary 1’s and 0’s should not cause a problem in time recovery.
• Low probability of bit error. Receivers can be designed that will recover the binary data
with a low probability of bit error when the input data signal is corrupted by noise or ISI.
The ISI problem is discussed in Sec. 3–6, and the effect of noise is covered in Chapter 7.
• A spectrum that is suitable for the channel. For example, if the channel is AC coupled, the
PSD of the line code signal should be negligible at frequencies near zero. In addition, the
signal bandwidth needs to be sufficiently small compared to the channel bandwidth, so
that ISI will not be a problem.