Digital Baseband Signal Transmitter - Digital Communication: Principles and System Modelling - page 79

4.4 Line Coding

As discussed, line codes (or transmission code for baseband communication) are
the representative techniques of digital sequences by pulse waveforms suitable for
baseband transmission. We here consider both binary and M-ary digital baseband
transmission technique. By binary we mean one of possible two level output of pulse
generator in response to each bit. In the baseband M-ary version of the system, the
output of the pulse generator may take on any of the possible M amplitudes with
M>2. The blocks of n-message bits are represented by an M-level waveform with
M 2 n .

The waveforms broadly can be classified into the following four groups.

=

1. Non-return to zero (NRZ)
2. Return to zero (RZ)
3. Phase encoded
4. Multilevel binary

The NRZ group is probably the most popular in practical usage. It can again
be partitioned into the following sub-groups: NRZ-L (L for level), NRZ-M (M for
mark), and NRZ-S (S for space). NRZ-L is used extensively in digital logic circuit.
Binary one is represented by one voltage level and binary zero by another. There
is a change in level whenever the data change from a one to zero or from zero
to one. With NRZ-M, the one, or mark is represented by a change in level and a
zero or space is represented by no change in level. This is sometimes called as
differential encoding. NRZ-M is used primarily in magnetic tape recording. NRZ-S
is the complement of NRZ-M: binary one is represented as no change in level and a
zero is represented as change in level.
The RZ waveforms consist of unipolar-RZ, and RZ-AMI. In magnetic tape
recording and baseband data transmission we can use these types of codes. With
unipolar-RZ, a 'one' is represented by a half-bit-wide pulse and a 'zero' is repre-
sented by absence of pulse. With bipolar-RZ, the one and zero is represented by
opposite level pulses that are one-half bit-wide as shown in the Fig. 4.4 . It means,
in each bit interval T B , there is a pulse present. RZ-AMI (AMI for 'alternate mark
inversion') is a signaling scheme used in telephone systems. Here also, the ones are
represented by the absence pf pulses.
The phase-encoded group consists of bi-phase-level ( bi L ); and delay

modulation or miller coding . The phase encoding schemes are used in magnetic
recording systems, optical communications and satellite telemetry systems. In bi

− φ −

−

φ − L , a one is represented by a half bit wide pulse positioned during the first half of

the bit interval; a zero is represented by a half bit wide pulse positioned during the
second half of the bit interval as shown in Fig. 4.4. In bi

M , a transition occurs
at the beginning of every bit interval. A one is represented by a second transition
one half bit interval later; a zero is represented by no second transition. In case of
bi

− φ −

− φ − S , a transition also occurs at the beginning of every bit interval. A one is