Page 305 - Satellite Communications, Fourth Edition

Digital Signals 285

Figure 10.2 Examples of binary waveforms used for encoding
digital data: (a) unipolar NRZ; (b) polar NRZ; (c) polar RZ; (d) split
phase or Manchester; (e) alternate mark inversion (AMI).

the term bipolar in North American usage is reserved for a specific wave-
form, described later). For a long, random sequence of 1s and 0s, the dc
component would average out to zero. However, long sequences of like
symbols result in a gradual drift in the dc level, which creates problems
at the receiver decoder. Also, the decoding process requires knowledge of
the bit timing, which is derived from the zero crossovers in the wave-
form, and these are obviously absent in long strings of like symbols.
Both the unipolar and polar waveforms shown in Fig. 10.2a and b are
known as non-return-to-zero (NRZ) waveforms. This is so because
the waveform does not return to the zero baseline at any point during
the bit period.
Figure 10.2c shows an example of a polar return-to-zero (RZ) waveform.
Here, the waveform does return to the zero baseline in the middle of the
bit period, so transitions will always occur even within a long string of