odulation are used. Line coding can be used to increase the data rate as shown in, for exam- ple, Figure 4.17, where each sequence of 2 data bits is encoded into four-level pulses for transmission. At the receiving end decoding is carried out and the original bits, 2 for each received symbol, are regenerated. Note that the sym- bol rate on the line is half of the bit rate seen by the data source and the desti- nation and thus the required bandwidth of the channel is reduced to half compared to binary transmission. The line code in Figure 4.17 also cancels dc and similar code is used in ISDN subscriber lines. Note that Gray coding, in which neighbor symbols differ by only one bit, is used in Figure 4.17. The symbol in error is typically a neighboring symbol of the transmitted symbol and with the help of Gray coding only one information data bit in error is generated. Transmission Binary information source Destination Encoder Decoder 4-level line signal Binary data Decoded binary data 10011100 Baud rate on the line is half of the bit rate seen by the source and the destination Figure 4.17 An example of the line coding. We often combine coding and modulation and instead of four or more pulse amplitude values we may transmit four symbol values in carrier wave- forms with, for example, four different phases. This SO called QPSK was dis- cussed in Section 4.2.4 and it can be seen to be a combination of four-level line coding followed by ordinary phase modulation. 4.4.2 Spectrum of Common Line Codes To determine what kind of impact line encoding has on the spectrum, we look at the characteristics of some common line codes. Figure 4.18 presents their power density spectrums showing how the signal power of random data is distributed over the frequencies. 4.4.2.1 Nonreturn to Zero (NRZ) NRZ is the most common form of digital signal used internally in digital sys- items. Each symbol has a constant value corresponding to binary symbol val- ues 1 and 0. The spectrum has a high dc component, and there are no discrete spectral 