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Part 2 book “Digital video broadcasting - The international standard for digital television” has contents: Digital modulation techniques, conditional access for digital television, the satellite standard and its decoding technique, the cable standard and its decoding technique, and other contents. | 7 Digital Modulation Techniques MPEG source coding, which achieves a data reduction in audio and video signals, has been discussed in chapters 3 and 4. As explained in chapter 5, the various elementary streams are combined in the MPEG transport multiplexer to form a single data stream. This is followed by a coding of the data stream in which redundant signal portions are inserted (see chapter 6). The entire processing of the baseband signals can take place in a computer (or a digital circuit) in which the data are available as a sequence of numerical values. For these values to be transmitted on .a channel they have to be converted into genuine data signals. The signals are output sequentially by an interface, synchronisation being provided by an internal processing clock. Each information bit possesses finite energy, which will be referred to as bit energy Eb. The methods by which data signals can be adapted to the respective transmission channel are the subject of the following section. 7.1 NRZ Baseband Signal The physical shape of the signal which is mainly used for the processing of digital signals in the baseband is called non-return-to-zero (NRZ). An ideal signal is usually described as a sequence of weighted Dirac pulses [LUKE I). The data are read out at the output interface of the computer, where they adhere to a rigid time-slot pattern nTB. Their shaping into an NRZ pulse is achieved by a hold unit, which holds the value of the information for the period of the timing pulse TB so that each ideal Dirac pulse is overlaid with a rectangular pulse. Figure 7.1 depicts an NRZ signal with the period TB. The amplitude assumes the value of A when state 1 is to be transmitted, and the value of 0 when state 0 is to be transmitted. If the signals, having assumed this form, are to be used for data transmission, it is important to know their spectral properties. The power spectral density (PSD or spectrum, for short) of the NRZ signal can be computed on the .
