5Pulse Modulation
Telecommunication systems aim to provide a faithful replica of transmitted signals to be successfully decoded by distant receivers. Transmitted signals may carry analog or digital information. For example, a speech or an analog TV signal is analog but a computer file is digital. A communication system may be designed for transmission of analog and/or digital signals. Analog (AM/FM) transmission of signals is continuous in time and amplitude. Accordingly, the demodulated received signals are also continuous in time and amplitude. However, in digital transmission, signals are discrete in time and amplitude. Therefore, for digital transmission of analog signals, they should be converted into digital by using analog‐to digital convertors (ADC). Similarly, received digital signals should be converted back into analog by digital‐to‐analog convertors (DAC) at the receiver. Digital communications is preferred mainly because it offers much improved performance compared to analog communications.
Figure 5.1 shows the role of ADC and DAC in a simple block diagram of a baseband communication system for digital transmission of analog signals. The modulator modulates the amplitude, the duration or the position of the information pulses to make best use of the baseband channel. Evidently, there is no need for ADC and DAC blocks for transmitting digital information.
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