Communication Systems

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Pulse modulation converts the sample analog information signals into a discrete pulses which will then be transported from its source to certain destination via a physical transmission media. With the pulse code modulation (PCM), in order to transmit the sample information analog signal as an output, the signals are converted to a series of n-bit binary codes.


In this step, the higher frequency of the signal is filtered out to make the conversion of the analog signal easier. When people speak, the energy in their voices is normally between the range of 200 to 300 hertz for the lower limits and 2,700 to 2,800 hertz for upper limits (Tomasi 275).
In the case of standard voice or speech communication, this reaches up to 3000 hertz bandwidth. The purpose of this bandwidth limiting is to eliminate aliasing or anti aliasing. Aliasing refers to the frequency of fold over distortion and in order to avoid aliasing, the following equation should be satisfied: fs 2fa , where fs=minimum Nyquist sample rate (hertz) and fa = maximum analog input frequency (hertz). Note that aliasing and antialiasing occurs due to under sampling of the input analog signals (voice). The Nyquist criterion fs 2fa happens because frequency of sample signals is less than the maximum frequency of the analog input signal (Cisco). Thus, an overlapping of the sampling frequency spectrum and the maximum frequency of the analog input signal. Overlapping happens due to inaccuracy of the output low-pass filter to detect the occurrence of overlapping between the frequency spectrum of samples and the analog input signals. The output low-pass filter is used in the reconstruction of the original input signal. ...
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