Q. Explain Pulse-Code Modulation?

PCM is the simplest and oldest waveform coding scheme for processing an analog signal by sampling, quantizing, and binary encoding. Figure shows a functional block diagram of a PCM system transmitter. In order to guarantee that the message is band-limited to the spectral extent for which the system is designed, a low-pass filter is introduced. The compressor is rather optional for better performance. Let us assume that the PCM signal is transmitted directly over the baseband channel. Corrupted by the noise generated within the receiver, the PCM signal is shown as the input to the PCM reconstruction function in Figure, which depicts a block diagram of functions (including an optional expandor) needed to receive PCM. The operations of the receiver are basically the inverse of those in the transmitter. The first andmost critical receiver operation is to reconstruct the originally transmitted PCM signal as nearly as possible from the noise-contaminated received waveform. The effect of noise is to be minimized through a careful selection of circuit implementation.

The only knowledge required of the receiver to reconstruct the original PCMsignal iswhether the various transmitted bits are 0s and 1s, depending on the voltage levels transmitted, assuming that the receiver is synchronized with the transmitter. The two levels associated with unipolar pulses of amplitude A are 0 and A, whereas those associated with polar pulses (of amplitudes ±A) are A and -A. It is, of course, better for the receiver if the ratio of the pulse-caused voltage to the noise rms voltage is the largest possible at the time of measurement. Figure shows PCM reconstruction circuits for unipolar, polar, and Manchester waveforms.