Pulse Code Modulation

Pulse Code Modulation

Section II Pulse code modulation (PCM) is the digital way to transmit analog data. The signals in PCM are binary; that is, there are only two possible states, represented by logic 1 (high) and logic 0 (low). This is true no matter how complex the waveform happens to be. Using PCM, it is possible to digitize all forms of analog data, including full-motion video, voices, music, telemetry, and virtual reality. The data is feed into an encoder that performs the following functions: sampling, quantizing, and encoding. Sampling is the process of reading the values of an analog signal in discrete time. This is done with the Nyquist Frequency, which states the analog wave must be sampled at a rate of at least twice its sampled rate. For example: if a analog signal contains a maximum frequency variation of 3000 Hz, the analog wave must be sampled at a rate of 6000Hz or more. The next function is the Quantizing process. Quantization is the process of assigning one of the possible discrete levels to each sample obtained. The total number of levels depends on the number of bits to used to represent each sample. This is done by either rounding (assigning the nearest level to the signal which may be greater than or less than the actual signal) or truncating (assigning the nearest level which is lower than the actual sample). This process gives a stepped waveform which resembled the original or sent signal. The difference between the actual sample and the value assigned to it is known as the quantization noise. The last function is the encoding process. In the encoder a binary number is assigned to the analog signal that was sampled. This is called the n-bit digital word, n is chosen as a power of 2 depending on the level of accuracy required. The output of a pulse code modulator is a series of binary numbers, each represented by the power of 2 bits. At the destination (receiver end) of the communications circuit, a pulse code …


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