Pulse Code Modulation. Pulse Code Modulation Analogue to Digital Conversion Quantizing Encoding. Pulse Code Modulation.
Pulse Code Modulation
Analogue to Digital Conversion
DEFINITION: Pulse code modulation (PCM) is essentially analog-to-digital conversion of a special type where the information contained in the instantaneous samples of an analog signal is represented by digital words in a serial bit stream.
The advantages of PCM are:
Relatively inexpensive digital circuitry may be used extensively.
PCM signals derived from all types of analog sources may be merged with data signals and transmitted over a common high-speed digital communication system.
In long-distance digital telephone systems requiring repeaters, a clean PCM waveform can be regenerated at the output of each repeater, where the input consists of a noisy PCM waveform.
The noise performance of a digital system can be superior to that of an analog system.
The probability of error for the system output can be reduced even further by the use of appropriate coding techniques.
111 111 001 010 011 111 011
The Analog-to-digital Converter (ADC) performs three functions:
Makes the signal discrete in time.
If the analog input has a bandwidth of W Hz, then the minimum sample frequency such that the signal can be reconstructed without distortion.
Makes the signal discrete in amplitude.
Round off to one of q discrete levels.
Maps the quantized values to digital words that are bits long.
If the (Nyquist) Sampling Theorem is satisfied, then only quantization introduces distortion to the system.
Quantized to 5-levels
The used of a non-uniform quantizer is equivalent to passing the baseband
Signal through a compressor and then appling the compressed signal to a
A particular form of compression law that used in practice is the so-called μ-law is defined by
XQ Natural coding Gray Coding
+7 111 110
+5 110 111
+3 101 101
+1 100 100
-1 011 000
-3 010 001
-5 001 011
-7 000 010
Levels are encoded using this table
Table: Quantization levels with belonging code words
Chart 2. Process of restoring a signal.
PCM encoded signal in binary form:
101 111 110 001 010 100 111 100 011 010 101
Total of 33 bits were used to encode a signal
Chart 1. Quantization and digitalization of a signal.
Signal is quantized in 11 time points & 8 quantization