29-06-2012, 03:13 PM
Pulse Code Modulation
Pulse Code Modulation.ppt (Size: 185.5 KB / Downloads: 61)
Analogue to Digital
After sampling, the analogue amplitude value of each sampled (PAM) signal is quantized into one of a number of L discrete levels. The result is a quantized PAM signal.
A codeword can then be used to designate each level at each sample time. This procedure is referred to as “Pulse Code Modulation”.
Encoding
After quantization, a digit is assigned to each of the quantized signal levels in such a way that each level has a one-to-one correspondence with the set of real integers. This is called digitization of the waveform.
Each integer is then expressed as an x-bit binary number, called codeword, or PCM word.
The number of codewords, L , is related to x by: 2x = L
Codeword
Quantization followed by digitization maps input amplitudes into PCM words.
A cell is the set of input amplitudes mapped to a codeword.
There are L integers, PCM words, or codewords to correspond to the L allowed output amplitudes of the quantizer.
Codebook is the set of all these L codewords.
RZ Coding may be unipolar or bipolar
Unipolar-RZ:
1 is represented by positive for the first half of T and zero for the second half.
0 is represented by 0
Bipolar-RZ:
1 is represented by positive for the first half of T and zero for the second half.
0 is represented by negative for the first half of T and zero for the second half.
Used in baseband data transmission, magnetic recording.
The transitions at T/2 may be used for synchronization.
Biphase Coding may be L, M, or S
Biphase-L (level) / Manchester:
0 is represented by +ve for first half and –ve for 2nd half
1 is represented by -ve for first half and +ve for 2nd half
used in digital logic circuits including IEEE 802.4 standard, Ethernet.
An alternate scheme adopted by some authors has the above coding for 0’s and 1’s in reversed manner.