03-07-2012, 02:52 PM
EEE358S Fundamentals of Communications Engineering
[attachment=26530]
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
NRZ-L Coding
NRZ-L (level):
1 higher level; 0 lower level
Used in SONET XOR bit sequence, and in early magnetic tape recording
Long sequence of same bit causes difficulty in clock recovery; also in detecting the average DC level
Biphase Coding may be L, M, or S
biphase-M (mark) / Differential Manchester:
Always transition at beginning of bit
1 (mark) is represented by second transition at T/2
0 (space) is represented by no second transition at T/2
Alternately:
always transition at center
1 is represented by additional transition at start of T.
biphase-S (space):
Always transition at beginning of bit
0 (space) is represented by second transition at T/2
1 (mark) is represented by no second transition at T/2
[attachment=26530]
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
NRZ-L Coding
NRZ-L (level):
1 higher level; 0 lower level
Used in SONET XOR bit sequence, and in early magnetic tape recording
Long sequence of same bit causes difficulty in clock recovery; also in detecting the average DC level
Biphase Coding may be L, M, or S
biphase-M (mark) / Differential Manchester:
Always transition at beginning of bit
1 (mark) is represented by second transition at T/2
0 (space) is represented by no second transition at T/2
Alternately:
always transition at center
1 is represented by additional transition at start of T.
biphase-S (space):
Always transition at beginning of bit
0 (space) is represented by second transition at T/2
1 (mark) is represented by no second transition at T/2