Linear and Non Linear PCM Codes
Linear codes
•
The magnitude change between any two successive steps is uniform. With linear
codes, resolution for higher amplitude analog signals is the same for lower-
amplitude signals
Non Linear Codes
•
The step size increases with the amplitude of the input signal
Companding
•
An operation in which the dynamic range of signals is compressed before
transmission and is expanded to the original value at the receiver.
•
The higher amplitude analog signals are compressed prior to transmission and then
expanded at the receiver
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Analog Companding
PCM system with analog companding
1. µ – Law
•
adopted in North America and Japan
Compression Characteristic:
V
out
=
V
max
x ln
(
1
+ µ
V
in
/ V
max
)
ln
(
1
+ µ
)
Where:
V
max
: maximum compressed analog input amplitude
V
in
: amplitude of the input signal at a particular instant of time
µ
: parameter used to define the amount of compression
V
out
: compressed output amplitude
Voice transmission requires a minimum dynamic range of 40 dB and a seven – bit
PCM code. A value of µ = 100 is normally used.
Most recent digital transmission systems use eight – bit PCM codes and µ = 255.
Relative output
amplitude
Relative input amplitude
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2. A – Law
•
Adopted in Europe as compression standard
Compression characteristic:
V
out
=
AV
in
/ V
max
1
+
ln A
0
≤
V
in
1
≤
V
max
A
V
out
=
V
max
Digital Companding
1
+
ln
(
AV
in
/ V
max
)
1
+
ln A
1
V
≤
in
≤
1
A V
max
The analog signal is sampled first and converted to a linear code, and then the
linear code is digitally compressed. At the receive end, the compressed PCM code is
received, expanded and decoded.
The most recently digitally compressed system uses a 12 – bit linear code and an 8 –
bit compressed code.
The 8 – bit compressed code consists of a sign bit, a 3 – bit segment identifier and a
4 – bit magnitude code that identifies the quantization interval within the specified
segment.
ermtiong
Digital Encoding
•
Process of converting digital information into digital signals
Types of Digital Encoding
1. Unipolar – representation of either binary 1 or 0 is a nonzero value
a. Non Return to Zero
b. Return to Zero
2. Polar – uses two levels (positive and negative polarities) of amplitude
a. NRZ
i. Non Return to Zero – Level
ii. Non Return to Zero – Inverted
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b. RZ
c. Biphase
i. Manchester
ii. Differential Manchester
3. Bipolar – uses the 0 V – amplitude to represent one binary value and the positive and
negative polarities to represent the other binary value
a. Alternate Mark Inversion
b. Pseudoternary
ermtiong
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