Transcript Chapter 4_part 3

Pulse Modulation
CHAPTER 4
Part 3
□Differential PCM (DPCM)
□Delta Modulation (DM)
□Noise in Delta Modulation
□Digital Modulation
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Differential Pulse Code Modulation
(DPCM)
Differential Pulse Code Modulation (DPCM) is a procedure of
converting an analog into a digital signal in which an analog
signal is sampled and then the difference between the actual
sample value and its predicted value (predicted value is based
on previous sample or samples) is quantized and then
encoded forming a digital value.
DPCM code words represent differences between samples
unlike PCM where code words represented a sample value.
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Cont’d
It often happens that in the analogical signals, which are
coded in PCM (voices, images, etc), that many next samples
show the same quantization level; as a consequence there is
the transmission of many equal PCM codes and this is
redundant for the reception signal reconstruction.
The DPCM coding exploits this redundancy between adjacent
samples.
DPCM requires fewer bits than the standard PCM.
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DPCM encoder

Quantizer input
• Quantizer output
; en - prediction error
; qn- quantization error
• Prediction filter is usually implemented by a linear predictor in
which the last r samples are used to predict the values of the next
sample.
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DPCM decoder
• Predictor input:
• Predictor output:
• PCM Decoder: to recover yn from the DPCM
sequence
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Delta Modulation (DM)
–
A single-bit PCM code to achieve digital transmission of
analog. Use only 1 bit either logic ‘1’ or ‘0’.
–
Logic ‘0’ is transmitted if current sample is smaller than
the previous sample
–
Logic ‘1’ is transmitted if current sample is larger than
the previous sample
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Cont’d…
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Operation of Delta Modulation
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Cont’d...
• Analog input is approximated by a staircase function
• Move up or down one level () at each sample interval
(by one quantization level at each sampling time) 
output of DM is a single bit.
• Binary behavior
– Function moves up or down at each
sample interval
• In DM the quantization levels are represented by two
symbols: 0 for - and 1 for +. In fact the coding
process is performed on eq.
• The main advantage of DM is its simplicity.
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Delta Modulation - Example
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Noise in Delta Modulation
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 Slope overload distortion
occurs when  is too small, the
staircase approximation mq(t)
can't follow closely the actual
curve of the message signal
m(t ).
 large  is needed for rapid
variations of m(t) to reduce the
slope-overload distortion
 granular noise occurs when 
is too large relative to the local
slope characteristics of m(t).
 granular noise is similar to
quantization noise in PCM
 small  is needed for slowly
varying m(t) to reduce the
granular noise
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Cont’d…
• To avoid slope overload,
σ = Amax ωm / fs
Where
σ=step size
Amax = Maximum signal
amplitude
ωm = = 2 πfm . For voice
signal, fm = 800 Hz.
fs =sampling rate
• average power of
granular noise,
N0=σ2 B / 3 fs
Where
σ=step size
B = signal bandwidth
fs =sampling rate
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Example 1
The specification of delta modulation (DM) system
used to transmit a voice signal is as below:
Sampling rate: 64 kHz.
Peak amplitude: 1 V.
Voice signal bandwidth: 3.5 kHz. Find
a)The minimum permissible value of the step size
delta in order to avoid slope overload distortion.
b)The average power of granular noise.
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Digital Modulation
• Using Digital Signals to Transmit Digital Data
– Bits must be changed to digital signal for transmission
– Unipolar encoding
• Positive or negative pulse used for zero or one
– Polar encoding
• Uses two voltage levels (+ and - ) for zero or one
– Bipolar encoding
• +, -, and zero voltage levels are used
Non-Return to Zero-Level (NRZ-L)
• 0 – High level
• 1 – Low level
Non-Return to Zero Inverted (NRZ-I)
• 0 – no transition
• 1 – transition at beginning of the interval
Multilevel Binary(Bipolar-AMI)
• 0 – No line signal
• 1 – positive or negative level, alternating for successive
ones
0
1
0
0
1
1
0
0
0
1
1
Pseudoternary
• 0 – positive or negative level, alternating for successive
zeros.
• 1 – No line signal.
0
1
0
0
1
1
0
0
0
1
1
Manchester
• There is a transition at the middle of each bit period.
• 1  low-to-high transition
• 0  high-to-low transition
Differential Manchester
• mid-bit transition is ONLY for clocking.
• 1  absence of transition at the beginning of the bit interval
• 0  presence of transition at the beginning of the bit interval
Example 2
• Sketch the data wave form for a bit stream
110100110 using
– NRZL
– Bipolar AMI
– Pseudoternary
– Manchester
– Differential Manchester