4.2 Digital Transmission
Outlines
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Pulse Modulation (Part 2.1)
Pulse Code Modulation (Part 2.2)
Delta Modulation (Part 2.3)
Line Codes (Part 2.4)
□ A single-bit PCM code to achieve digital
transmission of analog.
□ 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
Cont’d…
Operation of Delta Modulation
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.
Cont’d...
The transmitter of a DM System
The receiver of a DM system
Delta Modulation - Example
DM circuit’s problem
Cont’d…
•Slope overload distortion is due to the fact that the staircase
approximation mq(t) can't follow closely the actual curve of the
message signal m(t ). In contrast to 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.
•It seems that a large  is needed for rapid variations of m(t) to
reduce the slope-overload distortion and a small  is needed for
slowly varying m(t) to reduce the granular noise. The optimum  can
only be a compromise between the two cases.
•To satisfy both cases, an adaptive DM is needed, where the step
size  can be adjusted in accordance with the input signal m(t).
Cont’d...
□ In summary
□ Slope overload
□ Due to the input analog signal amplitude changes
faster than the speed of the modulator
□ to minimize : the product of the sampling step size and
the sampling rate must be equal to or larger than the
rate of change of the amplitude of the input analog
signal.
□ Granular noise
□ Due to the difference between step size and sampled
voltage.
□ To minimize : increase the sampling rate, decrease the
step size of modulator
DM Performance
□ Good voice reproduction
□ PCM - 128 levels (7 bit)
□ Voice bandwidth 4khz
□ Should be 8000 x 7 = 56kbps for PCM
□ Data compression can improve on this
□ e.g. Interframe coding techniques for video
Cont’d...
□ Adaptive Delta Modulation (ADM)
□ A Delta Modulation system where the step
size of the DAC is automatically varied
depending on the amplitude
characteristics of the analog signal.
□ A well designed ADM scheme can
transmit voice at about half the bit rate of
a PCM system with equivalent quality.
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Converting standard logic level to a form
more suitable to telephone line transmission.
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The line codes properties:
1. Transmission BW should be small as
possible
2. Efficiency should be as high as possible
3. Error detection & correction capability
4. Transparency (Encoded signal is received
faithfully)
Cont’d...
□ Six factors must be considered when
selecting a line encoding format;
1.transmission voltage & DC component
2.Duty cycle
3.Bandwidth consideration
4.Clock and framing bit recovery
5.Error detection
6.Ease of detection and decoding
Why Digital Signaling?
□ Low cost digital circuits
□ The flexibility of the digital approach
(because digital data from digital
sources may be merged with digitized
data derived from analog sources to
provide general purpose
communication system)
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)
□ Two different voltages for 0 and 1 bits.
□ Voltage constant during bit interval.
□ no transition, no return to zero voltage
□ More often, negative voltage for one value and positive for the
other.
Non-Return to Zero Inverted (NRZ-I)
□ Nonreturn to zero inverted on ones
□ Constant voltage pulse for duration of bit
□ Data encoded as presence or absence of signal
transition at beginning of bit time
□ Transition (low to high or high to low) denotes a binary 1
□ No transition denotes binary 0
□ An example of differential encoding
Multilevel Binary(Bipolar-AMI)
•
•
•
•
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
• No net dc component
• Lower bandwidth
• Easy error detection
0
1
0
0
1
1
0
0
0
1
1
Pseudoternary
□ One represented by absence of line signal
□ Zero represented by alternating positive and negative
□ No advantage or disadvantage over bipolar-AMI
0
1
0
0
1
1
0
0
0
1
1
Manchester
□ There is always a mid-bit transition {which is used as a
clocking mechanism}.
□ The direction of the mid-bit transition represents the
digital data.
□ 1  low-to-high transition
□ 0  high-to-low transition
□ Consequently, there may be a second transition at the
beginning of the bit interval.
□ Used in 802.3 baseband coaxial cable and CSMA/CD
twisted pair.
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
□ Differential Manchester is both differential and biphase.
[Note – the coding is the opposite convention from NRZI.]
□ Used in 802.5 (token ring) with twisted pair.
□ * Modulation rate for Manchester and Differential
Manchester is twice the data rate  inefficient
encoding for long-distance applications.
Example 5
□ Sketch the data wave form for a bit
stream 11010 using
□ NRZL
□ Bipolar AMI
□ Pseudoternary
END OF PART 2