4.2 Digital Transmission

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Transcript 4.2 Digital Transmission

Nonlinear Encoding
□ Quantization levels not evenly spaced
□ Reduces overall signal distortion
□ Can also be done by companding
• The process of compressing and then expanding.
• The higher amplitude analog signals are compressed
prior to transmission and then expanded in receiver.
• Improving the DR of a communication system.
Companding Functions
Method of Companding
□ For the compression, two laws are adopted: the -law in US
and Japan and the A-law in Europe.
□ -law
Vout 
□ A-law
Vmax ln( 1   Vin Vmax )
ln( 1   )
A Vin Vmax
 Vmax
1  ln A
Vmax )
 1  ln A
Vin 1
Vout A
1 Vin
A Vout
Vmax= Max uncompressed
analog input voltage
Vin= amplitude of the input
signal at a particular of
instant time
Vout= compressed output
A, = parameter define the
amount of compression
□ The typical values used in practice are: =255 and A=87.6.
□ After quantization the different quantized levels have to be
represented in a form suitable for transmission. This is done via
an encoding process.
Example 3
□ A companding system with µ = 255
used to compand from 0V to 15 V
sinusoid signal. Draw the characteristic
of the typical system.
□ Draw an 8 level non-uniform quantizer
characteristic that corresponds to the
mentioned µ.
PCM Line Speed
□ The data rate at which serial PCM bits are clocked out of the
PCM encoder onto the transmission line.
line speed 
second sample
□ Where
□ Line speed = the transmission rate in bits per second
□ Sample/second = sample rate, fs
□ Bits/sample = no of bits in the compressed PCM code
Example 4
□ For a single PCM system with a sample
rate fs = 6000 samples per second and
a 7 bits compressed PCM code,
calculate the line speed.
Virtues & Limitation of PCM
The most important advantages of PCM are:
□ Robustness to channel noise and
□ Efficient regeneration of the coded signal
along the channel path.
□ Efficient exchange between BT and SNR.
□ Uniform format for different kind of baseband signals.
□ Flexible TDM.
□ Secure communication through the use of
special modulation schemes of encryption.
□ These advantages are obtained at the cost of
more complexity and increased BT.
□ With cost-effective implementations, the cost
issue no longer a problem of concern.
□ With the availability of wide-band
communication channels and the use of
sophisticated data compression techniques, the
large bandwidth is not a serious problem.
Time-Division Multiplexing
□ This technique combines time-domain
samples from different message signals
(sampled at the same rate) and transmits
them together across the same channel.
□ The multiplexing is performed using a
commutator (switch). At the receiver a
decommutator (switch) is used in
synchronism with the commutator to
demultiplex the data.
□ TDM system is very sensitive to symbol dispersion,
that is, to variation of amplitude with frequency or
lack of proportionality of phase with frequency. This
problem may be solved through equalization of
both magnitude and phase.
□ One of the methods used to synchronize the
operations of multiplexing and demultiplexing is to
organize the multiplexed stream of data as frames
with a special pattern. The pattern is known to the
receiver and can be detected very easily.
Block diagram of TDM-PCM communication
□ 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
Operation of Delta Modulation
□ 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.
The transmitter of a DM System
The receiver of a DM system
Delta Modulation - Example
DM circuit’s problem
•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
□ 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
□ Granular noise
□ Due to the difference between step size and sampled
□ 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
□ 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.
Converting standard logic level to a form
more suitable to telephone line transmission.
The line codes properties:
1. Transmission BW should be small as
2. Efficiency should be as high as possible
3. Error detection & correction capability
4. Transparency (Encoded signal is received
□ 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
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
• No net dc component
• Lower bandwidth
• Easy error detection
□ One represented by absence of line signal
□ Zero represented by alternating positive and negative
□ No advantage or disadvantage over bipolar-AMI
□ 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
□ 0  presence of transition at the beginning of the bit
□ 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
□ Bipolar AMI
□ Pseudoternary