Digital signal is a sequence of discrete, discontinuous voltage pulses. Each pulse is a signal element. Binary data are transmitted by encoding the bit stream into signal elements. In the simplest case, one bit is represented by one signal element. - E.g., 1 is represented by a lower voltage level, and 0 is represented by a higher voltage level

1. Prepared By:
BIPLAP BHATTARAI

2. Introduction To Digital Data, Digital Signal
Digital signal is a sequence of discrete, discontinuous voltage pulses.
Each pulse is a signal element.
Binary data are transmitted by encoding the bit stream into signal
elements.
In the simplest case, one bit is represented by one signal element.
- E.g., 1 is represented by a lower voltage level, and 0 is represented by
a higher voltage level

3. Terminologies
 Unipolar
If all signal elements have the same algebraic sign (all positive or all negative), then the signal
is unipolar.
 Polar
One logic state represented by positive voltage, the other by negative voltage
 Data rate
Rate of data transmission measured in bps: bits per second
 Duration or length of a bit
Time taken for transmitter to emit the bit
 Modulation rate
- Rate at which the signal level changes
- Measured in baud: signal elements per second

4. Interpreting Signals at the Receiver
 The receiver needs to know
• The timing of each signal element, i.e., when a signal element begins and ends
• signal levels
• These tasks are performed by sampling each element position in the middle of the interval
and comparing the value to a threshold.
 Factors affecting successful interpreting of signals
• Signal-to-noise ratio (SNR)
• Data rate
• Bandwidth
 Another factor that can improve performance:
• Encoding scheme: the mapping from data bits to signal elements

5. Line Coding
 The process for converting digital data into digital signal is said to be Line
Coding.
 Line Coding Helps Error detection - errors occur during transmission due to
line impairments. Some codes are constructed such that when an error
occurs it can be detected.
 Noise and interference - there are line encoding techniques that make the
transmitted signal “immune” to noise and interference. This means that the
signal cannot be corrupted, it is stronger than error detection.
 Complexity - the more robust and resilient the code, the more complex it is
to implement and the price is often paid in baud rate or required bandwidth.

6. Uni-polar Encoding
 Unipolar encoding schemes use single voltage level to represent data. In this case, to
represent binary 1, high voltage is transmitted and to represent 0, no voltage is transmitted.
 It is also called Unipolar-Non-return-to-zero, because there is no rest condition i.e. it either
represents 1 or 0.

7. Polar Encoding
 Polar encoding scheme uses multiple voltage levels to represent binary values. Polar
encodings is available in four types:
 Polar Non-Return to Zero (Polar NRZ)
• It uses two different voltage levels to represent binary values. Generally, positive voltage
represents 1 and negative value represents 0. It is also NRZ because there is no rest
condition.
• NRZ scheme has two variants: NRZ-L and NRZ-I.
 NRZ-L changes voltage level at when a different
bit is encountered whereas NRZ-I changes voltage
when a 1 is encountered.

8. Polar Encoding
 PolarReturn to Zero (RZ)
• Problem with NRZ is that the receiver cannot conclude when a bit ended and when the next
bit is started, in case when sender and receiver’s clock are not synchronized.
• RZ uses three voltage levels, positive voltage to represent 1, negative voltage to represent 0
and zero voltage for none. Signals change during bits not between bits.
 Manchester
• This encoding scheme is a combination of RZ and NRZ-L. Bit time is divided into two
halves. It transits in the middle of the bit and changes phase when a different bit is
encountered.
 Differential Manchester
• This encoding scheme is a combination of RZ and NRZ-I. It also transit at the middle of the
bit but changes phase only when 1 is encountered.

9. Bipolar Encoding
 Bipolar encoding uses three voltage levels, positive, negative and zero. Zero
voltage represents binary 0 and bit 1 is represented by altering positive and
negative voltages.

10. Problems on Uni- Polar Encoding
If you encode a long series of ones using unipolar/on-off signaling,
then the DC component could fully charge a capacitor, resulting in
errors as the signal is brought to zero.
With bi-polar signaling, long series of ones are encoded as
alternating negative and positive voltages, so the average voltage is
zero, which eliminates the DC component issue.
The constant transitions on ones with bipolar signaling also allows
the circuit to be self clocking, which eliminates the need for a separate
clock source.

11. So Why Not Bi- Polar Encoding ?
Although, the constant transitions on ones with bipolar signaling
also allows the circuit to be self clocking, which eliminates the need
for a separate clock source.
But then you still have issues with runs of zeroes causing a lack of
transitions to clock on. This issue has been dealt with using several
methods.
 And that is Dealt with Scrambling Technique.

12. Scrambling
Scrambling is a technique used to create a sequence of bits that has the
required self clocking, no low frequencies, no wide bandwidth.
It is implemented at the same time as encoding, the bit stream is created on
the fly.
The best code is one that does not increase the bandwidth for
synchronization and has no DC components.
It replaces ‘unfriendly’ runs of bits with a violation code that is easy to
recognize.

13. Scrambling Cont..
Use scrambling to replace sequences that would produce constant voltage
these filling sequences must:
• Sequences that would result in a constant voltage are replaced by filling sequences that
will provide sufficient transitions for the receiver’s clock to maintain synchronization.
• Filling sequences must be recognized by receiver and replaced with original data
sequence.
• Filling sequence is the same length as original sequence.
Design goals
• have no dc component
• have no long sequences of zero level line signal
• have no reduction in data rate
• give error detection capability

14. Alternate Mark Inversion (AMI) used with
scrambling

15. Scrambling Techniques
 Bipolar With 8-Zeros Substitution
 Based on bipolar-AMI, whose drawback is a long string of zeros may
result in loss of synchronization.
 If octet of all zeros occurs and the last voltage pulse preceding this
octet was positive, encode as 000+-0-+ or 000VB0VB.
 If octet of all zeros occurs and the last voltage pulse preceding this
octet was negative, encode as 000-+0+-
 Causes two violations of AMI code:
• Unlikely to occur as a result of noise
• Receiver recognizes the pattern and interprets the octet as consisting
of all zeros.
A. B8ZS

16. Two cases of B8ZS scrambling technique
 The main formula for B8ZS is 000VB0VB
Where V
 stands for “violation bit”
 Follows the polarity of previous non-zero bit
B
 stands for “bipolar bit” or “balance bit”
 Inverses the polarity of previous non-zero bit

17.  High-Density Bipolar-3 Zeros
 Based on bipolar-AMI
 String of four zeros is replaced with sequences containing one or two
pulses.
B. HDB3
Number of Bipolar Pulses since last
substitution
Polarity of
Preceding Pulse
Odd Even
- 000- +00+
+ 000+ -00-

18.  HDB3 substitutes four consecutive zeros with 000V or B00V
depending on the number of nonzero pulses after the last
substitution.
 If no of non zero pulses is even the substitution is B00V to make total
non zero pulse even.
 If no of non zero pulses is odd the substitution is 000V to make total
non zero pulses even.
 Example 1 of HDB3 encoding
The pattern of bits " 1 0 0 0 0 1 1 0 “
encoded in HDB3 is " + 0 0 0 V - + 0 "
(the corresponding encoding using AMI is " + 0 0 0 0 - + ").
Different situations in HDB3 scrambling technique

19. Example 2 of HDB3 encoding
The pattern of bits " 1 0 1 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0 "
encoded in HDB3 is " + 0 - 0 0 0 V 0 + - B 0 0 V - + B 0 0 V 0 0 "
which is: " + 0 - 0 0 0 - 0 + - + 0 0 + - + - 0 0 - 0 0 "

20. B8ZS and HDB3

21. Special Thanks
Mr. Shree Krishna Khadka
KIST College Of Science And Technology