Digital to Digital

Multilevel Binary

Encoding more than 1 bit per 1 symbol. Achieved through having more than 2 voltage levels. The word is not implying 2 levels; it’s used as it is used in digital communication. Can carry more bits per symbol. Requires more signal power (approx. $3\text{dB}$, or $2\text{x}$).

Scrambling

The process of replacing the constant voltage levels with a filling sequence. This is done to prevent the receiver from detecting the signal incorrectly.

Filling sequence

Must produce enough transitions to sync. Must be recognized by the receiver, and replaced with original. Has same length as original.

Schemes

• Non-Return to Zero-Level (NRZ-L)
• Non-Return to Zero Inverted (NRZI)
• Bipolar -AMI
• Pseudoternary
• Manchester
• Differential Manchester
• B8ZS
• HDB3

NRZ-L

Short for Non-Return to Zero-Level. Two different voltage levels: 1 and 0. Either can be higher. Either can be positive or negative. Usually one is positive and the other is negative. Voltage is constant during bit interval.

NRZ-I

Short for Non-Return to Zero-Inverted. Constant voltage during bit interval, similar to NRZ-L. Transition denotes a binary 1. No transition denotes a binary 0. A differential encoding scheme.

Pros & Cons of NRZ

NRZ is used in magnetic recording. Not often used for signal transmission.

• Pros
  • Easy to implement
  • Efficient use of bandwidth (as there are no extra transitions)
• Cons
  • No built-in error detection
  • DC component won’t be transmitted
  • Receiver may lose timing due to lack of transitions

Note

For the same error rate, NRZ is more power efficient compared to multilevel binary.

Bipolar-AMI

Short for Alternate Mark Inversion. Most common bipolar encoding scheme. 0 is represented by 0. 1 is represented by non-zero level, with alternating polarity.

Pros:

• No DC component
• No loss of sync due to long sequences of 1s
• Uses less bandwidth compared to Manchester
• Can detect errors (bipolar violation)

Cons:

• Long sequence of 0s can cause loss of sync
• More complex compared to NRZ.
• Limited error detection (not all errors can be detected)

Pseudoternary

Opposite of AMI. No advantage or disadvantage compared to AMI.

Manchester

Aka. Phase encoding, or PE. A biphase encoding scheme. Each bit is represented by a transition at the middle part of the bit period.

• Low to high: 1
• High to low: 0

2 types of transitions:

• mid-bit transition
  Always present. Carries data and works as a clock signal.
• start of the bit transition
  Only present when consecutive bits are the same. Works as a timing cue.

Used by IEEE 802.3 which defines the physical and data-link layer’s media access control of ethernet.

Differential Manchester

Aka. Differential Phase encoding, or DPE. A biphase encoding scheme and a differential encoding scheme. Midbit transition is used for clocking. Non-midbit transitions denote the data. Transition at the start of a bit is 0. No transition means 1.

Used by IEEE 802.5 which is used to build local area networks.

B8ZS

Bipolar with 8-zero substitution. Based on Bipolar-AMI. Used to prevent long sequence of zeros in bipolar-AMI signals. Replaces 8 consecutive zeros with a special that intentionally includes 2 bipolar violations.

If the octet is full of zeros and:

• Last voltage pulse preceding was +ve then encode as 000+-0-+
• Last voltage pulse preceding was -ve then encode as 000-+0+-

The intentional violations are placed to make sure the replacement is detected correctly. They don’t mess up error detection because the specific 2 violations couple doesn’t occur because of noise.

HDB3

High Density Bipolar 3-level encoding. Similar to B8ZS. Replaces 4 consecutive zeros with patterns containing a non-zero pulse, to maintain synchronization. The exact pattern depends on the number of pulses in the last substitution, to keep it DC-balanced.

The substitution pattern includes:

• a violation pulse
• (optional) a balancing pulse