--- title: "Definitions of unipolar, polar, bipolar, RZ, Manchester codes" subject: "Communication Systems" chapter: 6 key: "semester_6/communication_systems:define-line-codes" source: https://www.bctengineer.com/q/semester_6/communication_systems/define-line-codes site: BCT Engineering --- # Definitions of unipolar, polar, bipolar, RZ, Manchester codes Communication Systems · Chapter 6: Baseband Digital Data Transmission - Asked in: 2079 Chaitra (CS I), 2×5 marks; 2067 Shrawan (CS I), 5×2 marks (asked 2 times) - Video: [Line Coding Techniques | RZ, NRZ, Unipolar, Polar, Bipolar & Manchester | Digital Communication](https://www.youtube.com/watch?v=daf9F9NC7zQ) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/define-line-codes - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-6 ## Question Define unipolar, polar, bi-polar, unipolar RZ and Manchester line codes. ## Short answer Unipolar: 1=+V, 0=0. Polar: 1=+V, 0= –V. Bipolar (AMI): 1 alternates ±V, 0=0. Unipolar RZ: 1 returns to zero half-bit, 0 is 0. Manchester: each bit has a mid-bit transition (1 high-low, 0 low-high), providing self-clocking and no DC at double bandwidth. ## Answer **Line codes** are electrical waveforms used to represent binary data for transmission over a baseband channel. Each code below is shown for the example bits **10110**, with bit period $T_b$ and level $V$. #### Unipolar (NRZ) code Binary 1 is sent as $+V$ for the full bit period, and binary 0 as zero voltage. It is simple, but it has a DC component, and long strings of 1s or 0s carry no timing information. **Unipolar NRZ** ```text 1 0 1 1 0 +V ────| |───────| 0 |───| |─── ``` #### Polar (NRZ) code Binary 1 is sent as $+V$ and binary 0 as $-V$ for the full bit period. There is no DC component for equally likely bits, and it has the best noise immunity of these codes for a given power. Long runs still give no transitions. **Polar NRZ** ```text 1 0 1 1 0 +V ────| |───────| -V |───| |─── ``` #### Bipolar (AMI) code Binary 0 is sent as zero voltage. Binary 1s are sent as pulses of **alternating** polarity, $+V, -V, +V, \dots$, usually half-width (RZ). It has no DC component and narrow bandwidth, and two 1s of the same polarity in a row show an error has occurred. A long run of 0s loses timing; HDB3 and B8ZS solve this. **Bipolar (AMI, RZ)** ```text 1 0 1 1 0 +V ──| |─| 0 |─────| |─| |───── -V |─| ``` #### Unipolar RZ code Binary 1 is sent as $+V$ for the first half of the bit, then returns to zero. Binary 0 is zero for the whole bit. Each 1 gives a transition that helps timing, but the bandwidth is double that of NRZ and there is a DC component. **Unipolar RZ** ```text 1 0 1 1 0 +V ──| |─| |─| 0 |─────| |─| |───── ``` #### Manchester (split-phase) code Each bit has a transition at its middle. Binary 1 is $+V$ for the first half and $-V$ for the second half; binary 0 is the reverse. It has no DC component and is self-clocking, which is why it is used in 10 Mb/s Ethernet. The cost is double the bandwidth of NRZ. **Manchester** ```text 1 0 1 1 0 +V ──| |───| |─| |─ -V |───| |─| |───| ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a key drawback of unipolar NRZ? A. It uses negative levels only B. It has a DC component and no timing info on long runs C. It needs double bandwidth D. It has the lowest noise margin because it is bipolar Answer: B. Unipolar NRZ sends 1 as +V and 0 as 0 V, giving a DC component, and long runs give no transitions. (Key point: Unipolar NRZ) 2. How does bipolar (AMI) code send binary 1s? A. Always as +V B. As pulses of alternating polarity C. As -V only D. As a transition in the middle of the bit Answer: B. Alternating polarity gives no DC component, and two same-polarity pulses in a row signal an error. (Key point: Bipolar (AMI)) 3. What is true of unipolar RZ? A. No DC component and same bandwidth as NRZ B. Each 1 gives a transition for timing, but it has double the NRZ bandwidth and a DC component C. 1 is -V and 0 is +V D. It sends 0 as a mid-bit transition Answer: B. A 1 is +V for half the bit then zero, which helps timing but doubles bandwidth and keeps a DC component. (Key point: Unipolar RZ) 4. Why is Manchester code used in 10 Mb/s Ethernet? A. It uses half the bandwidth of NRZ B. It has a mid-bit transition in every bit, so it is self-clocking with no DC component C. It has a DC component for power D. It has no transitions Answer: B. Every bit has a mid-bit transition, which gives clock recovery and zero DC, at the cost of double bandwidth. 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