AMI Line Coding

Alternate Mark Inversion (AMI) is a bipolar line coding scheme that uses three voltage levels to encode binary data, making it a member of the multilevel encoding family. Unlike NRZ and RZ which use two levels, AMI uses +V, 0V, and -V, which gives it properties that NRZ and RZ cannot provide: zero DC component and a limited form of error detection. AMI is studied in digital communications as the foundational bipolar scheme and appears frequently in GATE questions on line coding.

Core Concept of AMI Encoding

In AMI encoding, binary 0 is always represented by zero voltage. Binary 1 (called a mark) is represented by a non-zero voltage, but crucially, successive 1s alternate in polarity. The first 1 is encoded as +V, the second 1 as -V, the third 1 as +V, and so on. Binary 0 (called a space) always produces 0V regardless of what came before. This alternation rule is the defining property of AMI.

The alternating polarity of successive 1s has two immediate consequences. First, the positive and negative pulses cancel each other over time, resulting in zero average DC value for any data pattern. This is a significant advantage over unipolar NRZ and RZ, which always have a non-zero DC component. Second, if two consecutive 1s appear with the same polarity, this is a violation of the AMI rule and indicates a transmission error. This provides single-bit error detection capability built into the encoding scheme itself, without requiring additional error-detection bits.

Pseudo-Ternary is the inverse of AMI. In pseudo-ternary, binary 1 is encoded as 0V and binary 0 is encoded with alternating +V/-V polarity. The encoding logic is reversed but the three-level structure and zero DC property are identical. Pseudo-ternary was used in some ISDN BRI (Basic Rate Interface) applications.

Mathematical Expression and Bandwidth

AMI is a three-level (ternary) signaling scheme but it carries binary data. The effective signal power is reduced compared to NRZ because 0V is transmitted for all 0 bits, which contributes no power. The PSD of AMI has a very important property: it has no DC component (PSD is zero at frequency f = 0). This is because the alternating pulses create a correlation structure that cancels the zero-frequency component.

The bandwidth of AMI is the same as NRZ for the same bit rate: B_AMI = R_b (null-to-null bandwidth). This is because the pulse duration is T_b (full bit period), same as NRZ. AMI achieves zero DC without any bandwidth penalty compared to NRZ, which makes it more spectrally efficient than bipolar RZ alternatives. The fact that AMI has zero DC and NRZ bandwidth makes it an attractive practical choice for short-haul copper transmission.

The AMI signal can be analyzed as the difference of two unipolar NRZ sequences, one for positive pulses and one for negative pulses. The alternation rule ensures that the two sequences are interleaved such that they cancel at DC. For a balanced data stream with equal 1s and 0s, each polarity pulse occurs with probability 0.5 x p(1), where p(1) is the probability of a 1 bit. Since positive and negative pulses carry equal energy in opposite polarity, they average to zero.

Practical Understanding of AMI

AMI was the standard line coding scheme for T1 carrier systems in North America and E1 carrier systems in Europe (though E1 uses HDB3, an improved version of AMI). T1 transmits 1.544 Mbps of data over twisted pair copper wires. The zero DC component of AMI allows transformer coupling at the line interface, which provides electrical isolation between equipment and helps reject common-mode interference.

The major limitation of AMI is the synchronization problem caused by long strings of zeros. Since 0 bits produce 0V output, a sequence of many consecutive zeros produces no transitions in the signal, making clock recovery impossible. This problem is directly addressed by HDB3 (High Density Bipolar Order 3) and B8ZS (Bipolar with 8-Zero Substitution), which replace long zero sequences with special coded patterns that include deliberate AMI violations to force transitions. These schemes are studied in the next section.

Numerical Example

AMI error detection works because any two consecutive 1s must have opposite polarity. If a noise burst inverts one pulse, two pulses of the same polarity appear consecutively, which is detectable. The bandwidth calculation for AMI uses the same formula as NRZ since AMI also uses full-bit-period pulses.

Given:
Bit rate R_b = 1.544 Mbps (T1 carrier)
Encoding: AMI (Bipolar NRZ)
Voltage levels: +3V, 0V, -3V

Why this formula applies:
AMI uses full bit-period pulses like NRZ.
Null-to-null bandwidth = R_b (first null of sinc PSD).

Formula:
B_AMI = R_b

Substitution:
B_AMI = 1.544 x 10^6 Hz

Calculation:
B_AMI = 1.544 MHz

Data sequence: 1 0 1 1 0 1
AMI encoding: +V, 0, -V, +V, 0, -V
(Each successive 1 alternates polarity)

Final Answer:
Minimum bandwidth = 1.544 MHz
DC component = 0V (alternating pulses cancel)
Error detection: any two same-polarity consecutive pulses = error

Exam Tip: AMI uses three voltage levels but carries binary data. Its bandwidth equals NRZ (= R_b), not half. The zero DC property comes from alternating polarity, not from returning to zero. Long zero strings cause synchronization loss in AMI, which is why HDB3 and B8ZS were developed. These are among the most tested facts in GATE baseband coding questions.

AMI Mechanism Summary

• Binary 0 = 0V always. Binary 1 = alternating +V and -V for successive 1s. Three voltage levels total.
• DC component is zero for all data patterns due to polarity alternation of marks. AC-coupled transformer interfaces are fully supported.
• Single-bit error detection: two consecutive same-polarity pulses violate AMI rule and indicate an error.
• Bandwidth = R_b (same as NRZ). Full bit-period pulses used. No bandwidth penalty for zero DC property.
• Long runs of zeros produce no transitions, causing clock recovery failure. Maximum zero run before sync loss is typically 15 bits in T1 systems.
• Pseudo-ternary is the inverse: 1 maps to 0V and 0 maps to alternating +V/-V.
• HDB3 and B8ZS replace long zero sequences with special patterns that violate AMI rule deliberately to force transitions.

Quick Revision

• AMI uses three levels: +V, 0V, -V. Bit 0 = 0V. Bit 1 = alternating +V/-V for successive marks.
• DC component is always zero regardless of data pattern. Suitable for transformer-coupled channels.
• Bandwidth = R_b. Same as NRZ. No bandwidth penalty.
• Error detection: AMI violation (two same-polarity consecutive 1s) signals a single-bit error.
• Problem: Long zero sequences create no transitions. Clock recovery fails. HDB3/B8ZS solve this.
• Exam trap: AMI has three signal levels but it is still binary encoding. Signal level count does not equal bits per symbol here.
• Used in T1 (North America) and E1 (Europe, with HDB3). Foundational bipolar scheme for telephone carrier systems.