Alternate Mark Inversion (AMI)

Extra information for course - not included in assessment.

AMI (Alternate Mark Inversion) is a synchronous clock encoding technique that uses bipolar pulses to represent logical 1. The next logic 1 is represented by a pulse of the opposite polarity. Hence a sequence of logical 1s are represented by a sequence of pulses of alternating polarity. The alternating coding prevents the build-up of a d.c. voltage level down the cable.

Example of AMI coding of NRZ data and the corresponding clock signal

AMI (Alternate Mark Inversion) encoding was used extensively in first generation pulse code modulation networks, but suffers the drawback that a long run of 0's produces no transitions in the data stream (and therefore does not contain sufficient transitions to guarantee lock of a DPLL). Successful clock recovery therefore relies on the user not wishing to send long runs of 0's and this type of encoding is not therefore transparent to the sequence of bits being sent. A modified AMI code periodically inserts additional pulses of the same polarity as the last logical 1. These deliberate "violations" can be used to increase the clock content of the signal and help synchronisation of a remote receiver's DPLL. The HDB3 and 4b5b encoding schemes are two of many examples of methods that have been developed to provide regular transitions irrespective of the pattern of data being carried.

Example of AMI encoding

A logical 1 value is represented by high or low level - and a zero by no signal. The logical 1 by pulses use alternating polarity.

The pattern of bits " 1 0 0 0 0 1 1 0 " encodes to " + 0 0 0 0 - + ".

(Note: The HDB3 encoding is " + 0 0 0 + - + ").