1 Digital intensity modulations: Alternate Mark Inversion, Duobinary Modulation, Carrier Suppressed Return to Zero
1.4 Alternate Mark Inversion

In telecommunication, a paired disparity code is a line code in which at least one of the data characters is represented by two code words of opposite disparity that are used in sequence so as to minimize the total disparity of a longer sequence of digits.

The principle

The simplest example of a paired disparity code is Alternate Mark Inversion (AMI). It uses three logical levels corresponding to two bipolar levels: +,-, and zero.

A binary 0 is encoded as absence of power during the bit interval (zero voltage), while a binary 1 is encoded alternately as a positive voltage or a negative voltage. A binary 1 is referred to as a mark, while a binary 0 is known as a space.

Main benefits

General benefits of using a line code with bipolar encoding:

  • The use of a bipolar code prevents a significant build-up of DC, the cable may then be used for longer distances.
  • AMI helps to maintain synchronization between the transmitter and receiver, especially in case of long strings of binary 1 symbols, which are problematic for NRZ. An additional transmission medium for the clock signal is not necessary.
  • Error detection. AMI signal is regenerated at regular intervals. A signal with low Signal to Noise Ratio (SNR) can exhibit errors, as for example a mark can be interpreted as zero, or zero as positive or negative mark. AMI has the ability to detect single errors, which violate the rule of bipolarity (every following non-zero mark is different).

Disadvantages

  • Long sequences of binary zero are problematic in terms of synchronization.

The solution is to add a binary 1 after seven binary zeros to maintain synchronization. On the decoder side, this extra symbol is removed.