CMI Code Introduction

CMI, also known as sign-inversion code, is a two-level non-return-to-zero code. The "0" code is represented by fixed negative and positive levels, and the "1" code is represented by alternating positive and negative levels. It has the following advantages: (1) There is no DC component and the low-frequency component is small; (2) There is a strong clock component in the information code stream, which is convenient for extracting clock information from the signal; (3) It has a certain error detection capability. Therefore, it is used as an interface code in the PCM system of the high-order group and is recommended as a line transmission code in the optical fiber digital transmission system with a rate lower than 8848 kb/s.

The coding principle and circuit implementation of CMI code
CMI code is a 1B2B code (one-bit information code, two-bit code element), that is, the incoming one-bit code is represented by two-bit code. When the incoming "0" code is the code, the code outputs a fixed "01" code; when the incoming "1" code is the code, the code outputs "00" or "11" code, and they appear alternately.

According to the CMI coding principle described above, we must first separate the "0" and "1" codes in the incoming PCM code stream and then encode them separately. The principle block diagram of the coding implementation is shown in Figure 2.

According to the CMI coding principle, when the input code is "1", the code output alternates between "00" and "11". Therefore, a state must be set in the circuit to remember the last code output of "1"
. This function is realized by the JK trigger. When the input is "0", the code output is "01". The circuit diagram is shown in Figure 3.