# HDB3

`三阶高密度双极性码`是一种适用于基带传输的编码方式，应用于电信领域。它是为了克服AMI码的缺点而出现的，具有能量分散，抗破坏性强等特点。

HDB3码基于AMI码。

## AMI (Alternative Mark Inversion)

编码原则： `1`与`-1`交替出现

source: 1 0 1 1 0 1

encoded: 1 0 -1 1 0 -1

## HDB3编码流程

1. 碰到`0000`，替换为`B00V`
2. 第一个 `B` 为 `0`，第一个`1`为`-1`
3. `B`与前面`非零数的极性`相反（比如前面为`1`或`V`，后面应为`-B`）； `V`与你刚刚(之前)判断的`B的正负性`一致（比如前面为`-B`，后面应为`-V`）(如果前面的`B=0`，`V`与之前最近的一个`1` `正负性一致`)
4. 在重复`3`步骤的过程中，如果遇到其他的`1 or -1`，要严格按照`AWI`原则，再次做交替变化
5. B = 1 , -B = -1 , V = 1 , -V = -1

![](/files/-Lf5hgoyrSkFjttecqUW)

## 例题

1010000000011000011

to

101B00VB00V11B00V11

to

-101000VB00V11B00V11

to

-101000V-B00-V1-1B00V-11

to

-1010001-100-11-11001-11

## 手动解码

1. 如果`3个连零`前后`非零的数`极性相同，将`后一个非零数`变为0
2. 如果`2个连零`前后`非零数`极性相同，全都变为0


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