Editing Modified AMI code (section)

==HDB3 (European E-carrier) <span class="anchor" id="HDB3"></span>==
Used in all levels of the European [[E-carrier]] system, the '''high density bipolar of order 3 (HDB3)''' code replaces any instance of 4 consecutive 0 bits with one of the patterns "{{mono|000V}}" or "{{mono|B00V}}". The choice is made to ensure that consecutive violations are of differing polarity; i.e., separated by an odd number of normal {{mono|+ or −}} marks.
{|class=wikitable style="text-align:center;"
|+ HDB 3 coding of 0000{{sub|2}}
! Parity of +/− bits<br/>since previous V !! Pattern !! Previous pulse !! Coded
|-
|rowspan=2| Even || rowspan="2" | B00V
| + || −00−
|-
| − || +00+
|-
|rowspan=2| Odd || rowspan="2" | 000V
| + || 000+
|-
| − || 000-
|}

These rules are applied on the code as it is being built from the original string. Every time there are 4 consecutive zeros in the code they will be replaced by either 000−, 000+, +00+ or −00−. To determine which pattern to use, one must count the number of pluses (+) and the number of minuses (−) since the last violation bit V, then subtract one from the other. If the result is an odd number then 000− or 000+ is used. If the result is an even number then +00+ or −00− is used. To determine which polarity to use, one must look at the pulse preceding the four zeros. If 000V form must be used then V simply copies the polarity of last pulse, if B00V form must be used then B and V chosen will have the opposite polarity of the last pulse.

===Examples===
Here are some examples of bit streams codes with AMI and HDB3. All assume the same starting conditions: the previous 1 bit was −, and the previous violation was an even number of 1 bits ago. (E.g. the preceding bits could have been ++−.)
{|
| '''Input''' || {{mono|10000110}}{{sub|2}}
|-
| '''AMI'''   || {{mono|+0000−+0}}
|-
| '''HDB3'''  || {{mono|+B00V−+0}}
|-
|             || {{mono|+−00−+−0}}
|}
{|
| '''Input''' || {{mono|101000001100001100000001}}{{sub|2}}
|-
| '''AMI'''   || {{mono|+0−00000+−0000+−0000000+}}
|-
| '''HDB3'''  || {{mono|+0−000V0+−B00V−+B00V000+}}
|-
|             || {{mono|+0−000−0+−+00+−+−00−000+}}
|}
{|
| '''Input''' || {{mono|1010000100001100001110000111100001010000}}{{sub|2}}
|-
| '''AMI'''   || {{mono|+0−0000+0000−+0000−+−0000+−+−0000+0−0000}}
|-
| '''HDB3'''  || {{mono|+0-000V+000V-+B00V-+-000V+-+-B00V+0-B00V}}
|-
|             || {{mono|+0-000-+000+-+-00-+-+000+-+-+-00-+0-+00+}}
|}
{|
| '''Input''' || {{mono| 10000000000}}{{sub|2}}
|-
| '''AMI'''   || {{mono|+000000000}}
|-
| '''HDB3'''  || {{mono|+B00VB00V00}}
|-
|             || {{mono|+-00-+00+00}}
|}