Definition from ITU-T Rec. H.264:

A byte equal to 0x03 that may be present within a NAL unit. The presence of emulation prevention bytes ensures that no sequence of consecutive byte-aligned bytes in the NAL unit contains a start code prefix.

The sequence 0x00_00_01 inside a NALU payload can emulate a start code, but could be part of the payload itself, so we add a 0x03 b/w the second and third byte to prevent start code emulation.

() = start code
[] = NALU

[02 02 00 00 01 02 02] (00 00 01) [03...
       _uh oh__

emulates start code, but is not a start code

add a 0x03 byte b/w 2nd and 3rd byte

[02 02 00 00 03 01 02 02] (00 00 01) [03...
       __we good__

Note that emulation prevention doesn’t just happen for 0x00_00_01, but also for the following cases:

  • 0x00_00_00
  • 0x00_00_02
  • 0x00_00_03