Editing AES3 (section)

===Synchronisation preamble===
The synchronisation preamble is a specially coded ''preamble'' that identifies the subframe and its position within the audio block. Preambles are not normal BMC-encoded data bits, although they do still have zero [[DC bias]].

Three preambles are possible:
*X (or M) : 11100010{{sub|2}} if previous time slot was ''0'', 00011101{{sub|2}} if it was ''1''. (Equivalently, 10010011{{sub|2}} [[NRZI]] encoded.)  Marks a word for channel A (left), other than at the start of an audio block.
*Y (or W) : 11100100{{sub|2}} if previous time slot was ''0'', 00011011{{sub|2}} if it was ''1''. (Equivalently, 10010110{{sub|2}} [[NRZI]] encoded.)  Marks a word for channel B (right).
*Z (or B) : 11101000{{sub|2}} if previous time slot was ''0'', 00010111{{sub|2}} if it was ''1''. (Equivalently, 10011100{{sub|2}} [[NRZI]] encoded.)  Marks a word for channel A (left) at the start of an audio block.

The three preambles are called X, Y, Z in the AES3 standard; and M, W, B in IEC 958 (an AES extension).

The 8-bit preambles are transmitted in the time allocated to the first four time slots of each subframe (time slots 0 to 3).  Any of the three marks the beginning of a subframe.  X or Z marks the beginning of a frame, and Z marks the beginning of an audio block.
<pre>
 | 0 | 1 | 2 | 3 |  | 0 | 1 | 2 | 3 | Time slots
  _____       _            _____   _
 /     \_____/ \_/  \_____/     \_/ \ Preamble X
  _____     _              ___   ___
 /     \___/ \___/  \_____/   \_/   \ Preamble Y
  _____   _                _   _____
 /     \_/ \_____/  \_____/ \_/     \ Preamble Z
  ___     ___            ___     ___ 
 /   \___/   \___/  \___/   \___/   \ All 0 bits BMC encoded
  _   _   _   _        _   _   _   _
 / \_/ \_/ \_/ \_/  \_/ \_/ \_/ \_/ \ All 1 bits BMC encoded
 
 | 0 | 1 | 2 | 3 |  | 0 | 1 | 2 | 3 | Time slots
</pre>

In two-channel AES3, the preambles form a pattern of ZYXYXYXY..., but it is straightforward to extend this structure to additional channels (more subframes per frame), each with a Y preamble, as is done in the  [[MADI]] protocol.