Click here to purchase the entire book in PDF format. Channel Code This is information regarding the transmission itself - data that keeps the machines talking to each other. It consists of 5 bits making up the preamble (or sync code) and the parity bit. Preamble (also known as the Sync Code) These are 4 bits which tell the receiving device that the trasmission is at the beginning of a block or a subframe (and which subframe it is...) Different specific codes tell the receiver what's going on as is shown in Figure 8.40. Figure 8.40: The structure of the preamble at the start of each Sub-Frame. Note that each of these breaks the bi-phse mark rule that there must be a transition on every bit since all preambles start with 3 consecutive cells of the same value. Note that these codes violate the bi-phase mark protocol (because there is no transition at the beinning of the second bit.) but they do not violate the no-DC rule (because there is the same amount of high voltage as low voltage). Note as well, that these are sometimes called the X, Y, and Z preambles. An X preamble indicates that the sub-frame is an audio sample for the Left. A Y preamble indicates that the sub-frame is an audio sample for the right. A Z preamble indicates the start of a block. Parity Bit This is a single bit which ensures that all of the preambles are in phase. It doesn't matter to the receiving device whether the preambles start by going up in voltage or down (I drew the above examples as if they are all going up...) but all of the preambles must go the same way. The partity bit is chosen to be a 1 or 0 to ensure that the next preamble will be headed in the right direction.