3.10 Synchronizing

Once the PLL in the data separator has locked to the clock content of the transmission, a serial channel bitstream and a channel bit clock will emerge from the sampler. In a group code, it is essential to know where a group of channel bits begins in order to assemble groups for decoding to data bit groups. In a randomizing system it is equally vital to know at what point in the serial data stream the words or samples commence. In serial transmission, channel bit groups or randomized data words are sent one after the other, one bit at a time, with no spaces in between, so that although the designer knows that a data packet contains, say, 128 bytes, the receiver simply finds 1024 bits in a row. If the exact position of the first bit is not known, then it is not possible to put all the bits in the right places in the right bytes; a process known as deserializing. The effect of sync slippage is devastating, because a one-bit disparity between the bit count and the bitstream will corrupt every symbol in the block.

The synchronization of the data separator and the synchronization to the packet format are two distinct problems, although they may be solved by the same sync pattern. Deserializing requires a shift register fed with serial data and read out once per word. The sync detector is simply a set of logic gates arranged to recognize a specific pattern in the register. The sync pattern is either identical for every packet or has a restricted number of versions and it will be recognized by the receiver circuitry and used to reset the bit count through the packet. Then by counting channel bits and dividing by the group size , groups can be deserialized and decoded to data groups. In a randomized system, the pseudo-random sequence generator is also reset. Then counting derandomized bits from the sync pattern and dividing by the word length enables the receiver to deserialize the data words.

Even if a specific code were excluded from the transmitted data so that it could be used for synchronizing, this cannot ensure that the same pattern cannot be falsely created at the junction between two allowable data words. Figure 3.16 shows how false synchronizing can occur due to concatenation. It is thus not practical to search for a bit pattern that is a data code value. The problem is overcome in some synchronous systems by using the fact that sync patterns occur exactly once per packet and therefore contain redundancy. If the pattern is recognized at packet rate, a genuine sync condition exists. Sync patterns seen at other times must be false. Such systems take a few milliseconds before sync is achieved, but once achieved it should not be lost unless the transmission is seriously impaired.

Figure 3.16: Concatenation of two words can result in the accidental generation of a word which is reserved for synchronizing.

In run-length-limited codes false syncs are not a problem. The sync pattern is no longer a data bit pattern but is a specific waveform. If the sync waveform contains run lengths that violate the normal coding limits, such run lengths cannot occur in encoded data, nor can they be interpreted as data. They can, however, be readily detected by the receiver.

In a group code there are many more combinations of channel bits than there are combinations of data bits. Thus after all data bit patterns have been allocated group patterns, there are still many unused group patterns which cannot occur in the data. With care, group patterns can be found which cannot occur due to the concatenation of any pair of groups representing data. These are then unique and can be used for synchronizing.