Self-clocking signal

Self-clocking signal

In telecommunications and electronics, a self-clocking signal is one that can be decoded without the need for a separate clock signal or other source of synchronization. This is usually done by including embedded synchronization information within the signal, and adding constraints on the coding of the data payload such that false synchronization can easily be detected.

Most line codes are designed to be self-clocking.

Examples of self-clocking signals include:

Isochronous:
* Manchester code, where the clock signals occur at the transition points.
* PDH signals
* Eight-to-Fourteen Modulation
* 4B5B
* 8b/10b encoding
* HDLC
* Modified Frequency Modulation

Anisochronous:
* Morse code
* Asynchronous start-stop

Most of these codes can be seen as a kind of Run Length Limited code. Those constraints on "runs" of zeros and "runs" of ones ensure that transitions occur often enough to keep the receiver synchronized.

Such self-clocking signals can be decoded correctly into a stream of bits without bit slip.To further decode that stream of bits and decide which bit is the first bit of a byte, often a self-synchronizing code is used.


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