It is widely used in data communications; the similar concept used in analog systems like color television is known as carrier recovery.
The term is most often used to describe digital data transmission, in which case the entire signal is suitable for clock recovery.
For instance, in the case of early 300 bit/s modems, the timing of the signal was recovered from the transitions between the two frequencies used to represent binary 1 and 0.
For instance, the pioneering Wireless Set Number 10 used clock recovery to properly sample the analog pulse-code modulation (PCM) voice signals it carried.
[citation needed] The best bit error ratio (BER) is obtained when the samples are taken as far away as possible from any data stream transitions.
In order for this scheme to work, a data stream must transition frequently enough to correct for any drift in the PLL's oscillator.
The limit for how long a clock-recovery unit can operate without a transition is known as its maximum consecutive identical digits (CID) specification.