Jitter is an unwanted variation of one or more characteristics of a periodic signal in electronics and telecommunications. Jitter may be seen in characteristics such as the interval between successive pulses, or the amplitude, frequency, or phase of successive cycles. Jitter is a significant factor in the design of almost all communications links (e.g. USB, PCI-e, SATA, OC-48). In clock recovery applications it is called timing jitter. Wolaver, 1991, p.211]]

Jitter can apply to a number of signal qualities (e.g. amplitude, phase, [[pulse width or pulse position), and can be quantified in the same terms as all time-varying signals (e.g. RMS, or peak-to-peak displacement). Also like other time-varying signals, jitter can be expressed in terms of spectral density (frequency content). Jitter period is the interval between two times of maximum effect (or between two times of minimum effect) of a jitter characteristic, for a jitter that varies regularly with time. Jitter frequency, the more commonly quoted figure, is its inverse. Generally, very low jitter frequency is not of interest in designing systems, and the low-frequency cutoff for jitter is typically specified at 1 Hz.

CD

In the context of digital audio extraction from Compact Discs, seek jitter causes extracted audio samples to be doubled-up or skipped entirely if the Compact Disc drive re-seeks. The problem occurs during seeking because the Red Book (audio CD standard) doesn't require block-accurate addressing. As a result, the extraction process may restart a few samples early or late, resulting in doubled or omitted samples. These glitches often sound like tiny repeating clicks during playback. An approach that has produced good results is to do jitter correction in software, which involves performing overlapping reads, and then sliding the data around to find overlaps at the edges. Most extraction programs will perform seek jitter correction. CD manufacturers avoid seek jitter by extracting the entire disc in one continuous read using specific CD drive models at slower speeds so the drive will not re-seek.

Due to additional sector level addressing added in the Yellow Book (CD standard), CD-ROM data discs are not subject to seek jitter.

A jitter meter is a testing instrument for measuring clock jitter values, and is used in manufacturing DVD and CD-ROM discs.

Phase jitter metrics

For clock jitter, there are three commonly used metrics: absolute jitter, period jitter, and cycle to cycle jitter.

Cycle-to-cycle jitter is the difference in length of any two adjacent clock periods. Accordingly, it can be thought of as the discrete-time derivative of period jitter. It can be important for some types of clock generation circuitry used in microprocessors and RAM interfaces. dh All of these jitter metrics are really measures of a single time-dependent quantity, and hence are related by derivatives as described above. Since they have different generation mechanisms, different circuit effects, and different measurement methodology, it is still useful to quantify them separately.