In
electronics
The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
and
telecommunications
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
, jitter is the deviation from true periodicity of a presumably
periodic signal
A periodic function is a function that repeats its values at regular intervals. For example, the trigonometric functions, which repeat at intervals of 2\pi radians, are periodic functions. Periodic functions are used throughout science to des ...
, often in relation to a reference
clock signal
In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits.
A clock sign ...
. In
clock recovery
In serial communication of digital data, clock recovery is the process of extracting timing information from a serial data stream itself, allowing the timing of the data in the stream to be accurately determined without separate clock information. ...
applications it is called timing jitter.
Jitter is a significant, and usually undesired, factor in the design of almost all
communications link
A data link is the means of connecting one location to another for the purpose of transmitting and receiving digital information (data communication). It can also refer to a set of electronics assemblies, consisting of a transmitter and a recei ...
s.
Jitter can be quantified in the same terms as all time-varying signals, e.g.,
root mean square
In mathematics and its applications, the root mean square of a set of numbers x_i (abbreviated as RMS, or rms and denoted in formulas as either x_\mathrm or \mathrm_x) is defined as the square root of the mean square (the arithmetic mean of the ...
(RMS), or
peak-to-peak
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...
displacement. Also, like other time-varying signals, jitter can be expressed in terms of
spectral density
The power spectrum S_(f) of a time series x(t) describes the distribution of power into frequency components composing that signal. According to Fourier analysis, any physical signal can be decomposed into a number of discrete frequencies, o ...
.
Jitter period is the interval between two times of maximum effect (or minimum effect) of a signal characteristic that varies regularly with time. Jitter frequency, the more commonly quoted figure, is its inverse.
ITU-T
The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
G.810 classifies jitter frequencies below 10 Hz as wander and frequencies at or above 10 Hz as jitter.
Jitter may be caused by
electromagnetic interference
Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electros ...
and
crosstalk
In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, induc ...
with carriers of other signals. Jitter can cause a display monitor to flicker, affect the performance of processors in personal computers, introduce clicks or other undesired effects in audio signals, and cause loss of transmitted data between network devices. The amount of tolerable jitter depends on the affected application.
Metrics
For
clock
A clock or a timepiece is a device used to measure and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month and the ...
jitter, there are three commonly used metrics:
;Absolute jitter
:The
absolute difference
The absolute difference of two real numbers x and y is given by , x-y, , the absolute value of their difference. It describes the distance on the real line between the points corresponding to x and y. It is a special case of the Lp distance for a ...
in the position of a clock's edge from where it would ideally be.
;
Maximum time interval error
Maximum time interval error (MTIE) is the maximum error committed by a clock under test in measuring a time interval for a given period of time. It is used to specify clock stability requirements in telecommunications standards. MTIE measurements ...
(MTIE)
:Maximum error committed by a clock under test in measuring a time interval for a given period of time.
;Period jitter (a.k.a. ''cycle jitter'')
:The difference between any one clock period and the ideal or average clock period. Period jitter tends to be important in synchronous circuitry such as digital state machines where the error-free operation of the circuitry is limited by the shortest possible clock period (average period less maximum cycle jitter), and the performance of the circuitry is set by the average clock period. Hence, synchronous circuitry benefits from minimizing period jitter, so that the shortest clock period approaches the average clock period.
;Cycle-to-cycle jitter
:The difference in duration of any two adjacent clock periods. It can be important for some types of clock generation circuitry used in
microprocessor
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
s and
RAM
Ram, ram, or RAM may refer to:
Animals
* A male sheep
* Ram cichlid, a freshwater tropical fish
People
* Ram (given name)
* Ram (surname)
* Ram (director) (Ramsubramaniam), an Indian Tamil film director
* RAM (musician) (born 1974), Dutch
* ...
interfaces.
In
telecommunications
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
, the unit used for the above types of jitter is usually the
unit interval
In mathematics, the unit interval is the closed interval , that is, the set of all real numbers that are greater than or equal to 0 and less than or equal to 1. It is often denoted ' (capital letter ). In addition to its role in real analysis, ...
(UI) which quantifies the jitter in terms of a fraction of the transmission unit period. This unit is useful because it scales with clock frequency and thus allows relatively slow interconnects such as
T1 to be compared to higher-speed internet backbone links such as
OC-192 Optical Carrier transmission rates are a standardized set of specifications of transmission bandwidth for digital signals that can be carried on Synchronous Optical Networking (SONET) fiber optic networks. Transmission rates are defined by rate of t ...
. Absolute units such as ''picoseconds'' are more common in microprocessor applications. Units of ''degrees'' and ''radians'' are also used.
If jitter has a
Gaussian distribution, it is usually quantified using the
standard deviation
In statistics, the standard deviation is a measure of the amount of variation or dispersion of a set of values. A low standard deviation indicates that the values tend to be close to the mean (also called the expected value) of the set, while ...
of this distribution. This translates to an RMS measurement for a zero-mean distribution. Often, jitter distribution is significantly non-Gaussian. This can occur if the jitter is caused by external sources such as power supply noise. In these cases, ''peak-to-peak'' measurements may be more useful. Many efforts have been made to meaningfully quantify distributions that are neither Gaussian nor have a meaningful peak level. All have shortcomings but most tend to be good enough for the purposes of engineering work.
In
computer network
A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...
ing, jitter can refer to
packet delay variation
In computer networking, packet delay variation (PDV) is the difference in end-to-end one-way delay between selected packets in a flow with any lost packets being ignored.RFC 3393 The effect is sometimes referred to as packet jitter, although t ...
, the variation (
statistical dispersion
In statistics, dispersion (also called variability, scatter, or spread) is the extent to which a Probability distribution, distribution is stretched or squeezed. Common examples of measures of statistical dispersion are the variance, standard de ...
) in the delay of the
packets.
Types
One of the main differences between random and deterministic jitter is that deterministic jitter is bounded and random jitter is unbounded.
Random jitter
Random jitter, also called Gaussian jitter, is unpredictable electronic timing noise. Random jitter typically follows a
normal distribution
In statistics, a normal distribution or Gaussian distribution is a type of continuous probability distribution for a real-valued random variable. The general form of its probability density function is
:
f(x) = \frac e^
The parameter \mu ...
due to being caused by
thermal noise
A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
in an
electrical circuit
An electrical network is an interconnection of electrical components (e.g., batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e.g., voltage sources, ...
.
Deterministic jitter
Deterministic jitter is a type of clock or data signal jitter that is predictable and reproducible. The peak-to-peak value of this jitter is bounded, and the bounds can easily be observed and predicted. Deterministic jitter has a known non-normal distribution. Deterministic jitter can either be correlated to the data stream (
data-dependent jitter
Data-dependent jitter (DDJ) is a specific class of timing jitter. In particular, it is a form of deterministic jitter which is correlated with the sequence of bits in the data stream. It is also a form of ISI.
Properties
Depending on characteris ...
) or uncorrelated to the data stream (bounded uncorrelated jitter). Examples of data-dependent jitter are duty-cycle dependent jitter (also known as duty-cycle distortion) and
intersymbol interference
In telecommunication, intersymbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have a similar effect as noise, thus making ...
.
Total jitter
Total jitter (''T'') is the combination of random jitter (''R'') and deterministic jitter (''D'') and is computed in the context to a required
bit error rate
In digital transmission, the number of bit errors is the number of received bits of a data stream over a communication channel that have been altered due to noise, interference, distortion or bit synchronization errors.
The bit error rate (BER) i ...
(BER) for the system:
:,
in which the value of ''n'' is based on the BER required of the link.
A common BER used in communication standards such as
Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
is 10
−12.
Examples
Sampling jitter
In analog-to-digital and digital-to-analog conversion of signals, the sampling is normally assumed to be periodic with a fixed period—the time between every two samples is the same. If there is jitter present on the clock signal to the
analog-to-digital converter
In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provide ...
or a
digital-to-analog converter
In electronics, a digital-to-analog converter (DAC, D/A, D2A, or D-to-A) is a system that converts a digital signal into an analog signal. An analog-to-digital converter (ADC) performs the reverse function.
There are several DAC architec ...
, the time between samples varies and instantaneous signal error arises. The error is proportional to the
slew rate
In electronics, slew rate is defined as the change of voltage or current, or any other electrical quantity, per unit of time. Expressed in SI units, the unit of measurement is volts/second or amperes/second, but is usually expressed in terms of m ...
of the desired signal and the absolute value of the clock error. The effect of jitter on the signal depends on the nature of the jitter. Random jitter tends to add broadband noise while periodic jitter tends to add errant spectral components, "birdys". In some conditions, less than a nanosecond of jitter can reduce the effective bit resolution of a converter with a
Nyquist frequency
In signal processing, the Nyquist frequency (or folding frequency), named after Harry Nyquist, is a characteristic of a sampler, which converts a continuous function or signal into a discrete sequence. In units of cycles per second ( Hz), it ...
of 22 kHz to 14 bits.
Sampling jitter is an important consideration in high-frequency signal conversion, or where the clock signal is especially prone to interference.
In
digital antenna array
Digital antenna array (DAA) is a smart antenna with multi channels digital beamforming, usually by using fast Fourier transform (FFT).
The development and practical realization of digital antenna arrays theory started in 1962 under the guidance ...
s
ADC
ADC may refer to:
Science and medicine
* ADC (gene), a human gene
* AIDS dementia complex, neurological disorder associated with HIV and AIDS
* Allyl diglycol carbonate or CR-39, a polymer
* Antibody-drug conjugate, a type of anticancer treatm ...
and
DAC jitters are the important factors determining the
direction of arrival estimation accuracy
and the depth of jammers suppression.
Packet jitter in computer networks
In the context of computer networks, packet jitter or
packet delay variation
In computer networking, packet delay variation (PDV) is the difference in end-to-end one-way delay between selected packets in a flow with any lost packets being ignored.RFC 3393 The effect is sometimes referred to as packet jitter, although t ...
(PDV) is the variation in latency as measured in the variability over time of the
end-to-end delay
End-to-end delay or one-way delay (OWD) refers to the time taken for a packet to be transmitted across a network from source to destination. It is a common term in IP network monitoring, and differs from round-trip time (RTT) in that only path in t ...
across a network. A network with constant delay has no packet jitter.
Packet jitter is expressed as an average of the deviation from the network mean delay. PDV is an important
quality of service
Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network. To quantitat ...
factor in assessment of network performance.
Transmitting a burst of traffic at a high rate followed by an interval or period of lower or zero rate transmission may also be seen as a form of jitter, as it represents a deviation from the average transmission rate. However, unlike the jitter caused by variation in latency, transmitting in bursts may be seen as a desirable feature, e.g. in
variable bitrate
Variable bitrate (VBR) is a term used in telecommunications and computing that relates to the bitrate used in sound or video encoding. As opposed to constant bitrate (CBR), VBR files vary the amount of output data per time segment. VBR allows a ...
transmissions.
Video and image jitter
Video or image jitter occurs when the horizontal lines of video image frames are randomly displaced due to the corruption of synchronization signals or electromagnetic interference during video transmission. Model-based dejittering study has been carried out under the framework of digital image and video restoration.
Testing
Jitter in serial bus architectures is measured by means of
eye pattern
In telecommunication, an eye pattern, also known as an eye diagram, is an oscilloscope display in which a digital signal from a receiver is repetitively sampled and applied to the vertical input, while the data rate is used to trigger the horiz ...
s. There are standards for jitter measurement in serial bus architectures. The standards cover
jitter tolerance
In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a signific ...
,
jitter transfer function
In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a significa ...
and
jitter generation
In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a signific ...
, with the required values for these attributes varying among different applications. Where applicable, compliant systems are required to conform to these standards.
Testing for jitter and its measurement is of growing importance to electronics engineers because of increased clock frequencies in digital electronic circuitry to achieve higher device performance. Higher clock frequencies have commensurately smaller eye openings, and thus impose tighter tolerances on jitter. For example, modern computer
motherboard
A motherboard (also called mainboard, main circuit board, mb, mboard, backplane board, base board, system board, logic board (only in Apple computers) or mobo) is the main printed circuit board (PCB) in general-purpose computers and other expand ...
s have serial bus architectures with eye openings of 160
picosecond
A picosecond (abbreviated as ps) is a unit of time in the International System of Units (SI) equal to 10−12 or (one trillionth) of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000  ...
s or less. This is extremely small compared to parallel bus architectures with equivalent performance, which may have eye openings on the order of 1000
picosecond
A picosecond (abbreviated as ps) is a unit of time in the International System of Units (SI) equal to 10−12 or (one trillionth) of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000  ...
s.
Jitter is measured and evaluated in various ways depending on the type of circuit under test.
In all cases, the goal of jitter measurement is to verify that the jitter will not disrupt normal operation of the circuit.
Testing of device performance for jitter tolerance may involve injection of jitter into electronic components with specialized test equipment.
A less direct approach—in which analog waveforms are digitized and the resulting data stream analyzed—is employed when measuring pixel jitter in
frame grabber
A frame grabber is an electronic device that captures (i.e., "grabs") individual, digital still frames from an analog video signal or a digital video stream. It is usually employed as a component of a computer vision system, in which video fram ...
s.
Mitigation
Anti-jitter circuits
Anti-jitter circuits (AJCs) are a class of
electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow. It is a type of electrical ...
s designed to reduce the level of jitter in a clock signal. AJCs operate by re-timing the output pulses so they align more closely to an idealized clock. They are widely used in clock and data recovery circuits in
digital communications
Data transmission and data reception or, more broadly, data communication or digital communications is the transfer and reception of data in the form of a digital bitstream or a digitized analog signal transmitted over a point-to-point or ...
, as well as for data sampling systems such as the
analog-to-digital converter
In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provide ...
and
digital-to-analog converter
In electronics, a digital-to-analog converter (DAC, D/A, D2A, or D-to-A) is a system that converts a digital signal into an analog signal. An analog-to-digital converter (ADC) performs the reverse function.
There are several DAC architec ...
. Examples of anti-jitter circuits include
phase-locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...
and
delay-locked loop
In electronics, a delay-locked loop (DLL) is a pseudo-digital circuit similar to a phase-locked loop (PLL), with the main difference being the absence of an internal voltage-controlled oscillator, replaced by a delay line.
A DLL can be used to ch ...
.
Jitter buffers
Jitter buffers or de-jitter buffers are
buffers used to counter jitter introduced by queuing in
packet-switched networks to ensure continuous playout of an audio or video
media stream
Streaming media is multimedia that is delivered and consumed in a continuous manner from a source, with little or no intermediate storage in network elements. ''Streaming'' refers to the delivery method of content, rather than the content it ...
transmitted over the network. The maximum jitter that can be countered by a de-jitter buffer is equal to the buffering delay introduced before starting the play-out of the media stream. In the context of packet-switched networks, the term ''
packet delay variation
In computer networking, packet delay variation (PDV) is the difference in end-to-end one-way delay between selected packets in a flow with any lost packets being ignored.RFC 3393 The effect is sometimes referred to as packet jitter, although t ...
'' is often preferred over ''jitter''.
Some systems use sophisticated delay-optimal de-jitter buffers that are capable of adapting the buffering delay to changing network characteristics. The adaptation logic is based on the jitter estimates computed from the arrival characteristics of the media packets. Adjustments associated with adaptive de-jittering involves introducing discontinuities in the media play-out which may be noticeable to the listener or viewer. Adaptive de-jittering is usually carried out for audio play-outs that include
voice activity detection Voice activity detection (VAD), also known as speech activity detection or speech detection, is the detection of the presence or absence of human speech, used in speech processing. The main uses of VAD are in speech coding and speech recognition. I ...
that allows the lengths of the silence periods to be adjusted, thus minimizing the perceptual impact of the adaptation.
Dejitterizer
A dejitterizer is a device that reduces jitter in a
digital signal
A digital signal is a signal that represents data as a sequence of discrete values; at any given time it can only take on, at most, one of a finite number of values. This contrasts with an analog signal, which represents continuous values; at ...
. A dejitterizer usually consists of an
elastic buffer in which the signal is temporarily stored and then retransmitted at a rate based on the average rate of the incoming signal. A dejitterizer may not be effective in removing low-frequency jitter (wander).
Filtering and decomposition
A filter can be designed to minimize the effect of sampling jitter.
Jitter signal can be decomposed into
intrinsic mode functions (IMFs), which can be further applied for filtering or dejittering.
See also
*
Clock drift
Clock drift refers to several related phenomena where a clock does not run at exactly the same rate as a reference clock. That is, after some time the clock "drifts apart" or gradually desynchronizes from the other clock. All clocks are subject to ...
*
Dither
Dither is an intentionally applied form of image noise, noise used to randomize quantization error, preventing large-scale patterns such as color banding in images. Dither is routinely used in processing of both digital audio and digital vide ...
*
Jitterlyzer
The FS5000 Jitterlyzer performs physical layer serial bus jitter evaluation. It can inject controlled jitter and measure the characteristics of incoming jitter. When teamed with a logic analyzer or protocol analyzer, it can correlate these measu ...
*
Micro stuttering __notoc__
Micro stuttering is a term used in computing to describe a quality defect that manifests as irregular delays between frames rendered by the GPU(s), causing the instantaneous frame rate of the longest delay to be significantly lower than ...
*
Phase noise
In signal processing, phase noise is the frequency-domain representation of random fluctuations in the phase of a waveform, corresponding to time-domain deviations from perfect periodicity (jitter). Generally speaking, radio-frequency engineers ...
*
Pulse (signal processing)
A pulse in signal processing is a rapid, transient change in the amplitude of a signal from a baseline value to a higher or lower value, followed by a rapid return to the baseline value.
Pulse shapes
Pulse shapes can arise out of a process called ...
References
Further reading
* Li, Mike P
Jitter and Signal Integrity Verification for Synchronous and Asynchronous I/Os at Multiple to 10 GHz/Gbps Presented at International Test Conference 2008.
* Li, Mike P
A New Jitter Classification Method Based on Statistical, Physical, and Spectroscopic Mechanisms Presented at DesignCon 2009.
* Liu, Hui, Hong Shi, Xiaohong Jiang, and Zhe Li
Pre-Driver PDN SSN, OPD, Data Encoding, and Their Impact on SSJ Presented at Electronics Components and Technology Conference 2009.
*
* Zamek, Iliya
SOC-System Jitter Resonance and Its Impact on Common Approach to the PDN Impedance Presented at International Test Conference 2008.
External links
*
*
*
* , a Heuristic Discussion of Fibre Channel and Gigabit Ethernet Methods
Jitter in Packet Voice Networks
{{authority control
Electrical parameters
Packets (information technology)
Synchronization