In electronics and telecommunications, jitter is a deviation from the actual periodicity of a signal that may be periodic, often related to the reference clock signal. In the clock recovery application, this is called timing jitter . Jitter is a significant factor, and is usually undesirable, in the design of almost any communication link.
Jitter can be quantified in the same terms with all time-varying signals, for example, root mean square (RMS), or peak-to-peak displacement. Also like other time-changing signals, jitter can be expressed in terms of spectral density.
Jitter Period is the interval between two times the maximum effect (or minimum effect) of the signal characteristics that vary regularly with time. Jitter Frequency , the more frequently quoted number, is the reverse. ITU-T 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 and crosstalk with other signal carriers. Jitter can cause screen monitors to flicker, affect processor performance on personal computers, introduce clicks or other unwanted effects in audio signals, and cause loss of data transmitted between network devices. The number of jitter that can be tolerated depends on the affected application.
Video Jitter
Metrik jitter
For jitter hours, there are three commonly used metrics:
- Absolute jitter
- Absolute difference in the hour edge position from its ideal place.
- Period jitter (aka jitter cycle )
- The difference between a clock period and an ideal or average hour period. The jitter period tends to be important in synchronous circuits such as a digital state machine where error-free operation of the circuit is limited by the shortest possible clock period (average period less maximum jitter cycle), and circuit performance is regulated by the average clock period. Therefore, the synchronous circuit benefits from minimizing the period jitter, so that the shortest clock period approaches the average clock period.
- Cycle-to-cycle jitter
- The difference in duration of two adjacent clock periods. This can be important for some types of clock generation circuits used in microprocessors and RAM interfaces.
In telecommunications, the unit used for the above jitter types is usually the unit interval (UI) that quantifies the jitter in terms of the fraction of the transmission unit period. This unit is useful because it scales with clock frequency and thus allows relatively slow interconnections such as T1 compared to high-speed internet backbone links such as OC-192. 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 calculated using the standard deviation of this distribution. This translates to RMS measurements for the mean-zero distribution. Often, the distribution of jitter is significantly non-Gaussian. This can happen if the jitter is caused by an external source such as power supply interruption. In this case, the peak-to-peak measurement may be more useful. Much effort has been made to measure the distribution of significance that is not Gaussian or has a meaningful peak level. All have flaws but most tend to be good enough for engineering work. Note that usually, the reference point for jitter is defined such that mean jitter is 0.
In computer networks, jitter can refer to packet delay variations, variations (statistical spread) in packet delays.
Maps Jitter
Type
One of the main differences between random and deterministic jitter is that deterministic jitter is constrained and random jitter can not be limited in some circumstances.
Random Jitter
Random Jitter, also called Gaussian jitter, is an unpredictable electronic timing noise. Random jitter usually follows the normal distribution because it is caused by thermal noise in electrical circuits or due to central limit theorem. The central limit theorem states that the combined effect of many non-correlated noise sources, regardless of distribution, is near normal distribution.
Deterministic Jitter
Deterministic jitter is a predictable and reproducible type of clock or jitter data signal. The peak-to-peak value of the jitter is limited, and the limits can be easily observed and predicted. Deterministic jitter has a known non-normal distribution. Deterministic jitter can be correlated with data flow (data-dependent jitter) or uncorrelated to the bounded uncorrelated jitter. Examples of data-dependent jitter are jitter-dependent duty-cycle (also known as task-cycle distortion) and intersymbol interference.
Number of jitter
Total jitter ( T ) adalah kombinasi dari random jitter ( R ) dan jitter deterministik ( D ):
- T = D puncak-ke-puncak 2ÃÆ' n ÃÆ' - R rms ,
where the value n is based on the required bit error rate (BER) from the link.
The bit error rates commonly used in communication standards such as Ethernet are 10 -12 .
Example
Sampling jitter
In the conversion of analog signals to digital and digital to analog, sampling is usually assumed periodically with fixed periods - the time between any two samples is the same. If any jitter is present on a clock signal to an analog-to-digital converter or a digital-to-analog converter, the time between the samples varies and the onset of the signal error is instantaneous. This error is proportional to the rate of change in direction of the desired signal and the absolute value of the clock error. Various effects such as noise (random jitter), or spectral components (periodic jitter) may occur depending on the pattern of jitter in relation to the signal. In some conditions, less than one nanosecond of jitter can reduce the effective bit resolution of the converter with Nyquist frequency from 22 kHz to 14 bits.
This is a consideration in the conversion of high frequency signals, or where clock signals are highly prone to interference.
Package jitter in computer network
In the context of computer networks, jitter is a variation in latency as measured in variability over time from packet latency across the network. Networks with constant latency have no variation (or jitter). The jitter package is expressed as the average deviation from the network mean latency. However, for this use, the term is not appropriate. The standard-based term is "packet delay variation" (PDV). PDV is an important quality of service factor in network performance assessment.
Burst or burstiness transmission, which transmits high traffic bursts followed by low or zero rate intervals or transmission periods, can also be seen as a jitter form, as it represents the deviation from the average transmission rate. However, unlike jitter caused by variations in latency, transmission in bursts is usually seen as a desirable feature, eg. in variable bitrate transmission. The Usage/Network/Parameter Control (UPC and NPC) controls, as applied in the ATM network, allow both the Maximum Burst Size (MBS) on average or Sustained Cell Rate (SCR), and the tolerance of Cell Changes (CDVt) on the Cell Peak Rate (PCR) in which the bursts are transmitted. This MBS can be derived from or used to obtain the maximum variation between traffic arrival times in the bursts of time to arrive at the SCR, ie jitter about the SCR.
Compact disk search for jitter
In the context of digital audio extraction from compact discs, seek jitter causes the extracted audio samples to be doubled or fully skipped if the Compact Disc drive searches again. The problem occurs because Red Book does not require block-accurate addressing during search. As a result, the extraction process can resume some initial or final samples, resulting in a doubling or disposing of the sample. These glitches often sound like small repetitive clicks during playback. A successful approach to software correction involves overlapping reads and adjusting data to find overlap at the edges. Most of the extraction programs function to find the corrections of jitter. CD manufacturers avoid searching for jitter by extracting entire disks in one continuous read operation, using a special CD drive model at slower speeds so the drive does not re-search.
Jitter meter is a testing instrument to measure jitter clock value, and is used in making DVD disc and CD-ROM.
Due to the additional sector-level addressing added in Yellow Book, CD-ROM data discs are not charged for searching jitter.
Video and image multiplier
Video or jitter image occurs when the horizontal line of the video image frame is moved randomly due to synchronization signal corruption or electromagnetic interference during video transmission. Model-based dejittering studies have been performed under a digital image/video restoration framework.
Test
Testing for jitter and its measurement is increasingly important for electronic engineers because of the increasing clock frequency in digital electronic circuits to achieve higher device performance. Higher clock frequencies have smaller eyelets trivially, and thus impose tighter tolerances on jitter. For example, modern computer motherboards have a serial bus architecture with 160 picosecond or less eye openings. This is very small compared to parallel bus architectures with equivalent performance, which may have an eye opening on the order of 1000 picoseconds.
Performance testing of devices for jitter tolerance often involves the injection of jitter into electronic components with special test equipment.
Jitter is measured and evaluated in various ways depending on the type of circuit under test. For example, jitter in a serial bus architecture is measured using an eye diagram, in accordance with industry-accepted standards. The less direct approach - in which the analog waveform is digitized and the resulting data stream is analyzed - is used when measuring jitter pixels in the frame's hand. In all cases, the purpose of jitter measurement is to verify that the jitter will not interfere with the normal operation of the circuit.
There is a standard for jitter measurement in serial bus architecture. Standards include jitter tolerance, jitter transfer functions and jitter generation, with the required values ​​for these attributes varying between different applications. Where possible, appropriate systems are required to comply with this standard.
Mitigation
Anti-jitter circuit
Anti-jitter circuits (AJCs) are electronic circuit classes designed to reduce jitter levels in ordinary pulse signals. AJC operates by rearranging the output pulse so that it is aligned more closely to the ideal pulse signal. They are widely used in clock and data recovery circuits in digital communications, as well as for data sampling systems such as analog-to-digital converters and digital-to-analog converters. Examples of anti-jitter circuits include phase-locked loops and delayed-locked loops. In digital to analog converters, jitter causes undesirable high frequency distortion. In this case can be suppressed by the use of high clock fidelity signal.
Jitter buffer
Jitter buffers or de-jitter buffers are used to counter jitter introduced by queues in packet switched networks so that continuous audio (or video) playback transmitted over the network can be ensured. The maximum jitter that can be countered by the de-jitter buffer is equal to the buffering delay introduced before starting play-out from mediastream. In the context of packet-switched networks, the term delay delay package is often preferred over jitter .
Some systems use sophisticated de-jitter buffers that have an optimal delay capable of adapting delay buffering to alter the characteristics of network jitter. This is known as adaptive de-jitter buffer and adaptation logic based on the estimated jitter calculated from the arrival characteristics of the media package. Adaptive de-jittering involves the introduction of discontinuities in media play-outs, which may appear to offend listeners or viewers. Adaptive de-jittering is usually performed for audio playback featuring VAD/DTX encoded audio, allowing a period of silence to be adjusted, thus minimizing the perceptual impact of adaptation.
Dejitterizer
Dejitterizer is a device that reduces jitter in digital signals. A dejitterizer usually consists of an elastic buffer in which the signal is stored temporarily and then retransmitted at a rate based on the average rate of the incoming signal. A dejitterizer is usually not effective at handling low frequency jitter, such as jitter wait-time.
Filtering
Filters can be designed to minimize the effects of jitter sampling. For more information see the paper by S. Ahmed and T. Chen entitled, "Minimizing the effects of sampling anxiety in wireless sensor networks".
Decomposition
The jitter signal can be decomposed into Intrinsic Mode Functions (IMFs), which can be applied further for filtering or dejittering.
See also
- Buffer (telecommunications)
- Clock drift
- Either
- Maximum time interval error
- Phase noise
- Pulses (signal processing)
References
- This article incorporates public domain material from the "Federal Standard 1037C" Public Service Administration document (to support MIL-STD-188).
Further reading
- Levin, Igor. Terms and concepts involved with digital clocking related to Jitter issues in professional quality digital audio
- Li, Mike P. Jitter and Verification Signal Integrity for Sync and Asynchronous I/Os at Multiple to 10 GHz/Gbps. Presented at the 2008 International Test Conference.
- Li, Mike P. New Jitter Multiplier 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 Impact on SSJ. Presented at the 2009 Electronic Components and Technology Conference
- Miki, Ohtani, and Kowalski Jitter Requirements (Causes, solutions, and values ​​recommended for digital audio)
- Trischitta, Patrick R.; Varma, Eve L. (1989). Jitter in Digital Transmission System . Artech. ISBNÃ, 0-89006-248-X.
External links
- Defeat the jitter bug
- Jitter in VoIP - Causes, solutions, and recommended values ​​
- Jitter Buffer
- Definition of Jitter in the QoS Testing Methodology
- Introduction to Jitter in Communication Systems
- Jitter Specification Becomes Easy Heuristic Discussion of Fiber Channel and Gigabit Ethernet Methods
- Jitter in Packet Voice Networks
Source of the article : Wikipedia
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