Jitter is technically defined as a variation of a periodic signal or pulse from its ideal position in time. It is most commonly encountered in telecommunications (also called packet delay variation PDV – the variation in the time of arrival of packets in a high-speed data transmission) and in electronic systems (the deviation in the amplitude, phase, timing or width of high-frequency digital signals).
Data transmission is done in packets containing a payload (actual data) and a header (identifier) which indicates, among others, its correct position so that the receiving computer would know where to put it when re-assembling the data. Due to network congestion, timing drift or route changes, some of the transmitted packets may arrive late or are lost in transit.
A simplified example to understand PDV is to imagine a data consisting of three packets sent through a network, wherein one packet is sent every second (in reality, packets are sent in milli- or even nano-seconds). The receiving computer should receive the three packets in the same sequence in which these were sent and all the packets should be received three seconds after reception began, to enable the receiving computer to successfully reconstruct the data. Any of the following occurrences is considered a jitter:
a. delay in the arrival of packets, such as packet 2 arriving two seconds after packet 1 was received (it should be received one second after packet 1) – also called dispersion.
b. too fast arrival of packets, such as packet 2 arriving 0.5 second after packet 1 was received – also called clumping.
Jitter is not much of a problem when sending files or other forms of data, as the receiving computer usually compensates for delays using built-in jitter buffers. Even in streaming transmissions such as multimedia playback, jitter can be compensated by a bigger play-out buffer, wherein playback is delayed until sufficient data is stored in the local memory to seamlessly play the audio or video file.
It is in interactive real-time applications, such as online gaming and VoIP, where jitter problems are more pronounced. In VoIP, re-assembly of the data packets are done in real-time by the receiving computer and it is not possible to wait for delayed packets, resulting in noticeable gaps in the transmission. It is for this reason that VoIP requires Quality of Service (QoS) enabled networks that provide a guaranteed level of performance, including a low jitter rate.
The true jitters (as opposed to telecommunications jitter discussed above which is a misnomer) are deviations in pulses (waves) of high-frequency digital signals. Simply put, jitter refers to deviation of a signal characteristic (amplitude, phase or timing) from its ideal location at a given time, usually caused by electromagnetic interference or crosstalk with other signals. This deviation may cause a display monitor to blink, reduce computing ability of processors, add noise to audio/video or result to loss of transmitted data between devices. Jitter is therefore considered in the design of buses, interfaces, optical carriers and other communication links.
One particular jitter that is commonly encountered is in CDs, which is a time-axis variation in the digital bitstream. This is usually due to mechanical imperfections in the CD player, such as if the speed of the rotational servo varies even if minutely, as the recovered signal will have time-based errors (jitters). Another type of jitter occurs when converting from analog to digital signals or vice versa, due to imprecise timing of discrete samples. These jitters introduce noise and distortion into the signal, and sometimes produce tiny repeating clicks during playback.
Electronic engineers pay special attention to jitter in the development of digital circuits, to ensure that jitter will not disrupt normal circuitry operations. Specialized jitter measurements, such as eye diagrams and digitized waveform analysis are employed to ensure that jitter is within industry accepted standards. Engineers inject jitter into digital circuits to test jitter tolerance of electronic components during the testing phase.
Anti-jitter circuits (AJCs) were developed to reduce pulse signal jitters by re-timing output pulses to align with ideal pulse signals. A dejitterizer is another means of reducing jitter, wherein an elastic buffer temporarily stores digital signals then later re-transmits the signal at an average rate.
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