Jitter
:
Jitter is also the name of a set of
video processing and
matrix manipulation extensions to
Max/MSP.
In
telecommunication,
jitter is an abrupt and unwanted variation of one or more
signal characteristics, such as the interval between successive pulses, the amplitude of successive cycles, or the
frequency or
phase of successive cycles. Jitter has become a significant factor in the design of communications buses (e.g.
USB) for computers.
Jitter may be specified in qualitative terms (e.g. amplitude, phase,
pulse width or pulse position), or quantitative terms (e.g.
mean,
RMS, or peak-to-peak displacement). The low-frequency
cutoff for jitter is usually specified at 1 Hz. A jitter meter is a testing instrument for measuring jitter values and is used in manufacturing
DVD and
CD-ROM drives.
For
clock jitter, there are two main parameters:
phase jitter and
cycle to cycle jitter (or period jitter).
Phase jitter is a rapid, repeated
phase perturbation resulting in the intermittent shortening or lengthening of an electronic
signal. Phase jitter may be random or cyclic.
Peak to peak phase jitter is the difference between the maximum and minimum phase of the clock signal over all time.
RMS phase jitter is the standard deviation of peak to peak phase jitter.
The phase departure in phase jitter usually is smaller, but more rapid, than that of
phase perturbation. Phase jitter may be expressed in degrees, radians, or seconds. Phase jitter is usually random. However, if cyclic, phase jitter may be expressed in
hertz.
Cycle to cycle (or period) jitter is the variation from one period to the next adjacent period of the signal. In order to determine the variation between adjacent periods, all consecutive periods need to be measured. Peak to peak period jitter is the worst case of cycle to cycle jitter.
In
networking, in particular
IP networks such as the
Internet, jitter refers to the variation (
statistical dispersion) in the delay of the
packets (because of
routers' internal queues behaviour in certain circumstances,
routing changes, etc).
Anti-jitter circuits
Anti-jitter circuits (AJCs) are a class of
electronic circuits designed to reduce the level of jitter in a regular pulse
signal. AJCs operate by re-timing the output pulses so they align more closely to an idealised pulse signal.They are widely used in clock and data recovery circuits in
digital communications, as well as for data sampling systems such as the
analog-to-digital converter and
digital-to-analog converter. Examples of anti-jitter circuits include
phase-locked loop and
delay-locked loop. Inside digital to analog converters jitter causes unwanted high-frequency distortions. In this case it can be suppressed with high fidelity clock signal usage.
Jitter buffers
Jitter buffers or de-jitter buffers are used to counter jitter introduced by packet networks so that a continuous playout of audio (or video) transmitted over the network can be ensured. 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 mediastream.
Some systems use sophisticated delay-optimal de-jitter buffers which are capable of adapting the buffering delay to changing network jitter characteristics. These are known as adaptive de-jitter buffers and the adaptation logic is based on the jitter estimates computed from the arrival characteristics of the media packets. Adaptive de-jittering involves introducing discontinuities in the media play-out which may appear offensive to the listener or viewer. Adaptive de-jittering is usually carried out for audio play-outs which feature a VAD/DTX encoded audio, 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 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 is usually ineffective in dealing with low-frequency jitter, such as waiting-time jitter.
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Buffer (telecommunication)*
Drift*
Wander*
Federal Standard 1037C*
MIL-STD-188''
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Fibre Channel MJSQ (Methodology of Jitter Specification & Quality) Information*
Jitter Buffer*
An Introduction to Jitter in Communications Systems*
Jitter Specifications Made Easy A Heuristic Discussion of Fibre Channel and Gigabit Ethernet Methods
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Jitter Test Solutions Be A Jitter Master With Agilent Technologies
