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.

Jitter types

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).

Jitter prevention

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.

References

External links

Telecommunications | Packets

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