Jitter
Overview
The term "jitter" is used to describe variations in a periodic signal, which can be in the frequency, amplitude, or phase of the signal in relationship to the idea waveform or the waveform at the point of generation. In digital audio; one of the most problematic issues involving jitter is in "clock recovery" from signals transmitted between equipment. If jitter in the recovered clock signal affects the clocking of conversion, even small amounts of jitter can effectively reduce the resolution far below the theoretical limits of 16-24 bit conversion.
Basics
Because even "digital" signals are actually very high frequency analog signals, the receiving device must reconstruct the signal by means of some form of amplitude "threshold." Ideal digital signals have a "zero rise time and zero fall time" which means the change in voltage level between a "1" and a "0" requires no time. In digital circuitry, a "1" is represented by a voltage level very close to the power supply voltage; which in digital circuitry is typically a value like 5 Volts or 3.3 Volts. In the same circuitry, a “0” is represented by a voltage level very close to “0 Volts” or in other words ground. In order to reduce power dissipation, power supply voltages are being reduced further; but that is beyond the scope of this discussion.
Because of the difference between the ideal signal and the real signal, the transition between a “0” and a “1” (between 0 Volts and 5 Volts) takes a finite amount of time; which is referred to as the “rise time” or the reciprocal “fall time.” Thus a threshold is needed to decide when the signal has changed from a “0” to a “1” (or vice-versa) and this is a voltage threshold which does not change, by design. If either the amplitude of the signal or the “slope” (rise/fall time) of the signal changes; the signal will cross the threshold at a different time; thus effectively changing the amplitude variation into a timing variation.
Likewise, variations in the voltage of the power supply on which the digital circuitry operates, or ground noise can also affect the absolute voltage level of the threshold at any instant in time.
There are a number of sources of jitter, and in signal transmission between equipment the primary cause is “reflections” of the signal caused by impedance miss-matches in the signal path result from things like connectors or differences in the impedance of cable, wiring, or circuit board conductors on either side of a connector. Much like ripples on the surface of a pond, the “waves” add or subtract from each other in a complex changing pattern that results in changes to the shape of the transmitted waveform. These variations cause the waveform to cross the voltage threshold at different (relative) times than the transitions of the original signal; resulting in a time-domain distortion in the “recovered” output signal regardless of whether the distortion of the shape of the waveform was in the time domain or the amplitude domain.
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