The term 'jitter', in the context of a digital audio system, refers to the timing uncertainty of the data or clock signals. For numerous reasons, the clock of a 44.1 kHz digital signal is not rock stable. There is always a small degree of frequency modulation caused by noise which finds its way into the sensitive clocking circuitry. This modulation causes the timing to vary ever so slightly at the positive and negative going edges of the signal. The variation is called 'jitter' and it can cause major problems if it increases to more than a few nano-seconds.
Certain digital audio devices are VERY sensitive to jitter and they produce distortion effects which are clearly audible. Usually these devices are at the edge of the system, where analog meets digital, however even fully digital devices like sampling frequency converters can be sensitive to jitter. Analog to Digital and Digital to Analog converters are especially sensitive because they depend on the clock to precisely sample the analog signal at points which are equidistant. Any variation between points, even a few nano-seconds, results in amplitude errors which are audible.
Once jitter contaminates a digital signal, it propagates through the system, often increasing as it passes through various devices. Jitter is actually an analog signal which attaches itself to the digital waveform like a virus. Because the basic element of any digital device is an analog switching transistor, the jitter virus is everywhere. Power supply noise in the form of AC voltage and current spikes caused by finite switching times, in turn causes crosstalk noise etc..etc.
The following is a partial list of Jitter producing mechanisms:
- Noise riding on a digital signal causes digital circuitry to switch at different times than it would without the noise present.
- Noise coupled through the power supply rails into digital circuitry, causing switching thresholds to move.
- Transmission of digital signals over inadequate cables results in jitter producing phase distortion and high frequency attenuation.
- Phase Locked Loops (PLL's) used for digital clocking have analog filters and oscillators which are very sensitive to noise and cross-talk.
The PLL is the root cause of jitter in most digital audio equipment. The PLL is as sensitive to noise as a microphone preamplifier (or even more so). Good design practice dictates good isolation and heavy shielding of the PLL from the digital environment. The PLL generates the higher frequency clocks needed by the system from a reference clock e.g. the sync input of the device. It does this with an oscillator whose frequency is controlled by the phase and frequency difference of the reference signal and a signal derived from the oscillator output.The PLL is in fact a control loop. It has all the characteristics of a classical control loop: step response (how fast can it follow the input signal), overshoot(how much does the output signal go past the correct frequency), loop frequency response(determines its capability to suppress jitter), etc.
The PLL circuit can also attenuate jitter by using a very slow reacting control loop. Any jitter riding on the digital input signal gets smoothed out. It's the analogy of a flyweel.
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