Clock

Digital audio is PCM (Pulse Code Modulation) audio most of the time.
It consist of two components, the value of the signal (represented by 16 or 24 bits words) and the time step (sample rate).
We have two components, the signal and the time.

Sounds logical but pretty often you hear the 'bits are bits' theory, implying that if the bits are right, everything is right. This theory leaves the other half, the time step, out of the equation.

To play PCM audio, the bits has to be translate to a equivalent voltage and this must be done with a time step matching the sample rate.

This is done by a Digital to Analogue Converter, a DAC for short.

The sample rate is generated by a clock.

As absolute perfection don't exist, there is always some fluctuation in clock speed.

This is called clock jitter.

Interface jitter is jitter introduced in the transmission of digital signals.

Jitter could be induced by noisy power supplies, improper ground path and electromagnetic interference.

Crucial is the sampling jitter, deviations in the sampling interval in the DA conversion stage

According to the Redbook audio standard the clocks frequency should be within +/-100ppm (parts per million).

A deviation of 100 ppm means that a 440Hz tone deviates +/- 0.044Hz.

Important for audio is the cycle to cycle stability, each sample should be delivered with exactly the same interval. The deviation in this interval is the clock jitter.
Best results are obtained by using a crystal (XO).

A lot of designs requires the DAC to lock on the incoming stream.
In this case the clock frequency must vary to stay in sync.

This is called a VCXO (Voltage Controlled Crystal Oscillator).
Clock jitter of a VCXO can be below < 3ps rms.
Price for this marvel of precision:  € 30,- .

Master clock

If you need to synchronize a couple of devices, you can use a master clock.
This configuration is sometimes used to clock both a sound card delivering SPDIF and a DAC.
For playback only a high quality clock as close as possible to the DAC (chip set) in combination with asynchronous operation is probably a better solution