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 does not 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.

Noisy power supplies, improper grounding and electromagnetic interference could induce jitter.

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,- .

 

When I wrote this in 2011 3 ps was an excellent value.

Calyx audio was the first to introduce a DAC with a femto clock as its intrinsic jitter dropped below the 1 ps.
Today (2014) several manufacturers followed.

To give you an idea of the magnitude:
1 p = 0.000000000001, a  trillionth of a second.
1 f = 0.000000000000001, a quadrillionth of a second.

Wyred For Sound offers an upgrade of its existing models to a 82 fSec phase jitter clock for $150,- (labor included). Obvious this marvel of precision is not necessary expensive.

 

This no surprise as the femto clocks are needed in computers and networks.
If a processor is screaming at 3 MHz, precision of the clocking is crucial for a correct functioning.
As the femto clock is a computer part, it is mass produced hence cheap.

External master clock

If you need to synchronize a couple of devices, you can use a master clock.
For playback a high quality clock as close as possible to the DAC (chip set) in combination with asynchronous operation is probably a better solution.

 

Sound on Sound reviewed a couple of high quality master clocks.

Overall, it should be clear from these tests that employing an external master clock cannot and will not improve the sound quality of a digital audio system. It might change it, and subjectively that change might be preferred, but it won’t change things for the better in any technical sense. A-D conversion performance will not improve: the best that can be hoped for is that the A-D conversion won’t become significantly degraded. In most cases, the technical performance will actually become worse, albeit only marginally so.

Does Your Studio Need A Digital Master Clock? : SOS June 2010

It looks like an external clock will degrade the sound most of the time.

 

 

A €5,392 DAC

Clocked by a €7,338 Rubidium clock

Fed by a €1,675 PSU

Am I the only one who do thinks that if adding an external clock and an external PSU helps, this DAC must be equipped with both a lousy clock and a lousy PSU?

Master, bit and word clock

Word clock: the sample rate of the audio e.g. 44.1 kHz
Bit clock: the bit rate.
Assuming 16 bits words, 2 channel, 44.1 kHz the you have =16*2*44100= 1411200 bits per second.
The bit clock runs at double speed = 2822400 = 2.8224 MHz
The master clock runs a arbitrary number e.g. 4 times faster = 11.2896 MHz

References

Rock around the clock - What's Best Forum

Femto Clock in a Streamer - why ?? - Computer Audio Asylum