An introduction to computer audio
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,- .
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 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
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
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