The second form of interference we want to eliminate is typically associated with the clock signal of the system, and is usually referred to as “Jitter”. In essence, jitter is the result of time instability, and can be found in all signal processing and conversion processes.
In the case of an audio system, jitter may occur in the CD player when samples are being read off the compact disc. Since electronic functions are time-controlled by the pulses of a crystal oscillator, the so-called system clock; the actual reading time may vary from sample to sample if the pulses of system clock is unstable, and created what now known as jitter in the system. This undesirable phenomenon could be the result of a malfunctioning electronic component in the system, or due to excessive noise on a signal control line. Regardless of the root cause, jitter distorts the original signal and leads to degradation in the overall sound performance.
Imagine an analogue waveform produced by a musical instrument like the one shown in the figure below:
An analog waveform produced by a musical instrument
During the process of digital encoding, an Analog-to-Digital Converter (ADC) takes snapshots of the analog signal and measures the amplitude of the signal at discrete time intervals, to produce the digital samples. These samples taken by the ADC, labeled S1, S2, S3, are then stored digitally on a compact disc. When the CD is played back, these sample values are decoded back into an analog waveform that resembles the original musical signal. And for all practical purposes the higher the resemblance means the higher fidelity of the sound system.
As depicted in the diagrams below, the Analog-to-Digital converter relies on the clock signal to determine when each successive sample will be taken. As expected, a stable clock signal (denoted by the even spacing of the sampling intervals) will produce a waveform closely resemble the original signal.
And in contrast, an unstable clock signal (denoted by the uneven spacing of the sampling intervals) will produce a distorted waveform, as we will experience it as jitter.
The degradation in sound performance caused by jitter is both measurable and clearly audible. The sonic effect of jitter often stands out as harsh sound in high frequency signals, and ruins the natural harmony of the original audio signal.
As mentioned above, we use two highly effectively methods to make sure the jitter is totally eliminated both internally and externally:
1) Internal: utilize the ultra accurate oscillator to maintain the system clock accuracy. The main Oscillator of DAC2488 is used (TCXO) Temperature Compensated Crystal Oscillator. It can make sure the Jitter impact is minimal, the basic requirement of TCXO is: Freq. accuracy = +/- 1ppm, Aging (Year) Max +/- 1ppm year (0℃-50℃).
2) 2) External: we used the latest state-of-the-art receiver chip as an active jitter attenuator to reduce the out side jitter down to a mere 50 ps RMS.