Relationship between audio and crystal clear sound quality and clock phase noise
Analog → high resolution
Sound is essentially an analog signal. When processing analog signals, there are “fading, noise, and degradation” drawbacks. Digitization solves that deficiency. The original analog signal is digitized by an analog-to-digital converter (ADC) and distributed as a digital sound source via CD or a network. This digital sound source is processed by the digital-to-analog converter (DAC) of the user’s digital audio equipment and is finally output in analog.
To digitize an analog signal, sampling (* 1) is performed at a constant frequency. To reproduce the sound as closely as possible to the original sound, it is necessary to increase the sampling frequency (* 2) and the bit rate (* 3). Compared to CD sound sources, today’s high-resolution sound sources have an improved sample rate / bit rate, allowing sounds closer to the original sound to be digitized.
[Sampling frequency and bit rate of digital sound source]
Digital sound source Sampling frequency bit rate
CD sound source 44.1 kHz 16 bit
High resolution 96 kHz 24 bit sound source
192 kHz 24 bits
384 kHz 24 bits
Noise components and fluctuations that cause sound deterioration
To accurately reproduce a high-resolution sound source, it is necessary to suppress the deterioration of the sound source in the digital audio device and accurately convert (DAC) from digital to analog and output it. The conversion accuracy of this DAC depends on the noise characteristics (extra frequency components other than the required frequency) of the clock frequency of the audio equipment.
If there is no noise in the circuit, the clock frequency will be a single line ((1) in the right figure), but actually, it is modulated by noise as shown in (2) in the right figure , and the spectrum has an extra frequency component in the vicinity, it will be a characteristic that you will have. This additional frequency component is called “phase noise”.
The phase noise of this clock frequency affects the conversion accuracy of the DAC, resulting in irregular time intervals.
This is called “jitter”. (See figure below)
Noise-free and accurate clock source requirements
In digital audio, clock frequency phase noise affects the DAC function as jitter and contributes to deterioration of the sound source, making it difficult to reproduce faithful sound. Therefore, to improve sound reproducibility, a master clock crystal oscillator with excellent phase noise (small jitter) characteristics is required.
Phase noise is expressed as the level of the frequency component measured at a distance from the original frequency of the crystal oscillator. The distance from the reference frequency is called the offset frequency and is mainly measured in the range of 1 Hz to 1 MHz.
Also, frequency stability is generally considered important for crystal oscillators, but frequency stability is a measure that does not fluctuate in frequency over a long period of time. Audio equipment is required to have less short-term fluctuation than long-term stability. Therefore, SPXO (* 4), which has a frequency stability of ± 30 ppm to ± 100 ppm, is often used as the master clock. Also, in high-end digital audio, OCXO (* 5), etc. can be used in search of higher quality sound.
