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Relation between audio equipment and crystals

Relation between audio equipment and crystals -Sound quality and clock phase noise-

analog digital

Sound essentially consists of analog signals, whose processing is associated with the problems of attenuation, noise and deterioration. Digitization is the solution to these problems.

These issues are addressed by passing the original sound through an analog-to-digital converter (ADC), and the resulting data can be distributed on CDs or via networks as digital sound.

These digital sound data are then processed using a digital-to-analog converter (DAC) in the end-user's digital audio device and output as analog sound.

In the digitalization of analog signals, sampling(*1) is carried out at a certain frequency.

In order to reproduce sound with the highest fidelity possible, it is necessary to increase the sampling frequency (*2) and bit rate (*3).

Current high-resolution sound sources are characterized by sampling frequencies and bit rates superior to those used for CDs, enabling digitize sounds that are closer to the original.

(*1) Sampling:

A process of converting an analog signal into a digital signal at regular intervals.

(*2) Sampling frequency:

The number of conversions performed each second to digitalize analog signals.

(*3) Bit rate:

Amount of information processed per second.

[Sampling frequency and bit rate of digital sound source]

Digital sound source sampling frequency bit rate
CD sound source 44.1kHz 16bit
Hi-Res. sound source 96kHz 24bit
192kHz 24bit
384kHz 24bit

Noise Components and Jitter as Factors of Sound Degradation

In order to accurately reproduce high-resolution sound sources, the sound source must be accurately converted from digital to analog (DAC) and output with minimal degradation in the digital audio equipment.

This conversion accuracy of DAC depends on the noise characteristics (i.e., frequency components outside the target frequency) of the clock frequency of the audio device used.

The clock frequency spectrum for a circuit with zero noise has the form of a straight line (Figure 1, right).
However, real-life spectra are modulated by noise, and are characterized by an extra frequency component nearby (Figure 2, right) known as phase noise.

The phase noise of a clock frequency influences the conversion accuracy of the DAC and makes the time interval irregular.
This phenomenon is called jitter (see the figure below).

Demand for Accurate low-noise clock sources

In digital audio devices, the phase noise of the master clock influences DAC due to jitter, thereby impeding high-fidelity audio reproduction.
To enhance sound reproducibility, a crystal oscillator for a master clock with superior phase noise characteristics (i.e., low jitter) is necessary.

Phase noise is expressed as frequency component levels measured outside a crystal oscillator's original frequency, and is based on the component level of the original frequency.
Offset frequency is the departure from the original frequency, and is normally measured in the range of 1 Hz - 1 MHz.

Frequency stability (the characteristic by which frequency does not change over an extended period) is generally seen as an important property of crystal oscillators.
However, audio devices require short-term rather than long-term stability.
Against such a background, SPXOs(*4), which have a frequency stability of about ±30 - ±100 ppm, are commonly used for master clocks.
High-end audio systems may feature OCXOs(*5) instead for even higher-quality sound.

(*4) SPXO (Simple Packaged Crystal Oscillator)

A crystal oscillator with basic configuration and no temperature compensation or temperature control. Small SPXO products used for clock generation purposes are categorized as crystal clock oscillators.

(*5) OCXO (Oven-Controlled Crystal Oscillator)

A crystal oscillator that provides high accuracy based on a structure that maintains the temperature around the crystal unit in the oscillator using a thermostat chamber.

Low Phase Noise Crystal Oscillators for Digital Audio Master Clocks

In 2015, we developed the OCXO [DuCULoN®](*6) (Ducalon®) with the world's best ultra-low phase noise characteristics for audio crystal oscillators (according to our research in June 2015).

Examples of phase noise characteristics are introduced below.

Phase noise characteristic

(*6) DuCULoN®

Abbreviated form of "Dual Crystal Ultra Low Noise" categorized OCXO.

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