Clock Design

Which Metrics Truly Affect Digital System Sound Quality

As clock quality defines the sonic ceiling in digital audio systems, our extensive measurements uncover the core metrics shaping digital sound and their underlying design truths

01

Unified-Source Clocking is Paramount

During R&D, extensive testing by CelAudio demonstrated that using a single unified-source clock processed by a PLL system to supply all required clock signals across the entire system delivers the finest sonic performance.

02

Phase Noise Has Minimal Impact on Sound

During R&D, we collaborated closely with crystal oscillator suppliers. Extensive test data reveals that crystal oscillators with identical phase noise may exhibit different short-term and long-term stability characteristics, and conversely, oscillators with identical stability metrics may have different phase noise profiles. We have found that oscillators with superior short-term and long-term stability deliver better sonic performance, while phase noise itself has minimal direct impact. That said, oscillators with excellent phase noise typically also possess better stability — which explains why many tests mistakenly attribute sonic improvements to phase noise alone.

03

PLL Lock Bandwidth Significantly Affects Sound

Some PLL systems lack configurable lock bandwidth, but our testing confirmed that lock bandwidth significantly affects sonic performance. In the CelCLK II PLL system, lock bandwidth is software-configurable — influencing both PLL lock speed and overall system sound quality. A 1Hz lock bandwidth delivers the finest sonic performance; however, under certain conditions, this narrow bandwidth can result in excessively long clock output stabilization times, compromising system stability or even preventing startup. Generally, higher VCO clock precision enables faster lock acquisition. Therefore, when using standard VCO reference clocks, CelAudio initially adopts a 100Hz lock bandwidth for rapid clock signal output, then transitions to 1Hz via software for optimal sound quality.

04

The Truth About Short-Term and Long-Term Stability

Both short-term and long-term stability significantly affect sonic performance. In CelCLK II, two clock sources work in concert: the reference clock and the VCO clock. The reference clock primarily determines CelCLK II's short-term stability, while the reference clock, VCO clock, and lock bandwidth together govern the PLL system's long-term stability. Therefore, combining a 1Hz lock bandwidth with excellent VCO and reference clocks simultaneously achieves outstanding short-term and long-term stability — resulting in superior sonic performance.

The Solution

CelCLK II PLL Clock System

After extensively evaluating major PLL solutions across the industry, CelAudio ultimately selected a PLL architecture supporting 1Hz bandwidth lock. Using a dedicated VCO reference source paired with an external reference clock, we designed the second-generation CelCLK II PLL system — incorporating several key technical advances.

CelCLK II OCXO Oven-Controlled Crystal Oscillator

011Hz Bandwidth Lock

1Hz bandwidth lock is the key technology enabling the PLL system to achieve flat long-term stability curves, and forms the core of our sound quality improvement strategy.

02Independent VCO Reference Clock

Employing an independent VCO reference clock significantly increases overall PLL system cost, but yields advantages across several dimensions.

Significantly Improved System Stability

Unlike DACs and other products, digital systems such as switches/servers have explicit requirements for power-on time and clock input time. In traditional clock schemes, clock output time varies with different crystal oscillators, leading to insufficient system stability, and clock switching may cause failures in these digital systems. With an independent VCO clock, system clock input time is guaranteed, and stable clock output is maintained under any condition, ensuring stable system operation.

More Stable Clock Jitter

The presence of a VCO reference clock can help the PLL system achieve more stable clock jitter. When an external reference clock is present, it can leverage the 1+1>2 advantage, making the entire clock system's long-term stability test curve flatter, thereby ensuring sound quality.

Therefore, when using CelCLK II to switch between external reference clocks, the external reference and internal VCO clock operate in a hybrid working mode — rather than simply handing over to the external clock. This differs fundamentally from traditional internal/external clock switching mechanisms.

NW Series

Clock Design for NW Series HiFi Switches

All NW series HiFi switches employ the CelCLK II PLL system with an internal OCXO as the reference source. The NWX adopts a crystal with exceptional short-term stability of 8e-13@1s and -120dB@1Hz phase noise; the NW330 uses a 1e-10@1s, -95dB@1Hz OCXO. The VCO clock varies by product grade: NWX uses a -85dB@1Hz, 1e-11@1s, 50MHz OCXO as the VCO clock, while NW330 uses a 52MHz crystal.

For the NWX HiFi clock switch, it is worth noting that among its four clock outputs, two are SMA interfaces. In the audio clock domain, the debate between sine wave and square wave has long persisted. CelAudio believes that under appropriate conditions, square wave is superior to sine wave — provided the transmission bandwidth is sufficient. A typical BNC interface offers only 3GHz bandwidth, which is suboptimal for square wave transmission, whereas SMA interfaces provide 18GHz bandwidth — offering excellent support for square wave transmission. Therefore, when pairing NWX with NS series music servers, we recommend using the SMA interface with high-grade professional SMA clock cables for optimal square wave clock transmission.

NW Series Clock Design
NS Series Music Server Clock Design
NS Series

Clock Design for NS Series Music Servers

With the exception of the NS6, all NS series music servers employ the CelCLK II PLL system. To minimize system thermal noise, the NS system does not use a separate internal reference clock; instead, it uses a -85dB@1Hz, 1e-11@1s, 50MHz OCXO as the VCO clock, with the PLL lock bandwidth set to 1Hz.

NS series music server clock inputs provide not only the commonly used BNC interface, but also a dedicated SMA interface. This SMA interface does not perform input clock waveform conversion and accepts square wave input only — eliminating losses from signal conversion. Paired with high-grade professional SMA clock cables, this interface can receive the 10MHz square wave signal from NWX's SMA output with exceptional fidelity. Compared to traditional solutions, this connection method achieves outstanding clock transmission performance.

NA Series

Clock Design for NA Series Wireless Access Points

NA series wireless access points employ the CelCLK II PLL system. The NA adopts a ±2.5ppm, 52MHz passive crystal as the VCO clock. To meet the ARM system's strict timing requirements, the PLL lock bandwidth is initially set to 100Hz, then adjusted to 1Hz after stable clock signal output — simultaneously satisfying strict power-on timing requirements and delivering excellent playback quality.

NA series wireless access points provide the commonly used BNC interface for clock input. When receiving a high-quality external clock signal, even higher playback quality can be achieved.

NA Series Wireless Access Point Clock Design
Connection Guide

Clock Connection Recommendations

It is recommended to use NWX as the clock source in the system, connecting to NS series music servers via SMA interface, and to NA and other devices requiring clock input via BNC interface.

Clock Connection Topology