CelAudio

What is impact the sound quality?

A unified, single system clock source is paramount: Based on CelAudio's research, whether in complete HiFi systems or within individual devices like DACs and streaming devices, a unified singular clock source remains the fundamental cornerstone.

Phase noise has no impact on the sound quaility:Working with the supplier of the OCXO vendor, Celaudio has test lots of OCXO. The OCXO with the same phase noise may have different short-term and long-term stability, as well as same short-term and long-term stability may have different phase noise. The text result is that OCXO has better short-term and long-term stability has the better sound quality. Phase noise has no impact on the sound quality. But, when we select the OCXO, the better phase noise OCXO normally has the better short-term and long-term stability.

The influence of the PLL's locking bandwidth: Not all PLLs do have locking bandwidth support. However, we found that locking bandwidth impacts the sound quality significantly. In the CelCLK II PLL system, the lock bandwidth of the PLL system can be set by software. It will significantly affect the locking speed of the PLL system and the sound quality. A 1Hz bandwidth will provide the best sound quality. However, in some cases, a 1Hz bandwidth may take too long time to lock, which will affect the system stability.

Truth: In CelCLK II PLL system, we utilize two clock sources:a VCO and a reference clock. The VCO clock is a 48-52MHz clock source. As we know, high frequency clock exhibits higher phase noise compared to low frequency. And the CelCLK II PLL system's phase noise is determined by the worst clock resource. So, once the VCO clock is selected, the output phase noise of CelCLK II will be fixed. However, as previously mentioned, the phase noise DOES NOT impact the sound quality. During the CelCLK II PLL system is operating, Lock bandwidth, VCO clock and reference clock work together to impact the short-term and long-term stability. The sound quality is affected by both short-term and long-term stability. CelCLK II PLL system's short-term stability is determined by the best clock resource. Typically, the reference clock has the best short-term stability and mainly impacts the CelCLK II PLL system's short-term stability. Referenc clock, VCO clock and lock bandwidth mainly impact the long-term stability. When lock bandwidth is set to 1Hz, good reference clock and VCO clock are used, the sound quality is the best.

CelCLK II PLL System

1Hz bandwidth locking: 1Hz bandwidth locking is a key technology for the PLL system to achieve good long-term stability. It is a core technology for improving sound quality.

Independent VCO Reference Clock: After using an independent VCO reference clock, the cost of the entire PLL system will significantly increase, but several advantages are obtained.

The system stability has been significantly enhanced. Digital systems such as switches/servers are different from some devices like DACs. They have clear requirements for Power-Up Sequence, and clock input time is one of the key of the Power-up Sequence. Premature or delayed clock signal initialization may result in system boot failure. With VCO clock, the clock signal is guaranteed, and there is a stable clock output in any state. The stable operation of the system is ensured.
The VCO clock can work with reference clock to achieve better long-term stability

The Clock Design Of The NW HiFi Switch

The clocks of the NW series HiFi switches all use the CelCLK II PLL system, and an OCXO constant temperature crystal oscillator is provided internally as the reference source. Among them, NWX uses a low-second-stability series crystal oscillator with Allan Deviation at 1 second of 8e - 13, and the phase noise is -120dB@1Hz; NW330 uses an OCXO with 1e - 10 and -95dB@1Hz. The VCO clock varies depending on the level of the NW series switches. NWX uses an OCXO clock of -85dB@1Hz, 1e - 11@1s, and 50MHz as the VCO clock, while NW330 uses a 52M crystal as the VCO clock.

For the NWX HiFi clock switch, it is worth noting that two SMA interfaces are provided in the four-channel clock output. In the field of audio clocks, the debate between sine waves and square waves has always existed. According to CelAudio, under the right conditions, square waves are superior to sine waves, but the prerequisite is that the transmission bandwidth must be sufficient. Under the BNC interface, the usual transmission bandwidth is only 3GHz, and the transmission effect of square waves is not ideal. However, the transmission bandwidth of the SMA interface is as high as 18GHz, which provides a strong guarantee for the transmission of square waves. Therefore, when the NWX is paired with the NS series music server, it is recommended to use the SMA interface with a high-level SMA professional clock test line to achieve high-quality square wave clock transmission.

The Clock Design Of The NS Music Server

Except for NS6 which DOES NOT have independent clock, the NS series music servers use CelCLK II PLL system. Considering to minimize the system thermal noise as much as possible, the NS clock system only have VCO clock and PLL, a -85dB@1Hz, 1e-11@1s 50MHz OCXO is used as the VCO clock. 1Hz bandwidth locking. At the same time, the clock input of the NS series music servers not only provides the commonly used BNC interface, but also separately provides an SMA interface. This SMA input does not perform waveform conversion of the input clock and can only use a square wave input. The purpose is to reduce the loss caused by the clock signal conversion. The SMA connector is 12GHz bandwidth. With a high grade SMA professional clock test cable like 18GHz or 25.6GHz. The NS's SMA input can connect to the NWX SMA 10MHz reference clock output. This will provide high quality clock signal without loss.

Recommendations For Clock Connection

It is recommended to use NWX as the clock source in the system, and connect the NS series music server through the SMA interface, and connect other devices that require clock input, such as DAC, through the BNC interface.