Superclocks

[JohnSwenson]

18 hours ago, rickca said:

Many people are intrigued by the recently introduced superclock in the SOtM Ultra series.  Now Auralic says they will have a superclock in their G2 product line along with the G2 Aries, Vega and processor.

 

So what's a superclock?  Chances are that SOtM and Auralic don't mean precisely the same thing.  This is going to get interesting.  It's the new buzzword.

The term "superclock" means whatever the company using that term wants it to mean. There is no "official" definition of such a term. It is not even related to any specific measurement. The other term which has gotten a lot of press recently is "femtoclock", at least that is related to a measurement of jitter, but still doesn't give a threshold for what it means. It's like the term "fast car", at least you know it is referring to how fast it goes, but no clue where the threshold is between what is considered "fast" and what is "not fast".

 

As jabbr mentioned what does seem to have good correlation with sound quality is close in phase noise, so the phase noise plot is the most important piece of information when looking at clock specs. There are many other clock parameters, and some companies get caught up in optimizing those parameters which do not have correlation with good sound. Thus you can have the situation of a company saying their clock is a "superclock" when that is referring to an optimized parameter which has nothing to do with good sound.

 

A very favorite one is temperature coefficient. A really good oscillator might have something like 1ppm (parts per million) which sounds really good, it is something the marketing department can really highlight, but unfortunately has nothing to do with the phase noise, which is what really matters. As a matter of fact the circuitry which radically decreases the temperature coefficient in a TCXO (Temperature Compensated Crystal Oscillator) actually significantly increases the phase noise. So if you see someone using a TCXO in a DAC, you know they don't really know how to choose the right oscillator. 

 

There is another class of oscillator called the OCXO which you might have heard of (Oven Compensated Crystal Oscillator) which heats a special crystal to a high temperature which produces an extremely low temperature coefficient, say 1PPB, yep 1 part per BILLION. But again that does not guarantee very low phase noise. It does turn out that the lowest phase noise oscillators are OCXOs, but not all OCXOs are really low phase noise. There are a lot of inexpensive OCXOs that are much worse than the Crystek 575. To get an OCXO that has lower phase noise than the 575 takes a lot of money.

 

John S.

 

 

[JohnSwenson]

On 4/25/2017 at 4:11 PM, Hammer said:

Are these clocks different than a rubidium clock from say Stanford Reseach Systems?  I picked one up on the cheap off eBay and had been meaning to purchase a DAC such as a Mytek which accepts clock input to play around, but have not had the time.  Has anyone tried this with good result?  Thanks, hammer

Rubidium clocks are usually very bad to use for audio. They have very good long term stability, but high phase noise. The long term stability has nothing to do with audio but the close in phase noise is what is important. So a rubidium is exactly the wrong oscillator to use.

 

Another problem is that the rubidium is probably NOT going to be outputting a frequency that can be used directly by audio circuitry, so some for of frequency synthesizer is going to have to be used, and these ALWAYS increase the phase noise.

 

A rubidium is great for an actual clock (you can read the time) that you want to be accurate to the microsecond over years of run time, but not so good for audio.

 

John S.

[JohnSwenson]

23 hours ago, monteverdi said:

Thats quite interesting statement considering how much Antelope Audio charged for their external clocks

External clocks are useful in studio environments where you have to have say 32 channels and you are using 2 channel or 4 channel boxes, in order for things to work right you HAVE to feed the same clock into all the boxes.

 

But for a two channel system in a home setting where you do not need to synchronize multiple boxes an external clock is not needed. If a particular DAC sounds better with an external clock, that means the internal clock in that DAC is not that good.

 

This did happen (on purpose) with some professional equipment designed specifically for studio use. The designers assumed the equipment was going to be used in a studio with a master clock fed to multiple boxes, so they deliberately did not spend much money on the internal clock, assuming it would never be used.

 

For home use where you are not using multiple boxes together it is by far best to spend the money putting the clock inside the DAC box, you can get far better performance than spending the money on an external clocked system.

 

On rubidium clocks, there is a technique called "disciplinig" that uses a rubidium clock and an OCXO together. The OCXO frequency is very slowly changed (time period days or weeks) by comparison with the rubidium clock. This gives the best of both worlds, but is just wasted money for audio purposes, you get just as good results on sound quality by using the OCXO without the rubidium, for a lot less money.

 

BTW there is a program designed to analyze disciplined clock systems called Lady Heather, I leave it to the reader to figure out why.

 

John S.

[JohnSwenson]

3 hours ago, Elberoth said:

 

Thanks, that was the page I was looking for. The specs for their clock are as follows:

 

FemtoSecond Galaxy Clock Specifications:

Phase Noise at 0.1 Hz -67 db

Phase Noise at 1 Hz -99 db

Phase Noise at 10 Hz -134 db

Phase Noise at 1 kHz -157 db

Phase Noise at 10 kHz* -157 db

Phase Noise at 100 kHz -157 db 

 

If you get me the phase noise figures at the same freq for other oscillator (Crystek or other) I will calculate the jitter for you to make the comparo possible.  

Be very careful with these numbers, they are not directly comparable to any other clock phase noise numbers. Note the numbers are in dB, NOT dBc/Hz which is the standard for phase noise measurements. Looking at that page from MSB it looks like those numbers are from an FFT. Which means this is a sampling of some waveform with some clock. Exactly what is undefined. Did they use the Galaxy clock to sample the analog wave from a DAC? If so is the Galaxy clock also used in the DAC? These unknowns make it very difficult to convert these numbers into meaningful dBc/Hz numbers that can be compared to other clocks.

 

John S.