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24.6.7.22
In Reply to: RE: Seeking recommendations on Word Clock device... posted by Steve Cortez on June 08, 2007 at 04:55:11
First off what do you want to feed the wordclock into? The primary purpose for external wordclocks is to sync multiple devices together not achieve lower jitter per se. Whats going to happen in the system depends a lot on where the wordclock is going.
A quick bit on word clocks, a true word clock is a very slow signal, its switching at the sample rate (44.1KHz etc). Most DACs or other devices will use an internal clock that is at least 256 times higher than this in frequency. The only way to get the high frequency clock from the low one is a PLL frequency multiplier, which adds a significant amount of jitter. Thus even if the clock has extremely low jitter you are NOT going to see that in a device fed through a true word clock.
Now there ARE some clock boxes that can either send out a true word clock or a master clock (the high frequency one). If the box can accept a master clock as well as a word clock, THEN the external clock can be used as is and its low jitter characteristics can be taken advantage of.
So what kind of advantage you get from an external clock depends a lot on the devices themselves and how you have them connected. A few possibilities:
CDP with true wordclock only input, because of PLL on the wordclock line probably not a very big improvement no matter how good the external clock.
CDP with master clock input driven by good master clock. This CAN make a big improvement depending on how the clock is handled internally. Some STILL feed it into a PLL which negates a lot of the advantage, others feed it directly in.
Transport which takes true wordclock driving external DAC over S/PDIF. Again probably not that big of an improvement. If it takes a master clock then there is the possibility of a significant improvement, but not as big as the one above because there is still a PLL in the S/PDIF receiver in the DAC.
The one situation where the external clock can really shine is if you have a DAC that can take an external master clock and a transport that can take either a word clock or master clock. In this case the data is clocked into the DAC chip(s) by the good external clock and the word or master clock is used to sync the transport to that clock as well. I've worked with someone who has done this with very good results. The DAC took the master clock and the transport took a word clock. Fortunately the clock box he had would output both at the same time.
As to what parameters of a clock are important, I think you are barking up the wrong tree by going after rubidium clocks etc. Those have very high accuracy and very low long term drift, neither of which have much of anything to do with the sound. I've done lots of tests of this and absolute accuracy and very small long term drift make no difference to the sound. What DOES matter is short term differences in timing, commonly refered to as jitter or as phase noise. This DOES have a big impact on sound. Unfortunately just because an oscillator has very low long term drift does not mean it is going to have low jitter.
SOME low drift clocks do have low jitter, but they do NOT necessarily go hand in hand. As a matter of fact there is a certain type of inexpensive oscillator used in cell phones that have very low long term drift but have terrible jitter characteristics. Unfortunately some high end companies have become so enamered of the ultra low drift spec of these devices that they completely ignore the jitter specs and use them in their products so they can have very good numbers on their spec sheets.
So now that I have compeletely bored you with all that, let us know what devices you have and how they are connected and we can do a much better job of recommending a possible external clock.
John S.
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