Mutec REF 10 Masterclock

[BigAlMc]

Hi @romaz

 

Many thanks for the detailed reply. 

 

I've been following your "novel way" thread over the last few months on/off and all I can say is kudos! You are a trailblazer and some of the stuff you are discovering is nothing short of cutting edge. Keep up the great work as we truly appreciate it. 

 

I say on/off as I assumed the thread was only about bridging and dipped out for a while. I fully intended attempting bridging at some point but didn't follow the thread that closely. When I dipped back in - wow! It was a whole new thing (sCLK-EX, masterclocks, modded switches etc) and another amazing discovery. I was speechless dude! 

 

Anyway I've seen the sCLK-EX stuff and the @austinpopso call Trifecta posts (another great asset to the community by the way Austinpop).

 

I guess I was hoping for an easy/lazy way of hooking the Ref 10 to my SU-1. 

 

I've got the SU-1 singing sweetly and would be loathe to replace it so soon in. That said this whole masterclock business has me very, very intrigued. 

 

I'm 50/50 at this stage for trying some of the capacitor mods that reportedly further improve the SU-1 or trying to figure out how best to integrate a sCLK-EX (and perhaps Ref 10) approach. Assuming SOtM won't mod the Singxer that might not be simple. 

 

Two questions to clarify my understanding:

 

Without modding the SU-1 for the sCLK-EX then the upstream benefits would be mostly squandered, right? 

 

Is using the I2S output on the SU-1 to the detriment of the upstream clock benefits? 

 

Thanks again for the reply and all the inspiration on previous posts. 

 

Cheers, 

Alan 

 

[4est]

1 hour ago, romaz said:

 

Not a dumb question.

 

Your SU-1 has no master clock input option nor does its clock operate at 10MHz and so there is no direct way to associate the REF10 with the SU-1 although there is an indirect way.

 

This is where SOtM's revolutionary new sCLK-EX board comes in but there's a catch.  This new clock board has the option of a 10MHz master clock input (50 or 75ohm) and further has 4 independent (but synchronized) outputs that can be used to replace the clock of almost any component (i.e. router, switch, motherboard, LAN card, video card, USB card, etc).  I know this as I have done it already and the results have been spectacularly good, so good that I have purchased a 2nd sCLK-EX board and will be replacing 4 more clocks in my chain (a total of 8 clocks) from the router all the way to my final endpoint.  What this means is that I will have every clock in my audio path replaced and the REF10 will be used to synchronize all 8 clocks.  Moreover, since the REF10 will be powered by its well thought out linear PSU and since each of SOtM's clock boards (and its 4 clocks) can be powered by a high quality linear PSU (in my case, my Paul Hynes SR7), regardless of the dirty environment that each clock is placed into (ie a music server), each clock will be powered very cleanly.  What kind of improvement this leads to, I'm not yet sure, but thus far, the results with 4 clocks replaced have been breathtakingly good.  

 

In theory, the sCLK-EX can be used to replace the clock on the SU-1 and I'm willing to bet, based on my own experience so far and based on the comments made by others, that there will be an improvement even without the REF10 but the improvement will be even larger with the REF10.  The only problem is SOtM has suggested they will not replace clocks on products made by other audio companies.  Whether there are exceptions to the rule, I'm not sure.

 

If you are interested in my further impressions of the REF10, they are posted here:

 

 

Forgive my ignorance please, but how are these clocks synchronized? I do not understand how say a master clock at 24.567MHz gets sync'd to USB at 12MHz(or whatever it is).

[seeteeyou]

According to the discussions from that 100-page (and counting) megathread, the best practice would be picking the clock that's closet to the DAC itself if we're interested in replacing that with sCLK-EX.

 

However, we've gotta ask the $64,000 question in order to find out if the clocks inside DirectStream DAC were holding back the ones inside a modified Singxer SU-1 with sCLK-EX.

 

In other words, is DirectStream DAC operating in master mode or slave mode for I2S? My understanding could be incorrect so hopefully someone could chime in to correct me. If DirectStream DAC were operating in slave mode for I2S, it's taking the clock(s) from the source (i.e. Singxer SU-1 with sCLK-EX) and that's the only way to justify the investment IMHO.

 

Otherwise, DirectStream DAC could be accepting the inferior clock(s) inside the DAC itself if that were operating in master mode for I2S. Consequently the performance of sCLK-EX might be held back by the inferior clock(s) behind that and maybe we aren't exactly getting what we've paid for?

 

FYI - the differences between master mode and slave mode for I2S could be found here as well

 

https://www.allo.com/sparky/kali-reclocker.html

https://www.allo.com/sparky/boss-dac.html

[romaz]

4 hours ago, 4est said:

Forgive my ignorance please, but how are these clocks synchronized? I do not understand how say a master clock at 24.567MHz gets sync'd to USB at 12MHz(or whatever it is).

 

I am not a clock expert and so I have no choice but to accept statements made to me by clock designers and manufacturers at face value but according to Lee, SOtM's lead engineer and the gentleman responsible for the sCLK-EX clock board, the 4 clock outputs on this board are capable of independent frequencies but ultimately, the precision of their timing is "synchronized" to the performance characteristics of a single internal oscillator, an "internal master clock", if you will.  A similar analogy would be a computer motherboard's system clock operating at 24.567MHz but providing timing to various buses on the motherboard via DPLL even though those buses operate at frequencies other than 24.567MHz.  There is a language barrier when speaking with Lee since English is not his native language but he used the word "synchronized" repeatedly as the reason for why his clock board sounds so good and the primary reason why any audiophile should consider an external master clock.  Regardless of the exact technicalities involved, what matters more to me is the resultant improvement in SQ.

 

Isolating the impact of a clock in a component isn't always easy but with SOtM's standard endpoints, such as the sMS-200, tX-USBhubEX, or dX-USB HD and their corresponding "Ultra" versions, the only difference is the clock and so it becomes easy to appreciate the difference their new clock makes and the difference is surprisingly large with respect to increased air around voices and instruments resulting in a more 3D presentation but also resulting in the better layering of detail and discernment of subtle nuances that are easily and consistently differentiated with blind testing.  Even when their new clock was applied to an inexpensive $20 network switch, the improvement was shocking and when 4 Ultra endpoints were combined in series, all I could do was shake my head in disbelief with how my music had taken on new life while sounding smoother and more grain-free.  This wasn't any form of artificial romantic coloration that I could detect, it was simply realism taken to another level.

 

Now, when I got a chance to first listen to the REF10 in Munich, I was aware of all the criticism against external master clocks by the nay-saying experts and how the long clock cables involved would surely degrade the signal and based on my mediocre experience with the dCS Vivaldi master clock last year, I was admittedly skeptical but the REF10 has completely changed my mind about how impactful a well implemented external master clock can be.  In the same way that SOtM's sCLK-EX transformed their standard endpoints, the REF10 did the same with their MC-3+USB but perhaps to an even greater degree.   I hope to receive my REF10 and a pair of Habst clock cables in a few short weeks and so I will get a chance to hear for myself its impact in my system.  I will report my findings at that time.

[4est]

7 hours ago, romaz said:

 

I am not a clock expert and so I have no choice but to accept statements made to me by clock designers and manufacturers at face value but according to Lee, SOtM's lead engineer and the gentleman responsible for the sCLK-EX clock board, the 4 clock outputs on this board are capable of independent frequencies but ultimately, the precision of their timing is "synchronized" to the performance characteristics of a single internal oscillator, an "internal master clock", if you will.  A similar analogy would be a computer motherboard's system clock operating at 24.567MHz but providing timing to various buses on the motherboard via DPLL even though those buses operate at frequencies other than 24.567MHz.  There is a language barrier when speaking with Lee since English is not his native language but he used the word "synchronized" repeatedly as the reason for why his clock board sounds so good and the primary reason why any audiophile should consider an external master clock.  Regardless of the exact technicalities involved, what matters more to me is the resultant improvement in SQ.

 

Isolating the impact of a clock in a component isn't always easy but with SOtM's standard endpoints, such as the sMS-200, tX-USBhubEX, or dX-USB HD and their corresponding "Ultra" versions, the only difference is the clock and so it becomes easy to appreciate the difference their new clock makes and the difference is surprisingly large with respect to increased air around voices and instruments resulting in a more 3D presentation but also resulting in the better layering of detail and discernment of subtle nuances that are easily and consistently differentiated with blind testing.  Even when their new clock was applied to an inexpensive $20 network switch, the improvement was shocking and when 4 Ultra endpoints were combined in series, all I could do was shake my head in disbelief with how my music had taken on new life while sounding smoother and more grain-free.  This wasn't any form of artificial romantic coloration that I could detect, it was simply realism taken to another level.

 

Now, when I got a chance to first listen to the REF10 in Munich, I was aware of all the criticism against external master clocks by the nay-saying experts and how the long clock cables involved would surely degrade the signal and based on my mediocre experience with the dCS Vivaldi master clock last year, I was admittedly skeptical but the REF10 has completely changed my mind about how impactful a well implemented external master clock can be.  In the same way that SOtM's sCLK-EX transformed their standard endpoints, the REF10 did the same with their MC-3+USB but perhaps to an even greater degree.   I hope to receive my REF10 and a pair of Habst clock cables in a few short weeks and so I will get a chance to hear for myself its impact in my system.  I will report my findings at that time.

 

Thank you for your reply. I still do not understand, but then again there is a language barrier. At the risk of asking another stupid question, have you tried this on the DAC clocks?

[lmitche]

54 minutes ago, 4est said:

 

Thank you for your reply. I still do not understand, but then again there is a language barrier. At the risk of asking another stupid question, have you tried this on the DAC clocks?

Here is a paper on fractional N PLLs.

 

http://www.linear.com/solutions/5240

 

 I wouldn't be surprised to find something like the Linear chips in the SOTM clock.  It explains how any clock can be referenced to another.

[BigAlMc]

16 hours ago, romaz said:

In theory, the sCLK-EX can be used to replace the clock on the SU-1 and I'm willing to bet, based on my own experience so far and based on the comments made by others, that there will be an improvement even without the REF10 but the improvement will be even larger with the REF10.  The only problem is SOtM has suggested they will not replace clocks on products made by other audio companies.  Whether there are exceptions to the rule, I'm not sure.

 

So if I wanted to go big bang with this and pursue it the 'ideal' set up in my case might be:

 

SU-1 & LAN Switch modded for sCLK-EX, and SMS-200-Ultra and sCLK-EX

 

(assuming of course I can find someone to do the SU-1 mod. And if someone was lifting the hood then I could also get the other capacitor mods etc that are being recommended on the SU-1 thread. Tempting!)

 

If I wanted to go nuclear then I could add the Ref 10 as well, perhaps later.

 

Interesting and food for thought.

 

Thanks for the info and inspiration. Tho on behalf of my wallet - dammit guys!! Just as I thought I was 'nearly there'.....  

[Forehaven]

JC, how in the heck does one 'add' a clock to say a router?  Most routers I've seen don't have a clock input/output.  

[seeteeyou]

4 hours ago, 4est said:

At the risk of asking another stupid question, have you tried this on the DAC clocks?

 

I've been asking the same "stupid" question as well, those clocks are supposedly THE most effective ones once we've replaced them with sCLK-EX.

 

Nobody seemed to have try sCLK-EX with DAC clocks so far since there's only one way to get that done, warranty will be voided for sure since the existing clocks from Crystek and NDK etc. should be soldered under most circumstances. Consequently clock mods are more suitable for DACs that are already out of warranty or else we better have DIY DACs.

 

And then romaz mentioned that he wouldn't attempt to mod his own DAC:

 

On 6/17/2017 at 0:34 PM, seeteeyou said:

Not sure if anyone were having serious balls to mod Chord DAVE as well as DAVINA in the future?

IMHO we've gotta ask the same question for REF 10 as well, ideally both DAC and source already have 10 MHz master clock inputs IF we're connecting both of them to REF 10 for maximum benefits.

 

However, only quite a few reclockers from Mutec plus a fairly limited list of toys available are equipped with such inputs by default:

• ESOTERIC G-0、G-0s、G-0Rb、G-02、G-03X、G-25U、K-01、K-01X、K-03、K-03X、K-05、K-05X、K-07、K-07X、D1、D-02、D-02X、D-07X、P1、P-02、P-02X、N-05
• TASCAM CG1000
• dcs dcs992II、Vivaldi
• Antelope Audio Isochrone OCX HD、OCX-V、OCX、TRINITY
• BRAINSTORM DCD-8
• Phasetech HD-7A
• Phasemation HD-7A192
• SFORZATO DST-01、DSP-03、DSP-05
• TEAC UD-503、NT-503
• SPEC RMP-X1
• Sound Warrior SWD-CL10
• TechDAS D-7、D7i
• RATOC RAL-DSDHA2
• M2TECH EVO DAC TWO PLUS、HIFACE EVO TWO
• CH Precision C1
• Aurender W20

10 minutes ago, Forehaven said:

JC, how in the heck does one 'add' a clock to say a router?  Most routers I've seen don't have a clock input/output.  

 

I'm not JC but "adding" a clock would mean that crystal oscillator (X1 as shown in the picture below) is removed and then it's connected to sCLK-EX with a clock cable afterwards.

 

 

As always we've gotta feed quality power to sCLK-EX and fortunately a single LPS-1 is capable of providing enough juice to to power all four outputs.

[JohnSwenson]

Hi All,

I'm sorry I haven't responded in this thread earlier, but I have been very busy (mostly with moving stuff), I have some general comments on what has been brought up here.

 

But first I want to thank Julian for his posts, he has done an awesome job trying to explain some of the complexities and confusion on this subject.

 

First a little more on the subject of clock types, word, master, reference etc.

 

Word clocks are low frequency, they are the sample rate, 44.1KHz 96 KHz etc. Thus a 10MHz clock is not a word clock.

 

The term "master clock" can mean several different things which leads to a lot of confusion. I recommend not using the term at all to avoid the confusion.

 

As discussed in this thread a high frequency clock can have at least three different "uses".

1) integer multiple of a word clock, DACs frequently have two of these, one for each of the sample rate families.

2) clock used for "processing" not related directly to sample frequencies

3) reference for a frequency synthesizer (which may be producing #2 frequencies)

 

Very few DACs actually use word clocks any more. Most of them use some form of #2 clock.

 

The Ref10 is designed to be a #3 source. For most audio equipment it cannot be used directly since not very many boxes have frequency synthesizers built in.

 

#2 are clocks that are fed into things like processors, USB receivers, PCIe boards etc. They are not related to audio sample rate frequencies at all, but observations seem to indicated that they can effect ultimate sound quality. The mechanisms for this are not known. I do have a theory I have been working on that explains this but it is very preliminary at this point and I'm not willing to make it public yet.

 

Clock synthesizers come in in many forms, some good, some bad, some cheap, expensive etc. I'm not going to go into all these types now. Until fairly recently they have not been particularly good and a decent crystal oscillator at the exact needed frequency would outdo a synthesizer.

 

But that has changed. Modern frequency synthesizers can be quite good. Thus IF your system is using one of these very good synthesizers, a device like the Ref10 can be very effective, just not universally effective, it depends very much on the system, what synthesizer it is feeding and what the out of THAT is feeding. The MC3+ series and the SOTM board seem to be good synthesizers so they will probably benefit from a Ref10.

 

The the MC3+ and SOTM do different things so it is important to understand the differences.

 

The SOTM board uses an off the shelf synthesizer chip with 4 outputs, each output can be programmed to be a different frequency, but they need to programmed at the factory. It has its own reference oscillator for the synthesizer, but it also has an external port. The board has to be programmed at the factory for the frequency of the external reference.

 

The MC3+ is designed to work with external audio sources. Julian can correct me if I get this wrong, but I think I understand its functioning. There is one frequency synthesizer, but it can have its frequency adjusted on the fly in large or very tiny amounts.

 

Both USB and S/PDIF inputs have their own clocks which run their respective chips. The outputs from these fill a FIFO in the FPGA, clocked by their own clocks. The samples are read from the FIFO using the output of the synthesizer.  The FPGA reads information from the receiver and sets the synthesizer to the correct frequency for the sample rate being received. The FPGA keeps track of the FIFO, if it is getting too full or too empty it changes the output frequency of the synthesizer by a tiny amount to keep the FIFO around half full. This means that the overall average sample rate is controlled by the input source, the quality of the clock generating the output stream is determined by the reference to the synthesizer. Thus a better reference (such as the Ref10) will lower the phase noise of the output data.

 

But what about asynch USB, isn't the DAC in control? Overall yes, the DAC has its OWN FIFO and also checks it, but instead of changing a clock frequency it sends a command back to the computer which tells it to speed up or slow down the average sample rate. So even though the local DAC clock is in ultimate control of the sample rate, as far as the MC3+/USB is concerned the USB data stream is in control, it just passes it on down to the DAC.

 

Now on to cable length. For a properly implemented system the only thing that should matter is the attenuation of the signal with increase in length. Note the "properly" in there, it is very important. Two things primarily affect this: attenuation at different frequencies, and reflections caused by impedance mismatch.

 

Because the Ref10 has a square wave output, the frequency characteristics of the cable are very important. Most coax cables have quite low attenuation at 10MHz, BUT a square wave consists of LOTS of higher frequency components, most coax has MUCH higher attenuation at those higher frequencies. The result is that even for fairly short lengths the high frequency attenuation will cause the rise time of the signal to decrease significantly, which means an effective increase in jitter at the receiver. This can be mitigated somewhat by using very high frequency low loss cable.  (which is much more expensive, but might be worth it if you need a longer distance)

 

The affects of impedance mismatch are much more complex. The mismatch causes a "reflection" on the line which causes a change in amplitude on the signal which sits on top of the original signal. The length of the cable determines where that change is going to occur relative to the original signal. Some cable lengths will result in a reflection that can cause a problem for the receiver, but the same reflection may not cause a problem for a different receiver circuit.

 

If the source board design and layout, cable, receiver board design and layout are all at the same impedance this will not be a problem. BUT the practice of unsoldering an oscillator from a board and soldering some coax to the pads is almost going to guarantee that you have a massive impedance mismatch, meaning you WILL have cable length issues, but it is almost impossible to figure out in advance what they might be. Again it may cause a problem, it may not.

 

I guess that is enough for now.

 

John S.

 

[rickca]

51 minutes ago, JohnSwenson said:

I guess that is enough for now.

Terrific post, very helpful.

 

John, when you have some more time ... SOtM claims there's an advantage to having the 4 clocks on an sCLK-EX board share an internal reference oscillator.  IOW these clocks are in some sense synchronized even though the 4 outputs can be programmed to different frequencies.

 

Does this make sense to you?

 

@romaz please advise if I have misinterpreted what SOtM has said about this.

[Keith_W]

1 hour ago, JohnSwenson said:

If the source board design and layout, cable, receiver board design and layout are all at the same impedance this will not be a problem. BUT the practice of unsoldering an oscillator from a board and soldering some coax to the pads is almost going to guarantee that you have a massive impedance mismatch, meaning you WILL have cable length issues, but it is almost impossible to figure out in advance what they might be. Again it may cause a problem, it may not.

 

 

Interesting discussion, thank you everyone. That SOTM sCLK-EX board recommends desoldering the oscillator, which as you say, will cause a massive impedance mismatch which may or may not make cable length critical. Given that the SOTM board is meant to be mounted in a 3.5" bay within the PC, won't this mean that the cable lengths will be very short, which would reduce the effect of cable length? 

[JohnSwenson]

2 hours ago, rickca said:

Terrific post, very helpful.

 

John, when you have some more time ... SOtM claims there's an advantage to having the 4 clocks on an sCLK-EX board share an internal reference oscillator.  IOW these clocks are in some sense synchronized even though the 4 outputs can be programmed to different frequencies.

 

Does this make sense to you?

 

@romaz please advise if I have misinterpreted what SOtM has said about this.

Yes they definitely are synchronized, there is one PLL that syncs to the reference clock then four fractional/N dividers, since the dividers all run off the same PLL they definitely are all synchronized, whether this is a good thing is debatable. It is good in the sense that you will not have any low frequency beat frequencies between clock outputs due to outputs wandering relative to each other, but noise from each output will tend to directly add since they are all in the same point in the phase noise from the PLL. Independent sources do not directly add since they are not at the same point in the phase noise. I don't really want to go into more detail on this, it is a 20 page dissertation and I really don't have the time now to do that.  

 

Neither of these things is probably that important.

 

John S.

[Confused]

6 hours ago, JohnSwenson said:

1) integer multiple of a word clock, DACs frequently have two of these, one for each of the sample rate families.

2) clock used for "processing" not related directly to sample frequencies

3) reference for a frequency synthesizer (which may be producing #2 frequencies)

 

Very few DACs actually use word clocks any more. Most of them use some form of #2 clock.

 

[Confused]

6 hours ago, JohnSwenson said:

As discussed in this thread a high frequency clock can have at least three different "uses".

1) integer multiple of a word clock, DACs frequently have two of these, one for each of the sample rate families.

2) clock used for "processing" not related directly to sample frequencies

3) reference for a frequency synthesizer (which may be producing #2 frequencies)

 

Very few DACs actually use word clocks any more. Most of them use some form of #2 clock.

 

I was a little confused when I first read this.  in point 1, 'integer multiple of a word clock' you state 'DACs frequently have two of these', then you state 'very few DACs actually use word clocks any more'.  Am I correct in interpreting this as that point 1 is more generally referring to older DAC designs, with more modern DAC designs being typically per point 2?

[Confused]

6 hours ago, JohnSwenson said:

Both USB and S/PDIF inputs have their own clocks which run their respective chips. The outputs from these fill a FIFO in the FPGA, clocked by their own clocks. The samples are read from the FIFO using the output of the synthesizer.  The FPGA reads information from the receiver and sets the synthesizer to the correct frequency for the sample rate being received. The FPGA keeps track of the FIFO, if it is getting too full or too empty it changes the output frequency of the synthesizer by a tiny amount to keep the FIFO around half full. This means that the overall average sample rate is controlled by the input source, the quality of the clock generating the output stream is determined by the reference to the synthesizer. Thus a better reference (such as the Ref10) will lower the phase noise of the output data.

 

OK - This is the bit that really interests me!  I am in the position that adding any kind of external clock or clock reference to my DAC is not going to be a practical prospect.  In my case I am running dual mono Devialet amplifiers, so modifying these to accept some kind of external clock would require some serious modification work and a lot of specific expertise.  Maybe it could be done, and I would be fascinated if anyone has any insight into this, but for now I am considering this as not a practical option. 

 

So, this effectively leaves me, and many others, as someone with a DAC that will not accept a word clock or external clock reference.  However, my understanding is that if I feed the DAC by it's preferred AES input, the clock signal is imbedded the AES feed and the DAC has the job of 'recovering' this clock.  It is this interface between the clock in the feed to the DAC and the DAC itself that interests me.  I am presuming that it should generally be a case of the better and more accurate the clock in the feed is, the better the DAC will ultimately perform.  Pure speculation now, but I am assuming this could be very DAC / system dependant?  From my own subjective experience I believe this to be true with the Devialet DAC / ADH.  As an example I have listened to a Devialet D800 with a dCS Vivaldi Upsampler, both with and without the optional dCS 'Master Clock', I thought it sounded better with the clock.  Plus, I run my own Devialet's with a Mutec MC3+USB via AES.  This sounds a lot better than anything I have fed the Devialet via USB.  So subjectively I have some experience this stuff works, and I can see there is some hard science behind this.  What I do not know is how big an improvement might be achieved by adding the REF10 to the Mutec.  This will clearly improve the clock accuracy in the feed to the DAC, but does this definitively result in an improvement in the DAC performance?

[rickca]

7 hours ago, JohnSwenson said:

Yes they definitely are synchronized, there is one PLL that syncs to the reference clock then four fractional/N dividers, since the dividers all run off the same PLL they definitely are all synchronized, whether this is a good thing is debatable.

Thanks, John.  That helps me understand what's going on.  I don't have the technical background to appreciate a 20-page dissertation!

[austinpop]

14 hours ago, JohnSwenson said:

But what about asynch USB, isn't the DAC in control? Overall yes, the DAC has its OWN FIFO and also checks it, but instead of changing a clock frequency it sends a command back to the computer which tells it to speed up or slow down the average sample rate. So even though the local DAC clock is in ultimate control of the sample rate, as far as the MC3+/USB is concerned the USB data stream is in control, it just passes it on down to the DAC.

 

Thanks once again for your insight, John. 

 

This was an aspect of asynchronous USB that I had not fully understood. Let's consider the scenario:

• sMS-200 (the USB source) > ISO-Regen (USB reclocker) > DAC

To repeat back what I understood from your writeup:

• The USB source still generates the data clock, and embeds it in the data stream
• The reclocker reads in and regenerates the data stream with its own (better) clock
• The DAC is the "master" of this communication, not in terms of being the clock generator, but rather in terms of controlling the average sample rate. As far as reading the data itself, the DAC still needs to lock on to the embedded clock in the data stream.

Perhaps this explains why even asynchronous USB DACs are so profoundly affected by the quality of the clocks used by the USB source and any regenrator(s) in the path?

 

Question for you:

• When the DAC, as master controller, sends a command back - to "slow down" the ave. sample rate, say - what are the magnitudes we are talking about, in % terms of the nominal sample rate? 1%, 5%, 10%?
• When this command is sent back, presumably both the USB source and the reclocker register the command, and honor the new rate? Or does that only fall to the USB source, with the reclocker merely passing through what it receives?

I apologize if this is going off-topic for the Ref 10, in which case we can take this elsewhere.

[JohnSwenson]

9 hours ago, Confused said:

I was a little confused when I first read this.  in point 1, 'integer multiple of a word clock' you state 'DACs frequently have two of these', then you state 'very few DACs actually use word clocks any more'.  Am I correct in interpreting this as that point 1 is more generally referring to older DAC designs, with more modern DAC designs being typically per point 2?

In this point I'm talking about what is internal to the DAC box, not an external connection. Most modern DAC chip circuitry don't actually have a word clock. Remember a word clock is a clock at the actual sample rate. Instead they use a clock that is some multiple of the sample rate, 22.5792 MHz and 24.576 MHz are quite popular these days. These clocks are integer multiples of all the standard audio sample rates. If you take a scope and probe around the traces in most DACs today you will find these, but not the actual sample rates (44.1KHz 96KHz etc)

 

Older DACs (and a few modern ones) DO you use real word clocks. The problem today with an external word clock signal is that is that since the DAC doesn't use it, the word clock has to go through a complex circuit to generate the clock frequencies above which are actually used.

 

#1 and #2 are used for completely different things. Look carefully at what I wrote, ALL the numbered points are high frequency clocks, none of them refer to word clocks, I covered that in the paragraph before. #1 are the high frequency clocks used by the DAC chips etc, #2 is used for things like the CPU on the motherboard, USB receiver chips etc, #3 is a reference used by a clock synthesizer to generate #1 or #2 clocks.

 

John S.

[JohnSwenson]

9 hours ago, Confused said:

 

OK - This is the bit that really interests me!  I am in the position that adding any kind of external clock or clock reference to my DAC is not going to be a practical prospect.  In my case I am running dual mono Devialet amplifiers, so modifying these to accept some kind of external clock would require some serious modification work and a lot of specific expertise.  Maybe it could be done, and I would be fascinated if anyone has any insight into this, but for now I am considering this as not a practical option. 

 

So, this effectively leaves me, and many others, as someone with a DAC that will not accept a word clock or external clock reference.  However, my understanding is that if I feed the DAC by it's preferred AES input, the clock signal is imbedded the AES feed and the DAC has the job of 'recovering' this clock.  It is this interface between the clock in the feed to the DAC and the DAC itself that interests me.  I am presuming that it should generally be a case of the better and more accurate the clock in the feed is, the better the DAC will ultimately perform.  Pure speculation now, but I am assuming this could be very DAC / system dependant?  From my own subjective experience I believe this to be true with the Devialet DAC / ADH.  As an example I have listened to a Devialet D800 with a dCS Vivaldi Upsampler, both with and without the optional dCS 'Master Clock', I thought it sounded better with the clock.  Plus, I run my own Devialet's with a Mutec MC3+USB via AES.  This sounds a lot better than anything I have fed the Devialet via USB.  So subjectively I have some experience this stuff works, and I can see there is some hard science behind this.  What I do not know is how big an improvement might be achieved by adding the REF10 to the Mutec.  This will clearly improve the clock accuracy in the feed to the DAC, but does this definitively result in an improvement in the DAC performance?

The circuitry used to generate clocking in a DAC from a S/PDIF (AES3 etc) feed vary radically from DAC to DAC. There are many different ways to do it and they all have very different "clock transfer functions", how the quality of the input signal affects the clock feeding the DAC chip. I have no idea how the Devialet does it so I can't come up with even a speculation as to what that transfer function might be.

 

The only thing I can say is that the phase noise of the signal coming out of the source usually DOES have an impact on the phase noise of the clock fed to the DAC chip, exactly what the mechanism is and by how much will vary dramatically from DAC to DAC. There is no way to make generalizations here.

 

One note, you use the term "accuracy" several times, to me the accuracy of a clock refers to how close the actual frequency of the clock compares to what it SHOULD be. Generally in digital audio this has very little to do with anything. A little bit off is not going to make any audible difference. The phase noise of the clock (another way to talk about jitter) is what matters, not the accuracy.

 

John S.

[JohnSwenson]

1 hour ago, austinpop said:

 

Thanks once again for your insight, John. 

 

This was an aspect of asynchronous USB that I had not fully understood. Let's consider the scenario:

• sMS-200 (the USB source) > ISO-Regen (USB reclocker) > DAC

To repeat back what I understood from your writeup:

• The USB source still generates the data clock, and embeds it in the data stream
• The reclocker reads in and regenerates the data stream with its own (better) clock
• The DAC is the "master" of this communication, not in terms of being the clock generator, but rather in terms of controlling the average sample rate. As far as reading the data itself, the DAC still needs to lock on to the embedded clock in the data stream.

Perhaps this explains why even asynchronous USB DACs are so profoundly affected by the quality of the clocks used by the USB source and any regenrator(s) in the path?

 

Question for you:

• When the DAC, as master controller, sends a command back - to "slow down" the ave. sample rate, say - what are the magnitudes we are talking about, in % terms of the nominal sample rate? 1%, 5%, 10%?
• When this command is sent back, presumably both the USB source and the reclocker register the command, and honor the new rate? Or does that only fall to the USB source, with the reclocker merely passing through what it receives?

I apologize if this is going off-topic for the Ref 10, in which case we can take this elsewhere.

In USB data comes in packets, for digital audio using high speed mode there is a LOT of idle time in between the packets, this means that the actual rate at which bits are transmitted is actually irrelevant. For any given system the bit rate stays constant, the packet rate stays constant. The data rate is determined by how many samples are put in each packet. The source can vary this from packet to packet to achieve a very fine resolution in the average sample rate. This is important to understand, at no point is any clock frequency actually changed, it is all done by number of samples per packet.

 

In the actual transmitted signal each packet starts out with a number of "sync bits", this is a square wave at the bit frequency, the receiver uses this to lock its internal PLL to the bit rate in that packet. This is done for every packet, so even if there is some drift in clocks in either transmitter or receiver the receiver can lock onto the rate the data is coming over the wire.

 

None of these clocks are used to generate the "audio clock". The above clocks are used to put data into a FIFO, the local audio clock (a type #1 in my list) is used to grab the data from the FIFO.

 

Exactly how phase noise from the source gets into the audio clock is the subject of my current research which I am not ready to discuss yet so that is as far as I am going to go right now.

 

As far as how big is the change when the DAC tells the computer to change rates here are some actual numbers I have seen. These are from linux systems which usually let you know what the actual sample rate is. For example if the computer is sending a 44.1KHz signal, if no feedback has occurred it will think the actual rate is 44100. But if the computer's clock is a little off from the clock in the DAC, the DAC tells the computer to change it. I have frequently seen this go to 441001, or 44099, up to 441004 and 44096. 

 

John S.

[jelt2359]

I'd like to say a big THANK YOU for John in joining this thread; your insights have been simply invaluable. It looks like I'm not the only one who knows very little about clocks, so all this is incredible information. I understood from your reply earlier about cable lengths that what seems to matter most is not length, but the quality of the cable, and constant impedance throughout that matches the impedance of the clock (75 or 50 ohms). Is this right? 

[julian.david]

5 hours ago, jelt2359 said:

I'd like to say a big THANK YOU for John in joining this thread; your insights have been simply invaluable. It looks like I'm not the only one who knows very little about clocks, so all this is incredible information. I understood from your reply earlier about cable lengths that what seems to matter most is not length, but the quality of the cable, and constant impedance throughout that matches the impedance of the clock (75 or 50 ohms). Is this right?

 

Hi there,

 

I definitely agree regarding @JohnSwenson's input in this thread! Kudos for putting so much effort into explaining these complex topics in such a concise way. Can we hire you?

 

Regarding your question:

This part of John's explanations below is critical...

On 22/06/2017 at 3:22 AM, JohnSwenson said:

Because the Ref10 has a square wave output, the frequency characteristics of the cable are very important. Most coax cables have quite low attenuation at 10MHz, BUT a square wave consists of LOTS of higher frequency components, most coax has MUCH higher attenuation at those higher frequencies. The result is that even for fairly short lengths the high frequency attenuation will cause the rise time of the signal to decrease significantly, which means an effective increase in jitter at the receiver. This can be mitigated somewhat by using very high frequency low loss cable.  (which is much more expensive, but might be worth it if you need a longer distance)

 

The affects of impedance mismatch are much more complex. The mismatch causes a "reflection" on the line which causes a change in amplitude on the signal which sits on top of the original signal. The length of the cable determines where that change is going to occur relative to the original signal. Some cable lengths will result in a reflection that can cause a problem for the receiver, but the same reflection may not cause a problem for a different receiver circuit.

 

I would certainly agree with his statement to the effect that both quality and length of the cable are important to transmit the 10 MHz square wave of the REF 10 without significant losses. Ultimately though, the longer the cable needs to be, the higher the quality should be to avoid excessive losses!

 

Julian

[pam1975]

thanks @julian.david but what is considered long here? over 1m, 2m, 10m?

[julian.david]

1 minute ago, pam1975 said:

thanks @julian.david but what is considered long here? over 1m, 2m, 10m?

 

Here's an early post I wrote:

 

On 10/06/2017 at 10:18 PM, julian.david said:

 

 

 

Hi all,

 

Sorry I've been absent from this discussion for a few days, but I'm glad to see so much interest in the REF 10. I'll do a bit of catching up in the next couple of posts. 

 

Let's start with cable length:

It's extremely difficult to give a clear-cut answer to this question since cable length is not the only parameter here. The type of cable, connectors, as well as the impedances on both ends (sender & receiver) also matter. But let's focus on two aspects:

1. The 10 MHz signal of the REF 10 (or any other 10 MHz clock) is not the same as a "ordinary" word clock signal. The whole purpose of the REF 10 is to provide an ultra low phase noise, super precise clock signal that can be used to enhance existing clocks and DACs. Thus, you can't just apply the same rules that have been valid for word clock signals. 
2. While you can theoretically run a word clock signal up to 250 meters that doesn't mean that you should do so if you can avoid it. Antelope can claim what they deem appropriate, but you bet that there will be significant losses when you run a 250 m stretch and expect precise clocking. Sure, you'll still get some signal out of it and it may still be acceptable for broadcast, but we would never recommend a cable run like that for audiophile purposes.
3. It's not a fail/don't fail, binary type of situation here. So it's not like we can give you a specific length at which point it just "won't work anymore". But you will start to see negative effects of the cable building up as the length increases. Imagine a "rounding off" of the super sharp square wave that the REF 10 puts out which makes the lock point less precise. 

So while we have recommended a maximum cable length of 10 meters for the word clock (!) signals in the MC-3+ manual, we definitely recommend keeping the cables for the REF 10 as short as you possibly can. What exactly does 

mean? Well, again it depends, and we're still working on an official statement, but I'd try to keep the length to no more than 1-3 meters. Shorter is better. We were using 0.5 meter cables at High End recently and that worked remarkably well. 

 

I'm hoping we can show you guys some data to illustrate the effects of cable length, but our focus is shipping product at the moment, which I'm sure some of you will appreciate. 

 

Onwards...