Importance of clocking

@emergingsoul 

Perhaps I wasn't clear, I much much prefer a clock within the DAC than an external clock source. The better DACs have an extensive clock design. I believe something that separates a good DAC for a great one (both using the same DAC chip) is the (internal) clock circuitry, the power supply and the output stage of it.

@emergingsoul  +1. Any external clock will have to be connected by a cable (50 or 75Ohms) of course. This will cause smearing of the clock signal, it is not the frequency of the clock but the rise and fall times of the edges that are difficult to control without introducing several issues like overshoot/undershoot of the transitions (could cause more noise / jitter) and uncertainty (frequency / phase shift / linearity). In my book the clock source needs to be as close as possible to the DAC IC with proper PCB impedance control. An external clock source is required in a studio recording environment in order to sync several devices together.

DCS, Weiss, Meitner to name a few. But you are missing the point, the external clock is much further away from the DAC IC (assuming an integrated type) than the one inside the DAC unit. You have the extra cable to feed the external clock to the DAC unit, the length of the cable, the impedance, exact termination need to be taken into consideration. In the case of a square wave clock output, as I said before, the edges are extremely fast and need to be contolled.

Sure they do, they offer more options to the end user, you take your choice.

Andy has a very good point. In the case of SPDIF or AES the clock is embedded in the data stream is more prone to jitter therefore the clock of the source/streamer is critical and IMO could benefit from a good reclocker with a good clock cct and its associated PSU. There are some transports that have a clock input so that the clock from the DAC can be used to clock the data in the transport. As Andy says async USB is supposed to eliminate this issue since the data is clocked by the DAC. Again I agree that a good reclocker/DDC could clean up the USB jitter (noise) but it all depends on the quality of the equipment. The reclocker’s clock needs to be of superior quality to that of the DAC’s one. I believe I2S is the best interface available, it was first designed as an interchip protocol but I see lately is being used to connect sources to DACs. That is fine as long as the interconnects are kept as short as possible.

The issue with S/PDIF or AES "reclockers" is that you have two clocks arguing over what should the absolute clock rate be.

The DAC is forced to take one of two approaches: Abandon it’s internal clock or attempt to keep it’s internal metronome and "fix" upstream deviations from it’s own mechanism.

How does the DAC know? What do you mean by 'abandon it's internal clock'? Are you talking about the external clock input of a DAC substituting its internal one?   Don't you agree that the better (more accurate with less jitter/noise) the incoming data on AES/SPDIF, the easier the job of the DAC's internal clock?

@kijanki  Thanks for the explanation. My DAC is an older Weiss DAC202 that uses ARC. It has extensive fairly advanced PLLs (it uses a wider range digital PLL (I don't know its noise/jitter spectrum) and its output is fed to an further analog PLL to reduce the jitter of the first PLL. I believe both PLLs are implemented on a single DICE IC which was state of the art back then. I used to have a datasheet but I must have lost it during a move so I uncertain of the exact operation. The DAC uses older ESS converters and I think they always run at 1.536MHz. To complicate the clocking scheme further, the AES output of my source feeds a Trinnov ST2 Hifi (digital room correction device) which also uses a DICE generated clock and its AES output then goes to the DAC.  I need to refresh my memory!

My issue is that my DAC doesn't have a USB i/f and therefore I need a streamer with either an AES/SPDIF output or an additional DDC to convert the USB.

Your thoughts please?

@kijanki Thank you for the response. I enjoy reading your replies since you seem to understand electronics better than most on this site. Sometimes it is difficult to have a technical conversation with people who have little knowledge even about the basic concepts but they call themselves experts. I find audio a strange field... Your analysis on transmission line is spot on. For spdif I rather have proper 75 Ohms BNC terminated cable than with RCA plugs, I had an argument with somebody who was saying it is only audio, there is nothing high frequency so no need to worry about reflections, what he forgot is that reflections are caused by the rise/fall time of the edges and not the waveform frequency or data rate. Anyway, I diverted :)

Yes, yes, I much rather use Firewire than USB, in fact I still use a PC (with very little optimazation) with a PCIe Firewire card into the Weiss. But Firewire is getting old and new device of course don’t support it. I also use the Weiss AES i/f which is not too bad, almost as good as the Firewire. As I am interested in Qobuz, my next purchase is going to be a streamer, the way I see it is either fully optimize my server (both hardware and software), use the Firewire card and hopefully find some software that will support it, or buy a relatively decent streamer like Lumin/Innuos/Aurender and possibly a DDC as USB look like being the most common i/f these days. Unfortunately where I currently live there is no way to demo anything so I have to buy by what I read on forums and reviews... not the best way.

@kijanki Yeah, I think AES will be. Now I need to find a good streamer with AES output up to 4,000 Euro. I have been going round and round this subject for far too long without making a final decision...

@antigrunge2

I apologize for taking over your thread.  It seems that I am causing few disputes that  I didn't mean to.

@lalitk 

Although I rarely post over here I have read quite a few of your views and I respect your views and experience. Having said that you are saying my views are based on 'documentation' (whatever that means) and no direct experience. Ok if that's what you think. You have your opinion, I have mine and in this instance we disagree. Btw I don't dispute what you are hearing. It's all good.

I am out of this thread.

@antigrunge2 

I said I am out of this thread but I will answer you.

In my case I have an older DAC that is still excellent and I am reluctant to change it but it doesn't have USB i/f. therefore I am looking for AES or SPDIF i/fs. You say USB is the better i/f and you could be right but I think it really depends on how the DAC has implement the i/fs. With some DACs I still prefer AES over USB. Also some DACs are agnostic of the interface (please see kijanki's explanation further up this thread.

@lalitk

No problems, thanks for clarifying. I love it that people are so passionate about this hobby over here. In my engineering mind an external clock in the home environment could cause more trouble ie introduce jitter, impedance mismatch, etc BUT lots of things don't make engineering sense in our hobby but do work... like cables. So I hear you. I have tried a very good clock source from work in my system at home and the result was inconclusive, to be honest I don't think I could hear a difference. As you say it could depend on implementation and system specifics. I am glad both you and @antigrunge2 have improved your systems with an external clock source and you are enjoying them more.

Guys,

it all depends what is understood by the term 'clock accuracy'.

For example rubidium clocks have very poor short term stability, they jitter by design. This makes them totally unsuitable for a short term time base reference. Their big usage is due to their long term stability where the time has to be correct after a long time without synchronization (eg military applications). The long term frequency stability, the clock 'accuracy' is defined in ppm (part per million). The highest clock accuracy is not required for hifi applications.
An OCXO (or TCXO)  has a better short term stability (phase noise) but is doesn't have the accuracy of the rubidium. .
As in hifi the real long term stability isn't important, the better clock source is a very high quality OCXO or a TCXO.

My point is that the absolute accuracy of the clock is not as important as other aspects of it (phase noise, voltage stability, jitter, etc).

@antigrunge2 In your discussions with @nigeltheflash you are both  referring to clock accuracy, I was merely pointing out that in our hobby the clock accuracy is not the most critical parameter. That's all.

@lalitk Could you explain why the accuracy of the clock (which is defined by the long term stability in ppm) is as important? Genuine question.  For example the most accurate clock that I know of is cesium atomic clock which is approximately 1 -  3 part in 10^14. Is it relevant, does it matter for HiFi?