nubiann
This is a very long read, ignore if you wish. I have made this a longer more complete post on purpose, since it comes up several times each month.....and it gets old re-posting the same information in different ways. Most of this is from my personal experience and education, and not some form of conjecture. This also is a long post, so some discrepancies may exist….I’m not an electrical engineer (I’m mechanical and industrial)
I'd also suggest (highly) that anyone watch the video from "A British Audiophile" as I think Tarun does a nice job of explaining a bunch of these principles https://www.youtube.com/watch?v=3tKWWs-OMxw
As you can see from all the varying posts, there is disagreement as to the benefits of I2S, the use of a DDC, better DAC versus better streamer, which clock is most important, and whether I2S/SPDIF/AES-EBU/USB is the best connection to be used. In the end we all just want the cleanest, jitter free, most accurate, best clocked data to be used.....regardless of where it comes from, that is what provides the best sound quality. Many of these posts are not incorrect (though many are), because in that poster’s experience it is true.
There is NO sweeping generalization that can be made, the answer in almost all cases will be "it depends" or YMMV.....and to say that I2S is most important or unimportant connection is also incorrect, it depends. I however (correctly or incorrectly) feel that it is important to understand the underlying architecture, to learn why these variations exist.
And then we have external clocking devices that set a universal clock to be used across multiple devices, so that it remains a constant. I have no experience with external global clocks, so I'll make no comment.
Below is a post I made a few weeks ago about a DDC and DACs that another poster was asking about, that explains some of what I'm talking about. You can also see (from posts here and elsewhere), that the better the streamer and the better the DAC, the less need (or no need) for a DDC. But I will say that in each chance I've had to use I2S (all else being equal), it has been the best connection; sometimes by a lot. I would venture to say that most highly regarded inexpensive streamers (IE Bluesound, Wiim, iFi, Eversolo), could benefit from a DDC….many of us have found this to be the case.
However the more expensive (better) DACs and streamers are optimized for different connections. Some sound best with USB, some AES/EBU, some SPDIF......it varies by product and how they were designed, where the effort and expense was put. In other words the highest quality devices basically do internal DDC functions without the need of an external unit. This is why you see posters saying USB or I2S or AES/EBU or SPDIF is the best, in their experience; that is true. But it is not universally true.
On the lower end (read that as cost if you wish), none of the connections are properly optimized as that would add to cost. To say that I2s output from an average streamer negates the need for a better DAC would not be true. In fact that could be the worst case scenario, as the clock on I2S USUALLY comes from the sending device....garbage in, garbage out from a lower end streamer.
In my personal situation, I still own a Denafrips Pontus II DAC because 3 years later I still LOVE it's sonic signature; it still is one of the most fun, enjoyable, easy to listen to DAC I've ever heard, with excellent presence. It's connections (and clock) are not optimal, it's an inexpensive DAC at the $1,450 I paid for it, and there are better options (more expensive) available if level of detail and nuance is your criteria.
What does help my scenario (and many others), is that I/we use a DDC to take the signal from a cost effective streamer, convert it/clean it/de-jitter it/re-clock it, and send that much improved signal to the DAC via I2S where it remains clean. I2S has separate pins for the master clock, word clock, and data stream; this is known as a "synchronous" connection.
Other connections such as USB, SPDIF, and AES/EBU are usually "asynchronous" which means the master clock, word clock, and data are all combined in one interface. NOTE: Sometimes AES/EBU can be either “synchronous” or “asynchronous” depending on it’s implementation. This information must be separated at some point either by a DDC or by the DAC, into separate word clock, master clock, and data. Higher quality gear does this very well.
When we hear many (myself included) talking about the need or value of higher end gear, it is because they generally use much better power supplies and components to do this job very well. It is not to say that you HAVE to go that high, but if you have a “mid fi” or “hi fi’ system, the better gear will provide noticeable sound quality improvements. That is not a knock on cost effective gear, it’s a fact most of us recognize or have learned in listening.
Later this year I will be upgrading both my DAC and my streamer to rather expensive units, about $15,000 to $20,000ish combined for both units. In that instance, both the streamer and DAC will have optimized connections; though some will still sound better than others and what I'll have to discover. I suspect in this case that I may not have the need for a DDC any longer, in fact I expect that I will not need one. Also high end gear may have other benefits such as the ability to shut off certain aspects (ability to remain or turn off clocking in the DAC).
QUOTE: "You are in slightly different territory right now than my and Soix experience, as your Venus 15th DAC already uses the high end OCXO (oven controlled crystal oscillator), and Fred's Terminator 15th uses dual OCXOs. So if you were to add a DDC, it would have to be the Denafrips Hermes or Gaia so you don't downgrade the clock.
Typically (though not always) the DDC sets the clock, and the DAC is the slave to that clock when using I2S. With USB and the other connections, the DAC clock is most frequently used.....in my experience (and a bunch of others), I2S is the superior connection. I've found the same in other systems that I've helped friends set up.....using various Denafrips DDCs. The more expensive the streamer, there is possibly less of a need for a DDC; possibly.
Part of the reason that the Pontus is improved with the Iris, as the Iris has a TCXO (temperature compensated crystal oscillator) which is an upgrade from the Pontus' standard FEMTO (voltage controlled) clock (short for Femtosecond, or one quadrillionth of a second). Standard FEMTO clocks are not temperature or oven controlled, so their accuracy varies with temperature changes. The clock upgrade however is only part of the advantages of a DDC.....the Denafrips Iris ONLY has USB inputs; no AES, RCA or optical SPDIF, the Gaia and Hermes allow for those.
The other advantages of a DDC are: isolates the input signal (USB, AES/EBU, RCA or optical SPDIF), filters the signal both galvanically (magnetic) and optically, and re-clocks it with the superior crystal.
Digital devices use I2S internally to transmit data, using I2S to transmit from the DDC to the DAC allows for one less conversion. There is some debate on this, as I2S correctly was never designed to be transmitted over a cable.....but here we are. Also it is correct that there is no standard for I2S pinouts, but many use one of two more common formats.....and if not, most devices allow for I2S pin configuration. Most Chinese products use a similar I2S pin configuration. In my experience I2S cables should be of very high quality and as short as possible…..this is somewhere I’d absolutely look to spend money on better cables.
If it were me (and I will be upgrading soon), I'd buy an Innuous/Lumin/Aurender/Grimm/etc high end dedicated streamer and see where you land. With one of these high end streamers, you may find you do not need a DDC......and you can always add one later if you wish.