Apologies if I am digging up the same old subject, not only from other members, but from me too! Looking at getting the Innuos Stream 3? (For file storage, possible future Roon Core and the Sense App).
Without getting too technical, please could you guys give me your thoughts on adding the Phoenix USB upgrade Board to the Innuos Stream 3.
Apparently, it actively regenerates and re-clocks the USB signal using a high-precision 3ppb 24MHz OCXO clock for superior timing and signal integrity.
It also Incorporates ultra-low noise LT3045 linear regulators for independent voltage stages, supplying a clean 5V line directly to your DAC’s USB input without switching regulators on the main chip.
However, I am reliably told that dCS DACs, (in my case a Bartók Apex), does most of this anyway and doesn’t actually need a 5V line?
This isn’t a question about Streaming Transport products as such, but the £1,300 Phoenix USB Board upgrade.
My personal view is that the 24 MHz USB clock is effectively irrelevant to the final timing of the DAC output. It is a separate clock domain from the clock that determines when audio samples are converted in the DAC, so reducing jitter on that line will not have an impact on DAC output. The Innuos will be generating the reference clock for the USB PHY, but that is separate and completely unrelated to the audio clocks used by the DAC.
Note that the USB PHY frequency is 24 MHz, not 24.576 MHz as used by audio clocks, so unrelated to any audio frequency. Not that it matters in practice as it is abstracted from the audio clocks anyway.
There are some theories that lowering jitter on this USB line can create a better environment for the DAC, not because the jitter passes onto the DAC’s clocks, but because it would represent unwanted spectral content arriving at the DAC.
Ignoring for a moment the fact that the USB input on a dCS DAC is properly galvanically isolated and any 24 MHz USB activity is on the other side of this isolator, this theory is as far as I have been able to find, completely devoid of any valid evidence. The only source I have been able to find even remotely related to this topic was a Master’s thesis from Chalmers University on Digital Audio Interface Jitter, but it does not specifically talk to the impact of jitter on the USB PHY reference on resultant audio clock jitter or DAC output and makes no conclusions. It is simply the one and only published source that even touches the subject of USB jitter related to audio that I can find, which is in itself, very telling.
More than happy to be pointed at evidence I have not come across, but based on all current valid engineering sources that I have access to, I have not seen anything to suggest that improving the jitter performance of the 24 MHz USB reference clock, or improving the power supply used to drive the various USB elements, will have any impact - positive or negative - on a connected DAC that has an appropriate USB implementation.
To (mis-)use the OSI model just by way of explanation, USB Audio is asynchronous (at least as we and most others use it, it can actually operate in both synchronous and asynchronous modes) at the application level (OSI L7) but at the transport level (OSI L4) it is synchronous.
Wasn’t it Andy McHarg of dCS who pioneered the asynchronous use of USB for audio interfacing of a DAC and developed DOP? So yes, our use of it nowadays for DACs is generally asynchronous.
Exactly. This kind of product might be relevant for lesser products but the Bartok Apex is not going to gain much from the innuos board in respect of timing. likewise the 5v supply which is only appropriate for end products that need this as a power supply such as certain portable DACs ( e.g. Audioquest Dragonfly).
Also, while Gordon is often credited alone, if I’m not mistaken he collaborated with (the late) Charlie Hansen of Ayre Acoustics, and the initial implementation of Async USB appeared on both Wavelength and Ayre DACs at about the same time. Those days everyone was on Audio Asylum.
I owned the Scarlatti stack at the time, not sure if I’m remembering correctly but the Upsampler’s USB Type-B Port didn’t initially come with Async USB support, it came slightly later with a firmware update. At the time I remember thinking USB, like TOSLink, was for amateurs (and still do ).
If you plan to use USB out of Innuos to USB in of your DAC, go for it to get pleasantly surprised. Hygiene in the digital chain adds to sonic benefits always.
Looking at getting the Innuos Stream 3? (For file storage, possible future Roon Core and the Sense App).
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