Streaming Digital Audio Via the I²S Connection

Streaming Digital Audio Via the I²S Connection

Written by Tom Gibbs

I’ve talked a lot in recent articles about ditching the digital disc playback model and moving towards a strictly streaming-based setup for all my digital playback. I haven’t gotten rid of any of my discs (CD, SACD, DVD-Audio, Blu-ray), but I have ripped them all to either lossless FLAC (CD/DVD/BD) or DSF (SACD) files – which means that I’ve eliminated the need for any suboptimal playback from my disc player’s internal DAC. I simply stream them now via a USB connection from my music player/streamer directly to my DAC, using Roon as my library organizer, and the sound quality is outstanding, to say the least.

But a recent, out-of-the-blue occurrence had me rethinking my setup, when I was contacted by an artist rep with an offer to review an upcoming new album that was being released in a multitude of high-resolution formats. No physical media was yet currently available, but I was given a variety of download formats to choose from, and I chose the DXD (Digital Extreme Definition) 352.8 kHz, 32-bit PCM digital file. Which was a monstrous download (7.5 gigabytes!) that took freaking forever – and of course, this would be my very first experience with DXD files. I never gave the first thought as to whether my system would (or could) provide playback for the files.

Of course, the answer was “no,” found out immediately when I contacted PS Audio to confirm my Stellar GainCell DAC’s capabilities via the USB interface. It was limited to 24-bit via USB; the USB connection also restricted my DSD playback to DOP (DSD-over-PCM). The DOP part didn’t bother me too much, as it’s very commonplace. That is, until Dalibor Kasac, my contact at my music player/streamer manufacturer Euphony Audio informed me that they really didn’t care for DOP, and that any file conversion had losses. Native DSD was always preferable to DOP. On the positive side, he reassured me that the Euphony equipment was perfectly capable of handling the 32-bit DXD files. He also told me they were working with another European company on an outboard USB-to-I²S interface to add I²S functionality to the Euphony Summus and Endpoint equipment, but that it was still in the developmental stage and somewhere down the road.

The saving grace for me in the whole album review bit was that the release date wasn’t until mid-August, so I had some time to regroup and figure things out. I started digging around on the internet, and found several different Chinese-made USB-to-I²S converters available, but some of them looked really flimsy at best. I ended up settling on this model, the Douk Audio U2 Pro USB Digital Interface; they apparently rebadge their products for a number of Chinese resellers, and I’ve seen this same product under about at least a half-dozen different brand names online (and at a wide range of price points!). Douk Audio calls the device a USB Digital interface, and that is technically correct. There is no digital conversion going on, the device simply extracts and re-clocks the digital signal from the USB input, then presents the signal to your choice of I²S, coax, or toslink digital outputs. The I²S output  is essentially an HDMI connector, though there are varying viewpoints on the oversimplification of the differences between I²S and HDMI. HDMI seems to not really have anything to do with I²S, it just offers a really useful connection interface for the signal.


The Douk Audio U2 Pro isn’t a converter, it’s an interface that simply re-clocks the digital signal.


I²S apparently has been around for decades; I’ve included a link to a really informative video from PS Audio’s own Paul McGowan that talks at length about the I²S interface and why PS Audio has chosen it for all their DACs and transports. Paul’s explanation really helps to demystify the whole I²S connection thing.


The Douk Audio Digital Interface is actually a very useful device; it extracts and re-clocks the digital signal using a pretty impressive selection of high-end parts that you wouldn’t expect in a product that retails for just over $50 USD. And you have a choice of outputs available; you can use the interface for not only I²S, but also for coaxial and optical digital connections. All outputs are simultaneously active, so you can feed several different digital devices at the same time with no problems. The I²S output delivers a signal that is capable of PCM replay up to 384 kHz and 32-bit, and offers DSD playback natively in both DSD 64 and DSD 128. In my internet searches, I frequently came across an Italian-made device for this kind of interface that was more than double the price, but had frequent complaints from users. The Douk Audio unit actually had information on the site concerning the Italian device, and it appears to me that they probably reverse-engineered it to create their own. There’s also verbiage explaining how they improved upon the Italian model with better parts selection and implementation. For $56 and free shipping, I didn’t hesitate to pull the trigger; at the very worst, I’d just box it back up and return it to Amazon; no big deal, right?



It actually turns out that the Douk Audio device is a very big deal; upon its arrival, I unboxed it and marveled that it actually appeared to be very well made. However, expecting nothing but trouble right out of the gate, I took it downstairs anyway and inserted it into my system. The I²S interface requires power from either a 5V wall wart power supply (not included), or it can draw power from a USB connection (I wouldn’t recommend that setup). I have several linear power supplies in my system, and one of them, a Keces P8, has an available 5V USB power port, which has worked perfectly for my needs.

I had recently contacted Stephen Mejias at AudioQuest to ask him about HDMI cables and how they related to the I²S connection in various digital devices. While helping me wade through the technical details, he also essentially told me that virtually every HDMI cable made would work with I²S, but of course, you want to get a decent-quality HDMI cable. Stephen very generously sent me a couple of the latest AudioQuest 48G HDMI cables, a Cinnamon and a Carbon; I chose the Carbon because of its higher silver content. I connected everything to the Douk Audio interface, and then to the Stellar GainCell DAC, and to my great surprise, it immediately worked perfectly!


AudioQuest offers a huge selection of well-made cables; I’m currently using the Carbon 48 HDMI in my system.


I’ve read for years that the USB connection is an inferior one, and tends to really muck things up, but in general, it’s a good compromise for digital audio. And there are many recent advances in USB-related technology, and many ancillary devices are available to help remove noise from USB connections. I have USB isolators and a galvanic isolation device in my USB signal chain, but with the Douk Audio interface in place, those devices no longer offer any benefit. And are completely unnecessary; the sound quality without them in place is unaffected by their absence. The Douk Audio interface has actually simplified my playback significantly, by allowing me to eliminate extraneous devices, and by simultaneously eliminating unnecessary signal conversions.

I’ve only experienced one hiccup during the process. When playing through the Douk Audio interface to my Stellar GCD’s I²S input, the GCD doesn’t display any signal information. Again I contacted PS Audio about this, and they told me that it’s a quirk of the GCD; in certain circumstances, it won’t display the correct (or any) bit or sample rate information. Well, that’s unfortunate! As a fail safe, I contacted Euphony Audio in Croatia; my streaming system is network connected, and they can remote into my system at any time to observe playback protocols. Zeljko Vranic at Euphony was able to confirm for me that the throughput from my setup was presenting both 24-to-32 bit PCM and native DSD signals to the Gain Cell DAC, and that they were playing as delivered. Sweet!

PS Audio Stellar GainCell DAC.


So how does it sound? In a word, astonishing! If I had any doubts about the street cred of my PS Audio Stellar GainCell DAC, they’ve been completely eliminated by this experience, and the sound quality of the GCD has been totally legitimized. I’m now experiencing the cleanest, clearest, least-gimmicked and most musical digital sound my system has ever delivered; the addition of a $56 gadget from Amazon and China has been nothing short of transformational. All the DSD files I recently ripped from my SACD collection sound more impressive than ever, and that DXD file I mentioned earlier – it’s easily the best sounding digital file to ever play across my system. It takes the illusion of reality to an entirely new level of believability– it’s really that good. DXD was developed to allow for easy editing of SACDs as the one-bit DSD format wasn’t easily editable. As good as DSD is, DXD is on another level altogether. And believe it or not, my FLAC rips of 16-bit/44.1 kHz CD’s have taken on an improved level of clarity and transparency– via the I²S interface, they sound better than ever.

The I²S interface has its detractors, one of the principals being a guy named Amir who runs the Audio Science Review website. Trust me, you don’t want to get me started about what I consider the pseudo-science employed at ASR. I encourage you to make up your own mind. I²S connections are rapidly becoming the standard, especially in the excellent new crop of affordable DACS from the likes of Topping and Gustard. As far as I’m concerned, I²S is the ultimate connection for digital audio.

Header image courtesy of Wikimedia Commons/wdwd. Other images courtesy of the author and PS Audio.

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