I2S – pronounced eye-squared-ess for those unfamiliar with the term – is a serial digital signal bus scheme originally intended for board-level IC-to-IC transfer (integrated circuit). Now, an increasing number of audio equipment manufacturers are specifying I2S as a digital interconnect for use, for example, between a CD transport or a streamer and an associated DAC as an alternative to the much more commonly used S/PDIF (coaxial and optical) or AES/EBU (XLR) and USB schemes.
I2S is gaining popularity because it is said by some to be technically superior. Rather than multiplexing music and clock data for transmission down a a single conductor as S/PDIF and AES/EBU do, it separates the data streams and gives each of them its own conductor. This compartmentalisation, so the theory goes, can result in less jitter.
Basic I2S requires three conductors: one for the word select or left/right clock, one for the music data, and one for the bit clock. When used internally an I2S link is most likely to be soldered at both ends. When it is pressed into longer-distance service as an interconnect between components in an audio system it requires the cable to be terminated in plugs to facilitate DIY system configuration by users. Unhelpfully, there is no standard form-factor for I2S connection in this role. Some audio equipment manufacturers can be found using RJ45 plugs and sockets, but most review items that I have had through my hands in the past year and that supported I2S have been fitted with HDMI sockets.
There is a certain logic to this choice. HDMI cables are readily available and can support high definition data – in their conventional role it is of course video data. Moreover, inside their sheathing HDMI cables contain 23 conductors – plenty enough for use as an I2S conduit, even when, as is common, the three plus data lines are each mirrored with a minus line for noise-cancelling differential mode operation, thus using six conductors.
There is also the fact that unlike USB, S/PDIF and AES/EBU, I2S does not require a dedicated transmitter at the sending end and a receiver at the other end. However, the potential for cost savings are rarely if ever realised by audio manufacturers because most, if not all, also provide the traditional connections on their products as a matter of course.
The Denafrips Terminator Plus DAC that I own is unusually blessed with input options, among them I2S using the HDMI form-factor. I initially used the DAC’s S/PDIF and then AES/EBU inputs, and got excellent sonic results with Quiescent Apex, Gothic Audio and AudioQuest Diamond solid silver cables. Later, I robbed the household television of the HDMI cable that connected it to an adjacent PVR/Freeview box. That cable had cost all of £15 or thereabouts from Tesco of all places, and I was not too surprised to find that the resulting sound was rather tonally thin and dynamically constrained in comparison to the S/PDIF and AES/EBU connections.
Imagine my shock then, when an HDMI cable at over £2,000 a metre sounded smoother, quieter and more extended… but also almost as tonally thin and dynamically constrained. In fairness this was a regular, if highly specified HDMI cable, intended for video, not audio. Even so, something was clearly amiss, and I decided to investigate.
Longish story short, I tried HDMI cables from six manufacturers at a range of price-points and got results that in some instances were hard to tell apart and others where the sonic differences were impossible to discern. I began to wonder if the physical constraints imposed by the HDMI form-factor, the need to fit 23 conductors inside a narrow sheath, mandates wire gauges that are compatible with video but not conducive to the best audio reproduction.
How, I wondered, would an HDMI cable sound if it was built with only the conductors necessary for I2S, and therefore could use thicker gauge conductors? A web search led me to exchange emails with Aldwin Oomen who runs boutique cable-maker Tubulus in the south of the Netherlands, not far from the border with Belgium. Oomen makes three such I2S /HDMI cables, the most costly being the Concentus at €649 for a 50cm length terminated either in HDMI or RJ45 plugs (longer lengths are available).
Sound quality
Installed into the review system between a Denafrips Gaia re-clocker and the Terminator Plus DAC the Concentus delivered one of those rare aah… moments as my brain registered a much-more-like-it degree of tonal density and dynamic expression.
Before, solid silver AES/EBU interconnects by both Gothic Audio and Audioquest had been much preferred to assorted I2S/HDMI cables, and in pseudo blind tests with another member of the household toggling the input buttons on the Denafrips DAC it had been very easy to identify which connection was being used just by listening. Now, with the Concentus in the system, toggling the input showed that the previously wide gap between AES/EBU and I2S via HDMI had been closed
Oomen doesn’t want to make it too easy for the Tubulus recipe to be copied and is therefore coy about some materials and construction details, but confirmed that the Concentus has only the conductors necessary for I2S signal transfer and cannot therefore be used for HDMI video. Thermo-treated solid silver wire of 21 and 24 AWG is threaded down wide PTFE tubing to achieve a predominantly air dielectric. Solid copper is used for the grounds. The Concentus is configured for I2S /HDMI as implemented by PS Audio, Denafrips and others. It employs pins 1, 3, 4, 6, 7 and 9 for dual-differential working, as well as pins 10 and 12 for the MCLK signals used by some DAC/source combinations. The cable is likely to work with most, if not all, HDMI/ I2S schemes. The Concentus is necessarily quite a thick cable. It is also quite stiff, but not so unyielding that it cannot be dressed around reasonable, if not overly tight curves. Oomen uses good quality HDMI plugs that engaged firmly in the sockets on my re-clocker and DAC.
Martin Tingvall’s When Light Returns is stand-out solo piano jazz, not just because of Tingvall’s breathtaking lyricism, but also because the recording masterfully captures the sound of his piano with all its delicacy, its tonal complexity and its occasional power. Through the Concentus, when Tingvall used his left hand in the lowest octave to underpin an idea, the percussive and yet sonorous dynamic weight was given proper expression by the Concentus. On other material the Concentus continued to impress, sounding fast, open, tonally informative and with no hint of grain.
I wrote the above in December 2021, closing with the observation that my experience with the Concentus had left me impressed, but at the same time rather ambivalent about I2S as a component interconnect. Oomen’s cable had at last put I2S on a par with well executed AES, but no more.
2023 update
A little over a year on and I now realise that I had got the Concentus wrong, damming it with faint praise when, as I was to discover, in the right setting it makes a more emphatic case for itself. In my defence, it took a change of DAC to show me the error clearly, and to trigger this rather over-due addendum to my original review.
The Denafrips Terminator Plus really is a very competent DAC, for the money a must-audition contender able to see off a lot of the alternatives. But it is not in the same league (or price bracket) as the Mola Mola Tambaqui, the DAC that moved into my review system as the Terminator Plus moved out. Even before the departure of the Terminator Plus I had already set aside the AES digital interconnect and was using the Tubulus I2S cable as a matter of course. On extended listening, rather than within the inevitable time constraints imposed by a review deadline, it had eventually won me over, marginally compared to a solid silver AES alternative, but still sufficient to cause the permanent switch.
Moving the Concentus I2S cable over to the Tambaqui, the gains became clearer still. Tonal colours are heard to be slightly bolder, but it is in the rendering of dynamic energy, texture, timing and spatial detail that the Concentus I2S really pulls away from AES. Stand-up bass, for example, has notably more wood and gut, human voice more diaphragm and emotion, brass more body, glow and crackle, while instruments and performers are more sharply located in space on a deeper sound stage. The tonal detail gains tell us that the Concentus has, among other qualities, a low noise floor, while the sound stage specificity attests to low phase error. The Tubulus Concentus I2S interconnect is thoughtfully designed and able to turn the theoretical technical advantages of I2S into sonic reality. It is highly recommended.