Do HDMI cables make a difference for IIS/I2S/I²S-HDMI audiophile DACs?

It's taking a bit longer for my testing of the inexpensive I²S/IIS/I2S-HDMI DDC boxes due to some unexpected results which I'm chasing down currently; hopefully that gets resolved or understood soon.

In the meantime, let me address something that folks might be curious about related to this topic of the I2S-HDMI interface... Does the HDMI cable make a difference to the sound quality?

As with much of audiophilia wherein some members of the culture have the belief that "everything makes a difference", obviously this is not the case and I do not recommend this kind of mindset. There are many things that don't make much, if any difference (as discussed here and here) based on experience and understanding principles of the technology.

Since I have not seen measurements examining the effect of HDMI cables on I²S performance from a DAC, let's do some testing here.

First, let me show the set-up I'll be using:

As you can see, I'm using the Logitech Touch connected by USB to the DDC box (I'll use the SMSL PO100 PRO as shown), with the digital outputs (S/PDIF and I2S-HDMI) feeding the Topping DX9 DAC. Measurement ADC used will be the E1DA Cosmos ADCiso Grade A + Scaler in the background connected to my measurement MiniPC.

To explore for potential sonic differences objectively, it's best to measure a range of parameters so we can also contextualize the change (ie. is it a frequency domain change, noise, distortion...). So for expediency, let's use the venerable RightMark test suite which will give us a range of results to easily compare. Let me use a standard 24/48 test signal comparing USB direct from Touch to Topping DX9, then through the Touch to DDC box via USB over S/PDIF cables (TosLink and Coax) or through I²S-HDMI. The HDMI cable used here is a short 3' "good" generic cable I've used previously for 4K video with no issues:

Since these are all digital interfaces (as usual, Bits Are Bits isn't an unreasonable generalization), notice that the numerical results are all similar. In the case of this SMSL PO100 PRO, notice that the TosLink optical output results seem to be a tiny bit worse than the other interfaces.

Let's look at the graphs based on that data:

Yes, with digital interfaces, it's true that jitter and the master clock will differ between devices. For this test, the IIS interface uses the SMSL DDC's master clock, different from the Topping's internal DAC clock for USB and S/PDIF, and the measurement ADC will have its own reference. One of the details I need to chase down is that variation in frequency response between the IIS high frequency roll-off and the other interfaces. [I have an idea what it is but I'm not sure the culprit!]

What's most noticeable is that the optical interface of the SMSL PO100 PRO DDC seems to have significantly higher IMD+N distortion which we'll talk about another time when I publish more comparing the Douk and SMSL DDCs. But even with this anomaly, THD+N results below -90dB (0.0032%) for the SMSL optical output would not be of significance within the context of human audibility. Furthermore, in an audio system with other inherent distortions, phase shifts, and frequency responses (especially loudspeakers), what is measured above would be trivial.

What the data above shows is that the measurement system is sensitive enough to differentiate between the various digital inputs into the Topping DX9 DAC.

Given that level of sensitivity then, can we show any difference if we swapped between HDMI cables of various lengths, construction, and quality?

Let's put these cables to the test; all cables have been well-used through the years, so presumably all "broken in" already; unless you believe leaving cables unused for awhile reverses the "break in": 🤔

From top to bottom, we have the "good" 3'/1m generic plastic cable which is what I used in the "SMSL IIS" RightMark measurement above. I believe this was the free cable that came with my Oppo UDP-205 from a few years back. Below that, we have a more expensive 3' HDMI cable I bought about 5 years ago with thick braided black and white sleeve, metal connectors on each end, gold plated, with "Premium Certified" label - let's call this the "fancy" cable. Below that, we have an inexpensive 6' flat cable with swivel connectors on each end. Of all the cables, this one performs the worst when it comes to video quality - it can display 4K/HDR/60fps but tends to have occasional "blanking out" issues every ~10 minutes suggesting that it's more error prone when transmitting high-speed HDMI data. More than likely, this flat cable would not pass HDMI 2.0 18Gbps certification. Finally at the bottom, we have an Energy branded 15'/4.5m high-speed HDMI cable which is about 13 years old but of excellent construction and when I checked the other day, capable of 4K/HDR/60fps video on my living room TV connected to an nVidia RTX 5070Ti GPU.

Using the RightMark test as above, if I measure the Logitech Touch → SMSL PO100 PRO → HDMI cable (I²S) → Topping DX9 DAC, only changing which HDMI cable used, what do we see?

Basically nothing but sporadic variations, regardless of the cable length, type (flat vs. round), and construction quality.

This is of course to be expected since we are looking at digital data transfer down the cable through exactly the same interface and to the same DAC. While it's still possible that over the course of listening to music over time, errors might creep in with a lower quality HDMI cable (which might sound like noise through a bad USB cable), there is otherwise no reason to think that these cables would change the bass/treble tonality, resolution, soundstage imaging, or temporal transients!

Speaking of temporal changes, here's what the 24-bit J-Test (jitter) looks like with the different cables:

As you can see, the I²S-HDMI interface can indeed achieve low-jitter performance. The tallest jitter sidebands are less than -130dB below the 12kHz peak in this jitter-stimulation test.
However, I²S-HDMI jitter is still higher than asynchronous USB using the Topping DX9.

Digital cables clearly do not affect jitter in any meaningful way. This should be no surprise at this point as we have known this for more than a decade already - see USB cable tests and S/PDIF tests back in 2013 for example.

Apart from running the measurements, I did listen for an evening swapping between HDMI cables in my main system for about an hour and quickly got bored and just enjoyed some music. 🤣 As per the objective evidence from these measurements, there was no significant subjective audible difference whether I used the 3' cables, the 6' flat cable (which doesn't perform as well as the other cables in video applications), or the 15' Energy-branded wire.

In conclusion:

If you spend a lot of time online at the typical audiophile magazine sites and YouTube channels, you'll probably be exposed to claims about things that really make no difference. Articles or videos like this "Passion for Sound" gentleman who thinks he can hear all kinds of things from a fancy cable for example! Overt or not, we can see that he's doing a sales job on the Ricable Supreme HDMI Mk II - apparently "the revision for I²S audio". This simply doesn't make sense. That's typical casual "subjectivity" for ya... 😂

[I totally understand having a "passion for sound" quality. But how about we start by having foundational passion to pass on truthful ideas? Maybe then we can appreciate what is rather than what we want to believe... Or perhaps more accurately in videos like these, the individual's desire to sell a product to viewers.

Unless we're talking about being passionate about fictional superheroes, sci-fi, or the fantasy realms (which are all cool!), what is the point of having passion for technological products if the belief system and validation of these beliefs cannot be anchored with objectivity?! Beyond allowing the snake oil industry to extract money from audiophiles of course.]

The idea that HDMI cables make no difference for IIS/I2S/I²S should not be surprising at all. Realize that the intent of these HDMI cables are to be used for home theater AV purposes. For such audio-visual signals, the cables are expected to transmit much more data than 2-channel audio ever will need. Whereas a very high resolution (extreme resolution!) 2-channel 32/768 PCM audio requires about 50Mbps data rate, a typical standard HDMI stream carrying 4K/HDR/60fps video with audio can have bandwidth capabilities of 18Gbps with TMDS clock up to 600MHz. This is much higher than I²S, with the ~24MHz oscillator in the SMSL and Douk Audio DDC boxes.

Until proven otherwise, any decent HDMI cable that works for video will transmit hi-res stereo I²S/IIS/I2S audio to a DAC just fine. As usual, if you feel you're getting benefit from a more luxurious, more expensive item, that's totally fine to satisfy your desires; otherwise there's no reason to "upgrade" beyond an HDMI cable that fulfills standard specifications. Since they're cheap, might as well just get a modern HDMI 2.1 cable like this UGREEN Ultra High Speed cable (~US$10 for 6'). 

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Let's end off with some music.

Queen's "The March of the Black Queen" from Queen II (1974) - the new, significantly cleaner-sounding 2026 Mix was just released. It's also available on multichannel/Atmos streaming (DR9 stereo, DR13 multichannel/Atmos):

This track has often been considered as the precursor to the more popular "Bohemian Rhapsody" from A Night At The Opera (1975) with the dramatic operatic rock structure, vocal harmonies, and tempo changes.

Speaking of dramatics and vocal harmonies, I caught a performance of Les Misérables the other night. Gotta love the multipart vocals of the main characters in "One Day More":

Amazing to think it has been 41 years already since the musical opened in 1985!

Finally, here's some quirky modern indie "math-rock" dressed up in wild costumes:

That's "Sahardnieh" from the duo Angine de Poitrine ("angina pectoris" 😅) off their album Vol.1 (2024, DR8) hailing from Saguenay, Quebec. If you liked that, also check out "Sherpa".

Alright guys and gals, hope you're enjoying the music as we enter April.