Inexpensive USB2.0 Digital-to-Digital Converters (DDC) Pt.1: Douk Audio U2 PRO (Upgrade Version) and SMSL PO100 PRO - initial look, about I²S-HDMI. In praise of unofficial Atmos mixes!

This SMSL unit also does MQA decoding for those who want the feature. Unlike the Douk, this box only has an LED to indicate MQA decoding status but does not tell us the PCM or DSD samplerate.

While I've tested some inexpensive devices (like the Topping D10s DAC) which have the ability to convert USB 2.0 input into S/PDIF output, I have not specifically looked into these Digital-to-Digital Converter boxes which some audiophile reviewers (like this fellow, and this) seem to rave about.

For me, there just hasn't been any reason to be particularly interested since other than conversion from one interface type to another, the other promise of such devices - the reclocking capability - just hasn't been of great interest to me. As far as I can tell from listening, it's not like jitter has ever really been much of an issue unless severely bad for any decent modern device I've ever come across even at low price points (nor was jitter really an issue even with old CD players)!

However, since I have not tested I²S/I2S/IIS, I figure I'd buy both the Douk Audio U2 PRO 2025 upgrade version (US$63; Amazon Canada) and SMSL PO100 PRO (US$74; Amazon Canada) boxes to try out; have a listen and run some measurements. Unfortunately due to time limits, I'm not quite ready to show the measurements yet, so keep an eye out for those results, perhaps next week! For now let's just introduce you to these devices and talk about the I²S-HDMI connection.

As typical of the majority of my reviews/critiques, I bought these through retail channels.

SMSL PO100 PRO includes a cute HiRes sticker with the JAS logo if you like these things on your boxes...

The units come in typical cardboard boxes and inside we have the DDC device, 1m lengths of male USB-C to USB-A with the Douk unit having a switchable USB-A/C end and they also include a decent TosLink cable. There's also the multi-lingual manual/pamphlet. The Douk Audio has a 1-year warranty card.

The Douk Audio box is clearly larger than the diminutive SMSL. For context, the Douk U2 PRO's dimensions are 85mm x 79mm x 26mm, weighing 200g and the SMSL weighing only about 75g. They're both metal enclosures.

The shiny top of the SMSL is quite nice cosmetically though a fingerprint magnet.

Above we see pictures of the rear "business" end of the devices. They're both conveniently powered by the 5V USB-C input from your computer or streamer and convert the USB audio to S/PDIF Coaxial, S/PDIF TosLink (up to 24/192 PCM and DSD64 DoP), and I²S/I2S/IIS via the male HDMI-A-style physical port up to 32/768 and DSD512 native or DSD256 DoP. The internal USB audio interface for both devices use the high performance 16-core 3rd Gen XMOS XU316 chip. All digital outputs are simultaneously active so you can connect each one to the same DAC and switch inputs to compare sound if desired.

They both adopt the HDMI-style connector for I²S/IIS/I2S data transmission these days using LVDS (low-voltage differential signaling). This is NOT compatible with actual HDMI audio-visual data format that we use for our Bluray players or home theater receivers. This means anything that tries to decode an HDMI A/V signal will not work, including things like fiber optic HDMI cables that have to transcode the electrical data structure. In fact, be cautious and make sure you're not plugging "true" HDMI devices into this port just in case it might cause damage.

The I²S/IIS/I2S (Inter-Integrated Circuit Sound) bus was never intended to be used like this as a multi-feet cable component interface. Other than audiophiles and "high-end" folks doing this, the serial I²S signal was designed by Philips back in 1986 as a way to transmit 2-channel PCM across the inches between chips inside our audio boxes like CD players.

I believe it was PS Audio who introduced the IIS inter-component interface to the audiophile world based on the HDMI connector. The pin-out for the "PS Audio Standard" was released back in 2010 with the PerfectWave Transport, and it looked like this (here's a Paul McGowan video talking about it and the history):

The "core" I²S signal is sent through four differential wire pairs consisting of Serial Data (SDOUT, MSB format for PCM data), the Bitclock (BCK) that synchronizes the data transfer telling the receiver device the samplerate (2 channels x sample rate x bits/sample), the Master Clock (MCLK, not always needed by DAC) is a high-frequency multiple of the samplerate, and the Left/Right Clock (LRCLK, also known as Word Select or Frame Select) tells the receiver device which audio channel the data belongs to. In principle, assuming stable clock performance, jitter across the IIS transmission could/should be low.

As you can see, for the original PS Audio HDMI connector, pin 1 corresponded to SDOUT (data) NEGATIVE and pin 7 corresponds to LRCK (left-right clock) NEGATIVE as well. Without an actual formal I²S-HDMI standard, somewhere along the way, manufacturers like Gustard, Topping, Holo Audio among others decided to offer other configurations with pins 1 and 7 being + (thus inverted from PS Audio), adding an extra wrinkle to an already non-standard interface easily mistaken as something you can plug your computer's HDMI output into!

Beyond that basic pin configuration above, for native DSD (not DoP which is DSD data embedded in a PCM stream), the interface separates the channels with SDOUT and LRCLK carrying the individual left/right 1-bit data and added a "DSD Enable" pin to tell the DAC that it's DSD rather than default PCM.

Underneath the box we see this HDMI-pin diagram for the Douk Audio U2 PRO:

This corresponds to the "PS Audio Standard". You can also see how DSD left and right channels are split, and pins 14+15 to signal DSD data.

And for the SMSL, the bottom panel can actually be flipped to the top if you prefer to always be able to see the switch settings:

On both units, there's a small switch to set the box as either a UAC2.0 (USB 2.0) device for hi-res audio (default and recommended for most cases) or it can be put in UAC1.0 12Mbps compatibility mode capable of maximum 24/96 PCM if you're planning to plug this into something like the PS4/PS5/Nintendo Switch game machines without UAC2 driver support.

Notice that the SMSL PO100 PRO also has a switch for the "I2S Mode" which correlates to the two main pin configurations you'll run into for the majority of DACs:

Having this switch on the SMSL makes it more flexible than the Douk in case you're using a DAC that can only handle one specific pin configuration.

If you have the Douk U2 PRO and you're using something like my Topping DX9 DAC, check that your menu settings are correct for that configuration. Topping's "STD" default should be adjusted to "REV" phase and "DSDR using DATA":

Topping DX9 DAC set-up menu with options in red for Douk U2 PRO.
Default IIS DSD Flag and IIS MUTE settings are fine.

If your DAC does not have menu options for polarity, then the only option might be to swap L-R cables from your DAC to amplifier to correct for reversed channels.

[As an aside, I wondered why PS Audio used a -/+ polarity sequence for the SDOUT and LRCK wires? Conventionally, the + then - order makes sense. Also, why is DSD-Right first with the DATA line then further down DSD-Left with LRCLK instead of the other way around since again by convention we typically order Left-then-Right.

Intuitively, the Topping menu's default configuration makes more sense than what PS Audio uses IMO. I wonder, if PS Audio had just selected a more conventional scheme for the polarity across all the differential pairs (notice that BCK and MCLK are still ordered as +/- polarity) back in 2010, would there still be this hassle? 

I wondered if the folks at PS Audio had some kind of belief that if they interleaved the polarity between SDOUT/BCK/LRCK/MCLK that this made some kind of sonic difference due to noise or some other mythical audiophile reason?]

These USB audio DDC boxes are only 2-channel stereo devices. Apart from the SMSL PO100 PRO's automatic ability to decode MQA should you want that, they should be sending data to the DAC bit-accurately.

While I did not open up the box (not sure how since I don't see any easily accessible screws), Douk Audio states that the U2 PRO's internal clock oscillator can be upgraded:

The default is a small 24.000MHz +/- 20ppm 5032 SMD active oscillator as seen in the picture. It's replaceable with a 24MHz, 1.8-3.3V, DIP-14 rectangular plug-in module. We'll see in the measurements whether the stock +/-20ppm performance is adequate; some of the best oscillators can maintain temperature stability down to 0.1ppm with TCXO and even lower with OCXO. Presumably, a higher precision clock can improve performance and I see that Douk is advertising TXCO and femtosecond versions coming soon.

The manual for both units list typical jitter as 75ps which should be adequate - but not expected to be remarkable.

You might be asking yourself:
"Why do I even want or need a USB interface DDC?"

There are a few possible answers that range from a practical need to supposed subjective benefits:

1. If you don't have a modern asynchronous USB input on your DAC, these will serve as a bridge to convert to S/PDIF and I²S-HDMI.

2. You believe your DAC has poor temporal performance and expect that an external DDC box like these can reduce jitter because of reclocking.

3. You believe that an external USB DDC bridge box like these might reduce noise. (You believe that USB itself or the cable from streamer/computer to DAC directly significantly exacerbates noise.)

4. You just want to play with the I²S/I2S/IIS interface with your compatible DAC because it's what some people like Paul McGowan says "definitely sounds better than just straight USB".

[5. For the SMSL PO100 PRO, you can use it to add MQA decoding as a feature if your DAC does not support and you have need for it.]

From a functional perspective, answer 1 (and 5) certainly could makes sense. Say I have a good-sounding but older DAC with no USB input or have a known poor implementation (like an old UAC1 interface that peaks at 96kHz and I want to try 192kHz over S/PDIF Coax), clearly modern DDC boxes can provide the desired converter from USB to S/PDIF and I2S-HDMI. Answers 2 and 3 should be objectively demonstrable if these DDC boxes reduce jitter and/or noise. And answer 4 is clearly a subjective one that might or might not correlate with objective fidelity - or whether you have faith in Paul McGowan's claims in general. 😉

So, let's explore performance next time. And we can even try different cables because guys like this when talking about such boxes are not just selling the I²S-HDMI interface, but often also hyping up the cables as sounding very different! Hmmm, Ricable Supreme MkII HDMI cable designed for I²S priced at €88/m, eh? At least it's cheaper than most of these audiophile HDMI cables. Check out the flowery words in the video - the "possible optimization of electromagnetic fields" - along with noise and jitter anxiety around cabling! 

Here's a peek at my DDC testing rig. Let's use the Topping DX9 DAC (rare OG AKM AK4499 DAC), fed by an old-skool Logitech Touch through either the on-board S/PDIF or by using the EDO (Extended Data Out) applet, we can connect with USB either directly to the DAC or through the Douk/SMSL DDC boxes:

Can the modern Douk U2 PRO and SMSL PO100 PRO boxes improve performance/fidelity compared to directly connecting the old 2010 Squeezebox Touch's USB/TosLink/Coax to the Topping DX9 DAC? Should the modern XMOS-based Douk or SMSL interface "sound better"?

Let's have a look at actual data instead of mere impressions, feelings, and other subjective innuendo next time.

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As I end, I want to send a shout-out to the folks who have been working on "unofficial" multichannel/Atmos mixes of music using the tools we have relatively easy access to these days like DaVinci Resolve Studio I used last year to create the Atmos Walkthrough.

Roon does not decode/bitstream Atmos (unfortunately, as discussed). So I usually transcode to 5.1 AAC at 512kbps using the FDK encoder which sounds transparent. This removes the Atmos metadata but I can still use Dolby Surround Upmixer or Auro-Matic.
I keep a copy of the Atmos original elsewhere (typically as excellent-sounding 768kbps EAC3-JOC Atmos) for JRiver HDMI bitstreaming from my fanless miniPC to decoder (ie. AV receiver).

As per the Roon screenshot above, there is a growing library of unofficial spatial mixes out there being done by music lovers and hobbyists with experience in using the "immersive" tools. You'll need to find this stuff yourself since this is obviously underground unauthorized content, like unofficial vinyl rips or concert bootlegs spread across the corners of the Internet. Perhaps one day some of the music in the screenshot will be treated with official Atmos mixes. But until then, it's nice to hear "spatial" versions of Coldplay's "Viva La Vida", Madonna's "Ray of Light" or Lady Gaga & Bruno Mars' recent "Die With A Smile".

While quality will vary depending on the source from which the remix was created, many of these unofficial releases sound fantastic, especially the remixes created from official stems although AI stem separated remixes also can be compelling. In fact, I've come across a number of these that sound even more engaging than the professionally done, official, studio-released Atmos mixes!

For example, the other day, I came across a recent dump of Michael Jackson spatial remixes that sound very good. Let's end with MJ's "2000 Watts"; a track from his final studio album Invincible (2001) with terminology that audiophiles and AV tech geeks will appreciate - an example of music that could easily be turned multichannel and the unofficial Atmos remix of this is fun 🙂.

Well, spring is coming soon and this weekend is supposedly the last time we'll be "springing ahead" with daylight savings time turning permanently into "Pacific Time" here in British Columbia!

I hope you're all enjoying your music, audiophiles! May your listening sessions be like refreshing visits to the oasis in these uncertain times.