The ζ1 Audio Widget asynchronous USB-I²S module

History & acknowledgements

After the γ1 and γ2, I've been thinking about a bigger, more ambitious DAC project (the γ3). One important factor in the design is an upgraded USB interface. Rather than the limit of 16 bits, 48KHz sample rate (as it is with the PCM2707 chip used on the γ1), I would like to support up to 24 or 32 bits, 192KHz sample rate. Also, to achieve the lowest jitter, the new USB interface should operate in asynchronous transfer mode (compared to the adaptive mode on the PCM2707).

In the search for an alternative high-performance USB interface, I evaluated a number of chip-based and board-based solutions. Virtually all of the available ones that meet the functional and technical requirements are proprietary designs with hefty licensing fees and closed source firmware, which makes them unfit for DIY purposes. More specifically, the solution must satisfy the following requirements:
  • Supports all standard sample rates from 44.1KHz to 192KHz, including 88.2KHz and 176.2KHz, with 16 and 24/32 bit depths.
  • Runs in asynchronous mode.
  • Converts from USB to I²S, not to S/PDIF so that the low-jitter characteristic of async mode isn't compromised by the S/PDIF interface.
  • Supports all three major operating system environments (Linux, MacOSX, Windows).
  • Must be readily "buy-able" in small (or single) quantities.
  • Must not require a herculean development effort (e.g., writing complex firmware from scratch).
  • Must not require a licensing or royalty fee.
  • Must not require signing non-disclosure agreements (NDAs).
  • Open source highly desirable (hardware, firmware, software).
The only solution that meets these requirements is Audio Widget. I had been following the Audio Widget project for several years with interest, because it is open source, free of proprietary licensing, and developed by enthusiasts over the internet. Over this period of time the Audio Widget development team had made great progress and the result is now "production-ready". In fact I joined the Audio Widget team and hope to contribute to that project in the future.

I decided to create my own version of the Audio Widget, based closely on the USB-I2S module found in Børge Strand-Bergesen's Henry Audio (formerly known as QNKTC) DAC. My version is called the ζ1 Audio Widget asynchronous USB-I²S module, and would be offered as assembled and tested units, because it is very difficult to hand-solder.

Meanwhile, the ζ1 Audio Widget module sparked a new project, the γ1.5 miniature high-resolution USB DAC + headphone amplifier. The small size of the ζ1 module allowed the whole thing to be packed into the small Hammond C1455C80x extruded enclosure. It would be a truly high performance USB-powered, USB-only portable DAC and headphone amplifier, and probably the first and only high-volume portable DIY DAC + headphone amp capable of 24/192 with asynchronous USB.

With both γ1.5 and γ3 projects using the ζ1 module, the per-unit cost of the ζ1 will be reduced due to higher volume. Also, since the ζ1 is being developed concurrently with the two DACs, the γ1.5 served as a good testbed for the ζ1. I really like the idea of multiple projects sharing one or more common pieces.

I announced the ζ1 project to the community (at the AMB DIY audio forum) in August 2014, and solicited very useful feedback from various members. In early November a prototyping effort was launched for both the γ1.5 and ζ1, seven members signed up to build, evaluate and comment on the design, including myself. Prototype boards were sent to the team in mid-November. The prototype team members each built their ζ1s in SMD reflow ovens that were themselves DIY projects.

Both γ1.5 and ζ1 worked flawlessly "out of the box". I performed extensive bench measurements, the results were published in the AMB forum, and are also shown in the Specifications section of the γ1.5 website.

Production ζ1 modules (assembled units) were ordered, and became available in late January 2015.

The γ3 high resolution DAC, which spawned the thinking and adoption of the Audio Widget, was subsequently released in late 2015.

I would like to thank Børge Strand-Bersesen, George Boudreau and other contributors to the Audio Widget and SDR-widget projects. Their excellent work made the ζ1, γ1.5 and γ3 projects possible. In fact, the ζ1 module was designed to be pin-compatible (via the 2mm-pitch pin headers and receptacles) with the USB-I2S module in Børge's Henry Audio DAC.

Also thanks to Nikolay Kovbasa for his work on the Win-Widget ASIO driver. It made possible UAC2 operation on Windows, with an open source driver that is unencumbered by proprietary licenses.

Last, but not least, thanks to MisterX for collaborating on the γ1.5 project, to the prototype team members, and to people who contributed to the ζ1 development discussions in the forum. You made your mark in this project and its success is yours too.

Look for more exciting DIY projects from AMB in the future!

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