Element IV now Supports DSP over Optical

Last week we rolled out Core™ Firmware v1.2.0. It’s easy to gloss over changelogs like this:

• Added LED Mode: OFF
• Reimplemented the OLED DSP Menu, including Gain/Freq/Q for all 10 bands
• Added XMOS based decoding of S/PDIF (optical) input, with full 10-band EQ support
• Added ability to toggle between S/PDIF input modes from the OLED menu or Core Device Settings:
  • ESS: 44.1-192 kHz, no DSP
  • XMOS: 44.1- 96 kHz, full DSP support

What’s not clear from these brief notes is the incredible amount of thought and development that went into S/PIDF-to-I2S decoding. Implementation began years ago. If you had asked me in 2022, I would have told you we’d wasted months on software development—we abandoned the project after AKM’s factory fire. There was no reason to build an S/PDIF converter when ESS gives one away for “free” with every chip.

As Element IV took shape, the archived S/PDIF-to-I2S project suddenly made perfect sense. We could decode S/PDIF on Element IV, unlocking the ability to run its 10-band EQ in both USB and Optical modes.

In this post, we’ll walk through the high level architecture and performance considerations when using Element IV’s new S/PDIF modes.

Element IV’s Optical Modes

Element IV’s incoming optical signal is fed directly to its ESS DAC, as well as to a GPIO pin of its XMOS processor. As of firmware v1.2.0, you can now choose between the following modes from Core’s Device Settings or the OLED menu:

• XMOS S/PDIF Mode [default]: Optical decoding at up to 96 kHz with full DSP support
• ESS S/PDIF Mode: Optical decoding at up to 192 kHz (no DSP)

If you have no need for equalization, choose ESS mode. ESS’s patented algorithm decodes S/PDIF remarkably well—arguably the best in the industry.

Element IV’s XMOS S/PDIF mode is tuned in firmware to balance performance and computational resources. Keep in mind that Core firmware running on Element IV currently consumes 60-80% of available resources to run its 10-band EQ and general functionality.

Three of 16 XMOS cores are now dedicated to S/PDIF decoding. Under the current computational budget, jitter sidebands are slightly higher and THD is elevated by 1 dB when decoding optical in XMOS mode vs. ESS mode. These minor differences are imperceptible, and obviously more than compensated for by the ability to run the 10-band EQ in optical mode!

System Architecture: S/PDIF Reception to I2S Output

The S/PDIF-to-I2S engine uses a custom Asynchronous Sample Rate Converter (ASRC) implementation and a lock-protected ring buffer to hold I2S audio stream data at a nominal 50% fill level, protecting against underruns or overruns.

Precision & Buffer Management

Please keep in mind that v1.2.0 is our initial release of XMOS S/PDIF mode! While it’s been tediously tuned and auditions well, S/PDIF signals from some consumer devices can be unpredictable. Let us know if you encounter anything unexpected.

What’s Next

With DSP-over-optical complete, our devs are now hard at work on your feature requests! I’m pushing for on-device profile storage next (yes, it’s possible). One of our lead developers is optimizing the DSP stage this week, so stay tuned for spring Core firmware updates.