Project Skyverb Part 1: Building the Digital Wire

The Hardware Platform: Mercury 2

Our brain for this project is the Mercury 2, a development board powered by the Xilinx Artix-7 FPGA (XC7A35T). Unlike many “SoC” boards that come with Linux and pre-configured audio drivers, the Mercury 2 is bare-metal silicon. It gives us a 50MHz clock and a lot of pins. Everything else is up to us.

We are using the Mercury 2 Baseboard, which provides two critical components for audio:

1. Input: An MCP3008 Analog-to-Digital Converter (ADC).
2. Output: A passive Low-Pass Filter for Delta-Sigma conversion.

Step 1: The Input (Bit-Banging SPI)

The input jack connects to the MCP3008, a 10-bit ADC. To get audio from it, the FPGA acts as an SPI Master.

We didn’t use a library for this. We wrote a state machine in Verilog that:

1. Pulls the Chip Select (CS_n) low to wake up the ADC.
2. Sends a specific binary command (Start Bit, Single-Ended Mode, Channel 0) via the MOSI line.
3. Simultaneously listens to the MISO line to capture the 10 bits of audio data.

The Challenge: Synchronization. At one point during development, we had clean audio on the oscilloscope but “static” in the speakers. It turned out our SPI clock was misaligned by a single clock cycle, causing us to read the data bits while they were changing rather than when they were stable. A quick adjustment to the state machine fixed the timing, turning the static back into music.

Step 2: The Output (Delta-Sigma Modulation)

Getting sound out is even more interesting. The baseboard doesn’t have a dedicated DAC chip. Instead, we use the FPGA itself to create one.

We implemented a Delta-Sigma Modulator—a clever piece of logic that takes our 24-bit digital audio and converts it into a high-speed (50MHz) stream of 1s and 0s. The density of the 1s represents the voltage.

• Lots of 1s = High Voltage.
• Lots of 0s = Low Voltage.

When this 1-bit stream hits the analog Low-Pass Filter on the board, the jagged pulses are smoothed out into a continuous analog waveform. It’s a “hack” that became an industry standard because it sounds surprisingly good.

The Result: Loopback

By connecting our custom SPI Input driver to our Delta-Sigma Output driver, we created a “Digital Wire.” Sound comes in, gets converted to numbers, travels through the FPGA logic, gets converted back to voltage, and leaves.

When we plug a synthesizer into the Input and headphones into the Output, we hear… exactly what we played. Transparent, latency-free audio.

What’s Next?

Now that we have a working Digital Wire, the fun begins. In Part 2, we will cut this wire and insert our own custom DSP logic: the Strymon Bluesky Reverb algorithm. We’ll be translating floating-point math into hardware logic to create those lush, cloud-like shimmers.

Stay tuned.