WeebLabs/DSPi

Source: https://github.com/WeebLabs/DSPi

DSPi Firmware

DSPi transforms a Raspberry Pi Pico or other RP2040-based board into a very competent and inexpensive little digital audio processor. It acts as a USB sound card with an onboard DSP engine, allowing you to make use of essential tools like room correction, active crossovers, parametric EQ, time alignment, loudness compensation, and headphone crossfeed.

It is my hope that the RP2040 and RP2350 will garner a reputation as the “swiss army knife of audio for less than a cup of coffee”.

Feel free to join the official Discord server for development updates, discussion or to request assistance!

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Table of Contents

• Key Capabilities
• Platform Support
• Audio Signal Chain
• Hardware Setup
• DSP Features
  • Matrix Mixer
  • Parametric Equalization
  • Loudness Compensation
  • Headphone Crossfeed
  • Volume Leveller
  • Per-Channel Preamp
  • Master Volume
  • I2S Output
  • Subwoofer PDM Output
• User Presets
• Developer Reference
  • System Architecture
  • Performance Tuning
  • USB Control Protocol
  • System Telemetry
  • Data Structures
• Building from Source
• Detailed Specifications
• License

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Key Capabilities

• USB Audio Interface: Plug-and-play under macOS, Windows, Linux, and iOS. Supports 16-bit and 24-bit PCM input at 44.1, 48, and 96 kHz.
• 24-bit S/PDIF or I2S Outputs: Up to four independent stereo output slots (8 channels on RP2350, 4 channels on RP2040). Each slot can be switched at runtime between S/PDIF and I2S, enabling direct connection to any standard DAC. I2S slots share a common BCK/LRCLK and can optionally produce a 128×/256× master clock.
• Per-Channel Preamp: Independent gain control for each USB input channel (L/R), applied as PASS 1 of the DSP pipeline before any other processing.
• Matrix Mixer: Route either or both USB input channels to any output with independent gain and phase invert per crosspoint. 2x9 on RP2350, 2x5 on RP2040.
• Parametric Equalization: Up to 10 PEQ bands per channel with 6 filter types. 110 total filter bands on RP2350, 70 on RP2040. RP2350 uses a hybrid SVF/biquad architecture for superior low-frequency accuracy.
• Volume Leveller: RMS-based, stereo-linked, soft-knee upward compressor that lifts quieter content toward a target level without ever amplifying loud passages. Optional 10 ms lookahead, configurable speed and max-gain ceiling, with a -6 dBFS gain-reduction safety limiter.
• Loudness Compensation: Volume-dependent EQ based on the ISO 226:2003 equal-loudness contour standard. Automatically boosts bass and treble at low listening levels to maintain perceived tonal balance.
• Headphone Crossfeed: BS2B-based crossfeed with interaural time delay (ITD) reduces unnatural stereo separation for headphone listening. Three classic presets plus fully custom parameters.
• Master Volume: Device-side output ceiling (-128 to 0 dB, with a true-mute sentinel) applied at the very end of the signal chain, independent of USB host volume and DSP processing. Two persistence modes: stored independently of presets (default — survives reboots, unaffected by preset switching) or saved/restored as part of each preset.
• Per-Output Gain & Mute: Independent gain and mute controls for each output channel.
• Time Alignment: Per-output delay (up to 85ms) for speaker/subwoofer alignment with automatic latency compensation between S/PDIF/I2S and PDM output paths.
• Subwoofer Output: Dedicated mono PDM output channel with a high-performance 2nd-order delta-sigma modulator, enabling direct subwoofer output without the need for a second DAC.
• Dual-Core DSP: EQ processing is split across both cores on both platforms for maximum throughput when multiple outputs are active.
• Configurable Output Pins: All output GPIO pins (including I2S BCK/MCK) can be reassigned at runtime to suit custom PCB layouts, no reflashing required.
• 10-Slot Preset System: Save, load, and manage up to 10 complete DSP configurations with user-defined names. Includes per-channel naming, configurable startup slot, and bulk parameter transfer for fast state synchronization.
• Diagnostics: Per-channel peak/clip metering, USB PHY error counters (CRC, bit-stuff, timeout, overflow, sequence), buffer fill statistics, S/PDIF DMA starvation counters per output slot, and CPU load reporting per core.
• Firmware Update via USB: A vendor command reboots the device into the UF2 bootloader, allowing the host app to push new firmware without a physical BOOTSEL press.

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Platform Support

Feature	RP2040 (Pico)	RP2350 (Pico 2)
System Clock	307.2 MHz (overclock)	307.2 MHz
Core Voltage	1.15 V	1.15 V
Sample Rates	44.1 / 48 / 96 kHz	44.1 / 48 / 96 kHz
Audio Processing	Q28 Fixed-Point	Single-Precision Float
EQ Bands	10 per channel (70 total)	10 per channel (110 total)
Total Channels	7 (2 master + 4 S/PDIF·I2S + 1 PDM)	11 (2 master + 8 S/PDIF·I2S + 1 PDM)
Output Slots	2 stereo (each S/PDIF or I2S)	4 stereo (each S/PDIF or I2S)
Output Bit Depth	24-bit	24-bit
PDM Output	1 (subwoofer)	1 (subwoofer)
Max Delay	85ms per output	85ms per output
Math Engine	Hand-optimized ARM Assembly	Hardware FPU (hybrid SVF/biquad EQ)
Dual-Core EQ	Yes (Core 1 processes outputs 3-4)	Yes (Core 1 processes outputs 3-8)
User Presets	10 slots	10 slots
Status	Production	Production

Both platforms are fully tested and production-ready. The RP2040 reaches 307.2 MHz with a slight voltage bump; the RP2350 hits the same frequency at the same voltage. Clock is fixed (no rate-dependent switching), and PIO dividers are integer at every supported sample rate. The RP2350 offers significantly more processing headroom thanks to its hardware floating-point unit, enabling more output channels and a hybrid SVF/biquad filter architecture for improved low-frequency accuracy.

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Audio Signal Chain

DSPi processes audio in a linear, low-latency pipeline:

RP2350 (11 channels, 9 outputs):

USB Input (16/24-bit PCM Stereo, 44.1 / 48 / 96 kHz)
    |
PASS 1: Per-Channel Preamp (independent L/R gain) + USB Volume
    |
PASS 2: Master EQ (10 bands per channel, Left/Right)
    |
PASS 2.5: Volume Leveller (RMS upward compression, optional)
    |
PASS 3: Headphone Crossfeed (BS2B + ITD, optional) + Master Peak Metering
    |
        Loudness Compensation (volume-dependent EQ, optional)
    |
PASS 4: Matrix Mixer (2 inputs x 9 outputs, per-crosspoint gain & phase)
    |
PASS 5: Per-Output EQ -> Gain/Mute -> Delay -> Output Gain × Master Volume
    |
    +-- Out 1-2 --> S/PDIF or I2S slot 0 (data: GPIO 6 default)
    +-- Out 3-4 --> S/PDIF or I2S slot 1 (data: GPIO 7 default)
    +-- Out 5-6 --> S/PDIF or I2S slot 2 (data: GPIO 8 default)
    +-- Out 7-8 --> S/PDIF or I2S slot 3 (data: GPIO 9 default)
    +-- Out 9   --> PDM Sub               (data: GPIO 10 default)
                  (I2S BCK/LRCLK shared on GPIO 14/15 default; optional MCK on GPIO 13 default)

RP2040 (7 channels, 5 outputs):

USB Input (16/24-bit PCM Stereo, 44.1 / 48 / 96 kHz)
    |
PASS 1: Per-Channel Preamp + USB Volume
    |
PASS 2: Master EQ (10 bands per channel, Left/Right)
    |
PASS 2.5: Volume Leveller (RMS upward compression, optional)
    |
PASS 3: Headphone Crossfeed (BS2B + ITD, optional) + Master Peak Metering
    |
        Loudness Compensation (volume-dependent EQ, optional)
    |
PASS 4: Matrix Mixer (2 inputs x 5 outputs, per-crosspoint gain & phase)
    |
PASS 5: Per-Output EQ -> Gain/Mute -> Delay -> Output Gain × Master Volume
    |
    +-- Out 1-2 --> S/PDIF or I2S slot 0 (data: GPIO 6 default)
    +-- Out 3-4 --> S/PDIF or I2S slot 1 (data: GPIO 7 default)
    +-- Out 5   --> PDM Sub               (data: GPIO 10 default)
                  (I2S BCK/LRCLK shared on GPIO 14/15 default; optional MCK on GPIO 13 default)

Signal Chain Details

1. Input (USB): 16-bit or 24-bit PCM stereo audio at 44.1, 48, or 96 kHz. Bit depth is selected via USB alt setting; sample rate via the USB Audio Class rate-set request.
2. Per-Channel Preamp (PASS 1): Independent gain control for the USB Left and Right input channels in dB. Applied at the very start of the DSP chain so its setting affects all downstream processing.
3. Master EQ (PASS 2): Up to 10 bands of parametric EQ per channel (Left/Right). Supports peaking, low shelf, high shelf, low pass, and high pass filter types.
4. Volume Leveller (PASS 2.5): Optional feedforward, stereo-linked, single-band RMS compressor with soft-knee upward compression — quieter content is boosted toward a target level while content above the threshold passes through untouched. Configurable speed, max-gain ceiling, and noise gate. Optional 10 ms lookahead. A -6 dBFS gain-reduction safety limiter prevents output overshoots.
5. Headphone Crossfeed (PASS 3): Optional BS2B crossfeed that mixes a filtered, delayed portion of each channel into the opposite channel. Uses a complementary filter design with interaural time delay (ITD) via an all-pass filter. Three presets (Default, Chu Moy, Jan Meier) plus custom frequency and feed level. ITD can be independently toggled. Master peak metering taps into this stage.
6. Loudness Compensation: Optional ISO 226:2003 equal-loudness EQ that adapts to the current volume level. At low volumes, bass and treble are boosted to compensate for the ear’s reduced sensitivity. Configurable reference SPL and intensity. Driven by the USB host volume position so it remains correct regardless of master-volume attenuation downstream.
7. Matrix Mixer (PASS 4): Routes the two USB input channels (Left/Right) to all output channels. Each crosspoint has independent enable, gain (-inf to +12 dB), and phase invert. Outputs can be individually enabled/disabled to save CPU. RP2350 has a 2x9 matrix (9 outputs), RP2040 has a 2x5 matrix (5 outputs).
8. Output EQ (PASS 5): Independent 10-band EQ per output channel on both platforms. Ideal for crossover filters and per-driver correction. On RP2350, filters below Fs/7.5 use SVF topology for superior low-frequency accuracy; higher frequencies use traditional biquad.
9. Per-Output Gain & Mute: Independent gain (-inf to +12 dB) and mute for each output channel.
10. Time Alignment: Per-output delay for speaker alignment, up to 85 ms (4096 samples at 48 kHz). Automatic latency compensation between S/PDIF/I2S and PDM output paths.
11. Master Volume: Device-side output ceiling, -128 to 0 dB with a true-mute sentinel at -128. Folded into the per-output multiplier at PASS 5 so it’s effectively free CPU-wise. Independent of the USB host volume — the two multiply together. Does not affect loudness-compensation behavior.
12. Outputs: Each numbered slot is configurable as either 24-bit S/PDIF or 24-bit I2S (left-justified, MSB-first). I2S slots share a common BCK/LRCLK clock pair (LRCLK is always BCK + 1 due to a PIO side-set constraint). An optional master clock (MCK) at 128× or 256× Fs can be routed to a separate GPIO. PDM subwoofer is always on its own dedicated output and pin.

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Hardware Setup

Flashing the Firmware

1. Download the latest DSPi.uf2 release for your board.
2. Hold the BOOTSEL button on your Pico while plugging it into your computer.
3. A drive named RPI-RP2 will appear.
4. Drag and drop the .uf2 file onto this drive.
5. The Pico will reboot and appear as a “Weeb Labs DSPi” audio device.
6. Download and launch the DSPi Console application to control the DSPi.

Wiring Guide

RP2350 (Pico 2) — up to 8 output pins:

Function	Pin	Connection
Output Slot 0 (Out 1-2)	GPIO 6 (default)	S/PDIF or I2S data for main L/R or multi-way pair 1
Output Slot 1 (Out 3-4)	GPIO 7 (default)	S/PDIF or I2S data for multi-way pair 2
Output Slot 2 (Out 5-6)	GPIO 8 (default)	S/PDIF or I2S data for multi-way pair 3
Output Slot 3 (Out 7-8)	GPIO 9 (default)	S/PDIF or I2S data for multi-way pair 4
Subwoofer Out (PDM, Out 9)	GPIO 10 (default)	Active subwoofer or PDM-to-analog filter
I2S BCK (shared, I2S only)	GPIO 14 (default)	Bit clock for any slot configured as I2S
I2S LRCLK (I2S only)	GPIO 15 (BCK + 1, fixed)	Word/frame clock; always BCK + 1
I2S MCK (optional)	GPIO 13 (default)	128× or 256× Fs master clock when MCK is enabled
USB	Micro-USB	Host device (PC/Mac/Mobile Device)

RP2040 (Pico) — up to 6 output pins:

Function	Pin	Connection
Output Slot 0 (Out 1-2)	GPIO 6 (default)	S/PDIF or I2S data for main L/R or stereo pair 1
Output Slot 1 (Out 3-4)	GPIO 7 (default)	S/PDIF or I2S data for stereo pair 2
Subwoofer Out (PDM, Out 5)	GPIO 10 (default)	Active subwoofer or PDM-to-analog filter
I2S BCK (shared, I2S only)	GPIO 14 (default)	Bit clock for any slot configured as I2S
I2S LRCLK (I2S only)	GPIO 15 (BCK + 1, fixed)	Word/frame clock; always BCK + 1
I2S MCK (optional)	GPIO 13 (default)	128× or 256× Fs master clock when MCK is enabled
USB	Micro-USB	Host device (PC/Mac/Mobile Device)

Notes: S/PDIF output requires either a Toshiba TX179 optical transmitter or a simple resistor divider. I2S output is a standard 24-bit-in-32-bit left-justified frame — wires straight into most I2S DACs. PDM output is a 1-bit logic signal that requires a resistor and capacitor to form a low-pass filter for conversion to analog audio.

Custom Pin Assignments

All default pin assignments above work out of the box, but every output pin — including I2S BCK and MCK — can be reassigned at runtime through the DSPi Console application. No reflashing required. This is useful when designing custom PCBs or adapting to boards where the default GPIOs are inconvenient.

Pin assignments are saved to flash and restored automatically at boot. A few GPIOs are reserved and unavailable for output use: GPIO 12 (UART TX) and GPIOs 23-25 (power control and LED). LRCLK is always pinned to BCK + 1 due to a PIO side-set constraint.

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DSP Features

Matrix Mixer

The matrix mixer routes the USB stereo input to all output channels. RP2350 has a 2x9 matrix (9 outputs), RP2040 has a 2x5 matrix (5 outputs). Each crosspoint (input/output pair) has:

• Enable/Disable: Route active or inactive.
• Gain: -inf to +12 dB per crosspoint.
• Phase Invert: Polarity flip for driver alignment.

Each output channel also has:

• Enable: Disabled outputs skip all processing (EQ, delay, conversion) to save CPU.
• Gain: Per-output gain (-inf to +12 dB).
• Mute: Soft mute per output.
• Delay: Per-output time alignment.

Output Availability: Core 1 is shared between the PDM subwoofer modulator and the EQ worker that processes higher-numbered S/PDIF outputs. PDM and EQ worker modes are mutually exclusive:

RP2350:

Mode	Available Outputs	Core 1 Usage
PDM enabled (Out 9 on)	Out 1-2 (S/PDIF 1) + Out 9 (PDM)	Delta-sigma modulator
PDM disabled (Out 9 off)	Out 1-8 (S/PDIF 1-4)	EQ worker for Out 3-8

RP2040:

Mode	Available Outputs	Core 1 Usage
PDM enabled (Out 5 on)	Out 1-2 (S/PDIF 1) + Out 5 (PDM)	Delta-sigma modulator
PDM disabled (Out 5 off)	Out 1-4 (S/PDIF 1-2)	EQ worker for Out 3-4

When the PDM subwoofer is active, Core 1 is fully dedicated to the delta-sigma modulator, so higher-numbered S/PDIF outputs are unavailable. When PDM is off, Core 1 runs as an EQ worker processing those outputs in parallel with Core 0.

Common Configurations (RP2350):

Use Case	Routing	Mode
Stereo + Sub	L→Out1, R→Out2, L+R→Out9	PDM on (3 outputs)
2-Way Active	L→Out1(tweeter), L→Out3(woofer), R→Out2(tweeter), R→Out4(woofer)	PDM off (4 outputs)
3-Way Active	As above, plus mid-range on Out5-6	PDM off (6 outputs)
4-Way Active	As above, plus super-tweeter on Out7-8	PDM off (8 outputs)

Common Configurations (RP2040):

Use Case	Routing	Mode
Stereo	L→Out1, R→Out2	PDM off (2 outputs)
Stereo + Sub	L→Out1, R→Out2, L+R→Out5	PDM on (3 outputs)
2-Way Active	L→Out1(tweeter), L→Out3(woofer), R→Out2(tweeter), R→Out4(woofer)	PDM off (4 outputs)

Parametric Equalization

Each filter band supports 6 types:

Type	Description
Flat	Bypass (no processing)
Peaking	Parametric bell filter
Low Shelf	Low-frequency shelf
High Shelf	High-frequency shelf
Low Pass	Low-pass filter
High Pass	High-pass filter

On RP2040, all filters use biquad IIR (Transposed Direct Form II) with Q28 fixed-point arithmetic. On RP2350, the firmware uses a hybrid SVF/biquad architecture: filters below Fs/7.5 (~6.4 kHz at 48 kHz) use the Cytomic SVF (linear trapezoid) topology for superior numerical accuracy at low frequencies, while higher frequencies use traditional TDF2 biquad. All filters have configurable frequency, Q factor, and gain. Flat filters are automatically bypassed for zero CPU overhead.

Channel Layout:

RP2350 (11 channels):

Channel	Index	EQ Bands
Master Left	0	10
Master Right	1	10
Output 1-8 (S/PDIF)	2-9	10 each
Output 9 (PDM Sub)	10	10

RP2040 (7 channels):

Channel	Index	EQ Bands
Master Left	0	10
Master Right	1	10
Output 1-4 (S/PDIF)	2-5	10 each
Output 5 (PDM Sub)	6	10

Loudness Compensation

Based on the ISO 226:2003 equal-loudness contour standard. At low listening volumes, the human ear is less sensitive to bass and treble frequencies. Loudness compensation applies a volume-dependent EQ curve to maintain perceived tonal balance across all listening levels.

• Reference SPL: Configurable (40-100 dB). Set this to the SPL where your system sounds tonally balanced at full volume.
• Intensity: Adjustable from 0-200% of the standard ISO curve.
• Implementation: Precomputed coefficient tables for all 91 volume steps, double-buffered for glitch-free updates.

Headphone Crossfeed

Implements Bauer Stereophonic-to-Binaural (BS2B) crossfeed with a complementary filter design that reduces unnatural stereo separation for headphone listening. Each channel receives a lowpass-filtered, time-delayed mix of the opposite channel, simulating the acoustic crossfeed that occurs with loudspeaker listening.

• Complementary Design: Direct path = input - lowpass(input). Guarantees mono signals pass through at unity gain with no coloration.
• Interaural Time Delay (ITD): First-order all-pass filter adds ~220us of delay to the crossfeed path, modeling sound traveling around the head for 60-degree stereo speaker placement. ITD can be independently enabled/disabled.
• Presets:

Preset	Cutoff	Feed Level	Character
Default	700 Hz	4.5 dB	Balanced, most popular
Chu Moy	700 Hz	6.0 dB	Stronger spatial effect
Jan Meier	650 Hz	9.5 dB	Subtle, natural
Custom	500-2000 Hz	0-15 dB	User-defined

Volume Leveller

A feedforward, stereo-linked, single-band RMS dynamic range compressor that maintains consistent perceived volume across content with varying loudness.

• Upward compression: Boosts content below the threshold while leaving content above the threshold completely untouched. No makeup gain needed.
• RMS-based detection: Tracks root-mean-square envelope, which correlates with perceived loudness better than peak detection.
• Soft-knee: Gradual transition between full boost and unity gain for transparent, artifact-free behavior.
• Stereo-linked: The louder of the two channels determines gain for both, preserving the stereo image.
• Gain-reduction safety limiter: -6 dBFS ceiling enforced via gain reduction (instant attack, 100 ms release) rather than hard clipping. Rarely engages since loud content passes through at unity.
• Optional 10 ms lookahead for smoother transitions.
• Configurable: speed (attack/release), max-gain ceiling (cap on how much quiet content can be lifted), and gate threshold (below which the leveller stops boosting to avoid amplifying silence/noise).

The leveller sits at PASS 2.5 — after Master EQ, before crossfeed. Independent of Loudness Compensation; both can be enabled together without conflict.

Per-Channel Preamp

Each USB input channel (Left and Right) has an independent preamp gain in dB, applied at PASS 1 before any other processing. Useful for trimming channel imbalance, attenuating hot inputs ahead of EQ, or matching levels across sources. A legacy single-value command remains for backward compatibility (sets both channels to the same value).

Master Volume

A device-side output ceiling applied at the very end of the signal chain, independent of USB host volume.

• Range: -128 to 0 dB. -128 is a sentinel for true silence (mute).
• Independent of USB host volume: the two multiply together. The host slider operates within whatever range master volume permits.
• Independent of DSP processing: loudness compensation, EQ, leveller, and crossfeed are all driven by the raw USB volume, not the master volume — their behavior is unchanged regardless of the master setting.
• Two persistence modes (selectable at runtime, persists across reboots):
  • Mode 0 — Independent (default). Master volume is a stand-alone device setting. The app calls a save command to capture the current value into the directory; that value is applied at every subsequent boot. Preset save/load do not touch master volume — switching presets never moves the volume.
  • Mode 1 — With preset. Master volume is part of each preset. Saved with the preset, restored on preset load, like any other DSP parameter. Useful when different presets target speaker setups with different sensitivity / maximum-output requirements.
• Default at first boot: -20 dB (configurable via MASTER_VOL_DEFAULT_DB in firmware).

I2S Output

Each output slot can be switched at runtime between S/PDIF (default) and I2S, independently per slot. A single device can drive a mix — e.g., slot 0 as I2S into a DAC chip, slot 1 as S/PDIF over Toslink to an external receiver, all from the same audio pipeline.

• I2S format: 24-bit data, left-justified, MSB-first, 32-bit frames. Drop-in to most standard I2S DACs (PCM5102, ES9038Q2M, etc.).
• Shared clocks: All I2S slots share a single BCK/LRCLK pair. LRCLK is always BCK + 1 (PIO side-set hardware constraint).
• Optional MCK: When enabled, a 128× or 256× Fs master clock is generated on a configurable GPIO. Required by some DACs that don’t have an internal PLL. At 96 kHz, only 128× is selectable due to PIO clock-divisor limits.
• Sample-aligned start: I2S slots can be brought up together so multiple DACs stay phase-locked.

The DSP pipeline is identical for both output types — only the final encoding differs (BMC/NRZI for S/PDIF vs. raw left-justified PCM for I2S).

Subwoofer PDM Output

The subwoofer output uses a high-performance software-defined delta-sigma modulator running on Core 1.

• Modulation: 2nd-Order Delta-Sigma
• Oversampling Ratio: 256x (12.288 MHz bit clock at 48 kHz)
• Dither: TPDF (Triangular Probability Density Function) with noise shaping
• DC Protection: Leaky integrator design preventing DC offset accumulation

The objective was to use as much of Core 1 as necessary to produce an output that could be used full-range while sounding perfectly fine, even if it will only be used to feed a subwoofer. This implementation is very stable and without pops, clicks or idle tones.

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User Presets

DSPi includes a 10-slot preset system that stores complete DSP configurations in flash. A preset is always active — there is no “no preset” state.

• 10 Preset Slots: Each slot stores the full DSP state: per-channel preamp, EQ bands, delays, loudness, leveller, crossfeed, matrix mixer, output gains/mutes, output type (S/PDIF or I2S), I2S clock configuration, pin assignments, master volume (used in Mode 1), and per-channel names.
• Per-Channel Names: Each channel can be given a user-defined name (up to 31 characters) that is stored with the preset.
• Startup Configuration: Choose which preset loads on boot — either a specific default slot or whichever slot was last active.
• Pin Config Inclusion: Optionally include or exclude GPIO pin assignments when saving/loading presets (default: include — pin layout travels with the preset).
• Master Volume Mode: Selects whether master volume is part of each preset (Mode 1) or stored independently in the preset directory (Mode 0, default). See Master Volume.
• Preset-Switch Mute: Audio output is briefly muted (~10 ms) during preset transitions to prevent audible glitches.
• Legacy Commands: The original save/load/reset commands (0x51-0x53) redirect through the preset system, operating on the currently active slot.
• Bulk Parameter Transfer: The complete DSP state can be read or written in a single USB control transfer (~2.9 KB) for fast synchronization with host applications.
• Auto-Migration: Older preset directories are upgraded transparently on first boot of new firmware — slot names, startup config, and other persisted state are preserved.

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Developer Reference

System Architecture

• Core 0: USB communication, audio streaming, DSP processing (master EQ, crossfeed, loudness, matrix mixing, output EQ for S/PDIF pair 1), and control logic.
• Core 1 (three modes):
  • PDM Mode: Delta-sigma modulator for subwoofer output (when the PDM output is enabled).
  • EQ Worker Mode: Processes output EQ, delay, and S/PDIF conversion for higher-numbered outputs in parallel with Core 0. On RP2350: outputs 3-8. On RP2040: outputs 3-4. Activated when any of those outputs are enabled and PDM is disabled.
  • Idle Mode: When no outputs requiring Core 1 are enabled.
• PIO & DMA: Hardware offloading for S/PDIF encoding (PIO0) and PDM bitstream generation (PIO1) ensures zero CPU overhead for I/O.
• Math Engine:
  • RP2040: 32-bit fixed-point (Q28) processing with hand-optimized ARM assembly for the inner DSP loop.
  • RP2350: Single-precision float pipeline with hardware FPU. Hybrid SVF/biquad EQ — Cytomic SVF for low frequencies (below Fs/7.5), TDF2 biquad above. SVF provides superior numerical accuracy for low-frequency filters where biquad coefficient quantization becomes problematic.

Note: PDM mode and EQ Worker mode are mutually exclusive on Core 1. When the PDM output is enabled, Core 0 handles all S/PDIF output EQ processing. When PDM is disabled and higher-numbered outputs are active, Core 1 runs as an EQ worker for those outputs.

Performance Tuning

Both platforms run at a fixed 307.2 MHz system clock (VCO 1536 MHz / 5 / 1) so PIO dividers stay integer at every supported sample rate, eliminating sample-rate-dependent clock switching glitches.

Platform	System Clock	Core Voltage
RP2040	307.2 MHz (overclock)	1.15 V
RP2350	307.2 MHz	1.15 V

The RP2040 reaches 307.2 MHz with a slight voltage bump above the 1.10 V nominal; the RP2350 is comfortable at the same voltage at this clock. The voltage step is applied before the frequency change. Sample rate changes do not retune the system clock, only the PIO dividers, so transitions between 44.1 / 48 / 96 kHz are seamless.

Flash access is also tuned: PICO_FLASH_SPI_CLKDIV is set to 6 to keep XIP and erase/program operations safely below the W25Q080’s 104–133 MHz spec at this clock. On the RP2350, runtime QMI register management is handled by firmware/DSPi/flash_clkdiv.c since the bootrom does not honor the boot2 setting on that platform.

USB Control Protocol

Configuration is performed via Interface 2 (Vendor Interface) using Control Transfers under Windows and via Interface 0 under macOS. The device supports WinUSB/WCID for automatic driverless installation on Windows.

Request Table

Code	Name	Direction	Payload	Description
0x42	REQ_SET_EQ_PARAM	OUT	16 bytes	Upload filter parameters
0x43	REQ_GET_EQ_PARAM	IN	16 bytes	Read filter parameters
0x44	REQ_SET_PREAMP	OUT	4 bytes	Set global gain (float dB)
0x45	REQ_GET_PREAMP	IN	4 bytes	Get global gain
0x46	REQ_SET_BYPASS	OUT	1 byte	Bypass Master EQ (1=On, 0=Off)
0x47	REQ_GET_BYPASS	IN	1 byte	Get bypass state
0x48	REQ_SET_DELAY	OUT	4 bytes	Set channel delay (float ms)
0x49	REQ_GET_DELAY	IN	4 bytes	Get channel delay
0x50	REQ_GET_STATUS	IN	4-12 bytes	Get system statistics (wValue selects field)
0x51	REQ_SAVE_PARAMS	IN	1 byte	Save to active preset slot
0x52	REQ_LOAD_PARAMS	IN	1 byte	Reload active preset slot
0x53	REQ_FACTORY_RESET	IN	1 byte	Reset live state to defaults
0x54	REQ_SET_CHANNEL_GAIN	OUT	4 bytes	Set output channel gain (float dB)
0x55	REQ_GET_CHANNEL_GAIN	IN	4 bytes	Get output channel gain
0x56	REQ_SET_CHANNEL_MUTE	OUT	1 byte	Mute output channel (1=Muted)
0x57	REQ_GET_CHANNEL_MUTE	IN	1 byte	Get mute state
0x58	REQ_SET_LOUDNESS	OUT	1 byte	Enable/disable loudness (1=On)
0x59	REQ_GET_LOUDNESS	IN	1 byte	Get loudness state
0x5A	REQ_SET_LOUDNESS_REF	OUT	4 bytes	Set reference SPL (float, 40-100)
0x5B	REQ_GET_LOUDNESS_REF	IN	4 bytes	Get reference SPL
0x5C	REQ_SET_LOUDNESS_INTENSITY	OUT	4 bytes	Set intensity % (float, 0-200)
0x5D	REQ_GET_LOUDNESS_INTENSITY	IN	4 bytes	Get intensity
0x5E	REQ_SET_CROSSFEED	OUT	1 byte	Enable/disable crossfeed (1=On)
0x5F	REQ_GET_CROSSFEED	IN	1 byte	Get crossfeed state
0x60	REQ_SET_CROSSFEED_PRESET	OUT	1 byte	Set preset (0-3)
0x61	REQ_GET_CROSSFEED_PRESET	IN	1 byte	Get current preset
0x62	REQ_SET_CROSSFEED_FREQ	OUT	4 bytes	Set custom frequency (float Hz, 500-2000)
0x63	REQ_GET_CROSSFEED_FREQ	IN	4 bytes	Get custom frequency
0x64	REQ_SET_CROSSFEED_FEED	OUT	4 bytes	Set custom feed level (float dB, 0-15)
0x65	REQ_GET_CROSSFEED_FEED	IN	4 bytes	Get custom feed level
0x66	REQ_SET_CROSSFEED_ITD	OUT	1 byte	Enable/disable ITD (1=On)
0x67	REQ_GET_CROSSFEED_ITD	IN	1 byte	Get ITD state
0x70	REQ_SET_MATRIX_ROUTE	OUT	8 bytes	Set matrix crosspoint (MatrixRoutePacket)
0x71	REQ_GET_MATRIX_ROUTE	IN	8 bytes	Get matrix crosspoint
0x72	REQ_SET_OUTPUT_ENABLE	OUT	1 byte	Enable/disable output channel
0x73	REQ_GET_OUTPUT_ENABLE	IN	1 byte	Get output enable state
0x74	REQ_SET_OUTPUT_GAIN	OUT	4 bytes	Set per-output gain (float dB)
0x75	REQ_GET_OUTPUT_GAIN	IN	4 bytes	Get per-output gain
0x76	REQ_SET_OUTPUT_MUTE	OUT	1 byte	Mute output (1=Muted)
0x77	REQ_GET_OUTPUT_MUTE	IN	1 byte	Get output mute state
0x78	REQ_SET_OUTPUT_DELAY	OUT	4 bytes	Set per-output delay (float ms)
0x79	REQ_GET_OUTPUT_DELAY	IN	4 bytes	Get per-output delay
0x7A	REQ_GET_CORE1_MODE	IN	1 byte	Get Core 1 mode (0=Idle, 1=PDM, 2=EQ Worker)
0x7B	REQ_GET_CORE1_CONFLICT	IN	1 byte	Check if PDM vs EQ Worker conflict exists
0x7C	REQ_SET_OUTPUT_PIN	IN	1 byte	Change output GPIO pin (returns status)
0x7D	REQ_GET_OUTPUT_PIN	IN	1 byte	Get current GPIO pin for an output
0x7E	REQ_GET_SERIAL	IN	variable	Get unique board serial number
0x7F	REQ_GET_PLATFORM	IN	1 byte	Get platform ID (0=RP2040, 1=RP2350)
0x83	REQ_CLEAR_CLIPS	OUT	—	Clear clip detection latches
0x90	REQ_PRESET_SAVE	IN	1 byte	Save live state to preset slot (wValue=slot)
0x91	REQ_PRESET_LOAD	IN	1 byte	Load preset slot to live state (wValue=slot)
0x92	REQ_PRESET_DELETE	IN	1 byte	Delete preset slot (wValue=slot)
0x93	REQ_PRESET_GET_NAME	IN	32 bytes	Get preset name (wValue=slot)
0x94	REQ_PRESET_SET_NAME	OUT	32 bytes	Set preset name (wValue=slot)
0x95	REQ_PRESET_GET_DIR	IN	variable	Get preset directory (occupancy, startup config)
0x96	REQ_PRESET_SET_STARTUP	OUT	2 bytes	Set startup mode and default slot
0x97	REQ_PRESET_GET_STARTUP	IN	2 bytes	Get startup configuration
0x98	REQ_PRESET_SET_INCLUDE_PINS	OUT	1 byte	Set pin config inclusion (1=include)
0x99	REQ_PRESET_GET_INCLUDE_PINS	IN	1 byte	Get pin config inclusion setting
0x9A	REQ_PRESET_GET_ACTIVE	IN	1 byte	Get currently active preset slot index
0x9B	REQ_SET_CHANNEL_NAME	OUT	32 bytes	Set channel name (wValue=channel)
0x9C	REQ_GET_CHANNEL_NAME	IN	32 bytes	Get channel name (wValue=channel)
0xA0	REQ_GET_ALL_PARAMS	IN	~2896 bytes	Bulk read entire DSP state (multi-packet)
0xA1	REQ_SET_ALL_PARAMS	OUT	~2896 bytes	Bulk write entire DSP state (multi-packet)
0xB0	REQ_GET_BUFFER_STATS	IN	variable	Read buffer fill statistics
0xB1	REQ_RESET_BUFFER_STATS	IN	1 byte	Reset buffer statistics counters
0xB2	REQ_GET_USB_ERROR_STATS	IN	24 bytes	Read USB PHY error counters (CRC/bit-stuff/timeout/overflow/seq)
0xB3	REQ_RESET_USB_ERROR_STATS	IN	1 byte	Reset USB PHY error counters
0xB4	REQ_SET_LEVELLER_ENABLE	OUT	1 byte	Enable/disable Volume Leveller
0xB5	REQ_GET_LEVELLER_ENABLE	IN	1 byte	Get leveller enable state
0xB6	REQ_SET_LEVELLER_AMOUNT	OUT	4 bytes	Set leveller target/amount (float)
0xB7	REQ_GET_LEVELLER_AMOUNT	IN	4 bytes	Get leveller amount
0xB8	REQ_SET_LEVELLER_SPEED	OUT	1 byte	Set leveller attack/release speed
0xB9	REQ_GET_LEVELLER_SPEED	IN	1 byte	Get leveller speed
0xBA	REQ_SET_LEVELLER_MAX_GAIN	OUT	4 bytes	Set max upward gain (float dB)
0xBB	REQ_GET_LEVELLER_MAX_GAIN	IN	4 bytes	Get max upward gain
0xBC	REQ_SET_LEVELLER_LOOKAHEAD	OUT	1 byte	Enable/disable 10 ms lookahead
0xBD	REQ_GET_LEVELLER_LOOKAHEAD	IN	1 byte	Get lookahead state
0xBE	REQ_SET_LEVELLER_GATE	OUT	4 bytes	Set noise-gate threshold (float dB)
0xBF	REQ_GET_LEVELLER_GATE	IN	4 bytes	Get noise-gate threshold
0xC0	REQ_SET_OUTPUT_TYPE	OUT	1 byte	Set slot output type (0=S/PDIF, 1=I2S; wValue=slot)
0xC1	REQ_GET_OUTPUT_TYPE	IN	1 byte	Get slot output type (wValue=slot)
0xC2	REQ_SET_I2S_BCK_PIN	OUT	1 byte	Set shared I2S BCK GPIO (LRCLK auto = BCK + 1)
0xC3	REQ_GET_I2S_BCK_PIN	IN	1 byte	Get current I2S BCK pin
0xC4	REQ_SET_MCK_ENABLE	OUT	1 byte	Enable/disable I2S master clock output
0xC5	REQ_GET_MCK_ENABLE	IN	1 byte	Get MCK enable state
0xC6	REQ_SET_MCK_PIN	OUT	1 byte	Set MCK GPIO
0xC7	REQ_GET_MCK_PIN	IN	1 byte	Get MCK GPIO
0xC8	REQ_SET_MCK_MULTIPLIER	OUT	1 byte	Set MCK multiplier (0=128×, 1=256×)
0xC9	REQ_GET_MCK_MULTIPLIER	IN	1 byte	Get MCK multiplier
0xD0	REQ_SET_PREAMP_CH	OUT	4 bytes	Set per-channel preamp (wValue=channel, payload=float dB)
0xD1	REQ_GET_PREAMP_CH	IN	4 bytes	Get per-channel preamp (wValue=channel)
0xD2	REQ_SET_MASTER_VOLUME	OUT	4 bytes	Set master volume (-128 mute sentinel, -127..0 dB)
0xD3	REQ_GET_MASTER_VOLUME	IN	4 bytes	Get current live master volume
0xD4	REQ_SET_MASTER_VOLUME_MODE	OUT	1 byte	Set persistence mode (0=independent, 1=with preset)
0xD5	REQ_GET_MASTER_VOLUME_MODE	IN	1 byte	Get persistence mode
0xD6	REQ_SAVE_MASTER_VOLUME	IN	1 byte	Save live master volume to directory (mode 0 persistence)
0xD7	REQ_GET_SAVED_MASTER_VOLUME	IN	4 bytes	Read directory’s saved master-volume value
0xF0	REQ_ENTER_BOOTLOADER	IN	1 byte	Reboot into UF2 bootloader for firmware update

REQ_GET_STATUS (0x50) - System Telemetry

The REQ_GET_STATUS request returns data based on the wValue field:

wValue	Returns	Description
0	uint32	Peaks for channels 0-1 (packed 16-bit values)
1	uint32	Peaks for channels 2-3 (packed 16-bit values)
2	uint32	Peak for channel 4 + CPU0/CPU1 load (packed)
3	uint32	PDM ring buffer overruns
4	uint32	PDM ring buffer underruns
5	uint32	PDM DMA overruns
6	uint32	PDM DMA underruns
7	uint32	S/PDIF overruns
8	uint32	S/PDIF underruns
9	12 bytes	Combined: all 5 peaks + CPU loads
10	uint32	USB audio packet count
11	uint32	USB alt setting
12	uint32	USB audio mounted state
13	uint32	System clock frequency (Hz)
14	uint32	Core voltage (millivolts)
15	uint32	Sample rate (Hz)
16	int32	System temperature (centi-degrees C)
17	uint32	Total S/PDIF DMA starvations (all slots combined)
18	uint32	S/PDIF slot 0 starvations (Out 1-2)
19	uint32	S/PDIF slot 1 starvations (Out 3-4)
20	uint32	S/PDIF slot 2 starvations (Out 5-6, RP2350)
21	uint32	S/PDIF slot 3 starvations (Out 7-8, RP2350)

A starvation event means the S/PDIF DMA needed a buffer but the consumer pool was empty, so the firmware substituted a silence buffer for that transfer. This is a more direct output-side dropout signal than the older spdif_underruns USB-packet-gap heuristic.

Data Structures

Filter Packet (16 bytes):

struct __attribute__((packed)) {
    uint8_t channel;  // RP2350: 0-10, RP2040: 0-6
    uint8_t band;     // 0-9
    uint8_t type;     // 0=Flat, 1=Peak, 2=LS, 3=HS, 4=LP, 5=HP
    uint8_t reserved;
    float freq;       // Hz
    float Q;
    float gain_db;
}

Matrix Route Packet (8 bytes):

struct __attribute__((packed)) {
    uint8_t input;          // 0-1 (USB L/R)
    uint8_t output;         // RP2350: 0-8, RP2040: 0-4
    uint8_t enabled;        // 0 or 1
    uint8_t phase_invert;   // 0 or 1
    float gain_db;          // -inf to +12dB
}

Runtime Pin Configuration

Output GPIO pins can be reassigned at runtime without reflashing. This is useful for custom PCB layouts or when the default pin assignments conflict with other hardware.

REQ_SET_OUTPUT_PIN (0x7C) — IN transfer, returns 1-byte status:

• wValue = (new_pin << 8) | output_index
• RP2350: output_index 0-3 for S/PDIF outputs 1-4, 4 for PDM subwoofer
• RP2040: output_index 0-1 for S/PDIF outputs 1-2, 2 for PDM subwoofer
• S/PDIF outputs are automatically disabled and re-enabled during the pin change (~1ms audio dropout on that output only)
• PDM output must be disabled first (disable via REQ_SET_OUTPUT_ENABLE), otherwise returns PIN_CONFIG_OUTPUT_ACTIVE

Status Code	Value	Meaning
PIN_CONFIG_SUCCESS	0x00	Pin changed successfully
PIN_CONFIG_INVALID_PIN	0x01	Pin out of range or reserved (GPIO 12, 23-25)
PIN_CONFIG_PIN_IN_USE	0x02	Pin already assigned to another output
PIN_CONFIG_INVALID_OUTPUT	0x03	Output index out of range
PIN_CONFIG_OUTPUT_ACTIVE	0x04	PDM output must be disabled before changing its pin

REQ_GET_OUTPUT_PIN (0x7D) — IN transfer, returns 1 byte:

• wValue = output_index
• Returns the current GPIO pin number for that output

Pin assignments are stored in each preset and can optionally be included during preset save/load (controlled via REQ_PRESET_SET_INCLUDE_PINS).

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Building from Source

To build the firmware yourself, you’ll need a standard Raspberry Pi Pico C/C++ development environment.

1. Install Prerequisites

Ensure you have the following tools installed:

• CMake (3.13 or newer)
• Arm GNU Toolchain (arm-none-eabi-gcc, etc.)
• Python 3 (for Pico SDK scripts)
• Git

2. Clone the Repository

Clone the project recursively to include the Pico SDK and other submodules:

git clone --recursive https://github.com/WeebLabs/DSPi.git
cd DSPi

If you already cloned without --recursive, run:

git submodule update --init --recursive

3. Build the Firmware

You can build for either the standard RP2040 (Raspberry Pi Pico) or the newer RP2350 (Raspberry Pi Pico 2). The build system uses separate directories to avoid conflicts.

Option A: Build for RP2040 (Standard Pico)

mkdir build-rp2040
cd build-rp2040
cmake -DPICO_BOARD=pico -DPICO_EXTRAS_PATH=../firmware/pico-extras ../firmware
make

Output: DSPi/DSPi.uf2

Option B: Build for RP2350 (Pico 2)

mkdir build-rp2350
cd build-rp2350
cmake -DPICO_BOARD=pico2 -DPICO_EXTRAS_PATH=../firmware/pico-extras ../firmware
make

Output: DSPi/DSPi.uf2

4. Flash the Device

1. Hold the BOOTSEL button on your board while plugging it in.
2. Drag and drop the generated .uf2 file onto the RPI-RP2 (or RP2350) drive.

Alternatively, an already-running DSPi can be put into bootloader mode without a button press by sending REQ_ENTER_BOOTLOADER (0xF0). The DSPi Console application uses this for one-click firmware updates. See Documentation/Features/firmware_update.md for the protocol details.

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Detailed Specifications

In-depth specs for each subsystem are kept under Documentation/Features/. These are the authoritative source for protocol formats, wire layouts, edge cases, and host-app integration patterns.

Feature	Spec
Matrix Mixer	matrixmixer_spec.md
User Presets	user_presets_spec.md
Master Volume	master_volume_spec.md
Per-Channel Preamp	per_channel_preamp_spec.md
Volume Leveller	volume_leveller_spec.md
I2S Output	i2s_output_spec.md
Peak / Clip Metering	peak_clip_metering_spec.md
Buffer Statistics	buffer_statistics_spec.md
S/PDIF DMA Starvation	spdif_starvation_spec.md
USB Error Diagnostics	usb_errors_spec.md
Core 1 Modes	core1_modes_spec.md
Device Identification	device_identification_spec.md
S/PDIF Input (planned)	SPDIF_input_spec.md
Firmware Update via USB	Documentation/Features/firmware_update.md
Roadmap	roadmap.md

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License

This project is licensed under the GNU General Public License v3.0. See LICENSE for details.