TouchRTTY (RP2350 / Raspberry Pi Pico 2)

Professional-grade Radioteletype (RTTY) Decoder exclusively for the RP2350 microcontroller.

This project implements a high-performance, software-defined radio (SDR) style RTTY decoder on the dual-core RP2350 (ARM Cortex-M33). It utilizes the advanced DSP capabilities of the RP2350 to achieve highly stable reception even under severe selective fading and noise.

Important

Hardware Requirement: This project is specifically designed for the Raspberry Pi Pico 2 (RP2350). It will NOT run on the original RP2040 due to higher memory and DSP requirements.

🚀 Key Features (Phase 3 Complete)

• RP2350 Dual-Core Optimization:
  • Core 0 (DSP Engine): Dedicated strictly to hard-real-time audio processing at exactly 10,000 Hz using RP2350's floating-point unit (FPU).
  • Core 1 (UI & Rendering): Handles the 3.5" ILI9488 TFT touch display via 60MHz PIO DMA, rendering a 30+ FPS Waterfall, Spectrum, and Lissajous (XY) tuning scope without interrupting the DSP.
• Professional DSP Pipeline (Build 172):
  • 63-Tap FIR Bandpass Filter: Pre-filters the 10kHz ADC stream.
  • Quadrature (I/Q) Demodulator: Baseband mixing with hardware-generated sine/cosine tables, followed by Biquad Low-Pass Filters (Extended Raised Cosine). Eliminates Inter-Symbol Interference (ISI).
  • Automatic Threshold Correction (ATC): Fast-attack, slow-release (FASR) envelope detectors independently track Mark and Space fading, dynamically adjusting the decision threshold.
  • Digital Phase-Locked Loop (DPLL): A full Proportional-Integral (PI) synchronous loop tracks the zero-crossings of the bitstream, correcting both phase (timing jitter) and frequency error (baud rate mismatch). Allows continuous, gapless reception of 1.0 stop-bit streams without framing errors.
  • Automatic Frequency Control (AFC): 512-point FFT-based peak detection locks onto wandering signals within a ±100Hz window.
  • Strict Squelch: Intelligent noise-floor and SNR tracking ensures the decoder remains totally silent until a valid RTTY signal (SNR > 4dB) is present.
• Hardware Compatibility:
  • Optimized for the ILI9488 display quirk (Mode 11: 16-bit Endian Swapped, BGR out), rendering pure, artifact-free colors via native RGB565 manipulation.

📅 Development Roadmap

• PHASE 4: SD-Card Integration (exFAT) & Data Logging.
• PHASE 5: CW (Morse Code) Decoder & APF Filter.
• PHASE 6: FT8 / FT4 Mode Implementation.
• PHASE 7: WEFAX Decoder (HF Weather Fax).

📡 WebSDR / Real-World Testing Guide

You can now test this decoder with real over-the-air signals using a WebSDR (like the University of Twente WebSDR)!

Setup

1. Open a WebSDR in your browser.
2. Tune to a known RTTY frequency (e.g., German Weather Service DWD on 10100.8 kHz).
3. Set the WebSDR modulation to USB (Upper Sideband).
   • Note: DWD transmits in F1B/LSB. If you tune in USB, the Mark frequency will be higher than the Space frequency. You must press the INV button on the Pico's screen to swap them, OR tune the WebSDR to LSB and leave the Pico in Normal mode.
4. Connect your PC's headphone output using the audio adapter (read about it below) to the Pico's ADC input (GPIO 26) using an audio cable.
   • Ensure your audio level is correct. Watch the top-left SIG meter on the screen; it should peak around -15 dB to -5 dB without triggering the red clipping indicator.

Tuning on the Pico

1. DWD SYNOP (Weather):
   • Select B 50 (50 Baud).
   • Select S 450 (450 Hz Shift).
   • Select ST 1.5 (1.5 Stop bits).
2. Amateur Radio (Ham):
   • Select B 45 (45.45 Baud).
   • Select S 170 (170 Hz Shift).
   • Select ST 1.5 (1.5 Stop bits).
3. Tap the Waterfall to place the yellow/cyan markers over the two visible peaks.
4. The RTTY: WAIT indicator should turn green and say RTTY: SYNC.
5. Text will begin printing on the screen!

🔌 Hardware Wiring Guide

The project utilizes the Raspberry Pi Pico 2 (RP2350) and a 3.5" ILI9488 TFT Display with an XPT2046 touch controller. Below is the required pinout mapping.

Display (ILI9488) - SPI0

Display Pin	Pico GPIO	Physical Pin	Function
VCC	-	Pin 36	3.3V Power (3V3_OUT)
GND	-	Pin 38	Ground (GND)
CS	GP17	Pin 22	LCD Chip Select
RESET	GP21	Pin 27	Hardware Reset
DC/RS	GP20	Pin 26	Data / Command
SDI (MOSI)	GP19	Pin 25	SPI Data In
SCK	GP18	Pin 24	SPI Clock (60 MHz)
SDO (MISO)	GP16	Pin 21	Define in code, physically disconnect to reduce bus noise
LED	-	Pin 36	Backlight Power (3V3_OUT)

Touch Controller (XPT2046) - SPI1

Touch Pin	Pico GPIO	Physical Pin	Function
T_CLK	GP10	Pin 14	SPI Clock (2.5 MHz)
T_CS	GP15	Pin 20	Touch Chip Select
T_DIN	GP11	Pin 15	SPI TX (MOSI)
T_DO	GP12	Pin 16	SPI RX (MISO)
T_IRQ	GP14	Pin 19	Interrupt (Boot Calibration)

Audio Input

Component	Pico GPIO	Physical Pin	Function
Audio Signal	GP26	Pin 31	ADC0 (Biased Audio Input)
Audio Ground	-	Pin 33	Analog Ground (AGND)

Rotary Encoder (Future UI Expansion)

Currently, only the push-button (SW) is implemented to serve as a Hard Reset and UI interaction. Full rotary tuning (A/B pins) will be added in future phases.

Encoder Pin	Pico GPIO	Physical Pin	Function
SW (Switch)	GP4	Pin 6	Push Button to GND (Hold on boot to Factory Reset)
CLK / A	TBD	-	Reserved for future use
DT / B	TBD	-	Reserved for future use
GND	-	Any GND	Ground

SD Card Module - SPI1 (Phase 5 Logging)

The SD Card shares the SPI1 bus with the Touch Controller, but requires its own dedicated Chip Select (CS) pin.

SD Card Pin	Pico GPIO	Physical Pin	Function
MOSI / CMD	GP11	Pin 15	SPI1 TX (Shared with Touch T_DIN)
MISO / D0	GP12	Pin 16	SPI1 RX (Shared with Touch T_DO)
SCK / CLK	GP10	Pin 14	SPI1 Clock (Shared with Touch T_CLK)
CS / DAT3	GP13	Pin 17	Dedicated SD Chip Select
VCC	-	Pin 36 or 40	3.3V or 5V (Depends on your SD module)
GND	-	Any GND	Ground

🔌 Hardware Audio Input Adapter

To safely feed audio from a PC, radio, or WebSDR into the RP2350's ADC (Analog-to-Digital Converter), a simple DC-biasing circuit is required. The Pico's ADC reads voltages between 0V and 3.3V, so an AC audio signal centered around 0V will clip and potentially damage the pin if negative voltages are applied.

Required Circuit:

1. R1 (Input Level): A 10kΩ potentiometer to adjust the audio volume from your source.
2. C1 (DC Blocking): A 4.7µF capacitor to block any DC offset from the PC or radio.
3. R2 (Bias Voltage): A 10kΩ trimpot connected between 3.3V (Pin 36) and AGND to pull the ADC's resting voltage to exactly 1.65V (the center of the ADC's range).
4. R3 + C2 (Low-Pass Filter): A 1kΩ resistor and 47nF capacitor forming a simple RC low-pass filter. This suppresses high-frequency RF noise and anti-aliases the signal before it hits the ADC.

Important: For the cleanest reception with the lowest noise floor, connect all ground lines of this circuit to the Pico's AGND (Analog Ground, Pin 33) rather than a regular digital ground.

Connect the biased output to GPIO 26 (Pin 31).

🛠️ USB Serial Diagnostics (Tuning Mode)

By connecting the Pico via USB to a PC terminal (9600 baud), you gain access to the raw DSP telemetry every 500ms:

--- TUNING DIAGNOSTICS ---
Step 1 (ADC): V=1.94V (Range: 1585-2524)
Step 2 (ATC Level): Mark_Env=0.0974 Space_Env=0.0894
Step 3 (FFT Peaks): SNR=67.8 dB, Signal=-10.3 dB
Step 4 (RTTY Status): Squelch=OPEN DPLL_Phase=0.95
Params: Baud=45.45 ALPHA=0.0350 K=0.75 SQ=4.0
---------------------------------

You can send commands via the terminal to adjust the DSP on the fly:

• ALPHA 0.05 - Adjust DPLL tracking width (default 0.035).
• K 0.6 - Adjust Biquad LPF bandwidth multiplier (default 0.75).
• SQ 6.0 - Adjust Squelch SNR threshold (default 4.0).
• CLEAR - Hard reset the DSP state, AFC, and DPLL phase.

📜 Release History

• v1.72 (2026-03-31): Phase 3 Final. Professional DSP demodulator stability, build 172, ili9488 driver refactoring.

🏗️ Build Instructions

Compiled via the standard Raspberry Pi Pico SDK (v2.2.0+) and CMake.

mkdir build && cd build
cmake -G Ninja ..
ninja
picotool load TouchRTTY.uf2