aetherwave

aetherwave is a Software Defined Radio (SDR) receiver written in Go. This project is a learning laboratory designed to explore the fundamental concepts of Digital Signal Processing (DSP) and radio communications from the ground up.

🚀 Project Overview

The application interfaces with SDR hardware (specifically RTL-SDR based devices) to acquire raw I/Q signals, process them through a custom filtering and demodulation pipeline, and playback the resulting audio in real-time through the system's sound card.

🛠 Architecture & Organization

The project is modularly structured to separate hardware acquisition, mathematical processing, and output:

1. Core DSP (internal/dsp)

The mathematical heart of the project. It handles complex (I/Q) signals and implements:

• Signals & Samples: Abstractions for complex samples (Sample) and signal buffers (Signal).
• Filtering: FIR (Finite Impulse Response) filter implementation with coefficient calculation using Hamming windows.
• Fourier Transform (FFT): Engine for spectral analysis, featuring both recursive and iterative in-place implementations.
• Resampling: Logic to bridge the gap between high-frequency radio sample rates and standard audio rates.

2. Demodulation (internal/demod)

Algorithms to extract audio information from radio waves:

• AM (Amplitude Modulation): Envelope extraction via magnitude calculation and DC-offset removal.
• SSB (Single Sideband): Support for USB and LSB using heterodyning (frequency shifting via BFO) and real-part extraction.
• FM: Interface prepared in the codebase.

3. Hardware & Drivers (internal/hardware)

Low-level communication with physical devices:

• RTL-SDR: Integration with librtlsdr via CGO to configure center frequency, sample rate, and gain.
• Uses a buffer pool strategy to minimize memory allocations and reduce garbage collector pressure.

4. Audio (internal/audio)

• Manages acoustic output using the PortAudio library.
• Implements asynchronous callbacks to ensure smooth playback without blocking the DSP pipeline.

5. Orchestration (internal/radio & commands)

• Receiver: The "brain" that coordinates the pipeline. It manages concurrency using separate goroutines for hardware reading and signal processing.
• Commands: Provides a Command Line Interface (CLI) to start listening on specific frequencies and modes.

📡 Processing Pipeline

The data flows as follows:

1. Hardware: I/Q sampling from the RTL-SDR USB dongle.
2. DSP: Low-pass filtering to isolate the target channel.
3. Demodulation: Conversion of the radio signal into audio samples.
4. Resampling: Adjusting the rate to match the sound card (e.g., 48kHz).
5. Output: Real-time playback via PortAudio.

🔧 Requirements

• Go: version 1.26+
• System Libraries:
  • librtlsdr (for SDR hardware access)
  • portaudio (for audio output)
• Hardware: An RTL-SDR compatible dongle (e.g., Nooelec, RTL-SDR Blog V3/V4).

🏁 Getting Started

Note: Ensure the C libraries mentioned above are installed on your system.

To start the radio on a specific frequency (e.g., 100.1 MHz in AM mode):

aetherwave-cli listen -freq 100100000 -mode AM