Replay

Replay is a high-performance system audio recorder and player written in Go. It captures system audio output (loopback), compresses it in real-time using a hybrid Opus+Zstd pipeline, and provides a custom OpenGL-based GUI for managing playback segments.

🚀 Key Features

• System Loopback Recording: Automatically routes system audio (what you hear) to the recorder via PulseAudio monitor sources.

• High-Efficiency Compression: Implements a two-stage compression pipeline:

• Opus: For perceptual audio coding.

• Zstd: For additional lossless compression of the bitstream.

• Custom Audio Engine:

• Low-latency I/O using PortAudio.

• Lock-free Ring Buffers using atomic operations for thread-safe data transfer.

• Hardware Accelerated UI:

• Built from scratch using OpenGL 4.1 Core.

• Custom shader pipeline (GLSL) for rendering UI elements.

• No heavy UI frameworks — pure vertices and textures.

🛠 Tech Stack

• Language: Go (Golang)
• Audio I/O: PortAudio (gordonklaus/portaudio)
• Graphics: OpenGL 4.1 (go-gl), GLFW
• Compression: Opus (hraban/opus), Zstd (klauspost/compress/zstd)
• System Integration: PulseAudio (pactl CLI wrappers for routing)

🧠 Architecture Highlights

The Audio Pipeline

The application avoids GC pauses and blocking operations in the audio callback thread by utilizing a custom Ring Buffer implementation (internal/buffer).

1. Capture: Audio frames are captured via PortAudio.
2. Buffering: Raw PCM data is written to a lock-free Ring Buffer (buffer.go) using sync/atomic to manage read/write pointers safe across goroutines.
3. Processing: A separate goroutine drains the buffer, encodes audio chunks into Opus frames, and wraps them in Zstd for storage efficiency.
4. Storage: Data is written to disk in a custom binary format capable of handling multiple audio segments within a single file.

The Rendering Engine

Instead of using standard widgets, the UI is rendered directly via the GPU:

• Shaders: Custom Vertex and Fragment shaders handle texture mapping and color blending (shaders.go).
• Event Loop: Integrated with GLFW for handling mouse inputs and window events, manually mapping cursor coordinates to normalized device coordinates (NDC) for UI interaction.

📦 Usage

Prerequisites

• Linux (Due to pactl dependency for loopback routing)
• OpenGL drivers

Running

GUI Mode (Default):

./replay --path=session.rep

CLI Mode:

# Record strictly via CLI
./replay --path=music.rep --mode=record

# Replay strictly via CLI
./replay --path=music.rep --mode=replay

📝 Controls

• Record: Start capturing system audio.
• Stop: Finalize the current segment.
• Play/Pause: Playback the recorded segment.
• Prev/Next: Navigate between recorded segments (if multiple recordings exist in one session).

⚠️ Disclaimer

This project relies on pactl (PulseAudio Control) for automatic monitor source routing. It is designed primarily for Linux environments using PulseAudio or PipeWire-Pulse.

Licenses

This project is distributed under the MIT License. See the LICENSE file for details.

Third-party LICENSES