PC Audio Stream to Phone (AS2P)

PC Audio Stream to Phone (AS2P) is a specialized, open-source solution for real-time, low-latency audio streaming from your Windows PC to an Android device.

Security Notice

This project should currently be treated as a prototype-grade implementation rather than a hardened production audio transport. Use it only on trusted networks, and do not assume the current pairing or network behavior is suitable for hostile or shared environments without additional hardening. Review release-signing, network exposure, and deployment assumptions before distributing builds more broadly.

Screenshots

⚠️ Special Disclaimer

All code in this project (including the Windows Server and Android App) was automatically generated by the Gemini CLI.

• The developer has conducted basic functional testing and confirmed it is operational.
• The developer has not conducted a line-by-line manual code review or refactoring of the internal logic and security mechanisms.
• This project is currently in the Prototype stage and is intended for Proof of Concept (PoC) purposes.

---

Why AS2P?

There are several audio streaming solutions available on the market (e.g., AudioRelay, SoundWire, VBAN), but many are:

• Proprietary/Closed Source: You don't know what's happening under the hood.
• Paid/Subscription Based: Features like high-quality audio or low latency are often locked behind a paywall.
• Over-complicated: They often include complex routing features that users don't need for a simple PC-to-phone bridge.

AS2P is built to be a Free, Open-Source, and Lightweight alternative. It focuses on one thing and doing it well: Streaming high-quality audio with minimal latency and maximum stability.

---

Key Features

• Zero Bloat: No ads, no tracking, no complex setup. Just connect and listen.
• Modern Visual Interface:
  • Windows: Centralized, lightweight interface with "Run at Startup" support.
  • Android: Material Design 3 Dark Theme with a Dynamic Latency Chart, MediaStyle Notification, and Collapsible Advanced Tuning.
• Custom Setting Preset (v1.2.0):
  • Advanced Tuning UI: Complete control over Bitrate, Buffer Size, Catch-up Threshold, and PCM Pre-decode Target.
  • Demand-based Decoding: Tailor the CPU vs. Network stability balance by adjusting the PCM inventory target.
• Eco-Friendly Performance (v1.2.1):
  • Deep Power Optimization: Automatically stops non-essential background calculations and UI updates when the screen is off, maximizing battery life.
  • Static Notifications: Clean notification tray that only updates on state changes to reduce system overhead.
• High Resilience Architecture (v1.2.2):
  • Server Loop Decoupling: Completely separates audio capture/encoding from network handling. Prevents Discord or high network traffic from jittering the 5ms audio heart-beat.
  • Process Priority Elevation: Automatically elevates server.exe to HIGH_PRIORITY_CLASS on Windows to ensure CPU scheduling dominance during heavy system load.
  • Fixed Android UI Steps: Corrected slider logic for PCM Pre-decode Target to ensure smooth integer-only selection.
• High Performance Architecture:
  • Decoupled Worker Thread: Moves audio decoding out of the playback callback to eliminate glitches caused by CPU throttling.
  • Fine-grained Locking: Optimized mutex usage to prevent playback thread blocking during long decode operations.
  • Atomic Reset Mechanism: Includes a 100ms cooldown after preset switching to ensure seamless sequence synchronization.
  • PCM FIFO Buffer: Uses a thread-safe circular buffer to absorb system-level performance jitter.
  • Oboe Exclusive Mode: Direct hardware access for the lowest possible round-trip latency.
  • PLC Toggle: Runtime switch to enable/disable Packet Loss Concealment for audio artifact control.
  • Optimized Redundancy: Server introduces 1ms delay for duplicate packets to maximize time diversity against interference.
• Smart Persistence: Automatically remembers your last used Latency Preset.
• Battery Optimization: Proactively requests exemption to ensure glitch-free audio in background.
• Ultra-Low Latency (Pulse Architecture):
  • UDP Port Unification: Uses a single port (12345) for all traffic to maximize firewall penetration.
  • Jitter Buffer Catch-up: Automatically drops old packets to maintain a strict latency target.
  • Double-Send Redundancy: Server supports optional double-send mode to mitigate Wi-Fi interference.
• High Fidelity: Uses the Opus Codec at 48kHz Stereo with PLC (Packet Loss Concealment) support.
• Anti-Clipping: 10ms linear Fade-out/Fade-in on all transitions (Start/Stop/Device Change).
• CPU Efficient: Optimized Windows server; stops UI updates when hidden to consume < 0.1% CPU.
• Background Stable: Uses Foreground Service on Android to prevent system throttling, with persistent Media controls.
• Bluetooth-linked Lifecycle: Automatically stops the app and closes the connection if Bluetooth headphones are disconnected, whether searching or streaming.
• Smart Disconnect: Server automatically returns to Listening state if the connection is lost for 5 seconds.

Supported Platforms

Windows (Server)

• Tested: Windows 10/11 (x64).
• Optimized: Specifically designed to work flawlessly in background modes.

Android (Client)

• Minimum: Android 9.0 (Pie / API 28).
• Recommended: Android 13/14/15.
• Hardware: Fully supports 16KB page size environments (e.g., Pixel 6a).

---

Technical Stack

• Backend (Rust): cpal for capture, rubato for resampling, audiopus for encoding, Slint for the GUI.
• Mobile (Kotlin/C++): Oboe (C++) for low-latency playback, Jetpack Compose for UI.
• Protocol: AS2P Pulse V8 (Custom UDP-based protocol).

---

Initial Setup

1. Windows Audio Settings

To ensure the best audio quality:

• Open Sound Settings on your Windows PC.
• Set your default output device format to 48000 Hz (Studio Quality).

2. Firewall Configuration

The server communicates over UDP Port 12345.

• Ensure your firewall allows inbound/outbound traffic on this port for both Server and Client.

---

How to Use

1. Launch Server

1. Run server.exe. By default, the console window is hidden for a cleaner experience.
2. Debug Console: If you need to see logs or troubleshoot, open CMD or PowerShell and run:

   ./server.exe --debug

3. Arguments:
   • --debug: Enables detailed connection and audio logs.
   • --drop-rate <0.0-1.0>: Simulates packet loss for testing stability (e.g., 0.1 for 10% loss).

2. Connect Client

1. Open the Android app.
2. Click CONNECT. It will broadcast a discovery signal and link with the server instantly.
3. Background Streaming: You can now safely switch to other apps or turn off your screen. The connection will remain active until you click STOP or swipe away the app.
4. Spotify-style Notification: Control the stream directly from your notification tray or lock screen.

3. Adjust Presets

Choose between LOW_LATENCY, BALANCE, HIGH_QUALITY, and BEST_QUALITY depending on your Wi-Fi stability.

Troubleshooting

• Rapid Preset Switching: Switching presets too quickly (multiple clicks in a second) may cause the server to fall behind in reconfiguration, potentially leading to a temporary loss of audio streaming. If this happens, wait a few seconds or click STOP then CONNECT again.
• Server White Background: Occasionally, the server window may display a plain white background after running for a long period. This is a known rendering issue in the current prototype. Restarting the server or minimizing/restoring the window usually resolves it.
• Microsoft Defender Warning: On some systems, the "Run at Startup" feature might trigger a false positive in Defender. This is due to the combination of network activity and audio capture in an unsigned executable. We have implemented delayed initialization to mitigate this.
• Zombie Icon: Fixed in V8. Clicking Quit now removes the tray icon immediately.
• Mechanical Noise: Ensure Windows is set to 48kHz, though the server now resamples automatically.
• No Audio Captured: Ensure your PC is playing sound through the default output device.

License

Licensed under the MIT License.