VoiceBoostN

Real-time podcast audio normalizer for iOS. Normalizes audio to -14 LUFS with true-peak limiting, optimized for streaming playback.

Features

• Real-time processing — Works with streaming URLs, not just local files
• ITU BS.1770-4 compliant — Industry-standard LUFS measurement
• Low CPU usage — Optimized with Apple Accelerate/vDSP
• Complete processing chain:
  • Adaptive LUFS-based gain
  • 80Hz high-pass filter (removes rumble)
  • 2:1 compression (tames loud peaks)
  • True-peak limiter (-2 dBTP ceiling)

Processing Chain

Input → LUFS Measurement → Gain → High-Pass → Compressor → Limiter → Output (-14 LUFS)

Stage	Parameters	Purpose
LUFS Gain	Target: -14 LUFS	Normalize loudness
High-Pass	80 Hz, Q=0.707	Remove rumble/hum
Compressor	-8 dB threshold, 2:1 ratio	Reduce dynamic range
Limiter	-2 dBTP ceiling, 5ms lookahead	Prevent clipping

Project Structure

Normalizer/
├── VoiceBoostN/                    # Core C library
│   ├── VoiceBoostN.c               # Processing algorithms
│   ├── VoiceBoostN.h               # Public API
│   └── VoiceBoostNAnalysis.c       # Stats/analysis (optional)
├── AudioPlayer.swift               # Local files (AVAudioEngine)
├── StreamingAudioPlayer.swift      # Streaming (AVPlayer + MTAudioProcessingTap)
├── VoiceBoostNProcessorC.swift     # Swift wrapper for C code
└── ContentView.swift               # SwiftUI interface

scripts/
├── analyze_podcast.sh              # Process + quality report
├── process_podcast.sh              # Process audio file
└── audio_quality_test.sh           # Artifact detection

Usage

iOS App

1. Open Normalizer.xcodeproj in Xcode
2. Build and run (Cmd+R)
3. Add audio files to Normalizer/Files/ or use streaming URLs
4. Toggle VoiceBoostN on/off to compare
5. Use "Validate" for instant quality check

Command Line

# Process a podcast
./scripts/process_podcast.sh input.mp3 output.wav

# Process with full quality report
./scripts/analyze_podcast.sh input.mp3

# Test processed file for artifacts
./scripts/audio_quality_test.sh output.wav input.mp3

Requirements

iOS App:

• Xcode 14+
• iOS 15+ / macOS 12+

Command Line:

• ffmpeg / ffprobe
• python3, bc

Documentation

See CLAUDE.md for detailed technical documentation of the algorithm, AVPlayer integration, and optimization strategies.