docs: Add comprehensive Piper TTS integration documentation

- Created PIPER_TTS_INTEGRATION.md with complete integration summary
- Added test_engine_selection.py for testing engine switching
- Documented engine comparison, usage examples, and pitch guidelines
- Listed all modified/created files and git commits
- Included next steps for optional enhancements

Task 26 completed: Piper TTS integration with engine selection UI

PIPER_TTS_INTEGRATION.md

@@ -0,0 +1,209 @@
+# Piper TTS Integration Summary
+
+## Overview
+
+Successfully integrated Piper TTS as an alternative text-to-speech engine alongside Kokoro TTS, giving users the ability to choose between two engines with different strengths.
+
+## What Was Added
+
+### Backend Components
+
+1. **PiperTTSService** (`backend/services/piper_tts_service.py`)
+   - Handles Piper TTS synthesis with pitch control
+   - Supports pitch shifting from -12 to +12 semitones per character
+   - Automatic voice model downloading
+   - Audio effects processing (pitch and speed)
+
+2. **UnifiedTTSService** (`backend/services/unified_tts_service.py`)
+   - Manages multiple TTS engines (Kokoro and Piper)
+   - Runtime engine switching
+   - Unified API for both engines
+   - Handles engine-specific features (pitch control for Piper)
+
+3. **New API Endpoints** (in `backend/main.py`)
+   - `GET /api/tts/engines` - List available engines and current selection
+   - `POST /api/tts/engines` - Switch active TTS engine
+   - `GET /api/tts/voices` - Get available voices for all engines
+   - Updated `POST /api/tts` to support `pitch_adjustments` parameter
+
+4. **Installation Script** (`backend/install_piper.sh`)
+   - Automated Piper TTS installation for macOS/Linux
+   - Detects OS and architecture
+   - Installs to `/usr/local/bin`
+
+### Frontend Components
+
+1. **API Client Functions** (`frontend/src/api/tts.ts`)
+   - `getEngines()` - Fetch available engines
+   - `setEngine(engine)` - Switch active engine
+   - `getVoices()` - Get voices for all engines
+   - TypeScript interfaces for engine data
+
+2. **UI Components** (`frontend/src/components/VoiceAssignmentSection.tsx`)
+   - Engine selector buttons (Kokoro vs Piper)
+   - Dynamic voice loading based on selected engine
+   - Visual feedback for active engine
+   - Pitch control capability notice for Piper
+
+### Documentation
+
+1. **Backend README** (`backend/README.md`)
+   - Multi-engine setup instructions
+   - Engine switching examples
+   - Pitch control documentation
+
+2. **Piper TTS Guide** (`backend/PIPER_TTS_GUIDE.md`)
+   - Comprehensive installation guide
+   - Available voices documentation
+   - Pitch adjustment guidelines
+   - Usage examples and best practices
+   - Troubleshooting section
+
+3. **Integration Summary** (this file)
+
+## Engine Comparison
+
+| Feature | Kokoro TTS | Piper TTS |
+|---------|------------|-----------|
+| **Quality** | High | Medium-High |
+| **Speed** | Medium | Fast |
+| **Voices** | 54 voices, 9 languages | 5+ English voices |
+| **Pitch Control** | ❌ No | ✅ Yes (-12 to +12 semitones) |
+| **Model Size** | ~500MB | ~10-50MB per voice |
+| **Memory Usage** | Higher | Lower |
+| **Languages** | 9 languages | English (more available) |
+
+## How to Use
+
+### Backend Setup
+
+1. **Install Piper TTS** (optional):
+   ```bash
+   cd backend
+   ./install_piper.sh
+   ```
+
+2. **Set default engine** (optional):
+   ```bash
+   # In backend/.env
+   TTS_ENGINE=piper  # or kokoro (default)
+   ```
+
+3. **Install Python dependencies**:
+   ```bash
+   pip install -r requirements.txt
+   ```
+
+### Frontend Usage
+
+1. **Select Engine**: In the Voice Assignment screen, click either "Kokoro TTS" or "Piper TTS" button
+2. **Choose Voices**: Voice dropdown updates automatically with engine-specific voices
+3. **Generate**: Audio is generated using the selected engine
+
+### API Usage
+
+**Switch Engine:**
+```bash
+curl -X POST http://localhost:8000/api/tts/engines \
+  -H "Content-Type: application/json" \
+  -d '{"engine": "piper"}'
+```
+
+**Generate with Pitch Control (Piper only):**
+```bash
+curl -X POST http://localhost:8000/api/tts \
+  -H "Content-Type: application/json" \
+  -d '{
+    "text": "Alice: Hello!\nBob: Hi there!",
+    "lang": "a",
+    "roles": [
+      {"id": "Alice", "voice": "en_US-amy-medium"},
+      {"id": "Bob", "voice": "en_US-lessac-medium"}
+    ],
+    "pitch_adjustments": [
+      {"id": "Alice", "pitch": 3},
+      {"id": "Bob", "pitch": -2}
+    ]
+  }' \
+  --output audiobook.wav
+```
+
+## Pitch Control Guidelines
+
+When using Piper TTS, you can adjust pitch per character:
+
+- **+12 semitones**: One octave higher (very high-pitched)
+- **+6 semitones**: Noticeably higher (child-like)
+- **+3 semitones**: Slightly higher (feminine adjustment)
+- **0 semitones**: Original voice (no change)
+- **-3 semitones**: Slightly lower (masculine adjustment)
+- **-6 semitones**: Noticeably lower (deep voice)
+- **-12 semitones**: One octave lower (very deep)
+
+**Recommended ranges:**
+- Female characters: +2 to +5
+- Male characters: -2 to -5
+- Children: +5 to +8
+- Elderly/deep voices: -4 to -8
+
+## Testing
+
+Run the engine selection test:
+```bash
+cd backend
+python test_engine_selection.py
+```
+
+## Files Modified/Created
+
+### Backend
+- ✅ `backend/services/piper_tts_service.py` (new)
+- ✅ `backend/services/unified_tts_service.py` (new)
+- ✅ `backend/main.py` (modified - added endpoints)
+- ✅ `backend/requirements.txt` (modified - added piper-tts, pydub)
+- ✅ `backend/.env.example` (modified - added TTS_ENGINE)
+- ✅ `backend/README.md` (modified - added multi-engine docs)
+- ✅ `backend/PIPER_TTS_GUIDE.md` (new)
+- ✅ `backend/install_piper.sh` (new)
+- ✅ `backend/test_engine_selection.py` (new)
+
+### Frontend
+- ✅ `frontend/src/api/tts.ts` (modified - added engine functions)
+- ✅ `frontend/src/components/VoiceAssignmentSection.tsx` (modified - added engine selector)
+
+### Documentation
+- ✅ `.kiro/specs/text-to-audiobook/tasks.md` (modified - added task 26)
+- ✅ `PIPER_TTS_INTEGRATION.md` (new - this file)
+
+## Git Commits
+
+1. **feat: Add Piper TTS integration with pitch control support** (bfdcc94)
+   - Backend services and API endpoints
+   - Installation script and documentation
+
+2. **feat: Add TTS engine selector UI in frontend** (c310f11)
+   - Frontend UI components
+   - API client functions
+
+## Next Steps (Optional Enhancements)
+
+1. **Pitch Control UI**: Add pitch sliders in the frontend for Piper engine
+2. **Voice Previews**: Update preview functionality to work with Piper voices
+3. **More Piper Voices**: Add support for additional Piper voice models
+4. **Coqui TTS**: Integrate Coqui XTTS as a third engine option
+5. **Voice Cloning**: Add voice cloning capabilities with Piper/Coqui
+
+## Notes
+
+- Piper TTS requires separate installation (not included in Python packages)
+- Voice models are downloaded automatically on first use
+- Kokoro remains the default engine for backward compatibility
+- Both engines can coexist and be switched at runtime
+- Pitch control is only available with Piper TTS
+
+## Support
+
+For issues or questions:
+- See `backend/PIPER_TTS_GUIDE.md` for detailed Piper documentation
+- See `backend/README.md` for general backend setup
+- Check troubleshooting sections in both guides

backend/.env.example

@@ -1,3 +1,4 @@
 PORT=8000
 LOG_LEVEL=info
 OLLAMA_BASE_URL=http://localhost:11434
+TTS_ENGINE=kokoro

backend/PIPER_TTS_GUIDE.md

@@ -0,0 +1,292 @@
+# Piper TTS Integration Guide
+
+This guide covers the Piper TTS engine integration, including installation, configuration, and usage.
+
+## What is Piper TTS?
+
+Piper is a fast, local neural text-to-speech system that runs entirely on your machine. It offers:
+
+- **Fast synthesis**: Faster than Kokoro for most use cases
+- **Pitch control**: Adjust voice pitch per character (-12 to +12 semitones)
+- **Multiple voices**: Various English voices with different characteristics
+- **Low resource usage**: Smaller models and lower memory footprint
+- **Privacy**: All processing happens locally
+
+## Installation
+
+### Automated Installation (Recommended)
+
+```bash
+cd backend
+./install_piper.sh
+```
+
+This script will:
+1. Detect your OS and architecture
+2. Download the appropriate Piper binary
+3. Install it to `/usr/local/bin`
+4. Verify the installation
+
+### Manual Installation
+
+1. Visit [Piper Releases](https://github.com/rhasspy/piper/releases/tag/v1.2.0)
+2. Download the appropriate archive for your platform:
+   - **Linux x86_64**: `piper_amd64.tar.gz`
+   - **Linux ARM64**: `piper_arm64.tar.gz`
+   - **macOS**: `piper_macos_x64.tar.gz`
+3. Extract the archive
+4. Copy the `piper` binary to a directory in your PATH
+
+### Verify Installation
+
+```bash
+piper --version
+```
+
+You should see output like: `1.2.0`
+
+## Available Voices
+
+Piper supports multiple English voices:
+
+| Voice ID | Language | Gender | Quality | Description |
+|----------|----------|--------|---------|-------------|
+| `en_US-lessac-medium` | US English | Male | Medium | Clear, neutral male voice |
+| `en_US-libritts-high` | US English | Neutral | High | High-quality neutral voice |
+| `en_US-amy-medium` | US English | Female | Medium | Clear female voice |
+| `en_GB-alan-medium` | UK English | Male | Medium | British male voice |
+| `en_GB-alba-medium` | UK English | Female | Medium | British female voice |
+
+Voice models are automatically downloaded on first use (~10-50MB each).
+
+## Configuration
+
+### Set Piper as Default Engine
+
+**Option 1: Environment Variable**
+```bash
+export TTS_ENGINE=piper
+```
+
+Add to your `.env` file:
+```bash
+TTS_ENGINE=piper
+```
+
+**Option 2: Runtime API Call**
+```bash
+curl -X POST http://localhost:8000/api/tts/engines \
+  -H "Content-Type: application/json" \
+  -d '{"engine": "piper"}'
+```
+
+### Voice Model Storage
+
+Voice models are stored in:
+```
+backend/piper_models/
+```
+
+Models are downloaded automatically on first use. You can pre-download models manually if needed.
+
+## Usage Examples
+
+### Basic Text-to-Speech
+
+```bash
+curl -X POST http://localhost:8000/api/tts \
+  -H "Content-Type: application/json" \
+  -d '{
+    "text": "Hello, this is a test of Piper TTS.",
+    "lang": "a",
+    "roles": [
+      {"id": "Narrator", "voice": "en_US-lessac-medium"}
+    ],
+    "speed": 1.0
+  }' \
+  --output test.wav
+```
+
+### Multi-Voice with Pitch Control
+
+```bash
+curl -X POST http://localhost:8000/api/tts \
+  -H "Content-Type: application/json" \
+  -d '{
+    "text": "Alice: Hello Bob!\nBob: Hi Alice, how are you?\nNarrator: They greeted each other warmly.",
+    "lang": "a",
+    "roles": [
+      {"id": "Alice", "voice": "en_US-amy-medium"},
+      {"id": "Bob", "voice": "en_US-lessac-medium"},
+      {"id": "Narrator", "voice": "en_GB-alan-medium"}
+    ],
+    "speed": 1.0,
+    "pitch_adjustments": [
+      {"id": "Alice", "pitch": 4},
+      {"id": "Bob", "pitch": -3},
+      {"id": "Narrator", "pitch": 0}
+    ]
+  }' \
+  --output audiobook.wav
+```
+
+### Pitch Adjustment Guidelines
+
+Pitch is measured in semitones (half-steps on a musical scale):
+
+- **+12 semitones**: One octave higher (very high-pitched)
+- **+6 semitones**: Noticeably higher (child-like)
+- **+3 semitones**: Slightly higher (feminine adjustment)
+- **0 semitones**: Original voice (no change)
+- **-3 semitones**: Slightly lower (masculine adjustment)
+- **-6 semitones**: Noticeably lower (deep voice)
+- **-12 semitones**: One octave lower (very deep)
+
+**Recommended ranges:**
+- Female characters: +2 to +5
+- Male characters: -2 to -5
+- Children: +5 to +8
+- Elderly/deep voices: -4 to -8
+
+## Python API Usage
+
+```python
+from services.unified_tts_service import UnifiedTTSService
+
+# Initialize with Piper engine
+tts = UnifiedTTSService(engine="piper")
+
+# Generate audiobook with pitch control
+audio_bytes = await tts.generate_audiobook(
+    text="Alice: Hello!\nBob: Hi there!",
+    lang="a",
+    role_voice_map={
+        "Alice": "en_US-amy-medium",
+        "Bob": "en_US-lessac-medium"
+    },
+    speed=1.0,
+    pitch_adjustments={
+        "Alice": 3,
+        "Bob": -2
+    }
+)
+
+# Save to file
+with open("output.wav", "wb") as f:
+    f.write(audio_bytes)
+```
+
+## Performance Comparison
+
+| Feature | Kokoro TTS | Piper TTS |
+|---------|------------|-----------|
+| Speed | Medium | Fast |
+| Quality | High | Medium-High |
+| Pitch Control | ❌ No | ✅ Yes |
+| Model Size | ~500MB | ~10-50MB per voice |
+| Memory Usage | Higher | Lower |
+| Voice Variety | 10+ voices | 5+ voices (expandable) |
+| Languages | 4 languages | English (more available) |
+
+## Troubleshooting
+
+### Piper Not Found
+
+**Error:** `piper: command not found`
+
+**Solution:**
+```bash
+# Check if piper is in PATH
+which piper
+
+# If not found, reinstall
+./install_piper.sh
+
+# Or add to PATH manually
+export PATH="/usr/local/bin:$PATH"
+```
+
+### Voice Model Download Fails
+
+**Error:** Model download timeout or failure
+
+**Solution:**
+1. Check internet connection
+2. Manually download models from [Piper releases](https://github.com/rhasspy/piper/releases)
+3. Place `.onnx` and `.onnx.json` files in `backend/piper_models/`
+
+### Audio Quality Issues
+
+**Problem:** Distorted or robotic audio
+
+**Solutions:**
+- Reduce pitch shift amount (stay within ±6 semitones)
+- Try different voice models
+- Adjust speed (try 0.9 or 1.1)
+- Check for text formatting issues
+
+### Synthesis Timeout
+
+**Error:** Piper synthesis times out
+
+**Solutions:**
+- Break text into smaller chunks
+- Increase timeout in `piper_tts_service.py`
+- Check system resources (CPU, memory)
+
+## Advanced Configuration
+
+### Custom Voice Models
+
+To add custom Piper voice models:
+
+1. Download `.onnx` and `.onnx.json` files
+2. Place in `backend/piper_models/`
+3. Update `AVAILABLE_VOICES` in `piper_tts_service.py`:
+
+```python
+AVAILABLE_VOICES = {
+    "en_US-custom-voice": {"lang": "en", "gender": "female", "quality": "high"},
+    # ... existing voices
+}
+```
+
+### Adjusting Audio Effects
+
+Modify `_apply_audio_effects()` in `piper_tts_service.py` to customize:
+- Pitch shifting algorithm
+- Speed adjustment method
+- Audio processing pipeline
+
+## Best Practices
+
+1. **Voice Selection**
+   - Use distinct voices for different characters
+   - Match voice gender to character when possible
+   - Use British voices for variety
+
+2. **Pitch Adjustments**
+   - Start with small adjustments (±2-3 semitones)
+   - Test and iterate to find natural-sounding values
+   - Avoid extreme values (±10+) unless intentional
+
+3. **Performance**
+   - Pre-download voice models for faster first use
+   - Cache generated audio when possible
+   - Use appropriate chunk sizes for long texts
+
+4. **Quality**
+   - Use high-quality voice models when available
+   - Adjust speed slightly (0.95-1.05) for more natural pacing
+   - Combine with AI segmentation for best results
+
+## Resources
+
+- [Piper GitHub Repository](https://github.com/rhasspy/piper)
+- [Piper Voice Samples](https://rhasspy.github.io/piper-samples/)
+- [Voice Model Downloads](https://github.com/rhasspy/piper/releases/tag/v1.2.0)
+- [Piper Documentation](https://github.com/rhasspy/piper/blob/master/README.md)
+
+## License
+
+Piper TTS is licensed under the MIT License.

backend/README.md

@@ -13,6 +13,7 @@ The backend provides REST API endpoints for:
 
 - Python 3.10 or higher
 - espeak-ng (system dependency for grapheme-to-phoneme conversion)
+- Piper TTS (optional, for alternative TTS engine with pitch control)
 - Docker (optional, for containerized deployment)
 
 ## Local Development Setup
@@ -62,7 +63,33 @@ pip install --upgrade pip
 pip install -r requirements.txt
 ```
 
-### 3. Ollama Setup (Optional but Recommended)
+### 3. Piper TTS Setup (Optional)
+
+For an alternative TTS engine with pitch control support:
+
+**Installation:**
+```bash
+# Run the installation script
+./install_piper.sh
+```
+
+Or install manually:
+- Visit [Piper releases](https://github.com/rhasspy/piper/releases)
+- Download the appropriate binary for your platform
+- Extract and place in your PATH
+
+**Verify installation:**
+```bash
+piper --version
+```
+
+**Benefits of Piper TTS:**
+- ✨ Pitch shifting support (-12 to +12 semitones per role)
+- ✨ Fast, lightweight neural TTS
+- ✨ Multiple voice models available
+- ✨ Lower resource usage than Kokoro
+
+### 4. Ollama Setup (Optional but Recommended)
 
 For AI-powered speaker detection and text segmentation, install Ollama:
 
@@ -98,7 +125,7 @@ curl http://localhost:11434/api/tags
 - ✨ Handles narrative text without explicit role markers
 - ✨ Falls back to pattern matching if unavailable
 
-### 4. Environment Configuration
+### 5. Environment Configuration
 
 ```bash
 cp .env.example .env
@@ -109,9 +136,10 @@ Edit `.env` if needed:
 PORT=8000
 LOG_LEVEL=info
 OLLAMA_BASE_URL=http://localhost:11434  # Optional: Ollama API URL
+TTS_ENGINE=kokoro  # Options: kokoro, piper (default: kokoro)
 ```
 
-### 5. Run the Server
+### 6. Run the Server
 
 ```bash
 uvicorn main:app --reload --host 0.0.0.0 --port 8000
@@ -179,6 +207,43 @@ The Dockerfile includes:
 - Storage: ~500MB for Kokoro models
 - CPU: Multi-core recommended
 
+## TTS Engine Selection
+
+The backend now supports multiple TTS engines that can be selected at runtime:
+
+### Available Engines
+
+1. **Kokoro TTS** (default)
+   - High-quality neural TTS
+   - Multiple voices per language
+   - No pitch control
+   - ~500MB model size
+
+2. **Piper TTS**
+   - Fast, lightweight neural TTS
+   - Pitch shifting support (-12 to +12 semitones)
+   - Multiple voice models
+   - Lower resource usage
+
+### Switching Engines
+
+**Via Environment Variable:**
+```bash
+export TTS_ENGINE=piper  # or kokoro
+uvicorn main:app --reload
+```
+
+**Via API:**
+```bash
+# Get available engines
+curl http://localhost:8000/api/tts/engines
+
+# Set active engine
+curl -X POST http://localhost:8000/api/tts/engines \
+  -H "Content-Type: application/json" \
+  -d '{"engine": "piper"}'
+```
+
 ## API Endpoints
 
 ### GET /api/health
@@ -243,6 +308,71 @@ curl -X POST http://localhost:8000/api/tts/roles \
   -d '{"text":"Alice: Hello!\nBob: Hi there!"}'
 ```
 
+### GET /api/tts/engines
+
+Get list of available TTS engines and current selection.
+
+**Response:**
+```json
+{
+  "engines": [
+    {"name": "kokoro", "active": true},
+    {"name": "piper", "active": false}
+  ],
+  "current": "kokoro"
+}
+```
+
+**Example:**
+```bash
+curl http://localhost:8000/api/tts/engines
+```
+
+### POST /api/tts/engines
+
+Set the active TTS engine.
+
+**Request Body:**
+```json
+{
+  "engine": "piper"
+}
+```
+
+**Response:**
+```json
+{
+  "status": "ok",
+  "engine": "piper"
+}
+```
+
+**Example:**
+```bash
+curl -X POST http://localhost:8000/api/tts/engines \
+  -H "Content-Type: application/json" \
+  -d '{"engine": "piper"}'
+```
+
+### GET /api/tts/voices
+
+Get available voices for all TTS engines.
+
+**Response:**
+```json
+{
+  "voices": {
+    "kokoro": ["af_bella", "af_sarah", "am_adam", "am_michael", ...],
+    "piper": ["en_US-lessac-medium", "en_US-amy-medium", ...]
+  }
+}
+```
+
+**Example:**
+```bash
+curl http://localhost:8000/api/tts/voices
+```
+
 ### POST /api/tts
 
 Generate multi-voice audiobook with AI-powered text segmentation.
@@ -271,6 +401,7 @@ When Ollama is available, the system uses AI to intelligently split text and ass
 - `lang` (required): Language code (a=American English, b=British English, f=French, j=Japanese)
 - `roles` (required): Array of role-to-voice mappings
 - `speed` (optional): Playback speed (0.5-1.5, default: 1.0)
+- `pitch_adjustments` (optional): Array of role-to-pitch mappings (Piper only, -12 to +12 semitones)
 
 **Response:**
 - Content-Type: `audio/wav`
@@ -285,7 +416,7 @@ When Ollama is available, the system uses AI to intelligently split text and ass
 - Content-Disposition: `attachment; filename="audiobook.wav"`
 - Body: Audio file bytes
 
-**Example:**
+**Example (Kokoro):**
 ```bash
 curl -X POST http://localhost:8000/api/tts \
   -H "Content-Type: application/json" \
@@ -301,6 +432,26 @@ curl -X POST http://localhost:8000/api/tts \
   --output audiobook.wav
 ```
 
+**Example (Piper with pitch control):**
+```bash
+curl -X POST http://localhost:8000/api/tts \
+  -H "Content-Type: application/json" \
+  -d '{
+    "text": "Alice: Hello!\nBob: Hi!",
+    "lang": "a",
+    "roles": [
+      {"id": "Alice", "voice": "en_US-amy-medium"},
+      {"id": "Bob", "voice": "en_US-lessac-medium"}
+    ],
+    "speed": 1.0,
+    "pitch_adjustments": [
+      {"id": "Alice", "pitch": 3},
+      {"id": "Bob", "pitch": -2}
+    ]
+  }' \
+  --output audiobook.wav
+```
+
 ## Kokoro TTS Configuration
 
 ### Supported Languages

backend/install_piper.sh

@@ -0,0 +1,67 @@
+#!/bin/bash
+# Script to install Piper TTS
+
+echo "Installing Piper TTS..."
+
+# Detect OS
+OS="$(uname -s)"
+ARCH="$(uname -m)"
+
+case "${OS}" in
+    Linux*)
+        if [ "${ARCH}" = "x86_64" ]; then
+            PIPER_URL="https://github.com/rhasspy/piper/releases/download/v1.2.0/piper_amd64.tar.gz"
+        elif [ "${ARCH}" = "aarch64" ]; then
+            PIPER_URL="https://github.com/rhasspy/piper/releases/download/v1.2.0/piper_arm64.tar.gz"
+        else
+            echo "Unsupported architecture: ${ARCH}"
+            exit 1
+        fi
+        ;;
+    Darwin*)
+        if [ "${ARCH}" = "x86_64" ] || [ "${ARCH}" = "arm64" ]; then
+            PIPER_URL="https://github.com/rhasspy/piper/releases/download/v1.2.0/piper_macos_x64.tar.gz"
+        else
+            echo "Unsupported architecture: ${ARCH}"
+            exit 1
+        fi
+        ;;
+    *)
+        echo "Unsupported OS: ${OS}"
+        exit 1
+        ;;
+esac
+
+# Create temp directory
+TEMP_DIR=$(mktemp -d)
+cd "${TEMP_DIR}"
+
+# Download Piper
+echo "Downloading Piper from ${PIPER_URL}..."
+curl -L -o piper.tar.gz "${PIPER_URL}"
+
+# Extract
+echo "Extracting..."
+tar -xzf piper.tar.gz
+
+# Install to /usr/local/bin (requires sudo)
+echo "Installing to /usr/local/bin (may require password)..."
+sudo cp piper/piper /usr/local/bin/
+sudo chmod +x /usr/local/bin/piper
+
+# Cleanup
+cd -
+rm -rf "${TEMP_DIR}"
+
+# Verify installation
+if command -v piper &> /dev/null; then
+    echo "✓ Piper TTS installed successfully!"
+    piper --version
+else
+    echo "✗ Installation failed"
+    exit 1
+fi
+
+echo ""
+echo "To download voice models, they will be automatically downloaded on first use."
+echo "Or you can manually download from: https://github.com/rhasspy/piper/releases/tag/v1.2.0"

backend/main.py

@@ -11,7 +11,7 @@ from pydantic import BaseModel, Field
 
 # Import services
 from services.role_detector import RoleDetector
-from services.tts_service import TTSService
+from services.unified_tts_service import UnifiedTTSService
 
 # Configure logging
 logging.basicConfig(
@@ -54,6 +54,11 @@ class RoleVoiceMapping(BaseModel):
     id: str = Field(..., description="Role identifier")
     voice: str = Field(..., description="Kokoro voice ID")
 
+class RolePitchMapping(BaseModel):
+    """Model for role-to-pitch mapping."""
+    id: str = Field(..., description="Role identifier")
+    pitch: int = Field(0, description="Pitch shift in semitones (-12 to +12)")
+
 class TTSRequest(BaseModel):
     """Request model for TTS generation endpoint."""
     segments: Optional[List[ScriptSegment]] = Field(None, description="Optional edited script segments")
@@ -61,6 +66,7 @@ class TTSRequest(BaseModel):
     lang: str = Field(..., description="Language code (a, b, f, j)")
     roles: List[RoleVoiceMapping] = Field(..., description="Role to voice mappings")
     speed: Optional[float] = Field(1.0, description="Playback speed (0.5-1.5)")
+    pitch_adjustments: Optional[List[RolePitchMapping]] = Field(None, description="Optional pitch adjustments per role (Piper only)")
 
 class VoicePreviewRequest(BaseModel):
     """Request model for voice preview endpoint."""
@@ -75,6 +81,24 @@ class ErrorResponse(BaseModel):
     message: str = Field(..., description="Human-readable error message")
     details: Optional[dict] = Field(None, description="Additional error details")
 
+class EngineInfo(BaseModel):
+    """Model for TTS engine information."""
+    name: str = Field(..., description="Engine identifier")
+    active: bool = Field(..., description="Whether this engine is currently active")
+
+class EnginesResponse(BaseModel):
+    """Response model for available engines."""
+    engines: List[EngineInfo] = Field(..., description="List of available TTS engines")
+    current: str = Field(..., description="Currently active engine")
+
+class SetEngineRequest(BaseModel):
+    """Request model for setting TTS engine."""
+    engine: str = Field(..., description="Engine to activate (kokoro or piper)")
+
+class VoicesResponse(BaseModel):
+    """Response model for available voices."""
+    voices: dict = Field(..., description="Dict mapping engine names to voice lists")
+
 # Initialize FastAPI app
 app = FastAPI(
     title="Text to Audiobook API",
@@ -120,7 +144,7 @@ async def general_exception_handler(request: Request, exc: Exception):
 
 # Initialize services
 role_detector = RoleDetector()
-tts_service = TTSService()
+tts_service = UnifiedTTSService()
 
 # API Endpoints
 @app.get("/api/health")
@@ -129,6 +153,80 @@ async def health_check():
     logger.info("Health check requested")
     return {"status": "ok"}
 
+@app.get("/api/tts/engines", response_model=EnginesResponse)
+async def get_engines():
+    """
+    Get list of available TTS engines and current selection.
+    
+    Returns:
+        EnginesResponse with list of engines and current active engine
+    """
+    logger.info("Engines list requested")
+    
+    try:
+        available = tts_service.get_available_engines()
+        current = tts_service.engine_name
+        
+        engines = [
+            EngineInfo(name=engine, active=(engine == current))
+            for engine in available
+        ]
+        
+        return EnginesResponse(engines=engines, current=current)
+    except Exception as e:
+        logger.error(f"Error getting engines: {str(e)}", exc_info=True)
+        raise
+
+@app.post("/api/tts/engines")
+async def set_engine(request: SetEngineRequest):
+    """
+    Set the active TTS engine.
+    
+    Args:
+        request: SetEngineRequest with engine name
+        
+    Returns:
+        Success message with new engine
+        
+    Raises:
+        400: If engine name is invalid
+    """
+    logger.info(f"Engine change requested: {request.engine}")
+    
+    try:
+        tts_service.set_engine(request.engine)
+        return {"status": "ok", "engine": request.engine}
+    except ValueError as e:
+        logger.error(f"Invalid engine: {str(e)}")
+        return JSONResponse(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            content={
+                "error": "ValidationError",
+                "message": str(e),
+                "details": {"field": "engine"}
+            }
+        )
+    except Exception as e:
+        logger.error(f"Error setting engine: {str(e)}", exc_info=True)
+        raise
+
+@app.get("/api/tts/voices", response_model=VoicesResponse)
+async def get_voices():
+    """
+    Get available voices for all TTS engines.
+    
+    Returns:
+        VoicesResponse with dict mapping engine names to voice lists
+    """
+    logger.info("Voices list requested")
+    
+    try:
+        voices = tts_service.get_available_voices()
+        return VoicesResponse(voices=voices)
+    except Exception as e:
+        logger.error(f"Error getting voices: {str(e)}", exc_info=True)
+        raise
+
 @app.post("/api/tts/roles", response_model=RolesResponse)
 async def detect_roles(request: RoleDetectionRequest):
     """
@@ -420,6 +518,11 @@ async def generate_audiobook(request: TTSRequest):
     # Convert roles list to dictionary for TTS service
     role_voice_map = {role.id: role.voice for role in request.roles}
     
+    # Convert pitch adjustments to dictionary if provided
+    pitch_map = None
+    if request.pitch_adjustments:
+        pitch_map = {p.id: p.pitch for p in request.pitch_adjustments}
+    
     try:
         # Generate audiobook using segments if provided, otherwise use text
         if request.segments:
@@ -428,7 +531,8 @@ async def generate_audiobook(request: TTSRequest):
                 segments=[(seg.speaker, seg.text) for seg in request.segments],
                 lang=request.lang,
                 role_voice_map=role_voice_map,
-                speed=request.speed
+                speed=request.speed,
+                pitch_adjustments=pitch_map
             )
         else:
             logger.info("Generating audiobook from text with automatic segmentation")
@@ -436,7 +540,8 @@ async def generate_audiobook(request: TTSRequest):
                 text=request.text,
                 lang=request.lang,
                 role_voice_map=role_voice_map,
-                speed=request.speed
+                speed=request.speed,
+                pitch_adjustments=pitch_map
             )
         
         # Create streaming response with proper headers

backend/requirements.txt

@@ -6,3 +6,5 @@ python-multipart==0.0.6
 pydantic==2.5.3
 numpy==1.26.3
 httpx==0.26.0
+piper-tts==1.2.0
+pydub==0.25.1

backend/services/piper_tts_service.py

@@ -0,0 +1,219 @@
+"""
+Piper TTS service for generating multi-voice audiobooks.
+"""
+import logging
+import io
+import os
+import subprocess
+from typing import Dict, List, Tuple, Optional
+import numpy as np
+import soundfile as sf
+from pydub import AudioSegment
+from pydub.effects import speedup
+
+logger = logging.getLogger(__name__)
+
+
+class PiperTTSService:
+    """
+    Service for text-to-speech synthesis using Piper TTS.
+    
+    Piper is a fast, local neural TTS system that supports multiple voices
+    and allows pitch modification through audio processing.
+    """
+    
+    # Default voice for unmapped roles
+    DEFAULT_VOICE = "en_US-lessac-medium"
+    
+    # Sample rate for Piper TTS (typically 22050)
+    SAMPLE_RATE = 22050
+    
+    # Available Piper voices with their characteristics
+    AVAILABLE_VOICES = {
+        # English voices
+        "en_US-lessac-medium": {"lang": "en", "gender": "male", "quality": "medium"},
+        "en_US-libritts-high": {"lang": "en", "gender": "neutral", "quality": "high"},
+        "en_US-amy-medium": {"lang": "en", "gender": "female", "quality": "medium"},
+        "en_GB-alan-medium": {"lang": "en", "gender": "male", "quality": "medium"},
+        "en_GB-alba-medium": {"lang": "en", "gender": "female", "quality": "medium"},
+    }
+    
+    def __init__(self):
+        """Initializes Piper TTS service."""
+        self.models_dir = os.path.join(os.path.dirname(__file__), "..", "piper_models")
+        os.makedirs(self.models_dir, exist_ok=True)
+        logger.info("PiperTTSService initialized")
+    
+    def _check_piper_installed(self) -> bool:
+        """Check if Piper is installed and accessible."""
+        try:
+            result = subprocess.run(
+                ["piper", "--version"],
+                capture_output=True,
+                text=True,
+                timeout=5
+            )
+            return result.returncode == 0
+        except (subprocess.SubprocessError, FileNotFoundError):
+            return False
+    
+    def _download_voice_model(self, voice: str) -> bool:
+        """
+        Download a Piper voice model if not already present.
+        
+        Args:
+            voice: Voice ID to download
+            
+        Returns:
+            True if model is available, False otherwise
+        """
+        model_path = os.path.join(self.models_dir, f"{voice}.onnx")
+        config_path = os.path.join(self.models_dir, f"{voice}.onnx.json")
+        
+        # Check if model already exists
+        if os.path.exists(model_path) and os.path.exists(config_path):
+            return True
+        
+        logger.info(f"Downloading Piper voice model: {voice}")
+        
+        # Download using piper's built-in download
+        try:
+            subprocess.run(
+                ["piper", "--model", voice, "--download-dir", self.models_dir],
+                check=True,
+                timeout=60
+            )
+            return os.path.exists(model_path)
+        except subprocess.SubprocessError as e:
+            logger.error(f"Failed to download voice model {voice}: {e}")
+            return False
+    
+    def synthesize_with_pitch(
+        self,
+        text: str,
+        voice: str,
+        speed: float = 1.0,
+        pitch_shift: int = 0
+    ) -> np.ndarray:
+        """
+        Synthesizes text with Piper and applies pitch shifting.
+        
+        Args:
+            text: Text to synthesize
+            voice: Piper voice ID
+            speed: Playback speed (0.5-2.0)
+            pitch_shift: Semitones to shift pitch (-12 to +12)
+            
+        Returns:
+            Audio samples as numpy array
+        """
+        # Ensure voice model is available
+        if not self._download_voice_model(voice):
+            logger.warning(f"Voice {voice} not available, using default")
+            voice = self.DEFAULT_VOICE
+            self._download_voice_model(voice)
+        
+        model_path = os.path.join(self.models_dir, f"{voice}.onnx")
+        
+        # Generate audio with Piper
+        try:
+            # Run piper command
+            process = subprocess.Popen(
+                ["piper", "--model", model_path, "--output-raw"],
+                stdin=subprocess.PIPE,
+                stdout=subprocess.PIPE,
+                stderr=subprocess.PIPE
+            )
+            
+            # Send text and get audio
+            stdout, stderr = process.communicate(input=text.encode('utf-8'), timeout=30)
+            
+            if process.returncode != 0:
+                logger.error(f"Piper synthesis failed: {stderr.decode()}")
+                return np.array([], dtype=np.float32)
+            
+            # Convert raw audio to numpy array
+            audio_data = np.frombuffer(stdout, dtype=np.int16)
+            audio_float = audio_data.astype(np.float32) / 32768.0
+            
+            # Apply pitch shift and speed change using pydub
+            if pitch_shift != 0 or speed != 1.0:
+                audio_float = self._apply_audio_effects(
+                    audio_float,
+                    self.SAMPLE_RATE,
+                    speed,
+                    pitch_shift
+                )
+            
+            return audio_float
+            
+        except subprocess.TimeoutExpired:
+            logger.error("Piper synthesis timed out")
+            return np.array([], dtype=np.float32)
+        except Exception as e:
+            logger.error(f"Error in Piper synthesis: {e}")
+            return np.array([], dtype=np.float32)
+    
+    def _apply_audio_effects(
+        self,
+        audio: np.ndarray,
+        sample_rate: int,
+        speed: float,
+        pitch_shift: int
+    ) -> np.ndarray:
+        """
+        Apply speed and pitch effects to audio using pydub.
+        
+        Args:
+            audio: Audio samples
+            sample_rate: Sample rate
+            speed: Speed multiplier
+            pitch_shift: Semitones to shift
+            
+        Returns:
+            Processed audio samples
+        """
+        try:
+            # Convert numpy to pydub AudioSegment
+            audio_int16 = (audio * 32768.0).astype(np.int16)
+            audio_bytes = audio_int16.tobytes()
+            
+            audio_segment = AudioSegment(
+                audio_bytes,
+                frame_rate=sample_rate,
+                sample_width=2,
+                channels=1
+            )
+            
+            # Apply pitch shift
+            if pitch_shift != 0:
+                # Pitch shift by changing frame rate then resampling
+                new_sample_rate = int(sample_rate * (2.0 ** (pitch_shift / 12.0)))
+                audio_segment = audio_segment._spawn(
+                    audio_segment.raw_data,
+                    overrides={'frame_rate': new_sample_rate}
+                )
+                audio_segment = audio_segment.set_frame_rate(sample_rate)
+            
+            # Apply speed change
+            if speed != 1.0:
+                audio_segment = speedup(audio_segment, playback_speed=speed)
+            
+            # Convert back to numpy
+            samples = np.array(audio_segment.get_array_of_samples())
+            audio_float = samples.astype(np.float32) / 32768.0
+            
+            return audio_float
+            
+        except Exception as e:
+            logger.error(f"Error applying audio effects: {e}")
+            return audio
+    
+    def get_available_voices(self) -> Dict[str, Dict[str, str]]:
+        """
+        Returns dictionary of available Piper voices.
+        
+        Returns:
+            Dict mapping voice IDs to their characteristics
+        """
+        return self.AVAILABLE_VOICES.copy()

backend/services/unified_tts_service.py

@@ -0,0 +1,283 @@
+"""
+Unified TTS service that supports multiple TTS engines (Kokoro, Piper).
+"""
+import logging
+import os
+from typing import Dict, List, Tuple, Optional
+from enum import Enum
+
+logger = logging.getLogger(__name__)
+
+
+class TTSEngine(str, Enum):
+    """Available TTS engines."""
+    KOKORO = "kokoro"
+    PIPER = "piper"
+
+
+class UnifiedTTSService:
+    """
+    Unified service that delegates to specific TTS engines.
+    Allows runtime selection between Kokoro and Piper TTS.
+    """
+    
+    def __init__(self, engine: Optional[str] = None, ollama_service=None):
+        """
+        Initialize unified TTS service.
+        
+        Args:
+            engine: TTS engine to use ('kokoro' or 'piper'). 
+                   Defaults to TTS_ENGINE env var or 'kokoro'
+            ollama_service: Optional OllamaService for AI segmentation
+        """
+        self.engine_name = engine or os.getenv("TTS_ENGINE", "kokoro")
+        self.ollama_service = ollama_service
+        
+        # Lazy load engines
+        self._kokoro_service = None
+        self._piper_service = None
+        
+        logger.info(f"UnifiedTTSService initialized with engine: {self.engine_name}")
+    
+    @property
+    def kokoro_service(self):
+        """Lazy load Kokoro service."""
+        if self._kokoro_service is None:
+            from .tts_service import TTSService
+            self._kokoro_service = TTSService(self.ollama_service)
+        return self._kokoro_service
+    
+    @property
+    def piper_service(self):
+        """Lazy load Piper service."""
+        if self._piper_service is None:
+            from .piper_tts_service import PiperTTSService
+            self._piper_service = PiperTTSService()
+        return self._piper_service
+    
+    def set_engine(self, engine: str):
+        """
+        Change the active TTS engine.
+        
+        Args:
+            engine: Engine name ('kokoro' or 'piper')
+        """
+        if engine not in [e.value for e in TTSEngine]:
+            raise ValueError(f"Unknown engine: {engine}. Use 'kokoro' or 'piper'")
+        
+        self.engine_name = engine
+        logger.info(f"Switched TTS engine to: {engine}")
+    
+    def get_available_engines(self) -> List[str]:
+        """
+        Get list of available TTS engines.
+        
+        Returns:
+            List of engine names
+        """
+        return [e.value for e in TTSEngine]
+    
+    def get_available_voices(self) -> Dict[str, List[str]]:
+        """
+        Get available voices for all engines.
+        
+        Returns:
+            Dict mapping engine names to lists of voice IDs
+        """
+        voices = {}
+        
+        # Kokoro voices (from documentation)
+        voices["kokoro"] = [
+            "af_bella", "af_nicole", "af_sarah", "af_sky",
+            "am_adam", "am_michael", "bf_emma", "bf_isabella",
+            "bm_george", "bm_lewis"
+        ]
+        
+        # Piper voices
+        try:
+            piper_voices = self.piper_service.get_available_voices()
+            voices["piper"] = list(piper_voices.keys())
+        except Exception as e:
+            logger.warning(f"Could not load Piper voices: {e}")
+            voices["piper"] = []
+        
+        return voices
+    
+    async def generate_script_segments(
+        self,
+        text: str,
+        roles: List[str]
+    ) -> List[Tuple[str, str]]:
+        """
+        Generate script segments with speaker assignments.
+        Uses AI when available, falls back to pattern matching.
+        
+        Args:
+            text: Input text to segment
+            roles: List of detected role IDs/names
+            
+        Returns:
+            List of tuples: (speaker_name, text_content)
+        """
+        # This is engine-agnostic, use Kokoro's implementation
+        return await self.kokoro_service.generate_script_segments(text, roles)
+    
+    async def generate_audiobook(
+        self,
+        text: str,
+        lang: str,
+        role_voice_map: Dict[str, str],
+        speed: float = 1.0,
+        pitch_adjustments: Optional[Dict[str, int]] = None
+    ) -> bytes:
+        """
+        Generate audiobook using the selected TTS engine.
+        
+        Args:
+            text: Input text
+            lang: Language code
+            role_voice_map: Role to voice ID mapping
+            speed: Playback speed
+            pitch_adjustments: Optional dict of role to pitch shift (semitones)
+            
+        Returns:
+            Audio data as bytes (WAV format)
+        """
+        if self.engine_name == TTSEngine.KOKORO:
+            # Kokoro doesn't support pitch adjustment
+            return await self.kokoro_service.generate_audiobook(
+                text, lang, role_voice_map, speed
+            )
+        
+        elif self.engine_name == TTSEngine.PIPER:
+            # Use Piper with pitch support
+            return await self._generate_with_piper(
+                text, lang, role_voice_map, speed, pitch_adjustments or {}
+            )
+        
+        else:
+            raise ValueError(f"Unknown engine: {self.engine_name}")
+    
+    async def generate_audiobook_from_segments(
+        self,
+        segments: List[Tuple[str, str]],
+        lang: str,
+        role_voice_map: Dict[str, str],
+        speed: float = 1.0,
+        pitch_adjustments: Optional[Dict[str, int]] = None
+    ) -> bytes:
+        """
+        Generate audiobook from pre-segmented script.
+        
+        Args:
+            segments: List of (speaker_name, text_content) tuples
+            lang: Language code
+            role_voice_map: Role to voice ID mapping
+            speed: Playback speed
+            pitch_adjustments: Optional dict of role to pitch shift (semitones)
+            
+        Returns:
+            Audio data as bytes (WAV format)
+        """
+        if self.engine_name == TTSEngine.KOKORO:
+            return await self.kokoro_service.generate_audiobook_from_segments(
+                segments, lang, role_voice_map, speed
+            )
+        
+        elif self.engine_name == TTSEngine.PIPER:
+            return await self._generate_from_segments_with_piper(
+                segments, lang, role_voice_map, speed, pitch_adjustments or {}
+            )
+        
+        else:
+            raise ValueError(f"Unknown engine: {self.engine_name}")
+    
+    async def _generate_with_piper(
+        self,
+        text: str,
+        lang: str,
+        role_voice_map: Dict[str, str],
+        speed: float,
+        pitch_adjustments: Dict[str, int]
+    ) -> bytes:
+        """Generate audiobook using Piper TTS with pitch control."""
+        import io
+        import numpy as np
+        import soundfile as sf
+        
+        # Segment text using Kokoro's logic
+        segments = await self.kokoro_service.segment_text_with_ai(text, role_voice_map)
+        
+        if segments is None:
+            segments = self.kokoro_service.segment_text(text, role_voice_map)
+        
+        if not segments:
+            raise ValueError("No segments to synthesize")
+        
+        # Synthesize each segment with Piper
+        audio_segments = []
+        for role_id, voice, content in segments:
+            pitch_shift = pitch_adjustments.get(role_id, 0)
+            logger.debug(
+                f"Synthesizing with Piper: role='{role_id}', voice='{voice}', "
+                f"pitch={pitch_shift}"
+            )
+            
+            audio = self.piper_service.synthesize_with_pitch(
+                content, voice, speed, pitch_shift
+            )
+            audio_segments.append(audio)
+        
+        # Concatenate
+        final_audio = np.concatenate([s for s in audio_segments if len(s) > 0])
+        
+        # Convert to WAV bytes
+        buffer = io.BytesIO()
+        sf.write(buffer, final_audio, self.piper_service.SAMPLE_RATE, format='WAV')
+        buffer.seek(0)
+        
+        logger.info(f"Generated Piper audiobook: {len(buffer.getvalue())} bytes")
+        return buffer.read()
+    
+    async def _generate_from_segments_with_piper(
+        self,
+        segments: List[Tuple[str, str]],
+        lang: str,
+        role_voice_map: Dict[str, str],
+        speed: float,
+        pitch_adjustments: Dict[str, int]
+    ) -> bytes:
+        """Generate audiobook from segments using Piper TTS."""
+        import io
+        import numpy as np
+        import soundfile as sf
+        
+        if not segments:
+            raise ValueError("No segments provided")
+        
+        # Synthesize each segment
+        audio_segments = []
+        for speaker, text in segments:
+            voice = role_voice_map.get(speaker, self.piper_service.DEFAULT_VOICE)
+            pitch_shift = pitch_adjustments.get(speaker, 0)
+            
+            logger.debug(
+                f"Synthesizing segment: speaker='{speaker}', voice='{voice}', "
+                f"pitch={pitch_shift}"
+            )
+            
+            audio = self.piper_service.synthesize_with_pitch(
+                text, voice, speed, pitch_shift
+            )
+            audio_segments.append(audio)
+        
+        # Concatenate
+        final_audio = np.concatenate([s for s in audio_segments if len(s) > 0])
+        
+        # Convert to WAV bytes
+        buffer = io.BytesIO()
+        sf.write(buffer, final_audio, self.piper_service.SAMPLE_RATE, format='WAV')
+        buffer.seek(0)
+        
+        logger.info(f"Generated Piper audiobook from segments: {len(buffer.getvalue())} bytes")
+        return buffer.read()

backend/test_engine_selection.py

@@ -0,0 +1,58 @@
+"""
+Test script for TTS engine selection functionality.
+"""
+import asyncio
+import sys
+from services.unified_tts_service import UnifiedTTSService
+
+
+async def test_engine_selection():
+    """Test switching between TTS engines."""
+    print("Testing TTS Engine Selection\n" + "="*50)
+    
+    # Initialize service
+    print("\n1. Initializing UnifiedTTSService...")
+    tts = UnifiedTTSService()
+    print(f"   Default engine: {tts.engine_name}")
+    
+    # Get available engines
+    print("\n2. Getting available engines...")
+    engines = tts.get_available_engines()
+    print(f"   Available: {engines}")
+    
+    # Get available voices
+    print("\n3. Getting available voices...")
+    voices = tts.get_available_voices()
+    for engine, voice_list in voices.items():
+        print(f"   {engine}: {len(voice_list)} voices")
+        if voice_list:
+            print(f"      Examples: {voice_list[:3]}")
+    
+    # Test switching to Piper
+    print("\n4. Switching to Piper engine...")
+    try:
+        tts.set_engine('piper')
+        print(f"   Current engine: {tts.engine_name}")
+    except Exception as e:
+        print(f"   ⚠️  Error: {e}")
+        print("   Note: Piper may not be installed. Run ./install_piper.sh")
+    
+    # Switch back to Kokoro
+    print("\n5. Switching back to Kokoro engine...")
+    tts.set_engine('kokoro')
+    print(f"   Current engine: {tts.engine_name}")
+    
+    # Test invalid engine
+    print("\n6. Testing invalid engine...")
+    try:
+        tts.set_engine('invalid')
+        print("   ❌ Should have raised error!")
+    except ValueError as e:
+        print(f"   ✓ Correctly rejected: {e}")
+    
+    print("\n" + "="*50)
+    print("✓ Engine selection tests completed!")
+
+
+if __name__ == "__main__":
+    asyncio.run(test_engine_selection())

frontend/src/api/tts.ts

@@ -256,6 +256,31 @@ export async function generateAudiobook(payload: GenerateRequest): Promise<Blob>
   return await response.blob();
 }
 
+/**
+ * TTS Engine information
+ */
+export interface EngineInfo {
+  name: string;
+  active: boolean;
+}
+
+/**
+ * Response from engines endpoint
+ */
+export interface EnginesResponse {
+  engines: EngineInfo[];
+  current: string;
+}
+
+/**
+ * Response from voices endpoint
+ */
+export interface VoicesResponse {
+  voices: {
+    [engine: string]: string[];
+  };
+}
+
 /**
  * Request payload for voice preview
  */
@@ -266,6 +291,65 @@ export interface VoicePreviewRequest {
   text?: string;
 }
 
+/**
+ * Get available TTS engines
+ * @returns Promise resolving to engines list
+ * @throws ApiError if the request fails
+ */
+export async function getEngines(): Promise<EnginesResponse> {
+  const baseUrl = getApiBaseUrl();
+  const url = `${baseUrl}/api/tts/engines`;
+  
+  const response = await fetchWithErrorHandling(url, {
+    method: 'GET',
+    headers: {
+      'Content-Type': 'application/json',
+    },
+  });
+  
+  return await response.json();
+}
+
+/**
+ * Set the active TTS engine
+ * @param engine - Engine name to activate (kokoro or piper)
+ * @returns Promise resolving to success response
+ * @throws ApiError if the request fails
+ */
+export async function setEngine(engine: string): Promise<{ status: string; engine: string }> {
+  const baseUrl = getApiBaseUrl();
+  const url = `${baseUrl}/api/tts/engines`;
+  
+  const response = await fetchWithErrorHandling(url, {
+    method: 'POST',
+    headers: {
+      'Content-Type': 'application/json',
+    },
+    body: JSON.stringify({ engine }),
+  });
+  
+  return await response.json();
+}
+
+/**
+ * Get available voices for all engines
+ * @returns Promise resolving to voices by engine
+ * @throws ApiError if the request fails
+ */
+export async function getVoices(): Promise<VoicesResponse> {
+  const baseUrl = getApiBaseUrl();
+  const url = `${baseUrl}/api/tts/voices`;
+  
+  const response = await fetchWithErrorHandling(url, {
+    method: 'GET',
+    headers: {
+      'Content-Type': 'application/json',
+    },
+  });
+  
+  return await response.json();
+}
+
 /**
  * Generate a preview of a voice
  * @param payload - Preview request with voice, language, and optional speed/text

frontend/src/components/VoiceAssignmentSection.tsx

@@ -1,6 +1,6 @@
-import { useState } from 'react';
+import { useState, useEffect } from 'react';
 import type { Role } from '../api/tts';
-import { previewVoice } from '../api/tts';
+import { previewVoice, getEngines, setEngine, getVoices } from '../api/tts';
 
 type AgeCategory = 'young' | 'teen' | 'adult' | 'old';
 
@@ -161,6 +161,40 @@ export default function VoiceAssignmentSection({
 }: VoiceAssignmentSectionProps) {
   const [previewingRole, setPreviewingRole] = useState<string | null>(null);
   const [audioCache, setAudioCache] = useState<Map<string, string>>(new Map());
+  const [currentEngine, setCurrentEngine] = useState<string>('kokoro');
+  const [availableVoices, setAvailableVoices] = useState<VoiceOption[]>(KOKORO_VOICES);
+  const [isLoadingEngine, setIsLoadingEngine] = useState(false);
+
+  // Load current engine and voices on mount
+  useEffect(() => {
+    const loadEngineInfo = async () => {
+      try {
+        const [enginesData, voicesData] = await Promise.all([
+          getEngines(),
+          getVoices()
+        ]);
+        
+        setCurrentEngine(enginesData.current);
+        
+        // Update available voices based on current engine
+        if (enginesData.current === 'piper' && voicesData.voices.piper) {
+          const piperVoices: VoiceOption[] = voicesData.voices.piper.map(voiceId => ({
+            id: voiceId,
+            label: voiceId.replace(/_/g, ' ').replace(/-/g, ' - '),
+            gender: voiceId.includes('amy') || voiceId.includes('alba') ? 'female' : 'male',
+            accent: voiceId.includes('GB') ? 'British' : 'American'
+          }));
+          setAvailableVoices(piperVoices);
+        } else {
+          setAvailableVoices(KOKORO_VOICES);
+        }
+      } catch (error) {
+        console.error('Failed to load engine info:', error);
+      }
+    };
+    
+    loadEngineInfo();
+  }, []);
 
   const handleVoiceSelect = (roleId: string, voiceId: string) => {
     onVoiceChange(roleId, voiceId);
@@ -231,6 +265,36 @@ export default function VoiceAssignmentSection({
     }
   };
 
+  const handleEngineChange = async (engine: string) => {
+    setIsLoadingEngine(true);
+    try {
+      await setEngine(engine);
+      setCurrentEngine(engine);
+      
+      // Load voices for new engine
+      const voicesData = await getVoices();
+      
+      if (engine === 'piper' && voicesData.voices.piper) {
+        const piperVoices: VoiceOption[] = voicesData.voices.piper.map(voiceId => ({
+          id: voiceId,
+          label: voiceId.replace(/_/g, ' ').replace(/-/g, ' - '),
+          gender: voiceId.includes('amy') || voiceId.includes('alba') ? 'female' : 'male',
+          accent: voiceId.includes('GB') ? 'British' : 'American'
+        }));
+        setAvailableVoices(piperVoices);
+      } else {
+        setAvailableVoices(KOKORO_VOICES);
+      }
+      
+      // Clear audio cache when switching engines
+      setAudioCache(new Map());
+    } catch (error) {
+      console.error('Failed to switch engine:', error);
+    } finally {
+      setIsLoadingEngine(false);
+    }
+  };
+
   const handleLanguageSelect = (e: React.ChangeEvent<HTMLSelectElement>) => {
     onLanguageChange(e.target.value);
   };
@@ -250,6 +314,50 @@ export default function VoiceAssignmentSection({
           <p className="text-lg text-gray-600">Choose a voice for each character in your story.</p>
         </div>
 
+        {/* TTS Engine Selector */}
+        <div className="p-6 bg-gradient-to-br from-indigo-50 to-purple-50 rounded-2xl border-2 border-indigo-200">
+          <label className="block text-base font-bold text-gray-900 mb-3">
+            TTS Engine
+          </label>
+          <div className="flex gap-4">
+            <button
+              onClick={() => handleEngineChange('kokoro')}
+              disabled={isLoadingEngine}
+              className={`flex-1 px-6 py-4 rounded-xl font-semibold transition-all duration-200 ${
+                currentEngine === 'kokoro'
+                  ? 'bg-indigo-600 text-white shadow-lg shadow-indigo-500/30'
+                  : 'bg-white text-gray-700 border-2 border-gray-200 hover:border-indigo-300'
+              }`}
+            >
+              <div className="text-left">
+                <div className="font-bold">Kokoro TTS</div>
+                <div className="text-sm opacity-90">High quality, 54 voices</div>
+              </div>
+            </button>
+            <button
+              onClick={() => handleEngineChange('piper')}
+              disabled={isLoadingEngine}
+              className={`flex-1 px-6 py-4 rounded-xl font-semibold transition-all duration-200 ${
+                currentEngine === 'piper'
+                  ? 'bg-indigo-600 text-white shadow-lg shadow-indigo-500/30'
+                  : 'bg-white text-gray-700 border-2 border-gray-200 hover:border-indigo-300'
+              }`}
+            >
+              <div className="text-left">
+                <div className="font-bold">Piper TTS</div>
+                <div className="text-sm opacity-90">Fast, pitch control</div>
+              </div>
+            </button>
+          </div>
+          {currentEngine === 'piper' && (
+            <div className="mt-3 p-3 bg-blue-50 border border-blue-200 rounded-lg">
+              <p className="text-sm text-blue-900">
+                <strong>✨ Pitch Control:</strong> Piper allows you to adjust voice pitch per character (coming soon in UI)
+              </p>
+            </div>
+          )}
+        </div>
+
         {/* Voice Assignments */}
         <div className="space-y-5">
           {roles.map((role, index) => {
@@ -275,7 +383,7 @@ export default function VoiceAssignmentSection({
                     className="flex-1 p-4 border-2 border-gray-200 rounded-xl focus:border-indigo-500 focus:ring-4 focus:ring-indigo-100 focus:outline-none bg-white text-base font-medium transition-all duration-200"
                   >
                     <option value="">Select a voice...</option>
-                    {KOKORO_VOICES.map((voice) => (
+                    {availableVoices.map((voice) => (
                       <option key={voice.id} value={voice.id}>
                         {voice.label}
                       </option>