Songbird USB Audio Loopback Demo

An embedded audio processing demonstration using the Songbird hardware platform. This project showcases real-time USB audio loopback with visual feedback and interactive controls.

Table of Contents

• Project Overview
• Hardware Setup
• Software Installation
• Operation Guide
• Technical Details
• Troubleshooting
• Development
• Educational Resources

Project Overview

What is Songbird?

Songbird is a commercial audio demonstration device that showcases embedded audio processing capabilities. It features:

• Real-time USB Audio Processing: Direct loopback from USB input to output with minimal latency
• Visual Feedback: OLED display showing audio levels and system status
• Interactive Controls: Buttons for system state control and audio muting
• Professional Audio Quality: 16-bit/44.1kHz-48kHz audio processing
• Educational Value: Well-documented code demonstrating embedded audio concepts

Key Features

• USB Audio Interface: Appears as a standard USB audio device to any computer
• Zero-Latency Loopback: Direct audio passthrough for monitoring applications
• Real-time Level Metering: Visual audio level indication on display and LEDs
• State Machine Control: Professional state management for reliable operation
• Comprehensive Diagnostics: Built-in system monitoring and testing capabilities

Target Audience

• Audio Engineers: Understand embedded audio processing concepts
• Embedded Systems Students: Learn professional code organization and audio programming
• Makers and Hobbyists: Explore advanced Teensy Audio Library applications
• Educators: Demonstrate real-time audio processing principles

Hardware Setup

Songbird Device Overview

The Songbird device integrates several components:

• Teensy 4.1 Microcontroller: 600MHz ARM Cortex-M7 processor
• Audio Shield: SGTL5000 codec for high-quality audio processing
• 128x32 OLED Display: Real-time visual feedback with automatic rotation correction
• Two Color LEDs: Blue and pink LEDs for level indication
• Four Control Buttons: UP, DOWN, LEFT, RIGHT for system control
• USB-C Connector: Audio interface and power supply

Physical Connections

1. USB-C Connection: Connect Songbird to your computer using the included USB-C cable
2. Power: Songbird is powered entirely through the USB connection
3. Audio: No additional audio cables required - all audio is transmitted via USB

First-Time Setup Verification

1. Power LED Check: Confirm the Teensy power LED illuminates when connected
2. Display Test: The OLED display should show "Songbird Ready!" on startup
3. Button Test: Press the UP button to verify display changes
4. LED Test: LEDs should briefly illuminate during startup sequence

Audio Connection Setup

1. Connect Songbird: Plug the USB-C cable into your computer
2. Audio Device Recognition: Your operating system should automatically detect Songbird as a USB audio device
3. Audio Settings: Configure Songbird as both input and output device in your system audio settings

Software Installation

Prerequisites

• Arduino IDE: Version 1.8.15 or later
• Teensyduino: Latest version for Teensy 4.1 support
• Required Libraries: See library installation section below

Arduino IDE Setup

1. Download Arduino IDE:

   • Visit arduino.cc
   • Download and install the latest version

2. Install Teensyduino:

   • Visit pjrc.com/teensy/teensyduino.html
   • Download and install Teensyduino
   • Follow the installation wizard to add Teensy support to Arduino IDE

3. Board Selection:

   • Open Arduino IDE
   • Go to Tools → Board → Teensyduino → "Teensy 4.1"
   • Set USB Type to "Audio"
   • Set CPU Speed to "600 MHz"

Library Installation

Install the following libraries via Arduino IDE Library Manager (Tools → Manage Libraries):

Required Libraries

1. Adafruit SSD1306 (for OLED display)

   • Search for "Adafruit SSD1306"
   • Install latest version

2. Adafruit GFX Library (for graphics primitives)

   • Search for "Adafruit GFX"
   • Install latest version

3. Teensy Audio Library (built-in with Teensyduino)

   • Automatically included with Teensyduino installation

Library Versions

• Adafruit SSD1306: 2.5.7 or later
• Adafruit GFX: 1.11.5 or later
• Teensy Audio: Included with Teensyduino

Compilation and Upload

1. Download Source Code:

   • Extract all .ino files to a folder named usb_loopback_demo
   • Ensure all files are in the same directory

2. Open Project:

   • Open usb_loopback_demo.ino in Arduino IDE
   • Verify all other .ino files appear as tabs

3. Compile:

   • Press Ctrl+R (or Sketch → Verify/Compile)
   • Ensure compilation completes without errors

4. Upload:

   • Connect Songbird via USB-C
   • Press the program button on Teensy while plugging in (first time only)
   • Press Ctrl+U (or Sketch → Upload)
   • Wait for "Download Complete" message

Troubleshooting Installation

Common Issues:

• Board not found: Ensure Teensyduino is properly installed
• Library errors: Verify all required libraries are installed
• Upload fails: Try pressing the program button on Teensy
• Compilation errors: Check that all .ino files are present

Operation Guide

System States

Songbird operates in three main states:

1. STANDBY State

• Description: System ready, no audio processing
• Display: Shows "Press UP to start"
• LEDs: Off
• Audio: No processing active

2. ACTIVE State

• Description: Full audio loopback operation
• Display: Shows live audio levels and "USB Loopback Active"
• LEDs: Respond to audio levels
• Audio: Direct USB input to output passthrough

3. MUTED State

• Description: Input monitored, output muted
• Display: Shows input levels and "Output Muted"
• LEDs: Input LED responds, output LED off
• Audio: No output, input still processed for display

Button Controls

UP Button (Top)

• From STANDBY: Activates audio loopback (→ ACTIVE)
• From ACTIVE/MUTED: Returns to standby (→ STANDBY)

DOWN Button (Bottom)

• From ACTIVE: Mutes audio output (→ MUTED)
• From MUTED: Unmutes audio output (→ ACTIVE)
• From STANDBY: No effect

LEFT/RIGHT Buttons

• Currently Reserved: For future functionality

Display Elements

The 128x32 OLED display shows:

Status Line

• Top of display: Current system state and controls
• Sample Rate: Shows current USB audio sample rate (44k/48k)

Level Meters

• Format: I:[####----] for input, O:[####----] for output
• Segments: 8 segments representing audio level (0-8)
• Characters: # for active segments, - for inactive segments
• Update Rate: 10 times per second for smooth indication

Channel Indicators

• L: Left channel indicator, brackets show activity [L]
• R: Right channel indicator, brackets show activity [R]
• Active State: Brackets appear when audio is present
• Inactive State: Simple text without brackets

LED Indicators

Blue LED (LED1)

• Function: Input level indicator
• Brightness: Proportional to input audio level
• Behavior: Smooth transitions, logarithmic scaling

Pink LED (LED2)

• Function: Output level indicator
• Brightness: Proportional to output audio level
• Behavior: Mirrors input LED in ACTIVE state, off when muted

Using Songbird as USB Audio Interface

1. Connect: Plug Songbird into computer via USB-C

2. Configure Audio Settings:

   • Windows: Sound Settings → Choose Songbird for input/output
   • macOS: System Preferences → Sound → Select Songbird
   • Linux: Use alsamixer or PulseAudio to select Songbird

3. Activate Processing: Press UP button to enter ACTIVE state

4. Test Audio: Play music or speak into microphone

5. Monitor Levels: Watch display and LEDs for audio activity

Professional Audio Applications

• Audio Monitoring: Use as a reference loopback device
• System Testing: Verify audio paths and levels
• Educational Demonstrations: Show real-time audio processing
• Development Tool: Test USB audio applications

Technical Details

Audio Specifications

Specification	Value
Sample Rates	44.1 kHz, 48 kHz (host-determined)
Bit Depth	16-bit
Channels	2 (Stereo)
Latency	<10ms (typical)
Frequency Response	20Hz - 20kHz
Dynamic Range	>90dB

Processing Performance

Metric	Value
CPU Usage	<15% typical
Memory Usage	~182KB RAM, ~76KB Flash
Audio Memory	32 blocks (4KB total)
Display Update	10 Hz
Button Scan	20 Hz
Main Loop	50 Hz

Hardware Specifications

Teensy 4.1

• Processor: 600MHz ARM Cortex-M7
• RAM: 1MB
• Flash: 2MB
• Audio Processing: Hardware-accelerated

Audio Shield

• Codec: SGTL5000
• Input: Line/Mic configurable
• Output: Line/Headphone
• Control: I2C interface

Display

• Type: SSD1306 OLED
• Size: 128x32 pixels
• Interface: I2C (Wire1)
• Address: 0x3C
• Rotation: Automatically corrected for proper orientation

System Architecture

USB Audio ─→ Teensy 4.1 ─→ Audio Shield ─→ Physical Output
    ↓            ↓              ↓
    └─→ Level Detection ─→ Display/LEDs

Pin Assignments

Function	Pin	Description
Audio Interface
I2S_DIN	7	Audio data input
I2S_DOUT	8	Audio data output
I2S_SCLK	21	Serial clock
I2S_LRCLK	20	Left/right clock
SYS_MCLK	23	Master clock
User Interface
OLED_SDA	17	Display data (Wire1)
OLED_SCL	16	Display clock (Wire1)
BTN_UP	5	Up button
BTN_DOWN	4	Down button
LED_1	14	Blue LED
LED_2	15	Pink LED

Memory Usage

Component	RAM	Flash
Teensy Audio Library	~180KB	~50KB
Display System	512B	~8KB
Application Logic	~2KB	~15KB
Total	~182KB	~76KB

Troubleshooting

Audio Issues

No Audio Output

• Check: USB connection secure
• Verify: Songbird selected as audio device
• Confirm: System in ACTIVE state (press UP button)
• Test: Try different audio source

Audio Crackling/Distortion

• Reduce: Input levels on source device
• Check: USB cable quality
• Verify: Proper sample rate matching
• Monitor: CPU usage via diagnostics

High Latency

• Ensure: USB 2.0 or higher connection
• Check: Computer USB performance
• Verify: No other high-bandwidth USB devices
• Consider: Dedicated USB port

Display Problems

Display Blank

• Check: I2C connections (pins 16/17)
• Verify: Display power via USB
• Reset: Restart Songbird
• Test: Known working display

Display Upside Down

• Solution: Automatically corrected in software with setRotation(2)
• No action needed: Display should appear correctly oriented

Display Garbled/Corrupted

• Cause: Uses standard ASCII characters for compatibility
• Check: Level bars should show # and - characters
• Verify: No special Unicode characters displayed
• Reset: Restart Songbird if display corruption persists

Display Flickering

• Check: Loose connections
• Verify: Stable power supply
• Update: Display refresh rate in code
• Test: Different USB port

Button Issues

Buttons Not Responding

• Check: Physical connections
• Verify: Pull-up resistors enabled
• Test: Button diagnostic mode
• Clean: Button contacts

False Button Presses

• Adjust: Debounce timing
• Check: Electrical interference
• Verify: Proper grounding
• Test: Individual button response

LED Issues

LEDs Not Responding

• Check: PWM pin connections
• Verify: LED polarity
• Test: Manual LED control
• Check: Current limiting resistors

Incorrect Brightness

• Adjust: Level mapping algorithm
• Check: PWM resolution
• Verify: Audio level detection
• Test: Different audio sources

System Issues

System Won't Start

• Check: Power via USB
• Verify: Teensy 4.1 functionality
• Test: Basic Arduino sketch
• Check: Bootloader integrity

Random Crashes

• Monitor: Memory usage
• Check: Stack overflow
• Verify: Interrupt handling
• Test: Simplified configuration

USB Connection Problems

• Try: Different USB cables
• Test: Multiple computers
• Check: USB driver installation
• Verify: Teensyduino USB type setting

Diagnostic Tools

Built-in Diagnostics

• Serial Monitor: View system status and debug messages
• LED Test: Hold UP+DOWN during startup
• Button Test: Hold LEFT+RIGHT during startup
• Display Test: Automatic on startup

Development Tools

• Arduino Serial Monitor: Real-time system diagnostics
• Teensyduino Audio System Design Tool: Visual audio routing
• Logic Analyzer: For I2C and digital signal debugging
• Oscilloscope: For audio signal analysis

Development

Code Organization

The project is organized into modular components:

Core Files

• usb_loopback_demo.ino: Main control loop and state machine
• config.ino: Hardware configuration and constants
• audio_manager.ino: Audio processing and routing
• display_manager.ino: OLED display control
• led_manager.ino: LED brightness control
• button_manager.ino: Button input handling

Architecture Benefits

• Modular Design: Each component can be developed/tested independently
• Clean Interfaces: Well-defined functions between modules
• Educational Value: Clear separation of concerns
• Maintainability: Easy to modify individual components

Customization Options

Audio Processing

• Sample Rate: Modify for different rates
• Bit Depth: Upgrade to 24-bit processing
• Effects: Add reverb, EQ, or other processing
• Multi-channel: Expand beyond stereo

Display Customization

• Layout: Modify visual arrangement
• Graphics: Add custom indicators or patterns
• Information: Display additional system data
• Themes: Create different visual styles

Control Expansion

• Additional Buttons: Add more control options
• Rotary Encoders: Implement continuous controls
• Touch Interface: Add touch-sensitive controls
• MIDI: Add MIDI control capabilities

Performance Optimization

Memory Optimization

• Buffer Sizes: Adjust for memory constraints
• String Usage: Minimize dynamic string allocation
• Display Buffer: Optimize graphics memory usage
• Audio Memory: Balance quality vs. memory usage

CPU Optimization

• Update Rates: Adjust component update frequencies
• Interrupt Handling: Optimize ISR performance
• Floating Point: Consider fixed-point arithmetic
• Compiler Options: Use optimization flags

Testing Framework

Unit Testing

• Component Tests: Individual module verification
• Integration Tests: System-level functionality
• Performance Tests: CPU and memory usage validation
• Hardware Tests: Physical interface verification

System Testing

• Audio Quality: Frequency response and distortion analysis
• Latency Measurement: Real-time performance validation
• Stress Testing: Extended operation validation
• Environmental Testing: Temperature and power variation

Educational Resources

Learning Objectives

This project demonstrates:

• Real-time Audio Processing: Understanding of digital audio concepts
• Embedded Systems Design: Professional code organization and architecture
• State Machine Implementation: Robust system state management
• Hardware Integration: Combining multiple subsystems effectively
• User Interface Design: Creating intuitive control systems

Concepts Demonstrated

Audio Processing

• Digital Audio Fundamentals: Sample rates, bit depth, channels
• Real-time Constraints: Latency, buffering, interrupt handling
• Level Detection: RMS analysis and visualization
• Audio Routing: Signal flow and processing chains

Embedded Programming

• Modular Architecture: Separating concerns and creating reusable components
• State Machines: Managing system behavior and transitions
• Interrupt Handling: Real-time processing and timing
• Resource Management: Memory and CPU optimization

Hardware Integration

• I2C Communication: Display and sensor interfaces
• PWM Control: LED brightness and motor control
• GPIO Management: Button debouncing and digital I/O
• USB Communication: Host interface and protocol handling

Extended Learning

Advanced Audio Projects

• Audio Effects: Implement reverb, echo, or filtering
• Spectrum Analysis: Add FFT-based frequency visualization
• Multi-channel Processing: Expand to surround sound
• Synthesis: Generate tones and music programmatically

System Enhancements

• Wireless Control: Add WiFi or Bluetooth interfaces
• Data Logging: Record audio levels and system performance
• Remote Monitoring: Web-based control and status
• Configuration Management: Save/load system settings

Professional Applications

• Test Equipment: Audio analyzer and signal generator
• Monitoring Systems: Professional audio level monitoring
• Educational Tools: Classroom demonstration equipment
• Development Platform: Base for custom audio applications

License and Support

License

This project is provided as educational material with full source code and documentation.

Support Resources

• Arduino Community: arduino.cc/forum
• Teensy Community: forum.pjrc.com
• Audio Library Documentation: pjrc.com/teensy/td_libs_Audio.html
• Adafruit Learning: learn.adafruit.com

Contributing

Contributions to improve the code, documentation, or educational value are welcome. Please follow professional coding standards and include comprehensive documentation for any changes.

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Project Version: 1.0
Last Updated: July 2025
Hardware: Songbird Audio Demonstration Platform
Software: Arduino IDE with Teensyduino