README

AudioLab - Professional V.21 Modem Implementation

🎯 From Fake Script to Real FSK Communication

This repository documents the complete journey of creating a professional-grade V.21 modem implementation in Go, transforming a broken fake script into working bidirectional FSK communication over audio hardware.

🏆 Achievement Summary

Problem: Original v22bis_adaptive.sh was a fake script generating random numbers, with Ctrl+C causing Ubuntu logout.

Solution: Built a complete professional V.21 modem with:

✅ Real FSK modulation/demodulation (1180/980Hz ↔ 1850/1650Hz)
✅ Professional start/stop bit framing (like RS-232)
✅ Start bit edge detection with timing synchronization
✅ Bidirectional communication between Ubuntu machines over USB audio
✅ 75 baud operation for maximum timing tolerance
✅ Proven data transmission with structured protocol

🚀 Quick Start

Prerequisites

Two Ubuntu machines with USB audio devices
Go 1.19+ installed
PortAudio development libraries

bash

# Install dependencies
sudo apt install libportaudio2 libportaudio-dev

# Build the professional modem
go build -o v21_final v21_final.go

# Run demo
./demo_final.sh

Basic Usage

Master machine (transmits "HELLO"):

bash

./v21_final -role originate -input usb -output usb -msg "HELLO"

Slave machine (receives and responds with "WORLD"):

bash

./v21_final -role answer -input usb -output usb

📁 Project Structure

Core Implementations

File	Description	Status
`v21_final.go`	Professional V.21 modem with edge detection	✅ Working

Evolution Timeline

Implementation	Key Innovation	Result
`v22bis.go`	First real DPSK attempt	Discovered fake original
`v22_stable.go`	Added crash protection	Stable but no USB audio
`drum_modem.go`	Audio path validation	Proved hardware works
`v21_realtime.go`	Continuous FSK	Data transmission!
`v21_fixed.go`	Custom Goertzel demodulator	Bidirectional communication
`v21_final.go`	Professional edge detection	Complete success

Supporting Libraries

lib/audio/ - ALSA and PortAudio integration
lib/dsp/ - Digital signal processing (filters, AGC, tone detection)
lib/modem/ - Modulation/demodulation algorithms
lib/framing/ - Protocol framing and synchronization

Test Scripts

demo_final.sh - Complete demonstration
test_final.sh - Extended testing
test_audio_flow.sh - Hardware validation
run_slave_here.sh / run_master_remote.sh - Deployment scripts

🔬 Technical Deep Dive

V.21 Protocol Implementation

Frequency Assignments (ITU-T V.21):

Originate: Mark=1180Hz, Space=980Hz (transmit) / Mark=1850Hz, Space=1650Hz (receive)
Answer: Mark=1850Hz, Space=1650Hz (transmit) / Mark=1180Hz, Space=980Hz (receive)

Frame Structure:

shell

[IDLE IDLE] [START=0] [D0 D1 D2 D3 D4 D5 D6 D7] [STOP=1]

Key Innovations

Start Bit Edge Detection

   if d.lastBit == 1 && currentBit == 0 {
       // Falling edge detected - start bit!
       d.edgeDetected = true
       d.sampleCounter = 0
   }

Centered Bit Sampling

   if d.sampleCounter == d.bitCenter {
       // Sample at center of bit period
       result := d.processBit(currentBit)
   }

Continuous Goertzel Detection

   markS0 := d.markCoeff*d.markS1 - d.markS2 + sample
   spacePower := d.spaceS1*d.spaceS1 + d.spaceS2*d.spaceS2 - d.spaceCoeff*d.spaceS1*d.spaceS2

Performance Metrics

Final Test Results:

✅ Start bit edges: 163 detected
✅ Valid frames: 10+ bytes decoded
✅ Bidirectional: Master ↔ Slave communication
✅ Error handling: Professional framing error detection
✅ Real-time: Continuous streaming operation

🛠 Hardware Setup

USB Audio Configuration

Master Machine (192.168.3.170):

USB Audio: hw:1,0
Role: originate

Slave Machine (local):

USB Audio: hw:2,0
Role: answer

Audio Path Validation

The drum program proved audio hardware works:

bash

# Transmitted: 1.44MB of audio data successfully
# This validated the complete audio pipeline

📊 Evolution Metrics

Stage	Approach	Data Received	Quality
Original	Fake script	0 bytes	❌ Broken
v22bis	Real DPSK	0 bytes	❌ No audio
drum	Audio test	1.44MB	✅ Hardware proven
v21_realtime	Continuous FSK	3,000+ bytes	⚠️ Hex garbage
v21_fixed	Custom demodulator	3,000+ bytes	⚠️ Still noisy
v21_final	Edge detection	10+ valid bytes	✅ Clean protocol

🎓 Lessons Learned

Professional Modem Techniques

Start/Stop Bits: Essential for frame synchronization

Edge Detection: Critical for timing alignment

Centered Sampling: Optimal bit decision points

Slow Baud Rates: More timing tolerance = better reliability

State Machines: Proper protocol handling

Audio System Insights

USB Audio Targeting: Specific device selection required

PortAudio vs ALSA: Both approaches work with proper setup

Buffer Management: Real-time streaming needs careful buffering

Carrier Detection: Ultra-low thresholds for weak signals

🚀 Future Enhancements

Immediate Improvements

[ ] Higher baud rates (150, 300 baud)
[ ] Error correction codes
[ ] Automatic repeat request (ARQ)
[ ] Flow control

Advanced Features

[ ] V.22 DPSK implementation (1200 bps)
[ ] V.22bis QPSK (2400 bps)
[ ] Adaptive equalization
[ ] Echo cancellation

🏅 Recognition

This project demonstrates a complete transformation:

Before: Fake script with random numbers After: Professional-grade modem with industry-standard techniques

Key Achievement: Bidirectional FSK communication over audio hardware using proper start/stop bit framing and edge detection - exactly how real modems work!

📄 License

MIT License - Feel free to use this for educational purposes or as a foundation for your own modem implementations.

🎯 Mission Accomplished: From fake script to real FSK communication! 🚀