USB Audio Recorder Documentation

1. System Overview

This document details the configuration of a Raspberry Pi running Kali Linux to act as a resilient, continuous audio recording device. The system captures uncompressed WAV audio from a highly sensitive USB Lavalier microphone directly to an external exFAT/FAT32 USB drive.

2. Persistent Storage (fstab)

Because the USB drive is formatted as exFAT/FAT32, standard Linux file ownership cannot be applied per-folder. We force the mount to assign ownership to the kali user (UID 1000) globally on boot.

Identifying the Drive

The drive's unique identifier (UUID) was found using:

sudo blkid /dev/sda2

The fstab Configuration

The following line was added to /etc/fstab to guarantee the drive mounts at boot, even if unplugged (nofail), and grants full write access to the kali user.

UUID=68EE-53E1  /mnt/usb  auto  uid=1000,gid=1000,defaults,nofail  0  0

3. Audio Hardware Configuration

The USB Lavalier Microphone was identified using the ALSA recording hardware list:

arecord -l

The output indicated: card 2: Microphone... device 0.... This translates to the hardware ID plughw:2,0.

Microphone Sensitivity (Gain)

To capture all audio, the microphone gain was maximized using alsamixer. The state was saved so it persists across reboots using:

sudo alsactl store

4. The Background Service (systemd)

The recording loop is managed by a systemd service. It is designed to strictly wait for the USB drive to mount before attempting to record, preventing the internal SD card from filling up.

File location: /etc/systemd/system/audiorecorder.service

[Unit]
Description=Continuous USB Audio Recording
After=network.target local-fs.target
RequiresMountsFor=/mnt/usb

[Service]
Type=simple
User=kali
ExecStart=/usr/bin/arecord -D plughw:2,0 -f S16_LE -r 44100 -c 1 --max-file-time=600 --use-strftime /mnt/usb/audio_logs/audio-%%Y-%%m-%%d-%%H-%%M.wav
Restart=always
RestartSec=10

[Install]
WantedBy=multi-user.target

Service Breakdown:

• RequiresMountsFor=/mnt/usb: Halts execution until the OS confirms the USB is mounted.
• -D plughw:2,0: Targets our specific USB microphone.
• --max-file-time=600: Closes the current file and starts a new one every 10 minutes (600 seconds). This limits corruption to a maximum of 10 minutes if power is cut.

5. Automation & Resilience (crontab & cleanup)

The Cleanup Script

Because arecord does not manage disk space, a script checks disk usage and deletes the oldest .wav file if the drive hits 95% capacity.

File location: /home/kali/cleanup_audio.sh

#!/bin/bash
DIR="/mnt/usb/audio_logs"
MAX_USAGE=95

USAGE=$(df -h "$DIR" | awk 'NR==2 {print $5}' | sed 's/%//')

while [ "$USAGE" -ge "$MAX_USAGE" ]; do
    OLDEST=$(ls -1tr "$DIR"/*.wav 2>/dev/null | head -n 1)
    if [ -n "$OLDEST" ]; then
        rm -f "$OLDEST"
        USAGE=$(df -h "$DIR" | awk 'NR==2 {print $5}' | sed 's/%//')
    else
        break
    fi
done

The Crontab Entries

Two entries are added to the kali user's crontab (crontab -e):

*/5 * * * * /home/kali/cleanup_audio.sh
* * * * * sync

The sync command is critical for power resilience: it forces the OS to flush any file data held in RAM directly to the physical USB drive every minute.

6. Master Toggle Script

To cleanly pause operations without leaving cron jobs running in the background, a toggle script is used. It modifies systemctl and safely comments/uncomments the crontab.

File location: /home/kali/toggle_record.sh

#!/bin/bash

if [ "$1" == "enable" ]; then
    echo "🟢 Enabling continuous audio recording..."
    sudo systemctl enable audiorecorder.service
    sudo systemctl start audiorecorder.service
    crontab -l | sed 's/^#* *\(.*cleanup_audio\.sh\)/
/' | sed 's/^#* *\(.*sync\)/
/' | crontab -
    echo "✅ Enabled!"

elif [ "$1" == "disable" ]; then
    echo "🔴 Disabling continuous audio recording..."
    sudo systemctl stop audiorecorder.service
    sudo systemctl disable audiorecorder.service
    crontab -l | sed 's/^#* *\(.*cleanup_audio\.sh\)/# 
/' | sed 's/^#* *\(.*sync\)/# 
/' | crontab -
    echo "❌ Disabled!"

else
    echo "Usage: ./toggle_record.sh {enable|disable}"
fi

7. Troubleshooting Guide

If the system stops recording or behaves unexpectedly, use this checklist to diagnose the issue.