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Multi-Channel Recording Recipe

Learn how to capture audio from multiple sources simultaneously using 8-channel recording. Perfect for podcasts, interviews, or complex audio setups requiring source isolation.

Prerequisites​

  • macOS 14.0 or later
  • Swift 5.9 or later
  • Screen Recording and Microphone permissions
  • Multiple audio applications or input devices

Why Multi-Channel Recording?​

Multi-channel recording provides:

  • Source Isolation: Each audio source gets its own channel
  • Post-Production Flexibility: Edit each source independently
  • Professional Quality: Industry-standard approach for complex audio
  • Future-Proof: Easy to remix or remaster later

Channel Layout​

AudioCap's 8-channel layout:

ChannelsSourceDescription
1-2Process AudioStereo mix of captured applications
3-8Input DevicesUp to 6 microphones/input devices

Step-by-Step Implementation​

1. Enable Required Permissions​

Multi-channel recording requires both permissions:

let permissionManager = PermissionManager()

// Screen recording for process audio
guard permissionManager.checkScreenRecordingPermission() else {
    print("❌ Screen recording permission required")
    exit(1)
}

// Microphone for input devices
guard permissionManager.checkMicrophonePermission() else {
    print("❌ Microphone permission required for input device capture")
    exit(1)
}

2. Discover Multiple Audio Sources​

let processManager = ProcessManager()

// Find multiple audio applications
let musicProcesses = try? processManager.discoverProcesses(matching: "Music")
let safariProcesses = try? processManager.discoverProcesses(matching: "Safari")
let vlcProcesses = try? processManager.discoverProcesses(matching: "VLC")

var allProcesses: [RecorderProcessInfo] = []
if let music = musicProcesses, !music.isEmpty {
    allProcesses.append(contentsOf: music)
    print("   ✅ Music app found")
}
if let safari = safariProcesses, !safari.isEmpty {
    allProcesses.append(contentsOf: safari)
    print("   ✅ Safari found")
}

Process Discovery Tips:

  • Use multiple discoverProcesses() calls for different apps
  • Combine results into a single array
  • Check each result before adding to avoid empty arrays

3. Configure Multi-Channel Capturer​

let capturer = AudioCapturer(
    outputDirectoryPath: "multichannel-recordings",
    captureInputsEnabled: true,   // Enable 8-channel recording
    alacEnabled: true,           // ALAC for better compression
    logger: logger
)

Key Configuration:

  • captureInputsEnabled: true: Enables 8-channel mode with input devices
  • alacEnabled: true: ALAC compression reduces file size significantly
  • outputDirectoryPath: Custom directory for organized storage

4. Start Multi-Channel Recording​

print("⏺️ Starting 8-channel recording for 15 seconds...")
print("📊 Channel layout:")
print("   Ch 1-2: Process audio (stereo mix)")
print("   Ch 3-8: Input devices (up to 6 devices)")

Task {
    do {
        try await capturer.startCapture(for: allProcesses)
    } catch {
        print("❌ Capture error: \(error)")
    }
}

// Record for 15 seconds
try await Task.sleep(nanoseconds: 15_000_000_000)
capturer.stopCapture()

Complete Example Code​

import Foundation
import Core

func main() async throws {
    print("🎛️ Multi-Channel Recording Recipe")
    
    // Check permissions
    let permissionManager = PermissionManager()
    guard permissionManager.checkScreenRecordingPermission() &&
          permissionManager.checkMicrophonePermission() else {
        print("❌ Both screen recording and microphone permissions required")
        exit(1)
    }
    
    let processManager = ProcessManager()
    let logger = Logger(verbose: true)
    
    // Find multiple audio sources
    let musicProcesses = try? processManager.discoverProcesses(matching: "Music")
    let safariProcesses = try? processManager.discoverProcesses(matching: "Safari")
    
    var allProcesses: [RecorderProcessInfo] = []
    [musicProcesses, safariProcesses].compactMap { $0 }.forEach { processes in
        if !processes.isEmpty {
            allProcesses.append(contentsOf: processes)
        }
    }
    
    guard !allProcesses.isEmpty else {
        print("❌ No audio processes found")
        print("Please start Music, Safari, or another audio application")
        exit(1)
    }
    
    // Configure 8-channel recording
    let capturer = AudioCapturer(
        outputDirectoryPath: "multichannel-recordings",
        captureInputsEnabled: true,
        alacEnabled: true,
        logger: logger
    )
    
    print("⏺️ Starting 8-channel recording...")
    
    Task {
        try await capturer.startCapture(for: allProcesses)
    }
    
    try await Task.sleep(nanoseconds: 15_000_000_000)
    capturer.stopCapture()
    
    print("✅ Multi-channel recording complete!")
}

try await main()

Output File Analysis​

Multi-channel recordings produce larger files with more data:

📁 multichannel-recordings/
   📄 MultiApp_20240101_120000.m4a (12.5 MB)
      🎛️ 8-channel ALAC (lossless)

File Size Comparison (15 seconds):

  • Stereo CAF: ~2.5 MB
  • 8-channel CAF: ~10 MB
  • 8-channel ALAC: ~6 MB (compressed)

Post-Processing with Audio Tools​

Logic Pro X​

  1. Import the 8-channel ALAC file
  2. Each channel appears as a separate track
  3. Process channels independently (EQ, compression, effects)
  4. Mix down to stereo or surround format

Pro Tools​

1. Create new session with 8 inputs
2. Import 8-channel file
3. Route each channel to separate tracks
4. Apply individual processing chains

Audacity (Free)​

  1. File → Import → Audio
  2. Select 8-channel file
  3. Use Tracks → Mix and Render → Mix Down to Stereo for final mix

Command Line (FFmpeg)​

Extract individual channels:

# Extract channel 1 (left process audio)
ffmpeg -i multichannel.m4a -map 0:0 -ac 1 -filter:a "pan=mono|c0=c0" channel1.wav

# Extract channel 3 (first input device)
ffmpeg -i multichannel.m4a -map 0:0 -ac 1 -filter:a "pan=mono|c0=c2" channel3.wav

# Extract stereo process mix (channels 1-2)
ffmpeg -i multichannel.m4a -map 0:0 -ac 2 -filter:a "pan=stereo|c0=c0|c1=c1" process_stereo.wav

Create custom mixes:

# Mix channels 1, 3, 5 to stereo
ffmpeg -i multichannel.m4a -filter_complex \
  '[0:0]pan=stereo|c0<c0+c2+c4|c1<c1+c2+c4' custom_mix.wav

# Isolate input devices only (channels 3-8)
ffmpeg -i multichannel.m4a -filter_complex \
  '[0:0]pan=stereo|c0<c2+c4+c6|c1<c3+c5+c7' inputs_only.wav

Use Cases​

Podcast Production​

  • Host: Channel 3 (dedicated microphone)
  • Guest: Channel 4 (separate microphone)
  • Background Music: Channels 1-2 (from Music app)
  • Sound Effects: Channel 5 (from separate app)

Interview Recording​

  • Interviewer: Channel 3
  • Interviewee: Channel 4
  • Presentation Audio: Channels 1-2 (screen share audio)
  • Room Tone: Channel 5 (ambient microphone)

Live Streaming​

  • Game Audio: Channels 1-2 (from game process)
  • Streamer Voice: Channel 3 (primary microphone)
  • Discord/Chat: Channel 4 (from communication app)
  • Music/Alerts: Channel 5 (from streaming software)

Advanced Configurations​

Selective Process Recording​

// Record only specific high-priority processes
let criticalProcesses = try processManager.discoverProcesses(matching: "Zoom|Teams|Discord")

Custom Channel Mapping​

// Future enhancement: specify which input device goes to which channel
// Currently automatically assigns channels 3-8 based on device discovery order

Real-Time Monitoring​

// Add monitoring during recording
let timer = Timer.scheduledTimer(withTimeInterval: 5.0, repeats: true) { _ in
    let cpu = cpuMonitor.sampleUsedPercent()
    print("🖥️ CPU: \(String(format: "%.1f", cpu))% - Recording: ✅")
}

Troubleshooting​

"Only 2 channels recorded"​

  • Ensure captureInputsEnabled: true
  • Check microphone permission is granted
  • Verify input devices are connected and active

"Large file sizes"​

  • Use alacEnabled: true for compression
  • Consider shorter recording durations for testing
  • 8-channel files are inherently larger

"Some channels silent"​

  • Check input device volumes in System Preferences
  • Verify devices aren't muted or disabled
  • Test with fewer devices to isolate issues

"Audio dropouts"​

  • Monitor CPU usage during recording
  • Close unnecessary applications
  • Consider using CAF instead of ALAC for lower CPU usage

Next Steps​

Multi-channel recording opens up professional audio production workflows, giving you complete control over each audio source for maximum post-production flexibility.