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Audio Configuration

Before performing measurements or analyzing audio, LinFIR needs to be configured with the correct audio devices and parameters. This section covers the audio settings available in the application.

Accessing Audio Settings

Audio settings are accessed via:

  • Menu: View → Settings → Audio Settings
  • Keyboard: Cmd+, (macOS) or Ctrl+, (Windows), then select Audio Settings tab

Audio Settings Window Audio Settings Window

Device Selection

Input Device

The input device captures audio from microphones or measurement equipment:

  • Click the dropdown to see available input devices
  • Select your measurement microphone or audio interface input
  • Verify the device shows the correct channel count
  • Ensure the device isn’t in use by another application

Output Device

The output device plays back sweep signals or test tones:

  • Select the audio interface connected to your loudspeaker system
  • Verify the device supports the desired sample rate
  • Check that the device has sufficient channels for your application

Refreshing Devices

If your audio device doesn’t appear:

  1. Click “Refresh Devices” to rescan available hardware
  2. Ensure the device drivers are properly installed
  3. Check that the device is powered on and connected
  4. Try restarting the application if the device remains unavailable

Sample Rate Configuration

Choose a sample rate matching your needs:

  • 44.1 kHz: CD quality, most common for consumer audio
  • 48 kHz: Professional audio standard, video production
  • 88.2/96 kHz: High-resolution audio, improved time-domain precision
  • 176.4/192 kHz: Ultra-high resolution (use only when necessary for THD analysis)

⚠️ Note: The sample rate must be supported by both your input and output devices. If a rate is unavailable, try a different value.

Buffer Size

Controls latency vs. stability trade-off:

  • Smaller buffers (128-256 samples): Lower latency but may cause dropouts
  • Recommended (512-1024 samples): Good balance for measurements
  • Larger buffers (2048+ samples): More stable but higher latency

For sweep measurements, 256 or 512 samples is typically optimal.

Input Gain Compensation

Software-based gain adjustment applied to the input signal:

  • Range: -32 dB to +32 dB
  • Purpose: Compensate for fixed or unavailable hardware gain controls
  • Use case: Particularly useful for USB microphones like UMIK-1 on Mac OS

Why Use Input Gain Compensation?

Some measurement microphones have fixed gain or no hardware gain control, particularly on certain platforms:

  • Mac OS USB microphones: Mac OS enforces very low fixed gain for USB microphones
  • UMIK-1 example: No hardware control available in Mac OS system settings
  • Solution: Use positive gain compensation (+18 to +24 dB) to achieve optimal recording levels

Important Notes

  • Input gain compensation does not improve signal-to-noise ratio
  • It’s a convenience feature for level matching, not a replacement for proper gain staging
  • Optimal SNR still requires appropriate acoustic levels and microphone positioning
  • Use hardware gain controls when available for best results

Microphone Calibration

Import calibration files from measurement microphone manufacturers for accurate frequency response measurements.

Supported Format

Standard text file format with:

  • Sens Factor header line
  • Two-column data: Frequency (Hz) and Error/Correction (dB)
  • Example manufacturers: miniDSP, Dayton Audio, Earthworks, Beyerdynamic

How Calibration Works

When a calibration file is loaded:

  1. LinFIR computes a minimum-phase correction filter from the calibration data
  2. This correction filter is convolved with the recorded signal during capture
  3. The correction is baked into the captured impulse response
  4. This is not just a visual adjustment on graphs

Why Minimum-Phase Convolution?

  • Preserves causality: The corrected IR remains causal (no pre-ringing from correction)
  • Avoids artifacts: No truncation-induced artifacts from non-causal corrections
  • Accurate magnitude: Full-frequency correction of microphone response
  • Natural results: Maintains physical relationships between magnitude and phase

Calibration File Management

  • Loading: Click “Import Calibration” and select the manufacturer-provided file
  • Clearing: Click “Clear Calibration” to remove the active calibration

⚠️ Device Change Warning: When switching input devices, LinFIR warns you if a calibration is active. The calibration may not be valid for the new microphone.

Platform-Specific Considerations

Mac OS

  • Timing stability: Excellent (< 0.01ms typical jitter)
  • Timing reference: Optional when the same device is used as input and output but recommended for best accuracy
  • Multi-interface: Highly reliable even with different input/output devices
  • USB microphones: May require input gain compensation due to OS-enforced low gain with usb devices

Windows

  • Timing stability: Variable (1-150ms jitter due to scheduler instability)
  • Timing reference: Strongly recommended (Electric or Acoustic mode)
  • Timing inconsistency: IR arrival time may vary without timing reference
  • ASIO drivers: Provide improved stability compared to WASAPI
  • Measurement averaging: Requires active timing reference (Electric or Acoustic mode)

Without a timing reference on Windows, measurements may have inconsistent timing, making averaging and multi-position capture unreliable.

Configuration Checklist

Before starting measurements, verify:

  • Input device selected and recognized
  • Output device selected and functional
  • Sample rate matches project settings
  • Buffer size set to 256 or 512 samples
  • Input gain compensation configured (if needed)
  • Microphone calibration loaded (if available)
  • Timing reference configured (see Reference Timing)
  • Devices not in use by other applications

Troubleshooting

Device Not Listed

  • Click “Refresh Devices” to rescan
  • Ensure device drivers are installed
  • Check that device isn’t in use by another application
  • Try different sample rates or buffer sizes
  • Restart LinFIR

Configuration Invalid

  • Selected devices may have been disconnected
  • Sample rate may not be supported by device
  • Channel count may exceed device capabilities
  • Try selecting default devices and refreshing

Audio Dropouts

  • Increase buffer size
  • Close other applications using audio
  • Check for system resource constraints
  • Update audio interface drivers

No Sound During Sweep

  • Verify output device is selected and powered on
  • Check system audio settings
  • Ensure exclusive mode isn’t blocking access (Windows)
  • Try different buffer sizes
  • Verify output channel selection in IR Management window