Reference Timing for Sweep Measurements

Accurate timing is critical for consistent impulse response measurements, especially when capturing multiple measurements for averaging or multi-position analysis. LinFIR provides three timing reference modes to accommodate different hardware configurations and platform requirements.

Why Timing Reference Matters

Audio playback and recording systems introduce latency from:

• Digital-to-analog conversion (output)
• Analog-to-digital conversion (input)
• Buffer processing delays
• Operating system scheduler variability

Without a stable timing reference, each measurement may have slightly different latency, making:

• Driver alignment unreliable
• Directivity analysis inaccurate (time-of-flight variations)

Timing Reference Modes

None

No timing reference - relies solely on system scheduler timestamps.

How it works:

• Uses operating system audio clock
• Timestamps based on buffer callback timing
• No additional hardware or connections required

When to use:

• Mac OS systems (low scheduler jitter, typically < 0.01 ms)
• Single-shot measurements without averaging
• Simple setups where high precision isn’t critical

Limitations:

• Windows: Not recommended due to significant scheduler jitter (1-150 ms)
• IR arrival time may vary between captures

Mac OS: Reliable and accurate for most applications when using the same device as input and output. Windows: Avoid for professional work - use Electric or Acoustic mode instead.

Electric (Loopback)

Uses a physical cable connection between output and input to provide a stable timing reference.

How it works:

• Connect line output to line input with a cable
• Loopback signal captures exact OS and electrical latency
• Combined electrical and conversion latency measured
• Provides sample-accurate timing reference

Setup:

1. Connect output channel to spare input channel with audio cable
2. Select “Electric” timing reference mode in Audio Settings
3. Ensure loopback channel is not the measurement input channel
4. Run sweep - loopback captures timing automatically

Advantages:

• Most accurate timing reference
• Platform-independent reliability
• Compensates for interface-specific latency
• Essential for measurement averaging on Windows
• Eliminates scheduler jitter completely

When to use:

• Windows systems (strongly recommended)
• Designing crossovers
• Professional applications requiring highest accuracy
• Different input/output devices (aggregate devices)

Limitations:

• Requires spare input channel for loopback
• Requires physical cable connection
• May not be available on simple USB audio interfaces

Acoustic

Uses another driver (reference driver) as a timing microphone.

How it works:

• Reference driver captures sweep output acoustically
• Provides stable timing reference based on acoustic arrival
• Measures relative timing between reference and measured driver
• Does not capture true absolute acoustic + electrical travel time

Setup:

1. Select a reference driver (must reproduce high frequencies > 5 kHz)
2. Position reference driver microphone to capture sweep clearly
3. Select “Acoustic” timing reference mode in Audio Settings
4. Reference driver captures timing on each sweep

Advantages:

• No loopback cable required
• Works when electrical loopback unavailable
• Good alternative for Windows users without spare input channels
• Suitable for multi-driver directivity measurements

When to use:

• Loopback hardware not available
• Windows systems (recommended alternative to None)

Limitations:

• Only measures relative timing (not absolute acoustic delay)
• Reference driver must reproduce high frequencies cleanly
• Acoustic path must be clear and consistent
• Less accurate than Electric mode

Important: Reference driver should be full-range or at least capable of reproducing 5-10 kHz cleanly for reliable timing detection.

Timing Offset (Preferences)

Adjustable time offset to shift impulse position.

• Range: Typically ±100 ms
• Primary use: Acoustic mode when reference driver is farther than measured driver
• Purpose: Compensate for acoustic propagation delay differences

Example:

• Reference driver is 50 cm from its microphone
• Measured driver is 100 cm from its microphone
• Acoustic delay difference ≈ 1.5 ms (50 cm / 343 m/s)
• Apply negative offset of ~1.5 ms to align impulse peaks

Fine-tuning:

1. Run test measurement
2. Check impulse response time alignment
3. Adjust offset in small increments (1-2 ms)
4. Re-measure to verify alignment

Platform Requirements

Windows

• Averaging: Requires active timing reference (Electric or Acoustic)
• Scheduler jitter: Significant (1-150 ms variable latency)
• Recommendation: Use Electric mode whenever possible, Acoustic as second choice
• Single measurements: Work with “None” mode but may have timing variation

Without timing reference on Windows:

• Each measurement has unpredictable latency
• Directivity analysis compromised
• Time and phase alinment between drivers will lead to wrong results

Mac OS

• Scheduler jitter: Minimal (< 0.01 ms typical)
• Recommendation: Timing reference optional but still beneficial for best accuracy
• Multi-interface: More stable than Windows even without reference

Choosing the Right Mode

Configuration Workflow

1. Assess requirements:

   • Single measurement or averaging?
   • Mac OS or Windows?
   • Spare input channel available?

2. Select mode:

   • Windows averaging → Electric or Acoustic
   • Mac OS → None acceptable, Electric for best results
   • No loopback → Acoustic (if reference driver available)

3. Setup hardware (if Electric):

   • Connect output to spare input channel
   • Verify loopback signal path

4. Setup reference driver (if Acoustic):

   • Select driver with good high-frequency response
   • Position reference microphone to capture sweep clearly

5. Configure in Audio Settings:

   • Select timing reference mode
   • Adjust timing offset if needed (Acoustic mode)

6. Test:

   • Run test measurement
   • Verify timing consistency
   • Adjust offset if required

Troubleshooting

Inconsistent Measurements

Symptom: Each measurement has different arrival time
Cause: No timing reference active
Solution: Enable Electric or Acoustic mode

Reference Signal Not Detected

Symptom: Error message about missing reference
Cause: Loopback not connected or reference driver not capturing sweep
Solution:

• Electric: Verify cable connection and input channel selection
• Acoustic: Ensure reference driver can reproduce sweep frequencies

Timing Offset Not Working

Symptom: Impulse still misaligned after offset adjustment
Cause: Incorrect offset direction or magnitude
Solution:

• Try opposite sign (positive ↔ negative)
• Increase offset magnitude
• Verify reference driver is capturing sweep correctly

Best Practices

• Windows users: Always use Electric or Acoustic mode for professional work
• Loopback preferred: Most accurate, eliminates all timing variables
• Acoustic alternative: Good fallback when loopback unavailable
• Test before session: Verify timing consistency with test measurements
• Document settings: Record timing mode and offset in project notes
• Consistent hardware: Use same interface for all measurements in a session