Video summary

Electrical Audio How-To: Time Alignment of Multiple Sound Sources

Main summary

Key takeaways

Educational

Main ideas / lessons

  • Time alignment matters when combining signals from the same source. When you record a direct instrument signal and also record the amplifier/mic signal (e.g., mic on an amp cabinet), their arrival times differ, which creates phase differences when you combine them.

  • Acoustic and system delays are expected. The mic signal is delayed by the acoustic path length (sound traveling from loudspeaker to mic) and by electronics/mechanical latency inside the amplifier/speaker system.

  • Phase differences cause audible artifacts. Misalignment can lead to:

    • Cancellation of some frequencies
    • Thinning of bass
    • Comb filtering, especially noticeable on highs

  • Correct by delaying the earlier path (typically the direct signal). Insert a very short, precisely adjustable delay into the direct signal path so the waveforms line up with the mic signal.

  • Two-microphone alignment also benefits from the same principles. Even if microphones are carefully placed, arrival time differences and/or polarity issues can cause phase cancellation and muddy imaging. Use test displays and listening to correct.

Methodology / step-by-step instructions

A) Aligning a direct signal with an amp/mic signal (single sound source, two recordings)

  1. Identify the problem

    • You have:
      • A direct signal from the instrument through a direct box to the desk (nearly immediate arrival).
      • A microphone signal from a mic on the amplifier, which arrives later due to acoustic + amplifier/speaker latency.
    • The time difference creates phase differences and comb filtering when combined.

  2. Use a visual measurement (optional but demonstrated)

    • Set up an oscilloscope to compare both signals’ waveforms:
      • Bottom trace: direct signal
      • Top trace: microphone signal
    • Play notes and confirm that peaks/valleys do not line up (and in extreme cases can be nearly anti-phase).

  3. Choose an appropriate delay tool

    • The correction delay is very short, often < 1 ms.
    • You need microsecond-range adjustability.
    • Recommended options include:
      • A studio-quality analog delay
      • A digital delay with microsecond adjustment
      • An all-pass delay / all-pass filter type device (used for time compensation without changing overall level response in the same way typical EQ might)

  4. Apply delay to the direct path

    • Engage the delay in the direct signal path.
    • Adjust (scroll) the delay until the waveforms are in very good alignment on the oscilloscope.

  5. Validate

    • Confirm visually: waveforms align; phase mismatch is reduced.
    • Confirm audibly:
      • Better-supported low frequencies (less phase cancellation)
      • Cleaner high end (reduced comb filtering)

  6. Example result given

    • Required alignment delay: 774 microseconds (accounting for acoustic + speaker/microphone + mechanical/electrical contributions).

B) Aligning two microphones on the same sound source (e.g., guitar cabinet)

  1. Recognize the potential issues

    • Two mics will typically have arrival time differences, causing:
      • phase cancellation
      • reduced clarity
    • Also, polarity inversions (one mic wired opposite “hot”) can create severe anti-phase behavior.

  2. Use an oscilloscope phase display

    • Use a Lissajous (Lissau) pattern to observe phase relationship:
      • One microphone on the vertical axis
      • The other microphone on the horizontal axis
    • The trace shape (ellipse) indicates phase alignment:
      • A wide ellipse suggests strong mismatch / anti-phase behavior

  3. Set up the microphones

    • Demonstrated microphones:
      • An RCA ribbon microphone (after a cable replacement)
      • A Russian condenser microphone
    • Note: check microphones follow the modern pin 2 / pin 3 hot convention, since some older/oddball mics differ.

  4. Listen and diagnose

    • Listen for phase problems (described as “significant phase difference”).
    • If severe and clearly “out of phase,” suspect opposite polarity.

  5. Correct polarity if needed

    • Flip polarity for the microphone suspected to be reversed.
    • Re-check on the oscilloscope:
      • The ellipse should become narrower / closer to expected alignment.

  6. Correct time/placement by adjusting distance

    • After polarity correction, time differences may still affect imaging.
    • Use stereo image cues:
      • If the image “pulls” toward one mic side, the earlier-arriving mic is likely too close relative to the other.
    • Make small physical adjustments:
      • Move the mic a small amount (example given: ~1 to 1.5 inches closer for the condenser vs ribbon).
      • Fine-tune further:
        • Slide within the mic clip by a fraction of an inch (smaller correction once coarse distance is addressed).

  7. Validate

    • Confirm the oscilloscope ellipse becomes very narrow (near alignment).
    • Confirm by listening:
      • stereo image remains stable
      • some residual phase cancellation may still be audible at certain frequencies, but alignment is significantly improved.

  8. Key conclusion

    • Experiment with microphone placement and polarity.
    • Use tools (oscilloscope) plus your ears to guide corrections.

Speakers / sources featured (and who they are)

  • Steve Albini — presenter; explains time alignment, acoustic/system delay, oscilloscope techniques, and corrective delay methods.
  • Greg Norman — mentioned as the technician who replaced the ribbon mic cable and ensured wiring followed the modern pin 2 hot convention.
  • John San Paulo — performs guitar for the demonstrations (and provides/authorizes the listening context).
  • Even Tide (company) — referenced as the manufacturer of delay units available through Electrical Audio; described as providing single-microsecond-range adjustable delays.

Original video