Quick Tip: How to Sync Multi-Track Field Recordings in Pro Tools

2015/09/23

Rolls Royce

It’s dawn in late fall. Your team has arrived in the countryside, ready to record vintage Rolls Royce sound effects.

One person waits at each end of the road, another is in the middle, and you’re driving the car. The four of you are armed with your own microphone and recorder. Then it dawns on you: how will you match up a full day’s worth of sound fx when you get back to the studio, later?

Last week’s article explored this issue. It examined recording sound effects with multiple, unlinked audio recorders. It shared why sync slates are important to help create a point of reference in your field recordings to help align all of these sound effects together later. Today shares the the next step: how to sync those sound fx after returning to the edit suite.

So, today I’ll share a quick tip for syncing and mastering multi-track field recordings in Pro Tools.

The Challenge of Mastering Clips from Multiple Recorders

The most common field recordings are captured with one microphone and one recorder. What happens when you add more microphones and recorders, all spread across the field?

The result is that the clips captured on each recorder will have slightly different start times, and varying lengths.

This presents a challenge when mastering sound fx, later. When these clips are dragged into an editing session, by default, they will be lined up by their start time. Of course, since the recordings started at different times, this will mean the actual car recordings will be staggered, and not in alignment, or out of sync.

Here is an image of a recording with slightly different start times and durations. The next image shows what happens when these recordings are aligned by their start times: the sound effect is staggered, and out of sync.

Learn more about this in last week’s post.

The images above show the issue with just one recording. The issue becomes worse when you look at an entire day of field recordings.

I recorded fighter jets a few weeks ago using two recorders. There was only a slight delay between beginning and ending every sound effect on each recorder. Just the same, this has a huge effect on mastering.

Here is a screenshot of field recordings I gathered that day.

A day's worth of non-synced field recordings

A day’s worth of non-synced field recordings

Notice the difference in time when we look at the end of the tracks:

Second track, out of sync with shorter time

Second track, out of sync with shorter time

And, if we look at the waveforms of similar recordings, we can see they are quite far out of alignment:

The waveforms are mismatched

The waveforms are mismatched

So, what’s a better scenario?

The Importance of Editing in Sync

The goal is to edit clips with your sound effects in sync, from top to bottom, like this:

Recordings with different start times synchronized

Recordings with different start times synchronized

Why bother? There are many reasons:

  • Edit in sync. When every clip is in sync, all tracks can all be edited at once. This makes mastering much easier, and quicker.

    For example, let’s say you hear a bird chirp ten seconds after the gunshot in the image above. You’d like to end the sound file just before the chirp. When the gunshots are in sync, you can simply create an edit on all tracks in the same place, at once. That will slice out the chirp accurately and swiftly.

    Editing once across all tracks

    Editing once across all tracks

    If the tracks are out of sync, you must find the bird chirp on each file and create the edit, one after another, for every clip. Editing in sync avoids this hassle.

  • Increased precision. Of course, it’s possible to drag clips left and right so the gunshots roughly line up from top to bottom. This technique of “eyeballing it” will align the tracks crudely in place.

    However, manually dragging sounds into sync is frustrating. It’s difficult to match the sounds perfectly. It often requires zooming in to the most magnified setting to match the transients, zooming back out, and repeating. This isn’t a reliable way to align tracks. Even your most earnest efforts may be slightly out of sync. So, editing in perfect sync is a much better option to ensure precision.

  • Present editing options. Let’s imagine our gun recording above has a close, distant, and rear perspective, one on each track.

    This is helpful to an editor. He may want the close version at first for his scene, then the distant version for when the camera cuts away, an instant later. So, it’s best to master the sound effects in sync together. This provides a packaged set of similar sounds with the shot occurring at the same time in a file with identical lengths. That helps the editor find these related sounds and switch between these shots simply. It makes their lives much easier.

Of course, those sound like great benefits. But how do you get your mismatched takes in alignment? How do you get clips with different durations back in sync? How can you restore alignment to field recordings from different audio recorders?

Using Sync Points to Align Sound FX

The solution is to use Pro Tool’s sync points.

Sync points are a type of editing marker. However, they’re different than normal Pro Tools markers. Instead of being tied to a timeline, track size, and so on, sync points are clip-dependent. They create a marker within the clip itself, wherever the cursor is located.

Today’s tip will show how to use Pro Tools sync points to align mismatched field recordings captured from different audio recorders, and snap them perfectly in place.

How to Sync Sound FX From Different Recorders

For this example, I’ll explain how to sync two stereo files. Of course, you can extrapolate the instructions for additional track amounts.

I’ll also use the Mac shortcuts for Pro Tools 10. Learn the shortcuts for your own version and operating system.

Prep the Session

  1. Open Pro Tools.
  2. Create a new Pro Tools session by typing Command-n.
  3. Add your sound files by typing Command-Shift-i, then selecting your clips.

    A list of paired fx from two recorders

    A list of paired fx from two recorders

  4. Create two tracks by typing Command-Shift-n.
  5. Enter Pro Tools’ Slip mode by typing F2.
  6. Drag each recorder’s clips into their own track.

    A day's worth of non-synced field recordings

    All field recordings pulled into a session.

  7. Ungroup the tracks. You may have your tracks grouped. Ungroup them by typing Command-Shift-g.

Prep the Sound Clips

  1. Select the Grabber tool by typing Command–4.
  2. Select one clip.

    One paired recording selected and roughed in

    One paired recording selected

  3. Zoom to the track by typing Option-f.
  4. Rough in the sync by dragging the track left or right with the Grabber tool so that it is approximately in sync with the track below it.

    Rough in the sync

    Rough in the sync

Prep the Sync Points

  1. Find a spike in the waveform of the top track that is also visible in the bottom track.

    You’ll be able to find a spike easily if you created a sync slate as described in the previous article. Forgot to sync slate? Look for a sharp, characteristic waveform instead. This won’t be as reliable as a sync slate, but it will still work.

  2. Turn “Tab to Transient” on by typing Command-Option-Tab.

    "Tab to transient" selected

    “Tab to transient” selected

  3. Select the Edit tool by typing Command–3.

  4. Click in the first track anywhere before the spike.

  5. Press the tab key. The playhead cursor jump directly to the spike, courtesy of the “Tab to Transient” function.

    The cursor has jumped to the transient

    The cursor has jumped to the transient

  6. Zoom in and check that the cursor is located immediately at the transient or waveform spike.

  7. Drop an Identify Sync Point marker at the transient by typing Command-, (i.e., Command-comma).

    Drop a sync point

    Drop a sync point

  8. Repeat steps 3–6 for the second track.

    Drop a second sync point

    Drop a second sync point

Sync the Sounds

  1. Click before the sync point in the first track.
  2. Click tab to go to return to the sync point.
  3. Select the Grabber tool by typing Command–4.
  4. Hold control-shift, then click the second track (the one that doesn’t have the cursor). The sync point will jump the second track’s sync point into alignment to the first, snapping them together.

And that’s it! Your two tracks will be in sync, aligned by the sync slate, or other waveform transient or spike.

Testing Sync

So, how do you know that the tracks are actually in sync? There are three ways to test it:

  • Visual check. Zoom out so you see the entire clip. Do the waveforms match? Zoom in to extreme magnification at various places along the clip. Do the waveforms match at that level, too? Of course, the waveforms won’t be precisely identical. They sounds were captured with different microphones in different locations, so slight waveform variations are natural. However, they will be similar enough to get a rough impression.

  • Listen. Once the clips are sync, they will probably sound slightly out of phase. The clips will sound thinner when played together, with less bass. They will sound airy and “washy.” Check this by playing the tracks together, then soloed. You should notice a difference.

    This audible effect will be more drastic when the files are closely similar. It will still be apparent to some extent, even if the tracks are not an exact match.

  • Test with a plug-in. Use a plug-in like bx_solo on your master track to sum the files to mono. If the tracks are in alignment, you’ll notice phase issues.

Bonus Tips for Aligning Field Recordings

  • Manual sync points. Dropping sync points is easy when you recorded sync slates, or when transients are large. When that’s not the case, you may need to zoom in to maximum magnification, and drop in a sync point manually.
  • Ambiences and evolving tracks. It can be quite difficult to find a prominent transient if you have not recorded sync slates. Atmospheres usually have a bed of sounds with few spikes. Other tracks that swell, such as car passes, are similarly tricky. A clap, slap, or a even an intentional simultaneous mic bump may be all you need to find a point of reference for a sync point.
  • Create space. Snapping clips to sync points will make them jump to the left or the right as they align. Create space before and after the clips you are syncing to ensure they don’t overlap other clips.
  • Work backwards. A tip related to the previous point: sync clips from the last clip in the session to the first. This ensures snapping to sync does not overlap other tracks.
  • Shift clips. Running out of space? Want to create more space before or after your track, but are finding your tracks are too tightly packed? Use the shortcut shift-return to select all audio from the cursor to the beginning of the session. Or, you can type option-shift-return to select all clips from the cursor the end of the session. Select both tracks, then drag clips around to create more space.
  • Accommodating for distance. Do you have a microphone placed nearby, and another in the distance? Your gunshot will travel through the air at the speed of sound. It takes time for the sound of the shot to reach the distant microphone. So, the clips will naturally and properly be out of sync. Accommodate for this when mastering.

Read More

  • Learn more about understanding and identifying phase.
  • After last week’s article, reader Yohann shared a YouTube clip from David Farmer. The video explains how to line up sync slates with a slightly different method, using Elastic Audio:





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