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Mix Bus: Matching Out-of-Sync Analog Tape with DAWs

June 3, 2008
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After multiple playbacks and a few years, the stereo mix had deteriorated; fortunately, the vocals still sounded fresh. I rented a 1" machine and transferred the audio tracks into Pro Tools, where I planned to import my vocal tracks. I matched the waveform of the first vocal note to the downbeat of the verse. It sounded perfect—and then it began to drift. The tape with the vocals had stretched and, by the end of the song, it was off from the multitrack by a full beat!

The method shown here to resynchronize tracks from different tape recorders works equally well for one or more tracks of any kind. The main requirement is that you have at least one track from each recorder that shares musical material and plays virtually from beginning to end. You can use the same method if you have a timecode reference, though you may find spotting the matching frequency waveforms difficult. If the sync problems are due to major differences in machine speeds, you may also have to use pitch correction.

THE ANSWER

I first used Pitch ’n Time (see sidebar) to shorten the vocal, forcing the beginning of my last note to match the backing track. This didn’t work, as the 1/4" tape had stretched by different amounts in various places. I needed to break the vocals into sections, and perform multiple time compressions and/or expansions.

You may think it easy to just clip the vocal into smaller sections, and fix it piece by piece. But a vocalist’s job is to be expressive, and that doesn’t mean being overly accurate with regard to time. It helps to have a more rhythmic reference, like the stereo mix of the backing tracks.

BRING IN THE AUDIO

I imported the vocal and one of the backing tracks together. Both regions had the same start and end points. I grouped them so that moving either track would move the other. This avoided another sync problem. To find the right starting place for my vocal, I used the backing track, lining up the start note with the piano’s start note from the original 1" master multitrack.

Switching to Shuffle mode, I zoomed in and then moved the grouped tracks manually with the Grabber tool, set the Ruler to Samples, and set the Nudge to 10 samples, clicking the < or > key until the samples matched up. The start of the vocal lined up where expected.

I played through the song, listening for a disagreement between the parts and looking for a place where the vocal dropped out so I could make fixes where the vocal was absent. I found such a place at the end of the first chorus. I clicked on the transient on the piano part and dragged down across the backing track to the matching transient there. (You may try the Audio Zoom In tool to make the audio waveform taller; this is circled in Figure 2. Or, with recent versions of Pro Tools, grab a track’s lower boundary in the Edit view’s left-hand column, and drag up or down to resize.) I clicked the Enter key to bring up the Memory Location dialog, created a Marker titled “Drift 1,” and logged the selection edit length (Figure 1). You’ll need this number in case you can’t find a silence in your “keeper” track. A few times, I also needed to move my marker and find a different spot, so write that number down.

SLICING AND SQUEEZING

Switching to the Selector tool, I clicked at the right boundary of the selection on the two grouped tracks, zooming in to find a “zero crossing” just before the backing track note started. Find the zero crossing in the keeper track, as choosing a zero crossing helps to avoid creating a click in your edited tracks. (I wasn’t worried about clicks on the backing track, as it was to be discarded later.) I typed “B” to break the audio across the two grouped tracks.

Next I clicked and held down the Trim Tool icon to select the Time Compression/ Expansion Trim Tool (marked TCE in Figure 2). I dragged the right end of the grouped tracks to the left until their edge lined up with the Drift 1 marker. Go slowly, and keep your eye on the blue marker ruler for the gold diamond; but note that it’s not essential to get the exact right spot—even being 100 samples off is probably okay.

ABOUT STRETCHING

If your tracks are shorter (likely from slight differences in recorder speeds), you need to lengthen them. After you make your selection and create your marker, move the marker to the right edge of the selection. Then you’ll be working with a break to the left of your marker. To get the unprocessed regions out of the way, switch to Slip mode and, using the Grabber tool, click on the grouped regions to the right of the break and drag them further to the right so they’re out of the way. Use the Time Compression/Expansion Trim Tool to drag the right edge of regions you’re working on to the marker. Switch to Shuffle mode and drag the unprocessed regions up against the processed one.

Now listen back. Your reference track may contain transients from other instruments that aren’t an exact match for the solo instrument track. You might think you have the right trim point, but judge with your ears. Play around with it; you can always Undo.

Next, I zoomed out and put the selection tool on one of the tracks in a position about the same length as the first region, looking for a break in the vocal track to make my work easier. I zoomed back in, made a selection, added a marker titled Drift 2, and repeated the steps. Depending on the source material, you may have to run this process multiple times.

LAST STEPS

Doing the final alignment is tricky; the end of the region will most likely fade to silence, so you’ll want to match up with a transient earlier in the region. Try looking for the final silence before the last big note, split the audio there, and run the process. The little tag region should line up for the remainder of the song as it’s not likely to drift much.

Finally, I ungrouped the imported tracks and deleted the backing track. I kept only the vocal, which now matched perfectly with the original multitrack. Selecting all the vocal regions, I went to the Region menu and selected Group so the entire track would act as one audio region, then deleted the markers.

Working with analog tape in a digital world can make you wonder, “Can’t we all just get along?” Well, we can.

Time to Stretch

Not unexpectedly, I found that Pro Tools’ TC/E (Time Compression/  Expansion) algorithms gave unwanted audio artifacts when changing the audio’s length by any appreciable amount. Serato’s Pitch ’n Time (PNT) plug-in for Pro Tools has been the professional standard for some years; while at $799 list it’s not cheap, it can transpose well over a wider range than most time-stretch algorithms (Serato now has an LE version for $399, but I haven’t tried it). Download a free PNT demo at www.serato.com/products/pnt.

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