MOTU's Digital Performer (DP) provides a variety of tools for aligning the tempo of a sequence with the tempo of recorded audio. You can make the sequence's conductor track follow the audio tempo, or you can make audio follow the tempo of the conductor track. The program can even match up audio containing tempo variations such as ritards, accelerandi, and rubato, with audio that has a constant tempo, such as a typical drum loop.
When you match the sequence to the tempo of the audio, the audio remains unchanged. You are just moving the lines in the conductor track's onscreen grid to match the tempo of the performance. This method will give you useful visual markers for later editing but will have no impact on the sound or the tempo of the audio.
To illustrate this point, I assembled a mix with a rubato guitar track and a simple drum loop. The guitar part was recorded freely without a click track and with a tempo that varied from 70 to 76 bpm (see Web Clip 1). The drum loop had a fixed tempo of 95 bpm (see Web Clip 2; you can load both Web Clips into DP and follow along). To preserve the loose, spontaneous feeling of the guitar track, I made the sequence follow the guitarist's performance using DP's Beat Detection Engine and Adjust Beats feature to construct a custom tempo map.
Beating a Path
I dragged the guitar sound bite into a new DP track, lining it up with bar 1 of the sequence. At this point, DP's metronome click track had no relation to the guitar audio. The Adjust Beats feature lets you use the peaks in a sound bite's waveform as a guideline for lining up bar lines by dragging them on the time ruler. Adjusting one beat at a time can be time-consuming — especially on a long piece — but a number of techniques can help speed up the chore, including a feature recently introduced in DP 4.5 — the Beat Detection Engine.
The Beat Detection Engine looks at the peaks in an audio waveform and attempts to determine beat and tempo information. However, the feature has certain limitations. As the manual states, it works best on individual instrument parts that are rhythmic in nature and have a relatively steady tempo. My rubato guitar track was fairly rhythmic in nature, but its tempo varied. Before trying my usual method of constructing a tempo map with Adjust Beats, I decided to give Beat Detection a whirl.
The engine did a decent job of generating tempo information based on the guitar track's waveform peaks, but it misinterpreted the sound bite's tempo by doubling it instead. Fortunately, DP provides a handy command to correct this. With the sound bite selected, I chose Audio> Sound bite> Halve Sound bite Tempo and was back on track.
To check the results, I selected the guitar sound bite and chose Audio >Adjust Sequence To Sound bite Tempo. That adjusts the Conductor Track to match the tempo map created using Beat Detection. I set Tempo Control to Conductor track, enabled the metronome, and listened as the track played. The click tracks still didn't quite match up with the guitar playing. Beat Detection had gotten me in the ballpark, but I knew I would have to make further adjustments.
FIG. 1: The Adjust Beats window gives you options for adjusting entire measures or individual beats within the measure. The Snapping option can be used to snap bar lines to MIDI notes or audio beats.
Using Adjust Beats
Beat Detection is a valuable addition to Digital Performer and should be the first option to try when matching a sequence tempo to that of an existing sound bite. The Beat Detection Engine may be able to match the tempo automatically. Some of the new tempo maps, however, will require additional tweaking with the Adjust Beats option. In that case, use the following method:
- In the Sequence Editor, zoom in so that you can see the waveform peaks in the audio track.
- Make sure that Tempo Control is set to Conductor Track.
- Choose Project>Modify Conductor Track>Adjust Beats to bring up the Adjust Beats dialog window (see Fig. 1). Make sure that the “Drag beats in Graphic Editor” box is checked. Check the “Preserve realtime performance” box. Under Adjust, pull down Measures and click the “Apply adjusted beat's tempo to the end of sequence” option. Under Snapping, select the “Notes or Audio Beats” option (for more on snapping, see the section “Hot Tips for Beat Snappers”).
- Using the onscreen waveform peaks as a visual reference, drag the time ruler's bar lines so that they are better aligned with the music. The first bar already begins on time, so begin by clicking and dragging the start of the second measure. You'll see a red line appear that you can drag to line up with the correct spike in the audio (see Fig. 2). To check the results, enable the click track and listen to the first couple of bars. Press Stop and make further adjustments as needed. Continue to the next bar and always work from left to right. Don't skip to a later bar until the one before it is adjusted.
- Once you have the beginnings of measures following the audio, you can go back and adjust individual beats within the bar. Leave the Adjust Beats window open as you work and select the options Beats and “Move one beat at a time” for this.
FIG. 2: With Adjust Beats you can align the time ruler''s bar and beat marks by dragging them over peaks in audio waveforms.
With practice, you can quickly work your way through a tune, listening with the click track, pausing, and dragging. The result will be a custom tempo map that follows the variations of your playing precisely and leaves your rubato performance untouched.
The final step in my example was to sync up the 2-bar drum loop. I dragged it into a track, lining it up with the beginning of the sequence, and then option-dragged it to make copies every two bars until the end of the sequence. I selected all of the copies and chose Audio>Adjust Soundbites To Sequence Tempo. The fixed drum loop now matched the guitar part with all of its rubato tempo variation. The original track remained unchanged, and instead of the guitar being a slave to the click track, the sequence and drum loop grooved along with the free playing of the guitar (see Web Clip 3).
Hot Tips for Beat Snappers
Adjust Beat's little-known snapping feature has been one of Digital Performer's best-kept secrets. I first learned about it from MOTU's own “Magic” Dave Roberts, who explains how to use it, and why:
“I found that visually moving bar lines to match points on an audio waveform was tedious and inexact. The two fundamental problems are that audio cues may be slightly ahead or behind the beat for musical accent. If you line up a bar line on a kick that is supposed to be anticipated, the bar line is in the wrong place. The other problem is that if tempo is implied but not actually played, there's nothing to line up to at that point.
FIG. 3: Recording a MIDI click track by tapping and then using Adjust Beat''s note snapping feature on that track can help you adjust beats faster and more accurately.
“So I record a MIDI click track by listening to the track and tapping along. Once I've got a MIDI click track that follows the music, I use the Adjust Beats feature and turn on Note Snapping (see Fig. 3). With snapping on, you can click the mouse in the ruler in the general area where there is a MIDI note, and the bar line snaps to the MIDI note. Once I've got the first two or three bar lines in place, I can just go click, click, click down the ruler, and snap all the bar lines into place.”
Babz is a composer/multi-instrumentalist who has more than 15 years of experience using Digital Performer. She is a freelance writer on music technology and is based in New York City.