Six Degrees of Quantization

Quantizing is about more than moving notes to the bars-and-beats.
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Quantizing is about more than moving notes to the bars-and-beats.
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FIG. 1: The timeline markers delineate 12-bar sections of an audio file. The tempo event at each marker aligns the next marker to the bars-and-beats grid.

There goes the groove. That's what first comes to mind when I hear the word quantize. But quantizing is about more than moving notes to the bars-and-beats grid; it can be used to align that grid to the notes without affecting the groove. In this article, I'll describe how to apply those techniques to both audio and MIDI.

Start with a freely played audio or MIDI clip that you want to work on in your sequencer. For purposes of following along, record a MIDI clip without reference to a click or other rhythm track. Make the clip fairly long — a complete chorus of a song, for instance — and export it as both an audio and a MIDI file. For the examples in this article, I use Apple Logic Pro 7.2.1, which has a particularly robust set of tools for moving the grid around. You can, however, use most of these techniques with any full-featured digital audio sequencer.

Finding the Tempo

Whether you do it by hand or by using one of Logic's tools, aligning the grid to a freely played audio or MIDI clip amounts to time-locking the clip, and then adding tempo changes to shift the grid lines in time. That means creating a tempo track that matches the tempo changes in the audio or MIDI clip.

Creating a detailed tempo track is much easier when you trim the clip so that it starts and ends on a downbeat. If that cuts off material at the beginning or end of the clip, you can expand the clip at both ends to include that material after creating the tempo track. Once the clip is trimmed, note the number of beats it contains and its time signature. If there are changes in time signature, slice the clip into regions with no time-signature changes and align each region separately.

Place the trimmed clip at the beginning of a track, set Logic's time signature appropriately, and in the case of MIDI, set Logic's tempo to the recording tempo (if you know it). The recording tempo may have no relationship to the actual tempo of the MIDI clip, but it will allow you to recapture the actual tempo. If you're working with MIDI, also time-lock the clip by selecting Lock SMPTE Position from the Region menu. You don't need to time-lock audio clips, because their length remains fixed when you change Logic's tempo.

Before proceeding with finer alignment, place Logic's Left Locator at the beginning of the clip and set the Right Locator so that the cycle encloses the correct number of bars and beats. Then use Logic's Adjust Tempo feature using the Region Length And Locators command to calculate an average tempo for the clip. That will also adjust the grid so that the clip fits the cycle exactly.

The next steps change depending on the degree of refinement you need. If you want to align the grid in only a few places (say, to mark the choruses of a song for rehearsal or overdubbing purposes), it's easiest to proceed manually, and I'll describe how to do that first.

Marking Choruses

Ensure that Logic's Global Tracks are displayed and that the display includes the Marker and Tempo tracks. Place a marker at the beginning of each chorus and be sure to place one at the beginning of the clip. Logic's Create Marker Without Rounding key command is handy for dropping markers on the fly as the clip plays.

Open Logic's Marker list, and then time-lock all markers. Open Logic's Tempo list and adjust the tempo of its only tempo event until the second marker is at the correct grid position. The easiest way to zero in on the exact tempo is to click on individual digits of the tempo value, working from left to right, and then use the computer's Plus and Minus keys to increment and decrement each digit.

Next, move Logic's Song Position Line (SPL) to the second marker (the one you just aligned) and click on Create in the Tempo list to create a new tempo event at that location. The Go To Next Marker and Go To Previous Marker key commands are handy for moving the SPL between markers. Repeat this process for each marker. When you're done, the choruses will start at the correct grid positions, although within each chorus the grid probably won't line up exactly (see Fig. 1).

Marking Smaller Sections

Markers are great for aligning choruses or large sections, but for finer grid alignment, manually editing tempo events gets tedious. Logic's Beat Mapping feature streamlines the process.

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Fig 2: the MIDI tap track places notes at 4-bar intervals. Logic's Beat Mapping tool was used to align the taps to the correct bars-and-beats positions.

The easiest way to use Beat Mapping is to create a MIDI tap track. Logic's Klopfgeist (metronome) virtual instrument and the Capslock MIDI keyboard are handy tools for that. Create a Klopfgeist track and press the computer's Caps Lock key to display and activate the Capslock keyboard. Then start recording on the Klopfgeist track and play a note at the start of each of the desired sections. Finally, select the resulting MIDI clip, display the Beat Mapping Global track, and connect the taps (displayed at the bottom of the Beat Mapping track) to the appropriate grid lines (displayed at the top of the Beat Mapping track). Each new connection creates the necessary tempo event to align the tap with the grid line (see Fig. 2). If your taps are regularly spaced and no more than a bar apart, you can use the Beat Mapping track's Beats From Rgn button to connect them to the grid automatically rather than doing it manually.

There are two alternatives to creating a tap track manually. If you're working with audio, you can click on the Beat Mapping track's Analyze button to create transient markers along the bottom of the Beat Mapping track. For aligning sections that are a bar or more apart, however, ferreting out the correct transients can be more trouble than it's worth.

When working with a MIDI clip, you can often identify notes in the clip that mark the beginnings of sections. Copying those to a new MIDI clip and adding missing notes as needed is faster than creating a tap track from scratch.

The MIDI Edge

Aligning an audio clip to the grid helps make metric sense of the audio for transcribing, scoring, rehearsing, and overdubbing. You have several more options with MIDI clips, and if you convert an audio clip to Propellerhead REX file format, you can apply the same techniques to the resulting slices.

For scoring purposes, it's essential to have MIDI aligned with the grid. Depending on the amount of tempo variation in the playing, a fairly rough alignment, such as 4- or 8-bar sections, is often sufficient. Start by aligning the natural sections within the piece. Next, examine the score; if it needs more than a touch-up, refine the alignment and look again. If the score is still not looking good after a 1-bar alignment, try quantizing. That will probably ruin the feel, so you might want to save a copy of the original.

You can often use quantizing to tighten the feel after you've aligned the grid to 1-bar sections. You can quantize the entire clip using the Arrange window's Region Parameters box, and you can set the quantize strength, range, and other options in the Extended Region Parameters. Alternatively, you can quantize selections of notes in any of the note editors. In either case, quantizing is nondestructive, so you can change your mind later (see Web Clip 1).

Modifying the tempo map that results from grid alignment is another way to smooth things out. You can use that to reduce tempo variations and reduce or increase intentional speedups and slowdowns such as a ritard at the end of a piece. If you want to preserve the original tempo map, copy all the tempo events to a new tempo alternative and use that for the tempo alterations.

To reduce or increase tempo variations, open the Tempo Operations window, select Scale Existing Tempo Changes from the menu at the top, make note of the first tempo value, and then choose a negative scaling percentage to decrease variation and a positive percentage to increase variation. Next, select all tempo events and set the tempo of the first tempo event to the original value. That leaves the overall tempo unchanged while increasing or decreasing the tempo variations.

Once an audio or MIDI clip is aligned to the grid, you can often use step-entry and step-sequencing tools as well as tempo-synchronized effects. For example, you might find that you can use a drum sequencer to add percussion. As with quantizing, a 1-bar alignment works best with step-entry tools. The important point is that aligning the grid to a freely played clip brings a host of sequencing tools into play that would otherwise be unusable.

Len Sasso is an associate editor of EM. For an earful, visit his Web site