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Automatic Beat-slicing

Logic Notes
Published May 2004
By Len Sasso

The Strip Silence window's Threshold and Minimum Time To Accept As Silence parameters have been adjusted so that most of the events are caught in individual slices, but no single event is sliced in two.The Strip Silence window's Threshold and Minimum Time To Accept As Silence parameters have been adjusted so that most of the events are caught in individual slices, but no single event is sliced in two.

Although beat slicing is something more immediately associated with programs like Propellerhead Recycle or Bitshift Audio Phatmatik Pro, Logic also has facilities to chop up your loops automatically.

Unless you've been on holiday for the past decade, you're undoubtedly aware of the technique of slicing an audio file into segments in order to change the tempo of the audio without changing its pitch. Programs such as Propellerhead's Recycle, Bitshift Audio's Phatmatik Pro, Sonic Foundry's Acid, and Ableton's Live accomplish this with varying degrees of user intervention. With a little effort, you can accomplish the same thing completely within Logic using the Strip Silence function.

There are several digital signal processing (DSP) techniques for changing the length of an audio file while mostly preserving its pitch and timbre. Granular synthesis and analysis/resynthesis are two common methods. If time-stretching is your goal, you can use Logic 's Time Machine, either in the Sample Editor window or directly in the Arrange window, more easily than the event-slicing method I'll describe here. However, audio-file slicing allows you to do a number of things beyond simply changing tempo. Those include rearranging the order of events, applying different DSP processing to individual events, and replacing some events with other recorded or synthesized sounds. You can also load the individual slices into a sampler such as Logic 's EXS24 and play them from a MIDI keyboard or MIDI sequence.

Although there are several ways to approach slicing up an audio file, the underlying concept is the same in all cases. The duration of an audio file, and therefore its tempo if it is a rhythm-based file such as a drum loop, is fixed. If you slice the file into separate regions, usually corresponding to individual sound events (for example, kick drum hits) and leave the regions at their original beat-positions on Logic 's bars/beats Time Ruler, you can freely change Logic 's tempo without affecting the rhythmic relationship between the slices.

Logic Tips

The channels displayed in the Track Mixer window reflect the arrangement of tracks in the Arrange window. You can therefore use empty Folders with different Track Lists to access different Track Mixer views. If you put the Track Mixer in Contents Link mode, simply selecting the Folder will change the Track Mixer view. Len Sasso

When you're using multi-channel audio plug-ins, you need to set the Audio Instrument object's MIDI Cha parameter to All in order to play the instrument's different channels with MIDI data on different MIDI channels. Len Sasso

Preparation For Beat Slicing

Several preliminary steps are helpful in getting the slices automatically placed at the correct Time Ruler positions in Logic 's Arrange window. In the Sample Editor window, create a region within the audio file that starts and ends on a bar line and includes all the material you wish to slice. Then drag that region to the desired starting bar on an audio track in the Arrange window. Next, set Logic 's left locator to the beginning of the region and set the right locator to define a bars/beats Time Ruler selection which matches the region's length in bars. (Note that, unless Logic 's tempo already matches the audio, this will not correspond to the actual end of the region.) If you ultimately wish to preserve the Song's tempo, select an unused Tempo Alternative using the Options menu in Logic 's Tempo List window. Finally, select the region and choose Adjust Tempo Using Object Length & Locators from the Tempo submenu of Logic 's Options menu.

Your region will now fit perfectly between the locators, and will occupy the correct number of bars. The quickest, though not necessarily the most useful, way to slice up the region is to use the Scissors tool while holding the Alt key. Clicking at the desired slice-length from the left region boundary (one beat for example) will slice the region into equal-sized pieces. That may work well for precisely-played or quantised material, but in general the slices will bear no relation to the musical events within the audio file. More intelligent slicing, similar to that done in Recycle and Phatmatik Pro, can be accomplished with Strip Silence.

As its name suggests, the purpose of Strip Silence is to break an audio file into regions by removing the silence between them. It does that by searching for segments within an audio file that are below a specified threshold level for at least a specified minimum amount of time. It then splits the file into the regions between the 'silent' areas. That process is very handy for extracting loops and hits from audio sampling CDs, for example, where a track might contain several loops or many individual drum hits separated by silence. Beat-slicing programs work on a slightly different principle, detecting individual events by their attack transients and slicing the file up into the detected events. You can use Strip Silence to do much the same thing.

Cutting & Inserting Sections In The Arrange Window

One job that's particularly tedious when using Logic 's basic edit commands is the process of deleting a few bars from the middle of a Song and then closing up the gap. You can audition with a skipped section easily enough by marking a Skip Cycle in the Time Ruler at the top of the window (this shows up as a narrow white bar when you drag from right to left), but what's the fastest way to make this permanent?

It turns out that the exact command you need is available under the Arrange window's Functions menu. The Cut/Insert Time submenu has a Snip: Cut Time & Move By Locators option which works on the area of the Song between the two locators, though it also seems to work fine if you've set the locators up to do a Skip Cycle for auditioning purposes. However, you do have to remember to Select All before doing the operation, otherwise only the selected tracks will be affected, and most of the time you'll want to do it to all the tracks. Once you've carried out the Snip command, everything between the locators is removed and copied to the clipboard, and the end of the Song is moved back to fill the gap — easy! Furthermore, if you need to take the clipboard contents and insert them elsewhere in the Song, all you need to do is put the Song Position Line where you want the insert to start, Select All again, then use the Splice: Insert Snipped Part At Song Position option from the same submenu.

When carrying out these operations, it is worth knowing that any tempo changes, time-signature changes, and global Score-window symbols are also moved. If you don't select all objects at the edit point, Logic will open a dialogue box asking whether tempo changes and global score symbols should be affected or not. Any looped objects affecting the edit will be turned into real objects as required, but be aware that any frozen tracks will remain exactly as they were, and no warning dialogue will be issued. Paul White

Using Strip Silence

You invoke Strip Silence by selecting the unsliced audio file and choosing Strip Silence from the Arrange window's Audio menu. (You can also invoke Strip Silence in the Sample Editor window, but for our purposes the Arrange window is the place to be.) For practice, start with audio material which has clearly defined events — a basic drum loop, for example. The Threshold and Minimum Time To Accept As Silence settings are the most critical, so start by setting the Pre Attack Time and Post Release Time to zero. You'll notice that the number of regions varies inversely with Minimum Time To Accept As Silence (ie. a higher setting results in fewer regions) — a few 100ths of a second is usually a good starting point. You'll also notice that the number of regions at first increases as you raise the Threshold setting, then begins to decrease as the threshold gets higher. Depending on the material, settings between five and 15 percent tend to produce the best results.

Here the tempo has been increased, resulting in overlapping of the events produced by Strip Silence. Alternate events have been moved to a second track to make setting the fade-out times visually easier.Here the tempo has been increased, resulting in overlapping of the events produced by Strip Silence. Alternate events have been moved to a second track to make setting the fade-out times visually easier.The goal in adjusting the Threshold and Minimum Time To Accept As Silence settings is twofold. You want to get enough slices to separate most of the events without also slicing up individual events. Having some slices capture more than one event is tolerable, because you can fix those manually or by invoking Strip Silence again on the errant slices. Over-slicing is more of a nuisance, so err on the side of fewer slices.

The Pre Attack Time and Post Release Time settings allow you to extend all of the slices to the left and right respectively. Leave Pre Attack Time at zero and adjust Post Release Time until there are no gaps between the slices. That gives you back all the 'silence' that would otherwise have been left on the cutting-room floor, without changing the number of slices or their positions.

After you click the OK button, Strip Silence will slice the file into individual regions residing at the Time Ruler positions they originally occupied in the file. As previously mentioned, you will need to apply Strip Silence again or manually split those regions (if any) that contain multiple events. If you open the Audio window, you will see that the regions exist there as well, and you can use the Audio window's Audio menu to save the regions as individual audio files if so desired. (You might want to do that in order to use the regions in a sampler, for example.)

If your goal is simply changing tempo, you can now return to the Song's original Tempo Alternative and play the track to see how it sounds at the original tempo. If the difference in tempo is small, you may not need to make any adjustments, but the chances are that some tweaking will be in order. Since the regions are tied to the bars/beats Time Ruler, increasing the tempo reduces the time between regions, whereas lowering the tempo increases it. Put another way, increasing the tempo causes the regions to overlap whereas decreasing it produces gaps between the regions.

Have Your Say!

If you want to suggest changes or improvements to Logic, then here's your chance! The Emagic development team are inviting SOS readers to send in their suggestions of what they'd most like added or changed in Logic. Email your top five suggestions (in order of preference) to logicnotes@soundonsound.com, and we'll forward your lists on to the Logic team. We'll be asking them for feedback on which changes users deem most important and how these might be addressed.

Sorting Out Gaps & Overlaps

When increasing tempo, if the overlapping is not too severe, you can compensate in two ways. The simplest is to select all the regions and choose 'Tie Objects By Length Change' from the Object submenu of the Arrange window's Functions menu. That shortens the regions as necessary to make them fit end-to-end in the allotted space. If cutting off the end of each region produces unsatisfactory results, try fading out the regions rather than resizing them. To do that it is convenient to move every other region to an adjacent audio track. That allows you to visually judge the amount of fade required. Next select all regions on both tracks and input a Fade Out value in the Parameters box so that you fade out most of the overlaps. If the regions vary greatly in size, you may need to adjust some individual fades as well. About the only way to adjust for gaps when reducing the tempo is to add a little reverb to the track. If that turns out to be unsatisfactory, you will need to use a program such as Recycle that will fill out the slices using more sophisticated DSP processing.

There are a number of things you can do with the regions you've created beyond simply altering their timing. For one thing, you can allocate them among several audio tracks (retaining their time positions) in order to apply different plug-in processing. For another, you can replace some of the regions with other audio clips of similar or completely different sounds, for example replacing one kick drum with another, or replacing a hi-hat with a ride cymbal. You can also easily create a MIDI file corresponding to the timing of the slices, to be used as a Groove Template or as the rhythmic basis for another part played on a totally different synthesized or sampled instrument.

Published May 2004