Audio slicing in Cubase involves splitting an audio recording into a number of separate events based upon the locations of the attack transients in the signal. Events produced in this way are tempo sensitive — in other words, when you change the tempo of the project, the split audio events automatically change their positions along the timeline to play back correctly in time. The technique is advantageous because it does not use time-stretching and thus avoids the undesirable audio artifacts that the latter can sometimes produce, resulting in more natural-sounding loops when you adjust them away from their original tempos. However, audio slicing is not very useful for mixed material and legato-style performances: it is best suited to drums, percussion and musical performances with strong attack characteristics.
Propellerheads' ReCycle software was among the first to tackle the idea of automatically slicing drum loops and other rhythmic material into individual beats. Indeed, Cubase recognises ReCycle's REX file format, allowing you to load pre-sliced audio material. Anyone already familiar with ReCycle will find the slicing process similar in Cubase 4. The idea is based around an algorithm that recognises the main attack transients in the audio signal (attack transients are brief peaks in the signal where there is significant and rapid amplitude gain). Once recognised, markers are placed at the corresponding points, thus allowing you to split your audio into its constituent sonic events. The sensitivity of the process can be increased to detect peaks of lesser significance, thus producing more markers and constituent events of lesser duration.
In Cubase, these markers are called 'hitpoints', and each section marked by a pair of hitpoints is known as an audio slice. Based upon the hitpoint positions, audio events can be automatically divided up into audio slices using the Slice and Close button in the Hitpoints section of the Sample editor Inspector. Alternatively, 'Create audio slices from hitpoints' in the Hitpoints submenu of the Audio menu produces the same effect. In the case of drum loops, each audio slice usually contains a single sound or hit. Hitpoints can be calculated either over the whole event or within the current range selection.
News: Cubase Essential 4
Steinberg recently launched Cubase Essential 4, which they describe as "the ideal entry-level ticket to the world of Cubase" (it replaces the Cubase SE line). Although it's a 'cut-down' version of Cubase 4, it uses the same audio engine and includes many of its features — not least facilities such as time-stretching and pitch-shifting, and a VST3 plug-in set that includes the HALion One VST instrument and the Guitar Amp simulator. In all, Steinberg say there are over 60 new features. Cubase Essential 4 is available now and retails at £120 including VAT.
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Like many things in music technology, audio slicing is a combination of art and science. The main techniques to master are hitpoint calculation and hitpoint editing. Successful results rely as much on your ears and judgement as the accuracy with which you have chosen your hitpoints. Poorly chosen hitpoints result in poor audio slicing, so to get really accurate audio slices you will generally need to do a lot more than just move the hitpoint slider and slice up your audio. Automatically calculated hitpoints are not always sufficient. Luckily, calculating hitpoints in Cubase 4.1 got slightly easier with the new design of the Sample editor, but this step is just the beginning. This month, I'll work through the whole process of hitpoint calculation, hitpoint editing and subsequent audio slicing.
The following steps outline the audio-slicing process on a four-bar drum loop. For visual clarity, the example shown here uses a mono file but the same techniques are applicable to stereo files. Most of the action takes place in the Sample editor. Proceed in a similar manner to slice your own audio.
1. Double-click on the audio event containing the target drum loop, to open the Sample editor. Select the proposed range of the loop using the Range selection tool. Activate the local audition and loop buttons in the toolbar or use Shift+G to audition the loop. Make sure that the start point corresponds with the down beat of the first beat of the loop and adjust the end point so that the loop plays back over the required number of beats. If you're using a library loop that is already tightly edited and the clip fills the whole event in the Sample editor, you do not need to make a range selection and you would normally calculate hitpoints over the whole event (see screen 1).
2. Open the Definition section of the Sample editor Inspector. Select the desired resolution in the Grid menu (try 1/4, 1/8 or 1/16 note) and activate the Auto adjust button. Under normal circumstances, this automatically calculates the number of beats and the tempo of the audio (screen 2).
3. Open the Hitpoints section of the Sample editor Inspector and drag the Sensitivity slider to the right until one hitpoint appears for each hit in the rhythm (screen 3). The further you drag the Sensitivity slider to the right, the more hitpoints will be calculated.
4. To allow auditioning of the audio between each pair of hitpoints, activate the Edit Hitpoints button. Now when you move the mouse pointer between each pair of hitpoints it changes to a loudspeaker and clicking once plays the audio between the hitpoints concerned (screen 5, previous page). A slice turns blue as it is played. Attempt to produce audio slices where only one sound or hit is present in each slice. You can usually rely on the accuracy of the automatically generated hitpoints.
5. Once you're satisfied with the settings, click on the Slice & Close button in the Hitpoints section of the Sample editor Inspector to create the audio slices (screen 5).
6. When you click on the Slice & Close button, the Sample editor is automatically closed and the original audio event in the Project window is replaced by an audio part containing the new slices (screen 6).
The timing of the slices in the new audio part is automatically adjusted to match the tempo of the project. When you change the tempo, the sliced part follows.
To edit the audio slices in more detail, double-click on the sliced part to open the Audio Part editor (screen 7). Activate cycle playback to audition the part. Without stopping playback you can double-click on any problematic or 'clicky' slice to re-open the Sample editor. Try de-activating autoscroll in the Sample editor. This stops the display scrolling, thus helping you to focus on the selected problem slice at a high zoom level. To change the timing, or to ensure a clean attack, you can now drag the hitpoint at the start of the slice to a new position. After any editing, and while still in playback, click on the Slice and Close button again to re-calculate the slices. You can re-calculate the audio slices in this manner as many times as you wish. The new settings are immediately taken into account in the sliced part. All this can be achieved without dropping out of playback, allowing you to quickly verify the results. For fine-tuning and avoiding clicks, try activating the snap-to-zero-crossing button. Placing your hitpoints at zero-crossing points may help optimise the timing precision and the attack portions of your slices. If you need to manually alter the timing of the events, this too can be achieved in the Audio Part editor by dragging events to new positions along the timeline.
The solution to the first problem is to delete the unwanted hitpoints. Select the zoom tool and draw a box around the area of interest. When you can see the unwanted hitpoints clearly, select the Edit Hitpoints button and drag the corresponding hitpoint handles outside the Sample editor display. This deletes the hitpoints. Alternatively, Alt-click on the hitpoint handle to deactivate rather than delete the hitpoint. The hitpoint handles are the triangular-shaped markers just below the ruler. The Edit Hitpoints button is found in the Hitpoints section of the Sample editor Inspector.
The solution to the second problem is to manually insert two hitpoints at the positions of the hi-hat hits. Select the zoom tool and draw a box around the area of interest. When you can see the waveform clearly, select the Edit Hitpoints button and Alt-click in free space to insert hitpoints at the desired locations in the Sample editor display (the pointer changes to a pencil tool). Low-level hits like this are sometimes marked by changes in the shape of the waveform but more often than not they are masked by other elements in the audio. Finding the correct points may therefore require some trial and error. If required, click and drag on the hitpoint handles to drag the hitpoints to new positions along the timeline. Activate the Snap to Zero Crossing Point button in the toolbar to snap all editing to the nearest zero-crossing points. This helps find the appropriate positions in the waveform and avoids clicks. Manually entered hitpoints are displayed in blue. An alternative solution to manually entering the hitpoints is to temporarily increase the level of the hitpoint sensitivity slider until hitpoints appear at the relevant positions in the waveform. These hitpoints may then be locked by clicking once on the hitpoint handle (the hitpoint turns blue). You can now reduce the level of the sensitivity slider to where it was before and the locked hitpoints remain in the display. In all cases, carefully audition the slices at the new hitpoint positions to make sure they are accurate.
In addition to the issues already described, there are a number of other problems that may arise with audio slices. One of the most common is the loss of rhythmic precision when the material is pushed away from its original tempo. The primary cause of this is that the duration of one or more slices is too long and includes a number of hits within the same slice. The solution is to re-slice the audio into shorter segments, where each and every rhythmic hit is taken into account and contained within a separate slice. This requires re-calculation of the hitpoints at a higher resolution: drag the hitpoint sensitivity slider further to the right and then re-slice.
Clicks or other interference at the starts or ends of slices can also cause problems. This might be due to the start and end points themselves, but it may also occur when you increase the tempo by more than a few bpm, as this effectively bunches the events more closely together, causing them to overlap, and clicks may occur at the overlap points. The solution is to activate auto fades for the track. To open the Auto Fades dialogue, click on the 'Auto Fades Settings' button in the Inspector for the track containing the sliced part. De-activate 'Use Project Settings' and tick 'Auto Crossfades'. Somewhere between 10ms and 30ms would be a good starting point for the crossfade. Otherwise, to deal with clicks that are not due to overlapping, try activating the fade-in and fade-out options with a 5-10 ms fade time. This provides an alternative technique to manually editing the hitpoints and re-creating the slices. The chosen auto-fade settings are applied to all events on the track. Beware though: when using this technique, make sure that you do not suck the life out of the attack portions of your drum and percussion hits.
A third problem is posed by gaps of silence between events when you decrease the tempo. The duration of the gaps of silence increases in direct proportion to the amount by which you decrease the tempo below the original value. A solution is to select the sliced audio part in the Project window and activate Audio / Advanced / Close Gaps. This applies time-stretch to each individual slice in order to close the gaps, though, so it should only be used when you are sure that there will be no more tempo changes in the project. Remember also that silence between events may actually be a desirable effect, since it can give a sense of spaciousness and may help your mix breathe more easily!
You can judge the success of your audio slicing by the accuracy of the results when you audition the material at different tempos, and by the lack of clicks and interference. Well-sliced material maintains its rhythmic integrity regardless of extreme tempo changes, and the attack portion of each slice sounds crisp and click-free. By following the techniques outlined here and taking care of the finer details of your audio slices, you may be able to transform an ordinary slicing job into a slice of heaven!
Optimising The Accuracy Of Your Slices
For greater accuracy when using a range selection, you may need to readjust the range start point to the position of the first hitpoint. If you find that this is the case, you should reactivate the Auto adjust button in the Definition section to change the grid.
You may achieve better results by using a greater number of hitpoints than expected, thereby dividing up the audio into smaller slices. The more accurate the audio slices, the more accurate will be the result when the audio is played back at different tempos.
Obtaining Click-free Slices
Automatically calculated hitpoints are not always placed at zero-crossing points in the waveform, and this may result in clicks at the beginning of certain audio slices. For click-free slices, try manually dragging hitpoints to the nearest zero-crossing point with the 'Snap to Zero Crossing Point' button activated in the Sample editor. For this technique it is best to work at high magnification factors.