Sample editing

The manipulation of audio samples is an integral part of the music process. We describe the essential functions you'll find in most audio editors and explain how and when to use them...

In this Sample Edit feature we explain the following sample processing and manipulation functions. Read the whole article or use these hyperlinks to jump straight to the sections.

Cutting & pasting Snap To Zero crossing points DC offset Crossfading Time stretching Pitch shifting Pitch correction

Formants Pitch bending Channel conversion Phase and phase inversion Pseudo stereo Reverse Normalisation Resample/Sample conversion

Whether you want to edit an audio recording or tweak a sample, you need suitable tools for the job. And those tools are audio editors such as WaveLab, Sound Forge, Adobe Audition (formerly Cool Edit) or one of the many other audio editors bundled with hardware or available on the web. The main differences between the software are their editing and processing features. Here we'll look at common sample edit and manipulation functions and explain the part they play in sample editing.

The processes described here can be applied to short samples for use in a sampler, sample loop files, and to larger audio files such as instrumental recordings. The editing principles are exactly the same although some types of file lend themselves to certain functions more than others and we'll discuss those when we get to them.

Chop chop

The most basic edit functions are cutting, copying and pasting. In a sequencer, cutting is often done with a scissors tool. In most audio editors you highlight a section of audio and select Cut from the edit menu.

There are two important considerations here. If you are cutting part of a song file, try to cut at beat lines to keep cut sections in beat or rhythmic 'blocks'. Most sequencers have a Snap To function which restricts edit functions to certain divisions of the beat so use this. Audio editors may not have such a function so you need to be precise with your manual cuts. Some editors display material in a range of formats such as samples, minutes and even beats so zoom in close in order to cut or place a marker exactly on the required division.

Whatever editor you're using, check to see if it has any zero crossing points options. When you look at a waveform, the places it crosses the horizontal zero line are called, naturally, zero crossing points. At these points the waveform is at zero amplitude and, therefore, if you're going to cut and paste a sample these are the best places to do it to minimise potential glitches.

Off centre A DC Offset occurs when there is too large a DC (Direct Current) component in the signal. It shows up in a waveform when the wave is not centred above and below the zero line. However, a DC Offset may be problematic even if you can't physically see it (if you really want to see it, zoom in close on the sample). A DC Offset is most commonly caused by mismatched equipment such as a microphone or soundcard. DC Offset causes two problems. Firstly, it affects zero crossing points because any routine that scans for a these points will be looking close to the zero line for the crosses. Glitch-free looping, cutting and pasting will be difficult at best and impossible at worst. Secondly, many processing functions may not perform optimally due to the offset in the waveform. In a nutshell, you really ought to fix a DC offset problem before editing or processing a file. This is very easily done with most audio editors via a Remove DC Offset function. Most editors simply remove the offset completely which is usually what you want, although some may allow you to adjust the offset value by a specific amount. This may be necessary if the offset is not consistent throughout the sample.

If you cut at another point, above or below the zero line, and join this to another sample which starts at a different level, the change will produce a click. Selecting good zero crossing points is an essential part of the art of making sample loops. Zero crossing point options may simply be a 'snap to zero crossing points' setting or, as with Adobe Audition, for example, it may have options to move the boundaries of the selected area to crossing points to the left and right.

However, simply cutting at a zero crossing point is not always enough. If you try to join two contrasting timbres, for example, then there will inevitably be a click so you should also ensure that the two parts sounds similar. There are ways to join parts that are not similar or to loop a sample that is proving difficult (creating and manipulating drum loops) but for more amenable parts, most editors have some looping tools to help. And you should take advantage of any help that's offered!

Sound Forge's Loop Tuner and WaveLab's Crossfade Looper both butt the end of the loop up against the start so you can see and hear exactly how the join performs. You can easily adjust the start and end points and quickly hear the results. Both have a Crossfade Looper function that can crossfade across the join, too, which helps with difficult loops. WaveLab also has a Loop Tone Equalizer that goes even further by evening out differences in timbre and level.

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