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Loop File Formats
Understanding the world of loops...
By Darwin Grosse


Loop-based production has become part of the mainstream. Virtually every DAW application either features loop file support or offers a connection to looping applications. Sample CD sales continue to grow, and the amount of available content is staggering

However, when looking through loop CD catalogs, it is easy to be overwhelmed by a new vocabulary—there are several types of loop files, including AIFF and WAV files, REX files, ACID files and now Apple Loops. Each file format has its advantages and disadvantages, so an understanding of their differences will help when shopping for inspirational grooves.

ACID and beyond

The first sample CDs were simply audio CDs that you would record into a sampler. This was time-consuming, and few people had the programming skills to properly capture and loop the audio tracks. As sampling equipment became more computer-like, files were provided as standard AIFF and WAV files—file formats that are standard among computer audio systems. This helped get the files into a computer or sampler, but didn’t necessarily help make them usable.

A revolution in loop production appeared with Sonic Foundry’s (now Sony’s) groundbreaking ACID application. This computer program changed both how sample loops were used and how sample CDs were created. An ACID loop file is a special type of WAV file that contains extra information that makes pitch and time manipulation easier; soon sample loop CDs began appearing that supported the ACID format by including this extra information.

As new looping software was created by other companies, new sample formats appeared; the market has now become stabilized and we find three primary proprietary loop file formats: ACID files, REX files and the new Apple Loops.

Two concepts

To understand how these files are prepared for looping, there are two concepts that you need to understand: transient points and audio slices.

A transient is the onset of a significant audio event—most often it is a spike in level. A good example of a transient point is the very start of a drum hit. All of the common loop file formats are based on identifying transient points, and u sing them to create audio slices.

Slices are segments of a file that represent an audio “phrase”; for instance, a slice in a drum loop might be the snare drum at the second beat of a measure. This slice would contain all of the audio from the initial transient point of the snare until the kick drum begins at the next beat. Each of the file formats treat transient points and slices differently, but the concepts are identical among them all.

ACID files

Among vendor-created file formats, the “big dog” is the ACID Loop format. Created by Sonic Foundry (and now developed and sold by Sony), the ACID application crashed through barriers in establishing loop production, and became enormously successful as a result. The ACID file format has became the most popular vendor-developed standard for loop content, with Sony’s enormous ACID Loops catalog leading the way.

An “Acidized” file is a standard .WAV file (the most common PC audio format), but it takes advantage of an audio-file feature that allows non-audio data to be embedded into a file’s header. This extra information provides ACID with a loop’s length (in beats and bars), time signature, slice points and the base transposition key.

ACID files are normally edited using Sony’s ACID program itself. There is a track properties window (shown at left) used to set transient points and to select the loop duration, meter and key info. The slices match transients in the file, or can be set to musical time intervals. As an example, it is common (for some types of material) to set up an ACID File with slices on 16th notes, regardless of where the transients fall.

Of all the formats, this is the fastest and easiest editing system to use. Slices of sustained sounds can match musically useful boundaries, allowing for better stretching of these types of loops. Furthermore, ACID tends to have forgiving time-stretch algorithms, which produces tempo shifts that sound more natural than those of most other software.

The ACID file format received a shot in the arm with Cakewalk’s release of SONAR. This package combined DAW production with support for ACID loops, allowing for much more complex production. In addition to using ACID files, SONAR has a Loop Construction window that is a full-fledged slice editor, and is capable of saving files in ACID-compatible format. SONAR’s Loop Construction tool is becoming a preferred ACID file editor by loop developers, and provides exceptional support for the format.

There are a few limitations to the ACID file format. It is the least well-documented format, so developers have difficulty supporting it in their editing and sample playback software. There are currently no free editing utilities (although most ACID file fanatics are using either ACID or SONAR, so they have a built-in editor), and the ACID-specific data is unsupported in many software packages. Nevertheless, the popularity of ACID and SONAR, and the availability of ACID loops of almost any stylistic genre, has spurred many producers into using dedicated PCs solely for ACID file production.

ReCycle (REX) files

Another popular format, the REX file, is an output format created by Propellerhead Software’s ReCycle application. This software was among the first to focus on looped audio material as a creative musical device, and was initially developed to chop up beats into small, discrete files that were uploaded to hardware samplers. ReCycle has undergone many changes since its original release, and the REX file format was created to support software sequencers and samplers. A REX file contains a compressed version of the original audio loop, pre-sliced into individual audio regions.

REX files are specifically oriented toward percussion and drum loops. ReCycle is used to identify percussive transient points, which are then used to create the audio slices. Once the slices are identified, each slice’s relative time (from the start of the loop) is paired with the slice in the new file. When a REX file is imported by a host application, the individual slices appear as individual audio segments, with timing scaled to the original timing of the loop.

The reason for all this file manipulation is that, when played back, the feel of the loop is maintained even when the playback tempo is drastically altered. The individual slices are played unaltered, so the sound of the material is maintained—a difficult chore for most time-stretching algorithms.

The only software available for creating REX format files is ReCycle. Version 2.1, recently released, is fully cross-platform (Mac OS X and Windows XP) and supports stereo loops at 24-bit resolution. It also provides several interesting sound design tools, including an equalizer, a transient editor and a tempo/pitch remapper. ReCycle can be thought of as a creative tool as well as a REX file creator, adding to its value.

Propellerhead provides REX file development information and programming tools to all of its development partners, resulting in direct support by most major DAW programs. Emagic Logic, Steinberg Cubase SX and Nuendo, MOTU Digital Performer, and Cakewalk Project5 all support REX files without conversion, as do many software samplers (such as MOTU MachFive, Steinberg HALion, and Native Instruments Kontakt).

Perhaps the most sophisticated REX playback program is Propellerhead’s own software studio Reason, which includes a dedicated REX file player called Dr:rex. A feature-limited version of Reason (called Reason Adapted) is included with ReCycle 2.1, and includes Dr:rex as an available module.

REX files have a number of advantages over other loop formats. The ReCycle software is easy to use, with an impressive set of sound design tools for stretching, gating, re-pitching and re-timing percussive loops. More importantly, these loop files can be played back with little audible change, since the slices are not “stretched” when the host changes tempo. Many people consider the REX file format to be the best option for drum and percussion loops.

REX files also have their disadvantages. A REX file is not an audio file, so there is no way to audition a loop without loading it into an application; this is a real hassle in comparison to other formats, which are often directly supported by the computer’s operating system. There is also no free editor, so REX file creation requires the purchase of ReCycle. Finally, the REX format, because it literally “chops” the audio loop, is a poor choice for sustained pads or orchestral beds.

Apple Loops

The newest kid on the loop format block is the Apple Loops format. It was created by Apple to support its Soundtrack application, and gathered more steam as the sole supported loop format for GarageBand. In many ways it is similar to the ACID Loop, creating slices at either transients or time breaks, and storing its application-specific data in the non-audio header area of a standard AIFF audio file. Unlike other formats, however, it also stores a collection of information used to make loop selection easier for less sophisticated users.

When an Apple Loop file is created, the developer not only inserts transient markers for the loop’s audio, but also information related to the loop’s genre, music style, key and scale type (if tonal). There are also “descriptor” settings, where the developer will choose between such pairings as Dry or Processed, Melodic or Dissonant and Cheerful or Dark. When users want to select an Apple Loop from within GarageBand, they are presented with a search system that accesses any of the available descriptors. This is especially helpful for Apple’s core users: video editors (because Soundtrack is included with Final Cut Pro) and beginning musicians (since GarageBand is included free with every new Macintosh).

The software used to create Apple Loops is the Soundtrack Loop Utility, available with Soundtrack or free from Apple’s web site. It is fairly easy to use, with a simple transient selector and a full editing page for loop descriptors. You can audition the loop at various keys and speeds, but cannot batch convert files. ACID Loop files can be imported and saved as Apple Loops, but many more fields are maintained by Apple Loops than ACID, so each sample will require additional editing.

At this point, DAW software developers aren’t rushing to support this format, but sample CD creators are. To an extent, this is a low-risk venture for them, since the files are valid AIFF audio files and can be imported into most recording software. You should expect to see plenty of Apple Loops releases in the future, especially with the high visibility of GarageBand bringing a new generation of loopers into play.

If you use one of Apple’s software packages, you will find the Apple Loops format a necessity for your work. The inclusion of all the style and descriptor information makes track compilation a breeze; it is clear why these applications are popular for creating video beds and soloist demos. The availability of a no-cost editor is helping spread interest in the format, and Apple Loops are frequently popping up on Internet audio sites.

Disadvantages to the Apple Loops format do exist. The editing program is very AIFF-centric, so there is no way to embed this information in a WAV file without reformatting it. There is little support for this format outside of Apple, so there is no incentive to work with Apple Loops if you are not planning to use Soundtrack or GarageBand. Finally, the implication of the “style” settings can be somewhat limiting—what do you do if you are creating loops that fall outside the lines of the predefined options? You can select “none,” but may find that it will be more difficult for users to choose the loops you’ve created.

Why special formats at all?

In discussing the various formats, and the various positives and negatives about each, the question must be asked: Why use these vendor-created formats? Some examples might make it clearer. For GarageBand users, the reason is obvious—it makes appropriate loop selection much easier, and automates many of the tasks necessary to build a complete production. Without the Apple Loops embedded information, GarageBand wouldn’t be nearly as attractive to beginning musicians.

There are also creative reasons to use these formats. REX files are especially useful for creative manipulation; one of the common ways to make fresh new breakbeats is by shuffling the playback order of slices—stale loops can be made new by stuttering the snares, flipping a portion of a beat or pulling samples out of a loop for manual playback.

Also, in comparison to the time-stretching capabilities of most DAWs, these formats will cause time manipulation to sound much better. By giving the user some control over the slice points, better control is available for almost every kind of time adjustment.

The choice is yours...

So, what file format should you use and/or purchase? Actually, you probably shouldn’t decide on a format—you should decide on a software package that you are comfortable with, then use any format that is supported.

If you are a die-hard ACID fan, ACID loops are in your future. Is Reason or Cubase your bag? REX files will best serve your needs. Soundtrack and GarageBand users will certainly focus on Apple Loops, while Ableton Live users will avoid REX files (since REX files are not a standard audio file format). Once you have a software package that works for you, test all of its supported file formats and make informed decisions about your loop library purchases.

Darwin Grosse (talkback@recording is a recording engineer, musician, sound designer—and occasional contributor to the odd loop library here and there—living and working in the Colorado Rockies.

Kef America

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