Audio File Formats Demystified
I know what we need. It’s an ADA (another damn acronym)! It seems like the technology geeks who are reengineering our world know how to program and speak in code. The goal of this guide is to help you find your way through the maze of the most common file formats used in digital music making.
Uncompressed Audio Formats
When audio is digitized, it is transformed from analog sound pressure levels to numbers. Your computer, stereo system, or music player needs to understand the numbers and eventually turn all the little zeros and ones back into signals that your ears can hear and your brain can understand. This is where the world of audio file formats comes into play.
Audio files are broken down into two different groups — uncompressed (lossless) and compressed (lossy). Uncompressed file formats are used when maximum audio quality is desired. While analog purists will argue that digitizing any signal results in a loss in quality, lossless audio files try their best to keep all the digitized data in its full form. In general terms, you’ll want to use lossless formats when the audio may be further edited or passed through other computer processing. Files that have been compressed with lossy codecs will only be degraded further if they are subject to additional computer modifications. Compressed formats are used when the highest-quality audio is not necessary or when storage space and transfer time are important considerations. We’ll first take a look at the two most common lossless audio formats.
(Audio Interchange File Format). These files will often use the .aif extension. This is a noncompressed format that is used to store audio files primarily on Apple computers. However, most PC software can read AIFF files without a problem. The AIFF format was developed by Apple (some folks believe that the acronym stands for Apple Interchange File Format) and was based on the Electronic Arts Interchange File Format. One of the advantages of this format is the large amount of nonaudio data that can be stored along with the audio information. AIFF files often contain information such as name, author, copyright, and comments.
(Waveform Audio Format). The common extension for this type of file is .wav. This is a file format designed to store audio data on PCs, yet most Mac software will also open WAV files. While this format is a close cousin to AIFF, it makes some use of the special features available with Intel CPU machines. WAV files can hold audio that has been compressed, but they are most often used to house noncompressed audio. Due to the limitations of the format, WAV files can’t be larger than 2GB.
Compressed Audio Formats
Because uncompressed audio produces large files, the popularity of working with compressed file formats has grown. CD-quality sound (44.1kHz, 16-bit stereo) eats approximately 10MB per minute. That makes a 10-minute uncompressed audio file about 100MB in size. Some audio compression schemes can reduce file size by a factor of ten or more. Without audio compression, it wouldn’t be possible to get all those songs on your iPod!
All compression formats use a codec of some sort. An audio codec is a process or a software program that compresses (co-) and then decompresses (-dec) an audio signal based on a number of rules and assumptions about how we perceive music. Compressed audio is further divided into codecs that are lossy and those that are lossless. With lossless audio compression formats, the size of the file is reduced but the quality of the music remains the same. In theory, the decompressed signal is exactly the same as the original signal. While lossy compression formats can offer a huge reduction in the file size, you can expect only about a 50-percent reduction when using lossless codecs.
(Advanced Audio Coding). This is a compressed and lossy file format that was developed to be an improvement over MP3 and designed for audio streaming. Some of the features that make AAC more robust than MP3 are the ability to sample frequencies from 8 Hz up to 96 kHz, the use of up to 48 channels, and higher coding efficiencies for stationary and transient signals. The AAC file format was first published in 1997 but came into the popular consciousness when Apple computer’s iPod began using this file format in 2003.
(Advanced Streaming Format; later changed to Advanced Systems Format). This file format is patented in the US by Microsoft and is used by Windows Media for both audio and video. ASF files are more like containers that can hold data compressed by a number of different codecs. WMA (Windows Media Audio) and WMV (Windows Media Video) are the two most common file types held inside an ASF file.
(Apple Lossless Audio Codec). This format is also called Apple Lossless Encoder, or ALE for short. Similar to ASF files, ALAC files serve as a container to house files that have the extension m4a. This format was first introduced in a QuickTime upgrade in 2004 and implemented into Apple’s iTunes 4.5.
(Adaptive Transform Acoustic Coding). This algorithm was developed in 1991 by Sony as a file format for their Minidisc recorders. Two more recent versions, ATRAC3-LP2 and ATRAC3-LP4, enable the long play modes featured on many current Minidisc recorders. These two formats increase recording time up to 324 minutes on an 80-minute Minidisc.
(Core Audio Format). This is the newest addition to the audio format family and is supported natively in the Macintosh OS X operating system. Apple claims that this file format can record as many as a thousand channels of audio for as long as a thousand years (now that’s going to be a big file!). Like its cousins, it can serve as a container that can house both compressed and uncompressed files.
(Free Lossless Audio Codec). The “free” part of this file format’s name means that the FLAC process is not covered by any patent. Like other lossless compression formats, FLAC can reduce an audio file by 30 to 50 percent. FLAC files are becoming more popular as a way to archive CD collections and as a way to transfer high-quality audio over the Internet. FLAC files can be read by a number of different programs on all major computer platforms.
(MPEG-4 Audio Standard). This is the file extension of an audio file that uses MPEG-4 Advanced Audio Coding. Apple made this format famous by using it for their iTunes software. Most software that will read MPEG-4 files will support the m4a format.
(Moving Picture Experts Group-1 Audio Layer 3). Without a doubt, this is currently the most popular audio compression format on the planet. The MP3 audio compression format was first developed as early as 1987 and finalized in 1992. The first commercial software to use the MP3 format was Winplay3 in 1995. Since that time, MP3 files started appearing on the Internet, and as they say, “the rest is history.” Today, the number of applications that can read or write MP3 files is enormous. It is the ubiquitous standard for sharing audio files over the Internet, and for good reason. It’s fast, it’s free (for the user), and it can drastically reduce the size of an audio file without totally destroying the quality. Like other lossy compression schemes, it does this by removing portions of the audio image that the algorithm determines can’t easily be heard. MP3 files can perform their compression magic at a number of different bit rates that can adjust the trade-off between file size and audio quality. Common bit rates range between 32 and 320 kilobits per second (Kbps), with 128 Kbps and 192 Kbps being the most popular compromise. For most civilians, an MP3 file at 128 Kbps will sound just fine. If you’re planning on listening to music files in a moving car, 128 Kbps might satisfy your ear. But for most musicians who are familiar with the sonic fingerprint of high-quality instruments, a 192 Kbps format or higher is often necessary.
(Off Vorbis). Vorbis is an audio codec that is often placed inside an OGG container. Together, they use the .ogg file extension. This format has been gaining popularity with those that support open source software, and it’s been showing up on several web sites around the world.
(Windows Media Audio). WMA files are compressed files that are played using Microsoft’s Windows Media Player. Originally, the format was developed to be a competitor to MP3; it’s now a competitor to Apple’s AAC format. The newest version of WMA is 9.1 and contains codecs for multichannel surround sound and lossless support. WMA files are most often contained inside an ASF file.
There are a number of other lossy and lossless file formats, and it seems that new ones are being developed at a surprising pace. Some, like Monkey’s Audio’s APE format, are only supported on the PC platform. Others, like RealPlayer’s RealAudio Lossless format, are designed to be read by a single host program (although other software may be able to import the files).