Introduction to Computer Music: Volume One

8. Digital audio file formats

Digital audio files, which are the storable and editable collection of samples organized in a standard form, can be stored on computer drives, transferred to other computers or samplers, shared on the Internet to be downloaded or played-back in real time. They are different from audio CD or DAT tracks, which mostly contain only the raw sample data or items useless to a computer, such as error-correction and subcode data to help point the CD laser, etc. That is why a CD track must be "extracted" or "ripped" to an audio file format to be usable by a computer application. A standard 16-bit, 44.1K stereo file eats up about 10 megs of disk space per minute of sound.

Audio files come in a variety of types, which can influence their bit depth, multi-channel organization, compression scheme, sampling rate, organization of bytes high to low or visa versa (called "endian-ness") and amount of non-sample information stored in an area called the header, in units called chunks. Many audio programs are capable of opening and converting several file formats within limits. Some additional terms you may see when looking at a soundfile format is related to the bit-depth often tied to how computers store different sized numbers. Common sample sizes are often called 8-bit chars, 16-bit short integers, 32-bit unsigned long or 32-bit floating point (floats).

Stereo soundfiles can be organized as interleaved, where the sample bytes of respective channels alternate in a single stream (LRLRLR, etc), or as two separate files called split stereo, where one file contains the LEFT channel samples and another file contains the RIGHT channel samples. By convention, these are usually labeled with the same name with a .R or .L suffix (ex. myaudio.L, myaudio.R). Most programs will simultaneously open both files by default. Many programs, such as MOTU's Digital Performer and Digidesign's Pro Tools, work only with split stereo files—when importing an interleaved file, they will automatically split it into two files. However, some CD burning programs will burn only interleaved stereo files, so the separate files must be "bounced to disk" and then exported as an interleaved file to be burned.

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