AET 2050 - DAW Production

6. Audio File Formats

Objectives:

1. Describe the basic structure of audio files, including file descriptors and data storage methods
2. Identify the basic streaming file formats in terms of origin, application, platform compatibility, and type of audio data stored (bit depth, sample rate, channels, etc)

Reading:

Chapter 6.1-6.2, Desktop Audio Technology - Rumsey

Computer Files

What is a file?

• a series of bytes containing a grouping of data for a computer or program to use
• stored on a storage device (such as a hard drive)
• independent of the filing system - can be transferred between platforms, still exist as an identical set of data blocks
• Some features can relate to the OS that created them, but usually don't prevent their being transferred

File Structures

• Simple files (such as ASCII text files) are simple raw data files - the data begins at the beginning of the file.
• Many data file structures have 2 components:
• the actual data
• a Header
• description of the data
• section at the top of the file where certain important information is stored
• This can include information such as sampling rate, bit depth, timestamp info, etc.
• most audio formats contain such header information
• Mac files are unique in their file structure in that there can two parts to a file: a resource fork and a data fork.

• actually two files "joined together"
• separation of data from descriptions makes updating easier
• not ALL Mac files use both forks
• Windows cannot recognize a two-fork file
 
• Windows stores description info in the front area of a file
  • Header is a section of the file set aside for desriptor info
  • chunks - separate "containers" within one file sructure
  • Windows cannot automatically use Mac's resource information attached to a given file.
  • In some instances, the resource fork can be lost when transferring to other machines unless Mac-specific protocols are used.

Two methods of data byte ordering

• data words are generally made up of multiple bytes
• Computer archetecture defines the order in which data bytes are stored in memory
  • Big-endian: most significant byte comes first - Macintosh's legacy processors use this system
  • Little-endian: least significant byte comes first - Wintel processors use this system (new Macs, too!)
• All this means is that the data must be presented in the correct order for the appropriate system

Audio files

• Audio file is simply another type of data file
• most audio in high-quality audio formats is stored as two's complement form, and 16 or 24 bits (2 or three bytes) per sample

Background

• Historically, file formats were unique designs, getting the most out of slower hardware technology.
• Manufacturers worked in isolation, attempting to serve their own interests first; therefore, virtually every workstation would have its own file format designed to meet its needs.
• Hardware advancements provide enough speed and performance to where special format designs are no longer necessary or beneficial. Interchange between products is of much greater importance for designers and users alike.

Audio File formats

Sound Designer I (SD I)

• created by Digidesign
• designed for mono sounds, usually short sound bites
• designed for Macs
• no resource fork

Sound Designer II (SD II)

• next development from SD I
• multi-channel format (usually stereo)
• supports up to 24 bits
• supports up to 48K sampling rate- a limiting factor!
• separate resource fork; allows for storage of extra info
  • required info is sample rate, sample size (bit depth) and number of channels
  • other info, such as timestamp information, waveform diagram, region editing data, is usually included as well
• separate resource fork allows for updates of descriptive info w/o altering the actual audio data
• resource fork means that it is INCOMPATIBLE with Windows systems

• formerly one of the most commonly-used DAW file types (at least for Mac-based systems)
• because of sample rate limit and incompatability with PCs, SD-II has fallen out of popularity

Audio Interchange File Format (AIFF)

• audio interchange standard, conforms to IFF 85 for interchange format files
• one of the interchange standards in OMFI
• Apple standard (big-endian)
• designed for multiple resolutions and number of channels
• no resource fork - easier to transport
• AIFF-C allows for compressed audio data

WAVE (.wav)

• Microsoft equivalent of AIFF
• conforms to IFF 85
• little-endian
• no timestamp information stored

Broadcast WAVE (BWF)

• extension of WAVE format
• additional chunk holds description data, including Originator ID, Originator data and time, and most importantly, timestamp information
• multiple sample rates and bit depths, including newer "high-resolution" standards
• is now the defacto standard for high-performance audio workstations, since it supports higher sample rates

MPEG (MP3)

• compressed audio format
• not consistent within data like the previous formats - data parameters change for groups of data (frames)
• file format contains one main header, and a header for each frame
• processing or playback requires decoding of MPEG data

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