Audio Formats

Audio-based formats come in a variety of file extensions and can have a very wide range of file size depending on the clarity of the recording. This page is broken into a short explanation of how digital audio works, common format types to expect, and standards in SCERA.

Digital and analog audio

At its most basic level sound is a wave or vibration that you can hear. Sound creates a compression of the air (or other medium, like when hearing something while under water) around you that moves in a wave form that can vary in height and intensity depending on the pitch and volume of the thing creating the sound. If the wave form is written on a piece of paper, the wave will go equally above and below a middle line. At this middle line there is no sound present.

Analog audio recordings actually capture the audio wave form and stores that information on a physical media in some way. On a vinyl record, the wave form is etched in the vinyl disc as grooves that a needle passes through to recreate the sound. With a audio tape, the wave form is captured as changes in magnetic tape that is played back by passing the tape over a head that can read those changes.

Digital audio cannot capture the actual audio wave form like an analog recording, but it can simulate it. The simulation comes by creating small samples of the audio as it goes through the wave at different levels of detail (depth) of the wave above and below the middle line when no audio exists. The frequency of the samples is measured in Hertz and is done many thousands of times per second. The depth of the sample is done in multiples of 2, and is called bits. The two numbers are put together using a / to separate the numbers. So, a digital audio recording taken 96Khz/24 was sampled 96,000 times in 1 second at 24 bits. The typical CD-quality audio is taken at 44.1Khz and 16 bits, and the typical radio broadcast is set to 36Khz and 16 bits. MP3 files can vary in frequency, but are most often at 8 bits of depth.

Digital Audio formats

.aiff

.aiff stands for Audio Interchange File Format. This is a standardized format for digital audio to be shared between computer programs and was created in the 1980s. Quite often, audio CDs store their recordings as .aiff files. All, or almost all, audio players should be able to play this kind of file format.

.mp3

.mp3 is an abbreviation for Motion Picture Experts Group standard 1/standard2, part 3. The abbreviation MP3 is not the same as MPEG-3, which is a different MPEG standard. The MP3 standard was developed by the Motion Picture Experts Group. An MP3 uses a technique that compresses an audio recording and alters its sound to what it is expected that the average person is normally able to hear. Some quality and details of a recording, a tambourine for example, might be lost in an MP3 but the bulk of it is kept. As a defacto portable audio standard (as of 2015), .mp3 files can be played on all portable audio players, and is expected to work on all computer audio player programs.

.wav/Broadcast Wave

.wav stands for waveform audio (as in an audio wave) and is a file format originally developed by Microsoft. It is most common for stand-alone audio recordings above the MP3 level of quality, but can also be the format used on an audio CD. Perhaps due to its high frequency of use and developments by Microsoft, .wav files come in a wide variety of sub-types. There are too many variations of .wav to list them all on this page, but the most common is PCMWaveFormat.

Broadcast Wave is a subtype of the .wav format and has the .wav file extension. The Broadcast Wave format is the archival standard for digitizing and preserving digital audio. It differs from the standard .wav format because it saves additional descriptive information about the contents of the recording that is not kept by an average .wav file. In a Broadcast Wave file, for example, an archivist can write notes about the content and subject of a recording; whereas in the standard .wav file they cannot.

Digitized Audio in SCERA

Analog audio recordings that have been digitized by the South Carolina Department of Archives and History are captured in two formats: .wav and .mp3.

For general public access, a .mp3 file is created which includes only the portion of the original media that actually has recorded audio. The .mp3 file is freely available, but may not have the same level of detail as the preservation versions of digitized recordings.

For long-term preservation of the audio information, a .wav file that covers the entire duration of the media (regardless of length of actual recorded sound) is created at a 96kHz/24 bit level. A second .wav file is created for general use at the 44.1kHz/16 bit level, and is modified to the length of the actual duration of the recording with changes to help account for background noise or problems with the original recording.

Copies of the preservation-quality audio can be provided by the Archives on request for a fee. Please note that if a copy of the 96kHz/24 bit recording has been requested, the file size for a single recording can exceed 1 gigabyte so it may be impractical for most users.

For audio that was in digital format prior to arrival in SCERA, the Archives will attempt to create preservation and public access copies based on the standards provided above.