Introduction

There are a variety of digital audio file formats which may be used for ...

Computer systems (system sounds / alerts)
Audio CDs
Audio DVDs
Video DVDs
Multimedia projects
Web sites
MP3 players
DV camcorders
etc

Different file formats allow for different file sizes and qualities. All audio file have the following attributes ...

File format
Data rate
Channels (mono, stereo, or multichannel)
Sample rate quality
Bit depth quality
Compressed or uncompressed format

Compressed and uncompressed formats

It is possible to divide digital audio file formats into 2 categories ...

Uncompressed. Example include ... CD audio, DV audio, .aif and .wav files.
Compressed. Examples include ... MP3, Real Audio, Dolby AC3 (DVD video)

Audio quality (sample rate & bit depth)

Both uncompressed and compressed audio files can be of wildly varying qualities. Audio quality is usually defined by ...

Sample rate
Bit depth
Whether or not the file is compressed by an audio codec (eg MP3) and what the particular codec settings are.

A word about computer audio file extensions

When computer files are stored on a hard drive or CD/DVD-ROM, their file names usually end with extensions which identify the system on which they originated or the program that recorded (or created) them. For example ...

.wav ... PC interchange file format

.aiff ... audio interchange file format for all computers (used by Apple)

.mp3 ... popular compressed audio file format

.mov ... QuickTime can playback a wide range of compressed and uncompressed audio formats. Click here for a full list.

.sdII ... DigiDesign Sound Designer II, 2-track audio editor software

These extensions are independent of data rate, channels, sample rate and bit depth, They tell you nothing about audio quality. They are simply "envelopes" that contain data. Most .sd2, .wav and .aiff files contain mono or stereo 44.1kHz 16-bit audio data but they might be quite different.

1. Uncompressed audio files

Uncompressed audio files can vary in size and quality depending on 3 primary settings ...

Sample rate
Bit depth
Number of channels ... stereo, mono, surround sound or multi-channel

For example ... CD quality audio Sample Rate is 44.1KHz, Bit-depth is 16bit and it is 2 channels stereo.

By altering these settings, either before or after recording, you can achieve differing ...

Data rates
Files sizes
Audio quality

Uncompressed audio file formats (sometimes called PCM formats)

Here are the current most common uncompressed audio file formats ...

File format	Compression codec	Sample rate	Bit-depth	Channels	Data rate
Audio Interchange File Format (.aif) Used by computers, originated by Apple.	none	variable but most common are 11, 22 & 44.1KHz	variable but most common is 16bit	up to 8	variable
WAV (.wav) Used by computers, originated by Microsoft.	none	variable but most common are 11, 22 & 44.1KHz	variable but most common is 16bit	up to 8	variable
SDII (Sound Designer 2) Used by computers, originated by DigiDesign.	none	variable but most common are 11, 22 & 44.1KHz	variable but most common is 16bit	stereo or mono	variable
CD (Compact Disc) red book audio	none	44.1KHz	16bit	stereo	1.4 Mbits per sec. 10Mb per stereo minute (mono is therefore 5Mb per/min)
DVD audio	DVD video discs can use compressed or uncompressed audio	Uncompressed format allows for 44.1kHz, 48kHz, 88.2kHz, 96kHz, and 192kHz	24bit	stereo or multi channel (6)	1.5 - 9.6 Mbits per sec.
DV (Digital Video) audio	none	32KHz (4-channel) or 48KHz (stereo)	12 or 16bit	4 or stereo	1.4 Mbits per sec.
QuickTime	Files in the QuickTime .mov file format can be in both compressed and uncompressed formats. QuickTime can playback a wide range of compressed and uncompressed audio formats. Click here for a full list.

2. Compressed audio files

2 categories of audio compression

Lossless. This is a process whereby clever algorithms (such as Flac) are employed to "compact" the data in a file without losing any information. Losseless compression can reduce the file size but no-where near as much as lossy.
Lossy. These codecs find ways to throw away information in the file which it thinks the majority of listeners ears won't miss. File size reduction can be dramatic (10 times smaller) but the effect on audio quality is usually noticeable.

This section will focus on lossy compressions because it is by far the most used form of audio file reduction technology (iPod, cellphones, DVD, Freeview, Cable TV etc).

Why compress audio files?

Audio compression is employed for 3 primary reasons ...

to reduce file size so that more audio may be stored on a given media format (MP3 players, DVD-video discs, MiniDiscs etc)
to reduce file size so that files will download from a web site faster
to reduce data rate so that files will stream (broadcast) over a network such as the internet

Compressed audio file settings

Compressed audio files can vary in size and quality depending on 5 primary settings ...

Sample rate
Bit depth
Number of channels stereo, mono, surround sound or multi-channel
Choice of audio compression codec
Audio compression codec settings

By altering these settings, either before or after recording, you can achieve differing ...

Data rates
Files sizes
Audio quality

What is an audio codec?

Codec is short for Compression Decompression Algorithm. It is a piece of software that compress a file before storing it and uncompress it before playback.

Audio codecs are used to ...

Squeeze more information onto CD and DVD video discs and portable MP3 players
Reduce file sizes to enable files to download more quickly over networks
Reduce demands on computer processors

Compressed audio file formats

Here are the current most popular audio compression file formats ...

File format	Compression codec	Sample rate	Bit depth	Channels	Data rate
MiniDisc MiniDisc uses a sophisticated lossy compression codec to squeeze more audio onto an optical disc. MiniDisc audio format is incompatible with almost all computer systems / software and thus cannot be easily digitally transferred onto a hard disc.
MPeg Layer 3 (MP3)	MP3	variable but typically 44.1KHz	variable but typically 16bit	mono or stereo	variable but typically between 32 and 320 Kbits per sec. Average file size at 128Kbits per sec is 1Mb per stereo minute. Click here for advice on how to create MP3 files.
AAC Used by Apple for iTunes. AAC has the advantage of embedded anti-piracy encryption.	AAC	variable but typically 44.1KHz	variable but typically 16bit	mono or stereo	variable but typically between 32 and 320 Kbits per sec.
AC3 Dolby Digital Used for DVD video	AC3	variable but typically 44.1KHz or 96KHz	variable but typically 24bit	mono or stereo or multi channel (5.1)	variable but typically between 192 and 224 Kbits per sec for stereo. Click here to read more.
QuickTime QDesign Music 2 This is good for soundtrack music in internet and CDROM friendly video files	QDesign Music 2	n/a	n/a	n/a	n/a
QuickTime Qualcomm Purevoice This is good for soundtrack speech in internet and CDROM friendly video files	Qualcomm Purevoice	n/a	n/a	n/a	n/a
Other QuickTime codecs QuickTime has a wide number of audio file formats which it is able to replay. Click here for a full list.
Windows Media Microsoft thinks we should use their proprietary audio file formats, but most experts disagree.
Real Audio Used by Amazon, but aging, poor quality and cost of implementation has driven most users away from it. You need special server software for this one. Do a test for yourself, compare the quality of previews on Amazon (Real), iTunes (AAC) and this site (MP3).