Digital audio file formats by Ian Dustry

digital audio interfaces

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 multi-channel)
  • Sample rate quality
  • Bit depth quality
  • Compressed or uncompressed format

NOTE: You may wish to read our article on codecs, formats & optimising concepts first.

Compressed and uncompressed formats

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

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

Audio quality (sample rate & word length)

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

  • Sample rate
  • Word length
  • 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. For example, most .sd2, .wav, and .aiff files will contain mono or stereo 44.1kHz 16-bit audio data, but they could contain higher or lower quality data.

 

1. Uncompressed audio files

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

For example ... CD quality audio sample rate is 44.1KHz, bit-depth is 16-bit and it is 2 channels stereo.

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

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 Comments
Audio Interchange File Format (.aif) none variable but most common are 11, 22 & 44.1KHz variable but most common is 16bit up to 8 variable Used by computers, originated by Apple.
WAV (.wav) none variable but most common are 11, 22 & 44.1KHz variable but most common is 16bit up to 8 variable Used by computers, originated by Microsoft.
SDII (Sound Designer 2) none variable but most common are 11, 22 & 44.1KHz variable but most common is 16bit stereo or mono variable Used by computers, originated by DigiDesign.
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

What is a codec?

If you don't know the answer to this question you may wish to read our article on codecs, formats & optimising concepts now.

The 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. Lossless 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, cell phones, 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
  • Word length
  • 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 ...

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 Comments
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.
Flac (Free lossless audio codec) none variable variable - variable Can compress data to 40% of original size
Apple Lossless Codec None.         Similar to Flac. Files are stored within an MP4 container. Supported in iTunes
AAC
AAC variable but typically 44.1KHz variable but typically 16bit mono or stereo variable but typically between 32 and 320 Kbits per sec. Used by Apple for iTunes. AAC has the advantage of embedded anti-piracy encryption.
Ogg Vorbis   variable but typically 44.1KHz variable but typically 16bit mono or stereo variable but typically between 32 and 320 Kbits per sec. Similar to WMAPublic domain format (no license required)
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
QDesign Music 2 n/a n/a n/a n/a This is good for soundtrack music in internet and CDROM friendly video files
QuickTime Qualcomm Purevoice
Qualcomm Purevoice n/a n/a n/a n/a This is good for soundtrack speech in internet and CDROM friendly video files
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 Audio
n/a n/a n/a n/a n/a Microsoft thinks we should use their proprietary audio file formats, but most experts disagree.
Real Audio
Proprietary n/a n/a mono or stereo variable Used by Amazon, but aging, poorer quality than rivals and cost of implementation has driven most users away from it. You need special server software for delivery.
MiniDisc
Proprietary n/a n/a stereo n/a 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.

 

Optimising/compressing audio files

Click here for advice on how to compress audio files