Page 39 - DCAP303_MULTIMEDIA_SYSTEMS

Unit 3: Sound

3.4 audio file formats notes

A file format determines the application that is to be used for opening a file. There are a number
of different types of Audio files. The most common are Wave files (WAV) and MPEG Layer-3
files (MP3). There are, however, many other audio file types discussed below. The type is usually
determined by the file extension (what comes after the “.” in the file name). For example, “.wav”,
“.mp3” or “.dct”.
The way the audio is compressed and stored is called the codec which determines how small the
file size is. Some file types always use a particular codec. For example, “.mp3” files always use
the “MPEG Layer-3” codec. Other files like “.wav” and “.dct” files support selectable codecs.
Some file types just contain the audio. But other file types can contain additional header information
which can contain other information about the file (for example, .dct files have information about
the sender, priority, notes and other data in the file itself).

If you have a file which you cannot open in your program, try to convert it with the Switch Audio
File Format Converter. If it fails, you could also try by using the SoundTap Direct Streaming Audio
Recorder that can capture any audio you play.
A “.wav” file can be encoded with the “PCM”, “GSM6.10”, “MPEG3” and
many other codec’s. Be careful not to confuse the file type with the codec, a
“MPEG Layer-3” encoded “.wav” file.

3.4.1 List of audio file formats
open file formats (supported by and most likely to work with our software)
• .wav — standard audio file format used mainly in Windows PCs. Commonly used for
storing uncompressed (PCM), CD-quality sound files, which means that they can be large
in size — around 10 MB per minute of music. A list of common wave file codecs can be
found here. Sample .wav file.

• .mp3 — the MPEG Layer-3 format is the most popular format for downloading and storing
music. By eliminating portions of the audio file that are essentially inaudible, mp3 files are
compressed to roughly one-tenth the size of an equivalent PCM file while maintaining
good audio quality. We recommend the mp3 format for music storage. It is not that good
for voice storage.
• .ogg — a free, open source container format supporting a variety of codecs, the most popular
of which is the audio codec Vorbis. Vorbis files are often compared to MP3 files in terms of
quality. But the simple fact mp3 are so much more broadly supported makes it difficult to
recommend ogg files.
• .gsm — designed for telephony use in Europe, gsm is a very practical format for telephone
quality voice. It makes a good compromise between file size and quality. We recommend
this format for voice. Note that wav files can also be encoded with the gsm codec.
• .dct — a variable codec format designed for dictation. It has dictation header information
and can be encrypted (often required by medical confidentiality laws).
• .flac — a lossless compression codec. You can think of lossless compression as like zip but
for audio. If you compress a PCM file to flac and then restore it again it will be a perfect
copy of the original.
• .au — the standard audio file format used by Sun, Unix and Java. The audio in au files can
be PCM or compressed with the ulaw, alaw or G729 codecs.

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