Digital Audio Production 601The header provides Windows with all the information it needs. First off, it defi nes the
type of RIFF fi le, in this case, WAVEfmt. Notice the bytes, which are shown underlined.
The fi rst two, 22 and 56, relate to the audio sampling frequency. Their order needs
reversing to read; 5622 hexadecimal, which is equivalent to 22 050 in decimal—in other
words, 22-kHz sampling. The next two inform the Media Player that the sound fi le is
1 byte per sample (mono) and 8 bits per sample.
20.2.2 AU Files
AU (or μ -law—pronounced mu-law) fi les utilize an international standard for
compressing audio data. It has a compression ratio of 2:1. The compression technique
is optimized for speech (in the United States it is a standard compression technique for
telephone systems; in Europe, a-law is used). This fi le format is found most frequently
on the Internet where it is used for ‘ .au ’ fi le formats, alternately know as ‘ Sun audio ’ or
‘ NeXT ’ format. Even though it’s not the highest quality audio fi le format available, its
nonlinear, logarithmic coding scheme results in a relatively small fi le size, which is ideal
for applications where download time is a problem.
20.2.3 AIFF and AIFC
The Audio Interchange File Format (AIFF) allows for the storage of monaural and
multichannel sample sounds at a variety of sample rates. AIFF format is frequently found
in high-end audio recording applications. Originally developed by Apple, this format is
used predominantly by SGI and Macintosh applications. Like WAV, AIFF fi les can be
quite large; 1 min of 16-bit stereo audio sampled at 44.1 kHz usually takes up about 10
megabytes. To allow for compressed audio data, Apple introduced the new AIFF-C, or
AIFC, format, which allows for the storage of compressed and uncompressed audio data.
AIFC supports compression ratios as high as 6:1. Most of the applications that support
AIFF playback also support AIFC.
20.2.4 MPEG
The International Standard Organisation’s Moving Picture Expert Group (MPEG) is
responsible for one of the most popular compression standards in use on the Internet
today. Designed for both audio and video fi le compression, MPEG-I audio compression
specifi es three layers, and each layer specifi es its own format. The more complex layers
take longer to encode but produce higher compression ratios while keeping much of an