Analog sound Analog sound waves, like those in Figures 21-6 and 21-7,
are represented by a continuous signal that fluctuates with the frequency
and amplitude of the actual sound it represents.
Digital sound Digital sound files encode a characterization of the original sound
using a scheme that varies with the file format and compression techniques used.
The encoded data describes the frequency and amplitude fluctuations of the
original sound. Because it contains only samples of the sound, the original sound
cannot be completely reproduced.AIFF
AIFF (Audio Interchange File Format) is a digital audio file format developed by Apple
and used on the Macintosh. AIFF is the Macintosh equivalent of the Windows WAV for-
mat. It breaks sound objects into parts called chunks. The common chunk holds data such
as the sound’s sampling rate and size, and the sound data chunk contains the digitized
sound samples.
AU
The AU standard is very similar to the WAV file format. It was originally developed for
the Unix and NeXT platforms and is fairly common on the Internet. Nearly all Windows
audio players and Web browsers support the AU file format. AU is the audio file stan-
dard on UNIX systems. PC users may encounter AU (audio) files on Internet sites.
MIDI
The MIDI (Music Instrument Digital Interface) file format stores a synthesized sound that
is reproduced on the personal computer by sound cards equipped with a synthesizing
chip. A MIDI-capable sound card is able to accept commands that specify the instrument
originating the sound, the note being played, and the duration of the sound. MIDI is a
standard adopted by the electronic music industry for controlling devices, such as syn-
thesizers, keyboards, and other devices that create music. A MIDI file includes values
representing a note’s pitch, length, and volume, but it can also include additional charac-
teristics, such as attack and delay time. Because a MIDI file stores only how to reproduce
sounds, the actual sound’s data is not stored in the file, and MIDI files are much smaller
than other sound data files such as WAV or AIFF files.
Chapter 21: Audio/Visual Devices^551
Figure 21-7. An analog sound wave cut into samples