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musical instrument digital interface (MIDI) files. It’s important to note that a MIDI file is not an independent music file, unlike a WAV file that sounds more or less the same on many different PCs. A MIDI file is a text file that takes advantage of the sound processing hardware to enable the PC to produce sound. Programmers use these small files to tell the sound card what notes to play, how long, how loud, on which instruments, and so forth. Think of a MIDI file as a piece of electronic sheet music, with the instruments built into your sound card.

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NOTE MIDI files have the file extension .MID in the PC world.

The beauty of MIDI files is that they’re tiny in comparison to equivalent WAV files. The first movement of Beethoven’s Fifth Symphony, for example, weighs in at a whopping 78 MB as a high-quality WAV file. The same seven-minute song as a MIDI file, in contrast, slips in at a svelte 60 KB.

MIDI is hardware dependent, meaning the capabilities and quality of the individual sound card make all the difference in the world on the sound produced. Sound cards play MIDI files by using one of two technologies: FM synthesis or wave table synthesis.

FM Synthesis Early processors used electronic emulation of various instruments—a technique often called FM synthesis—to produce music and other sound effects. Software developers could tell the sound processor to reproduce a piano playing certain notes, for example, and a sound resembling a piano would pour forth from the speakers. The problem with FM synthesis is that although the modulation sounds okay for a single note, such as middle C, it sounds increasingly electronic the farther up or down the scale you go from that prime note.

Wave Table Synthesis To address the odd techno-sound of early sound processors, manufacturers began embedding recordings of actual instruments or other sounds in the sound card. Modern sound cards use these recorded sounds to reproduce an instrument much more faithfully than with FM synthesis. When asked to play a C note on a piano or on a viola, for example, the sound processor grabs a prerecorded WAV file from its memory and adjusts it to match the specific sound and timing requested. This technique is called wave table synthesis. The number of instruments a sound card can play at once is called the polyphony of that card—typically 64 sounds on better cards. Most modern sound cards have samples of 128 instruments—a veritable symphony orchestra on a chip!

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NOTE MIDI files are much less popular than other recorded formats on computers, but every Windows computer and every sound card still fully supports MIDI.

Other File Formats

The WAV, MP3, and MIDI formats may account for the majority of sound files, but plenty of other less common formats are out there. Here are the extensions of some other sound file formats you may run into in the PC world:

AAC Advanced Audio Coding is the native format for songs downloaded into the Apple iTunes music library. The AAC format is part of the MPEG-4 standard, offers better compression algorithms than MP3, and is freely distributed. Apple wraps downloaded songs in a Digital Rights Management (DRM) encapsulation called FairPlay that gives them control over distribution of those songs.

AIFF Audio Interchange File Format files are a popular sound format used on Macintosh computers. These files are often seen at Web sites, and you can use the well-known QuickTime Player to play them.

ASM Assembly Language Source files are compressed sound files often seen on the Internet and used in streaming sound (streaming media is discussed later in this chapter.)

ASX Microsoft created the ASX format to facilitate streaming audio over the Internet through Windows Media Player. It’s more than just a format, though; it acts like a super playlist and enables you to play other sound file types as well. The full name of the format is Microsoft Advanced Streaming Redirector.

AU This popular format is often seen in the Windows world. Many players can play these files, including players on non-Windows