Inside Macintosh: Sound

Wave-Table Data

To produce more complex sounds than are possible using square-wave data, your applications can use wave-table data. As the name indicates, wave-table data is based on a description of a single wave cycle. This cycle is called a wave table and is represented as an array of bytes that describe the timbre (or tone) of a sound at any point in the cycle.

Your application can use any number of bytes to represent the wave, but 512 is the recommended number because the Sound Manager resizes a wave table to 512 bytes if the table is not exactly that long. Your application can compute the wave table at run time or load it from a resource.

A wave table is a sequence of wave amplitudes measured at fixed intervals. For instance, a sine wave can be converted into a wave table by taking the value of the wave's amplitude at every 1/512 interval of the wave (see Figure 1-18 ).

A wave table is represented as a packed array of bytes. Each byte contains a value in the range $00-$FF. These values are interpreted as offset values, where $80 represents an amplitude of 0. The largest negative amplitude is $00 and the largest positive amplitude is $FF. When playing a wave-table description of a sound, the Sound Manager loops through the wave table for the duration of the sound.

Figure 18 A graph of a wave table