wave-table synthesizer
Sound Synthesizers Sound-producing code
using the available hardware to produce sounds. The Sound Manager
insulates an application from the underlying hardware primarily by selecting
the sound hardware drivers (or synthesizers) that are best suited to the
available audio hardware.
Synthesizers that drive the built-in audio hardware on Macintosh computers
are provided by Apple and are stored in the System file as resources of type
'snth'. Depending on the type of sound you wish to produce, the
Sound Manager uses either the square-wave synthesizer, the wave- table
synthesizer, or the sampled sound synthesizer to interpret commands and data
sent to it by your application. Sometimes you do not need to specify which
synthesizer you wish to use because some sound commands can be interpreted
by all three.
The Square-Wave Synthesizer
sound based on a square wave, and it is functionally equivalent to the
square-wave synthesizer contained in the old Sound Driver. Your
application can use the square-wave synthesizer to play a simple sequence of
sounds in which each sound is described completely by three factors: its
frequency or pitch, its amplitude (or volume), and its duration.
The square-wave synthesizer can play only one frequency at a time through a
sound channel, and it cannot play back a waveform sound description or a
recorded sound. The square-wave synthesizer requires very little CPU time,
however, and it is very easy for your application to use.
The Wave-Table Synthesizer
To produce more complex sounds than are possible using the square-wave
synthesizer, your applications can use the wave-table synthesizer. As the
name indicates, the wave-table synthesizer can produce sounds that are 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
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. You can open multiple
wave-table channels (up to a maximum of four).
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 the Figure
below).
A graph of a wave table
A wave table is represented as a packed array of bytes. Each byte contains a
value in the range 0x00-0xFF. These values are interpreted as offset values,
where 0x80 represents an amplitude of 0. The largest negative amplitude is
0x00 and the largest positive amplitude is 0xFF. When playing a wave- table
description of a sound, the wave-table synthesizer loops through the wave
table for the duration of the sound.
The Sampled Sound Synthesizer
You can use the sampled sound synthesizer to play back sounds that have
been digitally recorded (that is, sampled) as well as sounds that are computed,
possibly at run time. The sampled sound synthesizer is the most widely used of
all the available synthesizers, primarily because it is relatively easy to
generate a sampled sound and because such samples can contain a wide variety
of sounds. The sampled sound synthesizer is typically used to play back
pre-recorded sounds such as speech or special sound effects.
resources of type 'snd ' or as AIFF or AIFF-C format files. The structure of
short prerecorded sampled sounds, you should probably include the sound data
in an 'snd ' resource. If you want to allow the user to transfer recorded sound
data from one application to another (or from one operating system to
an other), you should probably store the sound data in an AIFF or AIFF-C file.
In certain cases, you need to store sampled sounds in files and not in resources.
For example, some sampled sounds might be too large to store as resources; in
those cases, you should store the sounds in an AIFF or AIFF-C file.
Regardless of how you store a sampled sound, you can use Sound Manager
routines to play that sound using the sampled sound synthesizer. If you choose
to store sampled sounds in files of type AIFF or AIFF-C, you can play those
sounds in resources, your application can play those sounds by passing the
play 'snd ' resources directly from disk.) There are three types of sampled
sound headers: the standard sound header (used for monophonic sampled
sounds), the extended sound header (used for stereo samples), and the
compressed sound header (used for compressed samples, whether monophonic
or stereo). The sampled sound header contains information about the sample
(such as the original sampling rate, the length of the sample, and so forth),
together with an indication of where the sample data is to be found. The data can
be stored in a buffer separate from the sound resource or as part of the sound
resource at the end of the sound header.
Note: The terminology sampled sound header can be conf using
because in most cases the sound header (and hence the 'snd '
resource) contains the sound data as well as information describing
the data.
You can play a sampled sound at its original rate or play it at some other rate
to change its pitch. Thus, once you install a sampled sound header into a
channel, it can be played at varying rates to provide a number of pitches and
hence can be used as a voice to play a series of sounds.