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Publisher17.2 MIDI Synthesis Methods
The process by which sounds cards
produce audio output from MIDI input is called
synthesis. There are three synthesis methods:
FM synthesis combines multiple sine waves of
differing frequency and amplitude to produce a composite wave that
resembles the native waveform of the instrument being synthesized.
How close that resemblance is depends on the instrument and the
quality of the FM synthesizer circuitry, and may vary from reasonably
close to only a distant approximation. Even the best FM synthesis
sound cards produce artificial-sounding audio, particularly for
"difficult" instruments. Until the
mid-1990s, most consumer-grade sound cards used FM synthesis, but FM
synthesis is now obsolete.
Wavetable synthesis uses stored waveform audio
samples of actual instrument sounds to reproduce music. The sample
may be used as is, or modified algorithmically to provide a sound for
which no sample is stored. For example, the wavetable may contain a
stored sample of an actual violin playing an A note at 1760 Hz. If
the MIDI score calls for a violin playing that A note, the sample is
used directly. If the MIDI score calls for a violin playing an A note
one octave higher (3520 Hz) and that note is not available as a
stored sample, the synthesizer creates the 3520 Hz A note based on
the data it has stored for the 1760 Hz A note. The quality of
wavetable synthesis depends on the number, quality, recording
frequency, and compression used for stored samples, and on the
quality of the synthesizer hardware. Early wavetable sound cards were
limited to the samples stored on their onboard ROM, which was
typically 512 KB to 4 MB. Many current wavetable sound cards have, in
addition to samples residing on onboard ROM, the ability to use
additional samples stored on disk and loaded into main system memory
as needed.
Although good wavetable sound cards produce music quality an order of
magnitude better than FM synthesis sound cards, they are still
limited by the finite number of stored samples, and so still must
simulate much of their output instead of using samples directly. For
example, whereas most wavetable sound cards store a total of perhaps
4 MB of samples for all instruments, fully sampling just a piano
would require 10 MB or more of samples. That means that some piano
sounds must be interpolated from existing samples, which in turn
means that the piano emulation is not as good as it might be because
interpolated sounds are less realistic than sampled sounds. The
latest synthesis method, waveguide synthesis,
also called physical modeling synthesis, creates
a virtual model of an instrument and produces sounds based on
calculations made against that model. Full waveguide synthesis for
all instruments is beyond the hardware capabilities of any current
PC. Some current high-end sound cards, however, use waveguide
synthesis in conjunction with wavetable synthesis to improve the
realism of emulation for some instruments.
