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Today's
more advanced sound cards include Wavetable's
with 32, 64 or 128 "voices" or instruments
(that are combined during creation and playback).
Some sound cards work with software that provides
additional voices. Wavetable's
are used as part of
music or sound synthesizers that use the musical
instrument digital interface (MIDI).
MIDI lets you capture sound and play it back based
on the commands in files that are essentially little
"scripts" to the "orchestra"
(one might think of it as a written description of
what the conductor is doing and which instruments
are being pointed to with the baton). A Wavetable
sound can be enhanced or modified using
reverberation or other effects before it is saved in
the table. Some Wavetable chips include a special
section for drum sounds to support rhythmic effects.
Many
sound cards take advantage of Direct Memory Access
(DMA). Many also include an FM synthesizer in order
to play back sounds from older applications or
files. A full-duplex sound card lets you record and
playback at the same time (or, if you're using
Internet telephony, talk and hear at the same time).
Full-duplex data transmission means that data can be
transmitted in both directions on a signal carrier
at the same time.
Wavetable
sound cards use digital signal processor (DSP)
chips. A Wavetable is a table of stored sound waves
that are digitized samples of actual recorded sound.
A Wavetable is stored in read-only memory (ROM) on a
sound card chip but it can also be supplemented with
software. Originally, computer sounds (digital
versions of analog waveforms) were generated through
frequency modulation (FM). Pre-storing sound
waveforms in a lookup table improved quality and
throughput.
MIDI
(Musical Instrument Digital Interface) is a
protocol designed for recording and playing back
music on digital synthesizers that is supported by
many makes of personal computer sound cards.
Originally intended to control one keyboard from
another, it was quickly adopted for the personal
computer. Rather than representing musical sound
directly, it transmits information about how music
is produced. The command set includes note-ons,
note-offs, key velocity, pitch bend and other
methods of controlling a synthesizer. The sound
waves produced are those already stored in a Wavetable
in the receiving instrument or sound card.
Since
a MIDI file only represents player information, it
is far more concise than formats that the sound
directly. An advantage is very small file size. A
disadvantage is the lack of specific sound control.
With
a program that provides this interface, you can
create music using a standard keyboard or other
input device. You or others can then play your
MIDI-conforming creation with the same or another
program and a sound card as a music synthesizer. The
MIDI program may come with a graphical user
interface that looks like a sound studio control
room. Many sound cards come as a package with MIDI
software (for example, Media Vision's Pro Audio
Studio 16).
The
MIDI protocol uses eight-bit serial transmission
with one start bit and one stop bit, has a 31.25 Kbs
data rate, and is asynchronous. Connection is made
through a five-pin DIN plug, of which three pins are
used. |
