PLAYING
MULTIPLE
TONES
AT
THE
SAME
TIME
You can play multiple tones either by using several channels independently
or
by sum-
ming the samples in several
data
sets, playing the summed
data
sets through a single
channel.
Since all four audio channels are independently programmable, each channel has its own
data
set;
thus
a different tone
or
musical note can be played on each channel.
MODULATING
SOUND
To
provide more complex audio effects, you can use one audio channel
to
modulate
another. This increases the range and type
of
effects
that
can be produced. You can
modulate a channel's frequency
or
amplitude,
or
do both types
of
modulation on a chan-
nel
at
the
same time.
Amplitude modulation affects
the
volume of the waveform.
It
is.
often used
to
produce
vibrato
or
tremolo effects. Frequency modulation affects the period of the waveform.
Although the basic waveform itself remains the same,
the
pitch is increased
or
decreased
by frequency modulation.
The
system uses one channel
to
modulate another when you
attach
two channels.
The
attach
bits in the ADKCON register control how the
data
from an audio channel is
interpreted (see the table below). Normally, each channel produces sound when
it
is
enabled. If
the
"attach"
bit for an audio channel
is
set,
that
channel ceases
to
produce
sound and its
data
is
used
to
modulate the sound
of
the next higher-numbered channel.
When a channel is used as a modulator, the words in its
data
set are no longer treated
as two individual bytes. Instead, they are used as
"modulator"
words.
The
data
words
from the
modulator channel are written into the corresponding registers
of
the
modulated
channel each time the period register of the modulator channel times out.
To
modulate only the amplitude of the audio
output,
you must
attach
a channel as a
volume modulator. Define the modulator channel's
data
set as a series
of
words, each
containing volume information in the following format:
Audio Hardware 149
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