Producing
High-quality
Sound
When trying to create high-quality sound, you need to consider the following factors:
o Waveform transitions.
o Sampling rate.
o Efficiency.
o Noise reduction.
o Avoidance of aliasing distortion.
o Limitations of the
low
pass filter.
MAKING
WAVEFORM
TRANSITIONS
To
avoid unpleasant sounds when you change from one waveform to another, you need
to
make the transitions smooth. You can avoid "clicks"
by
making sure the waveforms
start
and end
at
approximately the same value. You can avoid "pops" by starting a
waveform only
at
a zero-crossing point. You can avoid
"thumps"
by arranging the aver-
age amplitude of each wave to
be
about the same value. The average amplitude
is
the
sum of the bytes
in
the waveform divided by the number of bytes
in
the waveform.
SAMPLING
RATE
If you need high
preCISIon
in
your frequency output, you may
find
that
the frequency
you wish to produce is somewhere between two available sampling rates,
but
not close
enough to either rate for your requirements.
In
those cases, you may have to adjust the
length of the audio
data
table
in
addition
to
altering the sampling rate.
For higher frequencies, you may also need to
use
audio
data
tables
that
contain more
than one full cycle of the audio waveform to reproduce the desired frequency more accu-
rately, as illustrated
in
figure 5-4.
152
Audio Hardware
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