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Atari XL

Atari XL
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102 The
Ateri
XL
Handbook
reduce
the
clock
frequency
to 15
kHz
(1
kHz
is one
thousand
cycles
per
second):
20
POKE
53768,17
FILTERING
Bits
1
and
2
of
peR
control
what
are
known
as
high
pass
filters.
These
are
filters
which
cut
out
frequencies
lower
than
a
certain
value.
In
the
POKEY
chip,
the
filtering
frequency
(that
below
which
frequencies
are
cut
off) is
set
on
one
of
the
tone
channels
and
acts
on
another
channel.
As
shown
in
the
table
of
POKEY
control
registers
at
the
beginning
of
this
section,
if
bit
1
of
peR
is
set,
any
frequency
on
channell
which
is
lower
than
that
on
channel
3 is
filtered
out.
If
bit
2
is
set,
channel
2
acts
as
a
filter
on
channel
O.
The
principle
is
as
follows:
At
any
instant,
if
the
frequency
on
the
tone
channel
is
greater
than
that
of
the
filter,
the
tone
will
be
heard,
otherwise
it
will
not.
This
gives
rise
to
an
effect
similar
to,
but
much
more
regular
than,
distortion
as
illustrated
in
the
diagram
opposite.
Try
this
example
to
hear
it
in
action.
10
SOUND
0,0,0,0
20
POKE
53768,4
30
POKE
53760,240
40
POKE
53761,168
50
POKE
53764,120
60
POKE
53765,160
70
GOTO
70
If
the
volume
bits
of
the
filter
channel
are
set,
that
tone
channel
will
be
heard
as
well
-
change
line
60
to:

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