Radio Shack CCR-81 Technical Reference Manual

SOUND
OUTPUT
Another
important
usage of
the
D/A
converter
is
the sound
output.
This
D/A
converter
is the
primary
source
of
sound
effects
for
the
computer,
however
three
other
sound
sources
are
provided.
These
extra
sound
sources
are
a single
bit
sound
source,
sound from
the
cassette
tape
recorder,
and
sound
from
the
cartridge.
The
D/A
output
is
connected
directly
to the
MC14529B
analog
multiplexer
(U9).
This
chip
is
used
to
select one
of
three
sound
sources,
and
maybe
disabled
to allow
use
of
the
fourth
single
bit
sound
source.
Table
4
shows the
selection
of
the various
sound
sources.
Figure
1 1
shows
all of
the
circuitry
for
sound
generation.
The
cassette
sound
output
must
be
modified
before
being
connected
to
the analog
multiplexer.
For
this
purpose,
a 1 0^tf
non-polarized
capacitor
(C2)
is used
to level
shift
the
signal.
Also,
two
4.7K
resistors
(R31 and
R32)
assign
a DC
level of
2.5
volts
and
limit
the
signal
to
to
5 volts.
This
signal
is
then
connected
to
the
multiplexer
(U9).
The
output of
the
analog
multiplexer
is
connected
to
pin 3 of
the
modulator
(U5),
and
to the
single
bit
sound
source
(pin
11,
U4), which
is isolated
by
a
10K
resistor.
At any time,
only
one of
the two
sources
should
be
used, to
avoid
mixing
the
two
sources.
TABLE
4.
SOUND
SELECTION
SNDEIM
U9-15
SEL1
U9-6
SEL2
U9-7
SOUND SOURCE
SELECTED
1
6BITDIA
1
1
1
1
CASSETTE
CARTRIDGE
1
1
1
NOT
USED
0*
X
X
SINGLE
BIT
SOUND
*NOTE:
For
single
bit
sound, PI
A
U4
pin 1 1 must
be
programmed
as
an output.
It is
normally
programmed
as an input.
X
=
DON'T
CARE
0=
LOGIC
LOW
1
=
LOGIC
HIGH
D/A
CONVERTER
CASSETTE
I
INPUT
I
FIGURE
11. SOUND
CIRCUITRY
31

Radio Shack CCR-81 Specifications

General

Brand	Radio Shack
Model	CCR-81
Category	Desktop
Language	English