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Sel
B
Sel
A
Joystick
Input
Sound Source
1
1
I
I
Joy
Joy
1
Joy
2
Joy
3
DAC
Cassette
Cartridge
(no
sound)
The Digital
to
Analog Converter
employs
a
64-collector transistor
as
a
current source which gives good
linearity over
the entire
voltage
range.
In
order
to
determine
the
position
of the joystick, the
microprocessor
uses
a
technique
called "Successive Approximation".
The
microprocessor
first
selects
the
desired joystick
input by
means
of
the
select pins (which are connected
to
PIA
IC5).
The sound
multiplexing section
is
very simple. According
to
the
above
table,
different sound sources
are
selected
by the Sel
A
and Sel
B
inputs, and the selected input
is
routed
to
the sound output.
If
the
DAC
is
used
as
a
sound source, the
microprocessor
simply feeds
a
succession
of
values
to
the six bits
of
the
DAC
in
order
to
produce
the
desired waveshape. The output
of the
DAC
is
then
buffered and attenuated
to
provide approximately
3.9 volts
p-pj
which
is
the level
required
by
the
modulator
to
produce
maximum
volume.
If
the
cassette
is
the
selected
input, then sound
from
the
cassette recorder
is
routed
to the
sound output. This level follows
the
input level
up to 3.9
volts
p-p, at
which
point
it
clips
the input
waveform. Therefore,
the
volume
control
on the
cassette should
not be
set
higher than
the
level
which
provides 3.9 volts p-p
to
the
DAC
chip. Similarly, the
cartridge may
supply
the
sound source (from AC
coupled) since
the
SND IN
(2)
input
to the
DAC chip biases
the input at
the
midpoint
of the
allowable voltage
swing,
which
is
3.9
volts
p-p.
Any
greater signal amplitude will result
in
clipping (distortion) of
the
sound
waveform.
In
addition
to
the
Select inputs, the
sound must be enabled
by
bringing
SNDEN
to a
high
level.
This input
is
controlled
by
PIA
IC4.
If this
pin
is
at
a
low level, all sound (except
single-bit sound)
is
disabled.
The final function of
the
DAC chip
is
to
provide
the
output signal
for
recording
of
cassette
data. This
is,
quite simply,
a
buffered output
of
the DAC
which
is
attenuated
to
produce approximately
1
volt p-p into
a
2-kohm
load.
Therefore,
it is
up to
the
microprocessor
to
produce
the
necessary FSK signals through
the
DAC
and the
proper software.
-43-
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