Radio Shack VoxBox TRS-80 User Manual

Zero Crossing
Detector
1
he zero
crossing
detector is
a comparator
which
compares the
filter
output
with
a fixed
voltage and
generates a
+5
volt
output when
the filter
output
is less
than
the fixed
voltage,
and a zero
volt output
when the
filter
output is
greater than
the
fixed
voltage.
The
fixed voltage is
the
6
V
reference
voltage, so
that with
no
signal input
the
detector
output
is
a random square wave
pattern
generated
by random
low
level
noise in
the
circuitry.
When
a
slight signal
comes from
the filter
output,
the
detector output will
switch
from
zero
to
+5
volts
and back
to zero as the
filter
output swings
above
and below
the
fixed
voltage.
Rectifier/
Averager
The
output of each
filter also
drives
an
associated rectifier
averager
stage. The
input
to the
circuit is
through
a diode. Since
both
the inverting
and
non-inverting
inputs of
the
amplifier
are at the
6
volt
reference,
the
diode will
conduct
only when
the filter
output is
at
least one diode
drop
above its
rest output
of
6 volts. The
diode thus
rectifies the
signal
and the
associated
amplifier inverts
the
signal
and averages
it with
a 20 Hz
low
pass filter. The
output of the rectifier
averager
is thus
the
6 volt reference
in the
absence
of
any input
signal,
and moves
toward
ground when
an input
signal
is applied.
The higher
the
amplitude
of
the input, the closer
the
output is
to ground.
The
output of
the rectifier/
averager
stage
is
applied
to the negative
input
of a
voltage
comparator
whose
positive
input
is
a
fixed
voltage
just slightly less
than the
6
volt
reference.
The
normal
comparator
output is
thus
zero volts. When
a
small amount
of
speech
energy is
amplified,
filtered,
and
rectified,
the
out-
put of the
rectifier
stage trips the
comparator
producing
a 5 volt
output
from
the
comparator.
This signal
indicates
speech energy
is being
produced
in
the
bandwidth
of its
associated filter.
This
signal from
each
of the
rectifier
averages is
fed
to a bus driver,
along with
the two zero
crossing
detector
outputs
where it
is made
available
to
the
processor.
Address Decoding
From the viewpoint
of the TRS-80,
the
speech input
peripheral
is
a single
input
port
at
address
AFH
(
1
75
decimal).
Whenever
this
address is
accessed with
an input
instruction,
the
peripheral places its
data on the
computer
data bus in the
format
shown
in
Figure
4.
Power Supply
and
Regulators
The speech input peripheral
uses
2 voltages internally,
a
+5
volt
supply
for the
digital
circuitry
and a
+
1
2
volt supply
for the analog
circuitry.
These voltages
are
regulated
internally
using
the raw DC voltage
supplied
by the
battery eliminator
which
plugs
into
the wall.
D7-D4
D3
D2
D1
DO
NOT
USED
HIGH
BAND
AMPLITUDE
HIGH
BANK
ZERO
CROSSING
LOW
BAND
ZERO
CROSSING
LOW
BAND
ENERGY
1=ENERGY
0=NO
ENERGY
1=ENERGY
0=NO
ENERGY
FIGURE 4
-12-

Radio Shack VoxBox TRS-80 Specifications

General

Brand	Radio Shack
Model	VoxBox TRS-80
Category	Control Unit
Language	English