Radio Shack VoxBox TRS-80 User Manual

3.
Theory of Operation
This
section provides the theory of operation of the speech recognition
hardware functional
blocks, as
shown in block diagram
Figure 3.
Microphone
The first element
in
the
signal chain is
the microphone, a dynamic microphone
with a
push-to-talk switch. While there are several circuits in the microphone,
the only circuit
used is
the audio
circuit,
which
delivers
the 1 mv
—
40 mv signal from the microphone
when the
switch is
closed
and
is
an
open circuit when
the
switch is
open or released.
Microphone Preamp
The microphone preamp amplifies and frequency compensates the
microphone signal before
the signal is passed to the bandpass filters. The input from the microphone
is terminated
in a
1 K resistor to ground which keeps
the microphone input grounded when the
microphone switch is released. The first
stage of the microphone preamp
"pre-emphasizes"
the speech input, amplifying higher frequencies more than lower
frequencies, with the
rate of increase being 6 DB/ octave. This compensates for vocal
tract characteristics, which
produce less energy at higher frequencies than
at lower frequencies.
This pre-emphasis continues
up to about 1 .5 KHz after which the
preamp output rolls off,
or
decreases, at a
rate of 12
DB octave. The second
stage
of the
microphone preamp
provides additional gain. The microphone
preamp amplifies the 10
—
40 millivolt microphone
signal and produces
a
signal
of 1 to 10 volts peak
to peak output. Since all the
analog cir-
cuitry operates from
a
single
+
1
2
V
supply, the
output
of
the microphone preamp
is
biased for a no-signal
output of about
6V,
and swings
above and below this
rest level. This
amplified and frequency compensated
signal is then applied
to
the
inputs of
the two bandpass
filters.
Filters
The two bandpass filters
are each
implemented
as a low
pass filter
followed
by a high
pass
filter. The low
band filter formed
by IC2-1
and IC2-7 has
a passband
of 1 50 Hz
to 900
Hz. The
high band filter
formed
by I C
1
-8
and IC
1
-
1
4
has
a passband
of
1
KHz
to about 4 K Hz.
The
outputs of the filter
stages
are
biased
to a rest level of
about
6V,
through the
application
of the
6V
reference
level
to the non-inverting
inputs
of all the
amplifiers.
The
output of
each filter stage is
sine waves
whose
frequencies
fall within
the passband
of the
associated
filter.
The
frequencies of the
sinewaves are the
same
as the frequencies
contained
in
the input
signal from the
microphone,
and the
amplitude
of the sinewaves
is
proportional
to
the amplitude
of the signals
from the
microphone,
modified
by the frequency
shaping in
the
microphone
preamp. The
output
of each filter is
applied
to a zero-crossing
detector
and a
rectifier averager
circuit.
-10-

Radio Shack VoxBox TRS-80 Specifications

General

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