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Teac MT-20D-IO - Page 23

Teac MT-20D-IO
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3-2
Operation
of
data
recording/reproducing
system
(PCBA
Drive
Control)
3-2-1
Read
circuit
The
block
diagram
of
the
read
circuit
is
shown
in
Fig.
309.
The
read
circuit
consists
of
preamplifier,
track
select,
AGC
(Automatic
Gain
Control)
amplifier,
low-pass
filter,
differentiator,
zero
crossing
detector,
time
domain
filter,
threshold
detector,
and
read
gate,
etc.
The
outout
signal
of
several
mV
from
the
read
head
is
amplified
by
the
preamplifier
of
differential
input/output
type.
Two
channels
of
pre-
amplifiers
are
prepared
to
correspond
to
the
number
of
channels
of
the
read
head.
The
output
signals
of
the
preamplifiers
are
led
to
the
track
select
circuit
through
attenuators
to
adjust
the
read
preamplifier
level.
The
track
select
circuit
performs
changeover
between
the
track
0/2
side
and
track
1/3
side
according
to
the
track
to
be
accessed.
The
output
signal
of
the
track
select
circuit
is
amplified
by
a
one-stage
voltage-controlled
amplifier,
and
applied
to
the
AGC
amplifier.
The
AGC
amplifier
adjusts
automatically
its
own
gain
so
that
the
sensitivity
difference
of
the
magnetic
tape
used
is
corrected
and
the
read
level
is
kept
nearly
constant.
The
output
signal
of
the
AGC
amplifier
is
led
to
the
low-pass
filter
to
reject
the
unwanted
noise
components.
The
output
signal
of
the
low-pass
filter
is
phase
shifted
by
the
differentiator
so
that
the
peak
position
of
the
reproduced
signal
is
set
to
the
zero
cross
position,
and
the
phase-shifted
signal
is
led
to
the
zero
crossing
detector
and
time
domain
filter.
The
zero
crossing
detector
consists
of
two
comparators.
pulse
delay
circuit,
and
flip-flop
circuit,
etc.
to
convert
the
output
signal
of
the
differentiator
into
a
pulse
signal.
The
output
signal
of
the
zero
crossing
detector
is
made
free
through
the
time
domain
filter
from
the
bad
influence
of
the
saddle
generated
during
reading
of
IF
signal
(1001
data
pattern)
to
be
led
to
the
read
gate
circuit.
Meanwhile,
the
output
differentia
tor
and
at
signal
of
the
the
same
time,
low-pass
filter
is
applied
to
the
led
to
the
level
detect
amplifier
through
an
attenuator
to
adjust
the
read
level.
-
311
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