Somehow,
the
picture
and
sound
must
be
synchronized
together
(using
the
clapping
of
the
slate
as
both
audio
and
picture
cue)
so
when
someone
on
film
speaks,
the
words
spoken
match
the
movement
of
their
lips;
this
is
done
by
putting
the
mag-track
onto
the
same
rewinds
as
the
film.
They
may
start
rolling
together,
but
they
will
soon
fluctuate
in
actual
speed
if
they
are
not
locked
together
somehow.
To
correct
for
speed
deviations,
a
device
known
as
a
synchronizer
is
used
to
interlock
the
two
reels
of
film
and
sound
together.
A
synchronizer
has
several
sprocketed
wheels,
sharing
a
common
axle.
When
one
wheel
turns
10
frames,
the
others
follow
exactly
10
frames
(they
are
fixed
together).
Some
synchronizers
are
rigged
for
sound.
Directly
over
the
synchronizer
wheels,
audio
pick-up
heads
(just
like
on
a
quarter
inch
tape
recorder)
pick
up
the
sound
signal
from
the
mag-track
as
it
passes
underneath.
Always
have
the
"emulsion"
side
of
the
mag-track
facing
and
contacting
the
audio
head
if
you
expect
to
hear
anything.
Once
the
mag-track
exits
the
synchro,
it
goes
straight
to
its
reel
on
the
take-up
rewind.
The
picture
film
is
locked
to
the
synchro
too,
then
enters
the
viewer.
At
the
beginning
of
both
the
film
and
the
mag-track
reels,
a
common
"start"
cue
mark
is
made
(visually
with
a
grease
pencil).
These
two
marks
must
be
locked
together
in
the
synchro
using
the
"start"
frame,
usually
the
slate
or
clap
marker
sound
blob.
Once
these
two
marks
are
cued
up
and
locked,
picture
and
sound
will
stay
in
sync.
