<s>
An	O
audio	B-Protocol
synchronizer	I-Protocol
is	O
a	O
variable	O
audio	O
delay	O
used	O
to	O
correct	O
or	O
maintain	O
audio-video	B-Protocol
sync	I-Protocol
or	O
timing	O
also	O
known	O
as	O
lip	B-Protocol
sync	I-Protocol
error	I-Protocol
.	O
</s>
<s>
See	O
for	O
example	O
the	O
specification	O
for	O
audio	O
to	O
video	O
timing	O
given	O
in	O
ATSC	B-Device
Document	O
IS-191	O
.	O
</s>
<s>
This	O
effect	O
is	O
commonly	O
referred	O
to	O
as	O
A/V	B-Protocol
sync	I-Protocol
or	O
lip	B-Protocol
sync	I-Protocol
error	I-Protocol
and	O
can	O
cause	O
serious	O
problems	O
related	O
to	O
the	O
viewer	O
's	O
enjoyment	O
of	O
the	O
program	O
.	O
</s>
<s>
To	O
correct	O
audio	B-Protocol
video	I-Protocol
sync	I-Protocol
problems	O
,	O
the	O
video	O
processing	O
circuitry	O
outputs	O
a	O
DDO	O
(	O
digital	O
delay	O
output	O
)	O
signal	O
,	O
which	O
carries	O
information	O
about	O
the	O
amount	O
of	O
delay	O
the	O
video	O
signal	O
experiences	O
due	O
to	O
the	O
video	O
processing	O
.	O
</s>
<s>
The	O
DDO	O
may	O
,	O
for	O
example	O
,	O
be	O
provided	O
by	O
equipment	O
which	O
adheres	O
to	O
the	O
SMPTE	O
Audio	B-Protocol
to	I-Protocol
Video	I-Protocol
Synchronization	I-Protocol
Standard	O
.	O
</s>
<s>
The	O
audio	B-Protocol
synchronizer	I-Protocol
receives	O
the	O
DDO	O
signal	O
and	O
in	O
response	O
delays	O
the	O
audio	O
by	O
an	O
equivalent	O
amount	O
,	O
thereby	O
maintaining	O
proper	O
audio-video	B-Protocol
sync	I-Protocol
.	O
</s>
<s>
Modern	O
audio	B-Protocol
synchronizers	I-Protocol
operate	O
by	O
digitizing	O
and	O
writing	O
the	O
audio	O
signal	O
into	O
a	O
ring	O
memory	O
,	O
which	O
is	O
most	O
commonly	O
a	O
RAM-based	O
memory	O
having	O
independent	O
read	O
and	O
write	O
ability	O
.	O
</s>
<s>
To	O
maintain	O
proper	O
audio-video	B-Protocol
sync	I-Protocol
,	O
the	O
audio	O
delay	O
must	O
track	O
these	O
video	O
delay	O
changes	O
.	O
</s>
<s>
Some	O
audio	B-Protocol
synchronizers	I-Protocol
operate	O
by	O
making	O
repeated	O
,	O
very	O
small	O
jumps	O
that	O
cause	O
unwanted	O
(	O
but	O
less	O
annoying	O
)	O
distortion	O
and	O
noise	O
in	O
the	O
audio	O
signal	O
,	O
rather	O
than	O
pops	O
,	O
gaps	O
,	O
and	O
clicks	O
.	O
</s>
<s>
Other	O
audio	B-Protocol
synchronizers	I-Protocol
change	O
delay	O
by	O
changing	O
the	O
speed	O
of	O
the	O
reading	O
of	O
audio	O
from	O
the	O
ring	O
memory	O
.	O
</s>
<s>
Audio	B-Protocol
synchronizers	I-Protocol
that	O
use	O
variable	O
speed	O
reading	O
are	O
generally	O
preferred	O
in	O
professional	O
applications	O
.	O
</s>
<s>
Unfortunately	O
,	O
this	O
limits	O
the	O
rate	O
of	O
delay	O
change	O
and	O
when	O
large	O
video	O
delay	O
changes	O
occur	O
the	O
slow	O
tracking	O
rate	O
of	O
these	O
uncompensated	O
synchronizers	O
can	O
cause	O
the	O
audio-video	B-Protocol
sync	I-Protocol
to	O
be	O
off	O
for	O
several	O
seconds	O
or	O
minutes	O
until	O
the	O
audio	O
delay	O
catches	O
up	O
with	O
the	O
video	O
delay	O
.	O
</s>
<s>
In	O
higher	O
performance	O
audio	B-Protocol
synchronizers	I-Protocol
,	O
the	O
rate	O
of	O
delay	O
change	O
is	O
allowed	O
to	O
be	O
much	O
faster	O
,	O
generally	O
in	O
the	O
order	O
of	O
25%	O
,	O
and	O
the	O
resulting	O
pitch	O
error	O
is	O
corrected	O
with	O
a	O
pitch	O
correction	O
circuit	O
.	O
</s>
<s>
These	O
higher	O
performance	O
audio	B-Protocol
synchronizers	I-Protocol
allow	O
the	O
audio	O
delay	O
to	O
track	O
even	O
large	O
and	O
quick	O
video	O
delay	O
changes	O
without	O
generating	O
any	O
artifacts	O
that	O
are	O
perceptible	O
to	O
even	O
critical	O
listeners	O
for	O
most	O
audio	O
program	O
material	O
.	O
</s>
<s>
Recent	O
development	O
in	O
video	O
processing	O
devices	O
permit	O
those	O
devices	O
to	O
sense	O
when	O
a	O
large	O
video	O
delay	O
change	O
will	O
need	O
to	O
be	O
made	O
beforehand	O
and	O
allow	O
that	O
change	O
information	O
to	O
be	O
communicated	O
to	O
the	O
audio	B-Protocol
synchronizer	I-Protocol
.	O
</s>
<s>
The	O
"	O
advanced	O
notice	O
"	O
from	O
the	O
video	O
processing	O
device	O
allows	O
the	O
audio	B-Protocol
synchronizer	I-Protocol
to	O
anticipate	O
and	O
take	O
advantage	O
of	O
particular	O
audio	O
material	O
(	O
e.g.	O
,	O
periods	O
of	O
relative	O
silence	O
or	O
periods	O
without	O
music	O
)	O
to	O
facilitate	O
making	O
corresponding	O
large	O
audio	O
delay	O
changes	O
that	O
do	O
not	O
risk	O
generating	O
noticeable	O
audio	O
artifacts	O
.	O
</s>
<s>
Further	O
developments	O
permit	O
handshaking	O
between	O
the	O
video	O
processing	O
device	O
and	O
the	O
audio	B-Protocol
synchronizer	I-Protocol
to	O
control	O
when	O
the	O
video	O
delay	O
change	O
is	O
made	O
to	O
optimize	O
the	O
timing	O
of	O
the	O
tracking	O
audio	O
delay	O
change	O
thereby	O
further	O
reducing	O
the	O
risk	O
of	O
generating	O
noticeable	O
audio	O
artifacts	O
and	O
at	O
the	O
same	O
time	O
reducing	O
the	O
risk	O
of	O
missynchronization	O
due	O
to	O
rapid	O
video	O
delay	O
changes	O
.	O
</s>
