<s>
Audio-to-video	B-Protocol
synchronization	I-Protocol
(	O
AV	B-Protocol
synchronization	I-Protocol
,	O
also	O
known	O
as	O
lip	O
sync	O
,	O
or	O
by	O
the	O
lack	O
of	O
it	O
:	O
lip-sync	O
error	O
,	O
lip	B-Protocol
flap	I-Protocol
)	O
refers	O
to	O
the	O
relative	O
timing	O
of	O
audio	O
(	O
sound	O
)	O
and	O
video	O
(	O
image	O
)	O
parts	O
during	O
creation	O
,	O
post-production	O
(	O
mixing	O
)	O
,	O
transmission	O
,	O
reception	O
and	O
play-back	O
processing	O
.	O
</s>
<s>
AV	B-Protocol
synchronization	I-Protocol
can	O
be	O
an	O
issue	O
in	O
television	O
,	O
videoconferencing	B-Application
,	O
or	O
film	O
.	O
</s>
<s>
This	O
terminology	O
and	O
standardization	O
of	O
the	O
numeric	O
lip-sync	O
error	O
is	O
utilized	O
in	O
the	O
professional	O
broadcast	B-Operating_System
industry	O
as	O
evidenced	O
by	O
the	O
various	O
professional	O
papers	O
,	O
standards	O
such	O
as	O
ITU-R	O
BT.1359-1	O
,	O
and	O
other	O
references	O
below	O
.	O
</s>
<s>
Digital	O
or	O
analog	O
audio	O
video	O
streams	O
or	O
video	B-General_Concept
files	I-General_Concept
usually	O
contain	O
some	O
sort	O
of	O
synchronization	O
mechanism	O
,	O
either	O
in	O
the	O
form	O
of	O
interleaved	O
video	O
and	O
audio	O
data	O
or	O
by	O
explicit	O
relative	O
timestamping	O
of	O
data	O
.	O
</s>
<s>
Internal	O
AV-sync	O
error	O
:	O
Different	O
signal	O
processing	O
delays	O
between	O
image	O
and	O
sound	O
in	O
video	O
camera	O
and	O
microphone	B-Application
.	O
</s>
<s>
External	O
AV-sync	O
error	O
:	O
If	O
a	O
microphone	B-Application
is	O
placed	O
far	O
away	O
from	O
the	O
sound	O
source	O
,	O
the	O
audio	O
will	O
be	O
out	O
of	O
sync	O
because	O
the	O
speed	O
of	O
sound	O
is	O
much	O
lower	O
than	O
the	O
speed	O
of	O
light	O
.	O
</s>
<s>
If	O
the	O
sound	O
source	O
is	O
340	O
meters	O
from	O
the	O
microphone	B-Application
,	O
then	O
the	O
sound	O
arrives	O
approximately	O
1	O
second	O
later	O
than	O
the	O
light	O
.	O
</s>
<s>
Examples	O
of	O
transmission	O
(	O
broadcasting	B-Operating_System
)	O
,	O
reception	O
and	O
playback	O
that	O
can	O
get	O
the	O
AV-sync	O
incorrectly	O
synchronized	O
:	O
</s>
<s>
A	O
video	O
camera	O
with	O
built-in	O
microphones	B-Application
or	O
line-in	O
may	O
not	O
delay	O
sound	O
and	O
video	O
paths	O
by	O
the	O
same	O
number	O
of	O
milliseconds	O
.	O
</s>
<s>
charge-coupled	B-Algorithm
device	I-Algorithm
(	O
CCD	O
)	O
and	O
CMOS	B-Architecture
image	I-Architecture
sensors	I-Architecture
)	O
can	O
delay	O
the	O
video	O
signal	O
by	O
one	O
or	O
more	O
frames	O
.	O
</s>
<s>
An	O
AV-stream	O
may	O
get	O
corrupted	O
during	O
transmission	O
because	O
of	O
electrical	O
glitches	B-Error_Name
(	O
wired	O
)	O
or	O
wireless	O
interruptions	O
-	O
this	O
may	O
cause	O
it	O
to	O
become	O
out	O
of	O
sync	O
.	O
</s>
<s>
Pixelated	O
displays	O
require	O
video	O
format	B-General_Concept
conversion	I-General_Concept
and	O
deinterlace	O
processing	O
which	O
can	O
add	O
one	O
or	O
more	O
frames	O
of	O
video	O
delay	O
.	O
</s>
<s>
Some	O
transmission	O
protocols	O
like	O
RTP	B-Protocol
require	O
an	O
out-of-band	O
method	O
for	O
synchronizing	O
media	O
streams	O
.	O
</s>
<s>
In	O
RTP	B-Protocol
's	O
case	O
,	O
each	O
media	O
stream	O
has	O
its	O
own	O
timestamp	O
using	O
an	O
independent	O
clock	O
rate	O
and	O
per-stream	O
randomized	O
starting	O
value	O
.	O
</s>
<s>
A	O
RTCP	B-Protocol
Sender	O
Report	O
(	O
SR	O
)	O
is	O
needed	O
for	O
each	O
stream	O
in	O
order	O
to	O
synchronize	O
streams	O
.	O
</s>
<s>
The	O
necessary	O
RTCP	B-Protocol
packets	O
might	O
be	O
lost	O
(	O
since	O
RTP/RTCP	O
does	B-Protocol
not	I-Protocol
guarantee	I-Protocol
delivery	I-Protocol
)	O
or	O
not	O
sent	O
until	O
at	O
least	O
several	O
seconds	O
after	O
the	O
stream	O
has	O
begun	O
.	O
</s>
<s>
Many	O
software	O
clients	O
do	O
not	O
send	O
RTCP	B-Protocol
at	O
all	O
or	O
send	O
non-compliant	O
data	O
.	O
</s>
<s>
In	O
film	O
movies	O
these	O
timing	O
errors	O
are	O
most	O
commonly	O
caused	O
by	O
worn	O
films	O
skipping	O
over	O
the	O
movie	B-Application
projector	I-Application
sprockets	O
because	O
the	O
film	O
has	O
torn	O
sprocket	O
holes	O
.	O
</s>
<s>
AV-sync	O
is	O
commonly	O
corrected	O
and	O
maintained	O
with	O
an	O
audio	B-Protocol
synchronizer	I-Protocol
.	O
</s>
<s>
AV-sync	O
errors	O
are	O
becoming	O
a	O
significant	O
problem	O
in	O
the	O
digital	B-Application
television	I-Application
industry	O
because	O
of	O
the	O
use	O
of	O
large	O
amounts	O
of	O
video	O
signal	O
processing	O
in	O
television	O
production	O
,	O
television	O
broadcasting	B-Operating_System
and	O
pixelated	O
television	O
displays	O
such	O
as	O
LCD	B-Device
,	O
DLP	B-General_Concept
and	O
plasma	O
displays	O
.	O
</s>
<s>
In	O
the	O
television	O
field	O
,	O
audio-video	B-Protocol
sync	I-Protocol
problems	O
are	O
commonly	O
caused	O
when	O
significant	O
amounts	O
of	O
video	O
processing	O
is	O
performed	O
on	O
the	O
video	O
part	O
of	O
the	O
television	O
program	O
.	O
</s>
<s>
Particularly	O
troublesome	O
encoders	O
and	O
decoders	O
are	O
used	O
in	O
MPEG	O
compression	O
systems	O
utilized	O
for	O
broadcasting	B-Operating_System
digital	B-Application
television	I-Application
and	O
storing	O
television	O
programs	O
on	O
consumer	O
and	O
professional	O
recording	O
and	O
playback	O
devices	O
.	O
</s>
<s>
A	O
source	O
of	O
significant	O
video	O
delay	O
is	O
found	O
in	O
pixelated	O
television	O
displays	O
(	O
LCD	B-Device
,	O
DLP	B-General_Concept
and	O
plasma	O
)	O
which	O
utilize	O
complex	O
video	O
signal	O
processing	O
to	O
convert	O
the	O
resolution	O
of	O
the	O
incoming	O
video	O
signal	O
to	O
the	O
native	O
resolution	O
of	O
the	O
pixelated	O
display	O
,	O
for	O
example	O
converting	O
standard	O
definition	O
video	O
to	O
be	O
displayed	O
on	O
a	O
high	O
definition	O
display	O
.	O
</s>
<s>
In	O
broadcast	B-Operating_System
television	O
,	O
it	O
is	O
not	O
unusual	O
for	O
lip-sync	O
error	O
to	O
vary	O
by	O
over	O
100	O
ms	O
(	O
several	O
video	O
frames	O
)	O
from	O
time	O
to	O
time	O
.	O
</s>
<s>
The	O
EBU	O
Recommendation	O
R37	O
“	O
The	O
relative	O
timing	O
of	O
the	O
sound	O
and	O
vision	O
components	O
of	O
a	O
television	O
signal	O
”	O
states	O
that	O
end-to-end	O
audio/video	B-Protocol
sync	I-Protocol
should	O
be	O
within	O
+40ms	O
and	O
-60ms	O
(	O
audio	O
before	O
/	O
after	O
video	O
,	O
respectively	O
)	O
and	O
that	O
each	O
stage	O
should	O
be	O
within	O
+5ms	O
and	O
-15ms	O
.	O
</s>
<s>
The	O
result	O
typically	O
leaves	O
a	O
filmed	O
or	O
televised	O
character	O
moving	O
his	O
or	O
her	O
mouth	O
when	O
there	O
is	O
no	O
spoken	O
dialog	O
to	O
accompany	O
it	O
,	O
hence	O
the	O
term	O
"	O
lip	B-Protocol
flap	I-Protocol
"	O
or	O
"	O
lip-sync	O
error	O
"	O
.	O
</s>
<s>
The	O
resulting	O
audio-video	B-Protocol
sync	I-Protocol
error	O
can	O
be	O
annoying	O
to	O
the	O
viewer	O
and	O
may	O
even	O
cause	O
the	O
viewer	O
to	O
not	O
enjoy	O
the	O
program	O
,	O
decrease	O
the	O
effectiveness	O
of	O
the	O
program	O
or	O
lead	O
to	O
a	O
negative	O
perception	O
of	O
the	O
speaker	O
on	O
the	O
part	O
of	O
the	O
viewer	O
.	O
</s>
<s>
Television	O
industry	O
standards	O
organizations	O
,	O
such	O
as	O
the	O
Advanced	B-Device
Television	I-Device
Systems	I-Device
Committee	I-Device
,	O
have	O
become	O
involved	O
in	O
setting	O
standards	O
for	O
audio-video	B-Protocol
sync	I-Protocol
errors	O
.	O
</s>
<s>
Unfortunately	O
,	O
the	O
advent	O
of	O
high-definition	O
flat-panel	O
display	O
technologies	O
(	O
LCD	B-Device
,	O
DLP	B-General_Concept
and	O
plasma	O
)	O
,	O
which	O
can	O
delay	O
video	O
more	O
than	O
audio	O
,	O
has	O
moved	O
the	O
problem	O
into	O
the	O
viewer	O
's	O
home	O
and	O
beyond	O
the	O
control	O
of	O
the	O
television	O
programming	O
industry	O
alone	O
.	O
</s>
<s>
For	O
television	O
applications	O
,	O
the	O
Advanced	B-Device
Television	I-Device
Systems	I-Device
Committee	I-Device
recommends	O
that	O
audio	O
should	O
lead	O
video	O
by	O
no	O
more	O
than	O
15	O
milliseconds	O
and	O
audio	O
should	O
lag	O
video	O
by	O
no	O
more	O
than	O
45	O
milliseconds	O
.	O
</s>
<s>
The	O
Consumer	O
Electronics	O
Association	O
has	O
published	O
a	O
set	O
of	O
recommendations	O
for	O
how	O
digital	B-Application
television	I-Application
receivers	O
should	O
implement	O
A/V	B-Protocol
sync	I-Protocol
.	O
</s>
<s>
SMPTE	O
standard	O
ST2064	O
,	O
published	O
in	O
2015	O
,	O
provides	O
technology	O
to	O
reduce	O
or	O
eliminate	O
lip-sync	O
errors	O
in	O
digital	B-Application
television	I-Application
.	O
</s>
<s>
Presentation	B-Protocol
time	I-Protocol
stamps	I-Protocol
(	O
PTS	O
)	O
are	O
embedded	O
in	O
MPEG	O
transport	O
streams	O
to	O
precisely	O
signal	O
when	O
each	O
audio	O
and	O
video	O
segment	O
is	O
to	O
be	O
presented	O
,	O
to	O
avoid	O
AV-sync	O
errors	O
.	O
</s>
<s>
However	O
,	O
these	O
timestamps	O
are	O
often	O
added	O
after	O
the	O
video	O
undergoes	O
frame	O
synchronization	O
,	O
format	B-General_Concept
conversion	I-General_Concept
and	O
preprocessing	O
,	O
and	O
thus	O
the	O
lip	B-Protocol
sync	I-Protocol
errors	I-Protocol
created	O
by	O
these	O
operations	O
will	O
not	O
be	O
corrected	O
by	O
the	O
addition	O
and	O
use	O
of	O
timestamps	O
.	O
</s>
<s>
The	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
clocks	O
media	O
using	O
origination	O
timestamps	O
on	O
an	O
arbitrary	O
timeline	O
.	O
</s>
<s>
A	O
real-time	O
clock	O
such	O
as	O
one	O
delivered	O
by	O
the	O
Network	B-Protocol
Time	I-Protocol
Protocol	I-Protocol
and	O
described	O
in	O
the	O
Session	B-Language
Description	I-Language
Protocol	I-Language
associated	O
with	O
the	O
media	O
may	O
be	O
used	O
to	O
synchronize	O
media	O
.	O
</s>
