<s>
Reference	B-General_Concept
frames	I-General_Concept
are	O
frames	O
of	O
a	O
compressed	O
video	O
that	O
are	O
used	O
to	O
define	O
future	O
frames	O
.	O
</s>
<s>
As	O
such	O
,	O
they	O
are	O
only	O
used	O
in	O
inter-frame	B-Algorithm
compression	O
techniques	O
.	O
</s>
<s>
In	O
older	O
video	O
encoding	O
standards	O
,	O
such	O
as	O
MPEG-2	B-Algorithm
,	O
only	O
one	O
reference	B-General_Concept
frame	I-General_Concept
–	O
the	O
previous	O
frame	O
–	O
was	O
used	O
for	O
P-frames	O
.	O
</s>
<s>
Two	O
reference	B-General_Concept
frames	I-General_Concept
(	O
one	O
past	O
and	O
one	O
future	O
)	O
were	O
used	O
for	O
B-frames	B-General_Concept
.	O
</s>
<s>
Some	O
modern	O
video	O
encoding	O
standards	O
,	O
such	O
as	O
H.264/AVC	B-Application
,	O
allow	O
the	O
use	O
of	O
multiple	B-General_Concept
reference	I-General_Concept
frames	I-General_Concept
.	O
</s>
<s>
This	O
allows	O
the	O
video	O
encoder	O
to	O
choose	O
among	O
more	O
than	O
one	O
previously	O
decoded	O
frame	O
on	O
which	O
to	O
base	O
each	O
macroblock	B-General_Concept
in	O
the	O
next	O
frame	O
.	O
</s>
<s>
While	O
the	O
best	O
frame	O
for	O
this	O
purpose	O
is	O
usually	O
the	O
previous	O
frame	O
,	O
the	O
extra	O
reference	B-General_Concept
frames	I-General_Concept
can	O
improve	O
compression	O
efficiency	O
and/or	O
video	B-Device
quality	I-Device
.	O
</s>
<s>
Note	O
that	O
different	O
reference	B-General_Concept
frames	I-General_Concept
can	O
be	O
chosen	O
for	O
different	O
macroblocks	B-General_Concept
in	O
the	O
same	O
frame	O
.	O
</s>
<s>
The	O
maximum	O
number	O
of	O
concurrent	O
reference	B-General_Concept
frames	I-General_Concept
supported	O
by	O
H.264	B-Application
is	O
16	O
.	O
</s>
<s>
Different	O
reference	B-General_Concept
frames	I-General_Concept
can	O
be	O
chosen	O
for	O
each	O
8x8	O
partition	O
of	O
a	O
macroblock	B-General_Concept
.	O
</s>
<s>
Another	O
video	O
format	O
that	O
supports	O
multiple	B-General_Concept
reference	I-General_Concept
frames	I-General_Concept
is	O
Snow	B-Library
,	O
which	O
can	O
handle	O
up	O
to	O
eight	O
.	O
</s>
<s>
The	O
Theora	B-Algorithm
codec	I-Algorithm
provides	O
a	O
limited	O
form	O
of	O
multiple	B-General_Concept
reference	I-General_Concept
frames	I-General_Concept
,	O
allowing	O
references	O
to	O
both	O
the	O
preceding	O
frame	O
and	O
the	O
most	O
recent	O
intra	B-General_Concept
frame	I-General_Concept
.	O
</s>
<s>
Multiple	B-General_Concept
reference	I-General_Concept
frames	I-General_Concept
can	O
considerably	O
increase	O
encoding	O
time	O
because	O
many	O
of	O
the	O
decisions	O
,	O
such	O
as	O
motion	O
estimation	O
,	O
that	O
are	O
ordinarily	O
carried	O
out	O
only	O
on	O
one	O
reference	B-General_Concept
frame	I-General_Concept
have	O
to	O
be	O
repeated	O
on	O
all	O
of	O
the	O
reference	B-General_Concept
frames	I-General_Concept
.	O
</s>
<s>
Heuristics	B-Algorithm
can	O
be	O
used	O
to	O
reduce	O
this	O
speed	O
cost	O
at	O
the	O
cost	O
of	O
quality	O
.	O
</s>
<s>
Very	O
high	O
numbers	O
of	O
reference	B-General_Concept
frames	I-General_Concept
are	O
rarely	O
useful	O
in	O
terms	O
of	O
quality	O
for	O
live-action	O
material	O
because	O
frames	O
from	O
farther	O
back	O
in	O
time	O
generally	O
have	O
less	O
and	O
less	O
correlation	O
with	O
the	O
current	O
frame	O
.	O
</s>
<s>
This	O
is	O
not	O
as	O
true	O
for	O
animated	O
sources	O
,	O
where	O
repetitive	O
motion	O
can	O
make	O
high	O
numbers	O
of	O
reference	B-General_Concept
frames	I-General_Concept
more	O
useful	O
.	O
</s>
<s>
When	O
decoding	O
,	O
reference	B-General_Concept
frames	I-General_Concept
must	O
be	O
stored	O
in	O
memory	B-Architecture
until	O
they	O
are	O
no	O
longer	O
needed	O
for	O
further	O
decoding	O
.	O
</s>
<s>
This	O
can	O
considerably	O
raise	O
the	O
memory	B-Architecture
usage	O
of	O
the	O
decoder	O
for	O
videos	O
with	O
large	O
numbers	O
of	O
reference	B-General_Concept
frames	I-General_Concept
.	O
</s>
<s>
The	O
use	O
of	O
several	O
reference	B-General_Concept
frames	I-General_Concept
also	O
decreases	O
locality	B-General_Concept
of	I-General_Concept
reference	I-General_Concept
,	O
which	O
might	O
cause	O
a	O
speed	O
impact	O
.	O
</s>
<s>
Stand-alone	O
players	O
that	O
can	O
play	O
AVC/MKV	O
files	O
from	O
recorded	O
DVDs	O
or	O
USB	O
sticks	O
may	O
not	O
be	O
able	O
to	O
handle	O
the	O
maximum	O
possible	O
16	O
reference	B-General_Concept
frames	I-General_Concept
.	O
</s>
