<s>
In	O
telecommunications	O
,	O
average	B-General_Concept
bitrate	I-General_Concept
(	O
ABR	O
)	O
refers	O
to	O
the	O
average	O
amount	O
of	O
data	O
transferred	O
per	O
unit	O
of	O
time	O
,	O
usually	O
measured	O
per	O
second	O
,	O
commonly	O
for	O
digital	O
music	O
or	O
video	O
.	O
</s>
<s>
An	O
MP3	B-Application
file	I-Application
,	O
for	O
example	O
,	O
that	O
has	O
an	O
average	B-General_Concept
bit	I-General_Concept
rate	I-General_Concept
of	O
128	O
kbit/s	O
transfers	O
,	O
on	O
average	O
,	O
128,000	O
bits	O
every	O
second	O
.	O
</s>
<s>
It	O
can	O
have	O
higher	O
bitrate	O
and	O
lower	O
bitrate	O
parts	O
,	O
and	O
the	O
average	B-General_Concept
bitrate	I-General_Concept
for	O
a	O
certain	O
timeframe	O
is	O
obtained	O
by	O
dividing	O
the	O
number	O
of	O
bits	O
used	O
during	O
the	O
timeframe	O
by	O
the	O
number	O
of	O
seconds	O
in	O
the	O
timeframe	O
.	O
</s>
<s>
Bitrate	O
is	O
not	O
reliable	O
as	O
a	O
standalone	O
measure	O
of	O
audio	O
or	O
video	B-Device
quality	I-Device
,	O
since	O
more	O
efficient	O
compression	O
methods	O
use	O
lower	O
bitrates	O
to	O
encode	O
material	O
at	O
a	O
similar	O
quality	O
.	O
</s>
<s>
Average	B-General_Concept
bitrate	I-General_Concept
can	O
also	O
refer	O
to	O
a	O
form	O
of	O
variable	B-General_Concept
bitrate	I-General_Concept
(	O
VBR	O
)	O
encoding	O
in	O
which	O
the	O
encoder	B-General_Concept
will	O
try	O
to	O
reach	O
a	O
target	O
average	B-General_Concept
bitrate	I-General_Concept
or	O
file	B-General_Concept
size	I-General_Concept
while	O
allowing	O
the	O
bitrate	O
to	O
vary	O
between	O
different	O
parts	O
of	O
the	O
audio	O
or	O
video	O
.	O
</s>
<s>
As	O
it	O
is	O
a	O
form	O
of	O
variable	B-General_Concept
bitrate	I-General_Concept
,	O
this	O
allows	O
more	O
complex	O
portions	O
of	O
the	O
material	O
to	O
use	O
more	O
bits	O
and	O
less	O
complex	O
areas	O
to	O
use	O
fewer	O
bits	O
.	O
</s>
<s>
However	O
,	O
bitrate	O
will	O
not	O
vary	O
as	O
much	O
as	O
in	O
variable	B-General_Concept
bitrate	I-General_Concept
encoding	I-General_Concept
.	O
</s>
<s>
At	O
a	O
given	O
bitrate	O
,	O
VBR	O
is	O
usually	O
higher	O
quality	O
than	O
ABR	O
,	O
which	O
is	O
higher	O
quality	O
than	O
CBR	O
(	O
constant	B-General_Concept
bitrate	I-General_Concept
)	O
.	O
</s>
<s>
ABR	O
encoding	O
is	O
desirable	O
for	O
users	O
who	O
want	O
the	O
general	O
benefits	O
of	O
VBR	O
encoding	O
(	O
an	O
optimum	O
bitrate	O
from	O
frame	O
to	O
frame	O
)	O
but	O
with	O
a	O
relatively	O
predictable	O
file	B-General_Concept
size	I-General_Concept
.	O
</s>
<s>
Two-pass	O
encoding	O
is	O
usually	O
needed	O
for	O
accurate	O
ABR	O
encoding	O
,	O
as	O
on	O
the	O
first	O
pass	O
the	O
encoder	B-General_Concept
has	O
no	O
way	O
of	O
knowing	O
what	O
parts	O
of	O
the	O
audio	O
or	O
video	O
need	O
the	O
highest	O
bitrates	O
to	O
be	O
encoded	O
.	O
</s>
