<s>
AAC	O
Low	O
Delay	O
,	O
or	O
AAC-LD	B-Algorithm
)	O
is	O
audio	O
compression	O
standard	O
designed	O
to	O
combine	O
the	O
advantages	O
of	O
perceptual	O
audio	O
coding	O
with	O
the	O
low	O
delay	O
necessary	O
for	O
two-way	O
communication	O
.	O
</s>
<s>
It	O
is	O
closely	O
derived	O
from	O
the	O
MPEG-2	O
Advanced	B-Application
Audio	I-Application
Coding	I-Application
(	O
AAC	O
)	O
standard	O
.	O
</s>
<s>
It	O
was	O
published	O
in	O
MPEG-4	B-Algorithm
Audio	I-Algorithm
Version	O
2	O
(	O
ISO/IEC	O
14496-3:1999/Amd	O
1:2000	O
)	O
and	O
in	O
its	O
later	O
revisions	O
.	O
</s>
<s>
AAC-LD	B-Algorithm
uses	O
a	O
version	O
of	O
the	O
modified	O
discrete	O
cosine	O
transform	O
(	O
MDCT	O
)	O
audio	O
coding	O
technique	O
called	O
the	O
LD-MDCT	O
.	O
</s>
<s>
AAC-LD	B-Algorithm
is	O
widely	O
used	O
by	O
Apple	O
as	O
the	O
voice-over-IP	B-Application
(	O
VoIP	B-Application
)	O
speech	B-Application
codec	I-Application
in	O
FaceTime	B-Operating_System
.	O
</s>
<s>
The	O
AAC-LD	B-Algorithm
coding	O
scheme	O
bridges	O
the	O
gap	O
between	O
speech	B-Application
coding	I-Application
schemes	O
and	O
high	O
quality	O
audio	O
coding	O
schemes	O
.	O
</s>
<s>
Two-way	O
communication	O
with	O
AAC-LD	B-Algorithm
is	O
possible	O
on	O
usual	O
analog	O
telephone	O
lines	O
and	O
via	O
ISDN	O
connections	O
.	O
</s>
<s>
Compared	O
to	O
known	O
speech	B-Application
coders	I-Application
,	O
the	O
codec	O
is	O
capable	O
of	O
coding	O
both	O
music	O
and	O
speech	O
signals	O
with	O
good	O
quality	O
.	O
</s>
<s>
Unlike	O
speech	B-Application
coders	I-Application
,	O
however	O
,	O
the	O
achieved	O
coding	O
quality	O
scales	O
up	O
with	O
bitrate	O
.	O
</s>
<s>
AAC-LD	B-Algorithm
can	O
also	O
process	O
stereo	O
signals	O
by	O
using	O
the	O
advanced	O
stereo	O
coding	O
tools	O
of	O
AAC	O
.	O
</s>
