<s>
Constrained	B-Algorithm
Energy	I-Algorithm
Lapped	I-Algorithm
Transform	I-Algorithm
(	O
CELT	B-Algorithm
)	O
is	O
an	O
open	O
,	O
royalty-free	O
lossy	B-Algorithm
audio	O
compression	O
format	O
and	O
a	O
free	B-Application
software	I-Application
codec	B-Algorithm
with	O
especially	O
low	O
algorithmic	O
delay	O
for	O
use	O
in	O
low-latency	O
audio	O
communication	O
.	O
</s>
<s>
Development	O
of	O
the	O
format	O
was	O
maintained	O
by	O
the	O
Xiph.Org	B-Application
Foundation	I-Application
(	O
as	O
part	O
of	O
the	O
Ogg	O
codec	B-Algorithm
family	O
)	O
and	O
later	O
coordinated	O
by	O
the	O
Opus	B-Algorithm
working	O
group	O
of	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
.	O
</s>
<s>
CELT	B-Algorithm
was	O
meant	O
to	O
bridge	O
the	O
gap	O
between	O
Vorbis	B-Algorithm
and	O
Speex	B-Application
for	O
applications	O
where	O
both	O
high	O
quality	O
audio	O
and	O
low	O
delay	O
are	O
desired	O
.	O
</s>
<s>
It	O
borrows	O
ideas	O
from	O
the	O
CELP	B-Algorithm
algorithm	O
,	O
but	O
avoids	O
some	O
of	O
its	O
limitations	O
by	O
operating	O
in	O
the	O
frequency	O
domain	O
exclusively	O
.	O
</s>
<s>
The	O
original	O
stand-alone	O
CELT	B-Algorithm
has	O
been	O
merged	O
into	O
Opus	B-Algorithm
.	O
</s>
<s>
Therefore	O
,	O
CELT	B-Algorithm
as	O
stand-alone	O
format	O
is	O
now	O
abandoned	O
and	O
obsolete	O
.	O
</s>
<s>
Development	O
is	O
going	O
on	O
only	O
for	O
its	O
hybridised	O
form	O
as	O
a	O
layer	O
of	O
Opus	B-Algorithm
,	O
integrated	O
with	O
SILK	O
.	O
</s>
<s>
This	O
article	O
covers	O
the	O
historic	O
,	O
stand-alone	O
format	O
;	O
for	O
the	O
integrated	O
form	O
and	O
its	O
evolution	O
since	O
the	O
integration	O
into	O
Opus	B-Algorithm
see	O
the	O
article	O
on	O
Opus	B-Algorithm
.	O
</s>
<s>
CELT	B-Algorithm
's	O
central	O
feature	O
is	O
low	O
algorithmic	O
delay	O
.	O
</s>
<s>
CELT	B-Algorithm
supports	O
mono	O
and	O
stereo	O
audio	O
and	O
is	O
applicable	O
to	O
both	O
speech	O
and	O
music	O
.	O
</s>
<s>
There	O
are	O
no	O
known	O
intellectual	O
property	O
issues	O
pertaining	O
to	O
the	O
CELT	B-Algorithm
algorithm	O
,	O
and	O
its	O
reference	O
implementation	O
is	O
published	O
under	O
a	O
permissive	O
open-source	O
license	O
(	O
the	O
2-clause	B-Operating_System
BSD	I-Operating_System
)	O
.	O
</s>
<s>
Like	O
Vorbis	B-Algorithm
,	O
CELT	B-Algorithm
is	O
a	O
fullband	O
(	O
entire	O
human	O
hearing	O
range	O
)	O
general-purpose	O
codec	B-Algorithm
,	O
i.e.	O
</s>
<s>
not	O
specialized	O
for	O
special	O
types	O
of	O
audio	O
signals	O
and	O
therefore	O
different	O
from	O
its	O
sibling	O
project	O
Speex	B-Application
.	O
</s>
<s>
The	O
format	O
enables	O
for	O
transparent	B-Algorithm
results	O
at	O
high	O
bitrates	O
,	O
as	O
well	O
as	O
very	O
decent	O
quality	O
at	O
lower	O
bitrates	O
.	O
</s>
<s>
All	O
in	O
all	O
,	O
the	O
compression	O
capabilities	O
are	O
said	O
to	O
be	O
significantly	O
superior	O
to	O
those	O
of	O
MP3	B-Application
,	O
and	O
as	O
another	O
useful	O
feature	O
for	O
realtime	O
applications	O
like	O
telephony	O
,	O
CELT	B-Algorithm
's	O
audio	O
quality	O
at	O
lower	O
bitrates	O
are	O
even	O
on	O
par	O
with	O
HE-AACv1	O
,	O
thanks	O
to	O
the	O
band	O
folding	O
.	O
</s>
<s>
It	O
has	O
a	O
comparably	O
low	O
computational	O
complexity	O
that	O
resembles	O
that	O
of	O
the	O
low-delay	O
variant	O
of	O
AAC	B-Application
(	O
AAC-LD	O
)	O
and	O
stays	O
significantly	O
below	O
the	O
complexity	O
of	O
Vorbis	B-Algorithm
.	O
</s>
<s>
It	O
enables	O
for	O
constant	B-General_Concept
and	O
variable	B-General_Concept
bitrate	I-General_Concept
.	O
</s>
<s>
Loss	O
of	O
whole	O
packets	O
as	O
well	O
as	O
bit	O
errors	O
can	O
be	O
masked	O
with	O
a	O
steady	O
degradation	O
of	O
audio	O
quality	O
(	O
packet	B-Protocol
loss	I-Protocol
concealment	I-Protocol
,	O
PLC	O
)	O
.	O
</s>
<s>
CELT	B-Algorithm
is	O
a	O
transform	B-Algorithm
codec	I-Algorithm
based	O
on	O
the	O
modified	O
discrete	O
cosine	O
transform	O
(	O
MDCT	O
)	O
and	O
concepts	O
from	O
CELP	B-Algorithm
(	O
with	O
a	O
code	O
book	O
for	O
excitation	O
,	O
but	O
in	O
the	O
frequency	O
domain	O
)	O
.	O
</s>
<s>
The	O
initial	O
PCM-coded	B-Algorithm
signal	O
is	O
handled	O
in	O
relatively	O
small	O
,	O
overlapping	O
blocks	O
for	O
the	O
MDCT	O
(	O
window	B-Language
function	I-Language
)	O
and	O
transformed	O
to	O
frequency	O
coefficients	O
.	O
</s>
<s>
The	O
coefficients	O
are	O
grouped	O
to	O
resemble	O
the	O
critical	B-Application
bands	I-Application
of	O
the	O
human	O
auditory	O
system	O
.	O
</s>
<s>
The	O
entire	O
amount	O
of	O
energy	O
of	O
each	O
group	O
is	O
analysed	O
and	O
the	O
values	O
quantised	B-Algorithm
for	O
data	B-General_Concept
reduction	I-General_Concept
and	O
compressed	O
through	O
prediction	O
by	O
only	O
transmitting	O
the	O
difference	O
to	O
the	O
predicted	O
values	O
(	O
delta	B-Algorithm
encoding	I-Algorithm
)	O
.	O
</s>
<s>
The	O
coefficients	O
of	O
the	O
resulting	O
residual	O
signal	O
(	O
so-called	O
“	O
band	O
shape	O
”	O
)	O
are	O
coded	O
by	O
Pyramid	B-Algorithm
Vector	I-Algorithm
Quantisation	I-Algorithm
(	O
PVQ	O
,	O
a	O
spherical	O
vector	B-Algorithm
quantisation	I-Algorithm
)	O
.	O
</s>
<s>
This	O
encoding	O
leads	O
to	O
code	O
words	O
of	O
fixed	O
(	O
predictable	O
)	O
length	O
,	O
which	O
in	O
turn	O
enables	O
for	O
robustness	O
against	O
bit	O
errors	O
and	O
leaves	O
no	O
need	O
for	O
entropy	B-Algorithm
encoding	I-Algorithm
.	O
</s>
<s>
Finally	O
,	O
all	O
output	O
of	O
the	O
encoder	O
are	O
coded	O
to	O
one	O
bitstream	O
by	O
a	O
range	B-Algorithm
encoder	I-Algorithm
.	O
</s>
<s>
In	O
connection	O
with	O
the	O
PVQ	O
,	O
CELT	B-Algorithm
uses	O
a	O
technique	O
known	O
as	O
band	O
folding	O
,	O
which	O
delivers	O
a	O
similar	O
effect	O
to	O
spectral	B-Algorithm
band	I-Algorithm
replication	I-Algorithm
(	O
SBR	O
)	O
by	O
reusing	O
coefficients	O
of	O
lower	O
bands	O
for	O
higher	O
ones	O
,	O
but	O
has	O
much	O
less	O
impact	O
on	O
the	O
algorithmic	O
delay	O
and	O
computational	O
complexity	O
than	O
the	O
SBR	O
.	O
</s>
<s>
This	O
works	O
against	O
“	O
birdie	O
”	O
artifacts	B-Algorithm
by	O
preserving	O
more	O
richness	O
in	O
the	O
appropriate	O
frequency	O
bands	O
.	O
</s>
<s>
The	O
decoder	O
unpacks	O
the	O
individual	O
components	O
from	O
the	O
range	O
coded	O
bitstream	O
,	O
multiplies	O
the	O
band	O
energy	O
to	O
the	O
band	O
shape	O
coefficients	O
and	O
transforms	O
them	O
back	O
(	O
via	O
iMDCT	O
)	O
to	O
PCM	B-Algorithm
data	O
.	O
</s>
<s>
The	O
individual	O
blocks	O
are	O
rejoined	O
using	O
weighted	O
overlap-add	B-Algorithm
(	O
WOLA	O
)	O
.	O
</s>
<s>
For	O
the	O
channel	O
coupling	O
CELT	B-Algorithm
may	O
use	O
M/S	O
stereo	O
or	O
intensity	O
stereo	O
.	O
</s>
<s>
Blocks	O
can	O
be	O
described	O
independent	O
from	O
adjacent	O
frames	O
(	O
Intra-frame	B-General_Concept
)	O
;	O
for	O
example	O
to	O
enable	O
a	O
decoder	O
to	O
jump	O
into	O
a	O
running	O
stream	O
.	O
</s>
<s>
With	O
transform	O
codecs	O
so-called	O
pre-echo	O
artifacts	B-Algorithm
can	O
get	O
audible	O
,	O
because	O
the	O
quantisation	B-Algorithm
error	O
of	O
sharp	O
,	O
energy-heavy	O
sounds	O
(	O
transients	O
)	O
can	O
spread	O
over	O
the	O
entire	O
DCT	O
block	O
and	O
the	O
transient	O
does	O
n't	O
mask	O
them	O
backward	O
in	O
time	O
as	O
well	O
as	O
forward	O
.	O
</s>
<s>
With	O
CELT	B-Algorithm
each	O
block	O
can	O
be	O
further	O
divided	O
to	O
thwart	O
such	O
artifacts	B-Algorithm
.	O
</s>
<s>
First	O
work	O
on	O
plans	O
and	O
drafts	O
for	O
a	O
Vorbis	B-Algorithm
successor	O
was	O
done	O
in	O
2005	O
at	O
Xiph.org	B-Application
as	O
part	O
of	O
the	O
Ghost	O
project	O
(	O
initially	O
talked	O
about	O
as	O
“	O
Vorbis	B-Algorithm
II	O
”	O
)	O
.	O
</s>
<s>
This	O
discussion	O
together	O
with	O
Vorbis	B-Algorithm
creator	O
Christopher	O
Montgomery	O
led	O
to	O
Jean-Marc	O
Valin	O
′s	O
interest	O
in	O
a	O
particularly	O
low-latency	O
codec	B-Algorithm
.	O
</s>
<s>
Valin	O
has	O
worked	O
on	O
CELT	B-Algorithm
since	O
2007	O
.	O
</s>
<s>
In	O
December	O
2007	O
,	O
the	O
first	O
draft	O
version	O
of	O
libcelt	B-Algorithm
was	O
published	O
as	O
version	O
0.0.1	O
,	O
initially	O
named	O
“	O
Code-Excited	O
Lapped	O
Transform	O
”	O
.	O
</s>
<s>
CELT	B-Algorithm
was	O
established	O
as	O
an	O
IETF	O
technology	O
in	O
July	O
2009	O
under	O
the	O
"	O
ietfcodec	O
"	O
working	O
group	O
.	O
</s>
<s>
In	O
May	O
2009	O
,	O
a	O
draft	O
of	O
RTP	B-Protocol
payload	O
format	O
for	O
the	O
CELT	B-Algorithm
Codec	B-Algorithm
was	O
published	O
.	O
</s>
<s>
With	O
CELT	B-Algorithm
0.11	O
from	O
February	O
4	O
,	O
2011	O
the	O
format	O
was	O
tentatively	O
frozen	O
(	O
“	O
soft	O
freeze	O
”	O
)	O
–	O
reserving	O
the	O
possibility	O
of	O
unexpectedly	O
necessary	O
last	O
changes	O
.	O
</s>
<s>
Shortly	O
after	O
the	O
advent	O
of	O
the	O
CELT/SILK	O
hybrid	O
codec	B-Algorithm
Opus	B-Algorithm
(	O
formerly	O
known	O
as	O
Harmony	O
)	O
,	O
the	O
development	O
of	O
CELT	B-Algorithm
as	O
a	O
separate	O
project	O
was	O
halted	O
,	O
instead	O
living	O
on	O
the	O
basis	O
of	O
Opus	B-Algorithm
,	O
which	O
aims	O
to	O
treat	O
the	O
lower	O
part	O
of	O
the	O
spectral	O
range	O
in	O
the	O
time	O
domain	O
with	O
linear	O
prediction	O
(	O
SILK	O
)	O
and	O
the	O
higher	O
part	O
in	O
the	O
frequency	O
domain	O
with	O
the	O
MDCT	O
.	O
</s>
<s>
The	O
draft	O
for	O
Opus	B-Algorithm
has	O
been	O
registered	O
at	O
the	O
IETF	O
since	O
September	O
2010	O
.	O
</s>
<s>
The	O
software	B-Library
library	I-Library
libcelt	B-Algorithm
serves	O
as	O
the	O
reference	O
implementation	O
for	O
CELT	B-Algorithm
,	O
written	O
in	O
C	B-Language
and	O
published	O
as	O
free	B-Application
software	I-Application
under	O
Xiph	B-Application
's	O
own	O
3-clause	O
BSD-ish	O
license	O
.	O
</s>
<s>
Despite	O
the	O
format	O
not	O
being	O
finally	O
frozen	O
,	O
it	O
was	O
being	O
used	O
in	O
many	O
VoIP	B-Application
applications	O
such	O
as	O
Ekiga	B-Language
and	O
FreeSWITCH	B-Language
,	O
which	O
switched	O
to	O
CELT	B-Algorithm
upon	O
entering	O
soft-freeze	O
in	O
January	O
2009	O
,	O
as	O
well	O
as	O
Mumble	B-Language
,	O
TeamSpeak	B-Language
and	O
other	O
software	O
.	O
</s>
<s>
In	O
April	O
2011	O
,	O
support	O
for	O
CELT	B-Algorithm
was	O
included	O
in	O
FFmpeg	B-Library
.	O
</s>
<s>
CELT	B-Algorithm
is	O
also	O
supported	O
or	O
used	O
by	O
:	O
</s>
